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101. Mesosomes in bacteria are best described as:
ⓐ. True respiratory organelles in bacteria
ⓑ. Golgi-like secretion stacks in bacteria
ⓒ. Nucleus-bound pore complexes in bacteria
ⓓ. Fixation-related membrane infoldings
Correct Answer: Fixation-related membrane infoldings
Explanation: Mesosomes were historically described as inward foldings of the bacterial plasma membrane, but strong evidence indicates that many “mesosome-like” structures arise during chemical fixation used for electron microscopy. Because they are not consistently observed under near-native preservation methods, they are not treated as stable, normal organelles like eukaryotic structures. Bacteria also lack Golgi bodies and nuclear envelope pore complexes, so those descriptions are not valid. The most scientifically accurate modern description is that mesosomes are commonly fixation-related membrane infoldings seen in prepared micrographs. Hence, fixation-related membrane infoldings is the best description.
102. A commonly stated functional association of mesosomes is:
ⓐ. Protein translation on 80S ribosomes
ⓑ. Photosynthesis in thylakoid grana only
ⓒ. Respiration enzyme attachment sites
ⓓ. mRNA splicing inside a true nucleus
Correct Answer: Respiration enzyme attachment sites
Explanation: In older descriptions of prokaryotic cells, mesosomes were linked with respiration because bacterial respiratory enzymes are located on the plasma membrane. The logic was that membrane infoldings could increase membrane surface area and therefore appear to provide more sites for respiratory enzyme complexes. While modern interpretation emphasizes that mesosome-like structures are often preparation artifacts, this respiration association is still commonly stated as a traditional functional link in exam-oriented summaries. The other options describe eukaryotic-only processes or plant chloroplast structures, which do not apply to bacteria. Therefore, the commonly stated association is respiration enzyme attachment sites.
103. Mesosomes are derived as an invagination of the:
ⓐ. Capsule (glycocalyx) outer layer
ⓑ. Peptidoglycan cell wall layer
ⓒ. Outer membrane of Gram-negative
ⓓ. Plasma membrane layer
Correct Answer: Plasma membrane layer
Explanation: Mesosome-like structures, as described in classic bacterial cell studies, appear as inward foldings continuous with the plasma membrane. They are not formed from the capsule, which is an external coating, and they are not invaginations of the peptidoglycan wall. The outer membrane in Gram-negative bacteria is a separate layer, but the traditional mesosome description specifically concerns plasma-membrane-associated invaginations. Even when interpreted as fixation-related artifacts, the origin of the observed folds is still the plasma membrane. Therefore, the correct layer of origin is the plasma membrane layer.
104. Mesosomes have been linked to bacterial cell division mainly via:
ⓐ. Chloroplast stroma reactions only
ⓑ. Septum formation during division
ⓒ. Nuclear pore transport control only
ⓓ. Ribosome assembly in nucleolus only
Correct Answer: Septum formation during division
Explanation: In traditional descriptions, mesosomes were proposed to assist in bacterial cell division by participating in septum formation and membrane organization at the division site. The idea was that membrane foldings could help coordinate wall and membrane synthesis where the cell pinches inward. Modern interpretation treats mesosomes as largely method-dependent appearances during sample preparation, so these “functions” are best understood as historical associations rather than proven organelle roles. However, when asked about the commonly linked division process, septum formation is the classic association. Hence, septum formation during division is the correct linked process.
105. A bacterium performing aerobic respiration without mitochondria may rely on:
ⓐ. Mesosomal membrane infoldings
ⓑ. Golgi cisternae stacks in cytosol
ⓒ. Endoplasmic reticulum network sheets
ⓓ. Nuclear envelope folds with pores
Correct Answer: Mesosomal membrane infoldings
Explanation: Since bacteria lack mitochondria, the plasma membrane is the main site for respiratory electron transport and ATP generation in aerobic conditions. Mesosomal infoldings are traditionally cited as membrane extensions that increase the available surface for these membrane-associated enzyme systems. This provides a structural explanation for how high rates of respiration can be supported in a small prokaryotic cell. In contrast, Golgi, ER, and a nuclear envelope are membrane-bound eukaryotic structures that bacteria do not possess. Therefore, mesosomal membrane infoldings best match the prokaryotic strategy for aerobic energy processes.
106. Mesosome-like structures are reported mainly in electron micrographs of:
ⓐ. Mammalian red blood cells only here
ⓑ. Plant sieve tubes with nuclei here
ⓒ. Chemically fixed bacterial cells
ⓓ. Fungal hyphae with septa present
Correct Answer: Chemically fixed bacterial cells
Explanation: Mesosome-like structures are most commonly described from electron micrographs of bacteria prepared using chemical fixation, where membrane distortions can occur. They are not characteristic structures of mammalian red blood cells, plant sieve tube elements, or fungal hyphae. The key scientific point is that the appearance of mesosomes is strongly dependent on preparation methods and is typically associated with chemically fixed bacterial samples. This helps students avoid treating mesosomes as universal organelles or as normal eukaryotic structures. Therefore, chemically fixed bacterial cells is the correct context.
107. A suggested role of mesosomes related to the nucleoid is that it:
ⓐ. Creates nucleolus for rRNA synthesis
ⓑ. Helps DNA segregation at division
ⓒ. Builds spindle microtubules for mitosis
ⓓ. Packages proteins in Golgi cisternae
Correct Answer: Helps DNA segregation at division
Explanation: Classical explanations proposed that mesosomes could serve as membrane attachment points for the nucleoid and assist in DNA replication or segregation during bacterial division. This idea was used to explain how bacteria, lacking a true nucleus and mitotic spindle, could organize genetic material during cell division. Modern evidence indicates mesosome-like structures are often artifacts of preparation, so the “role” should be understood as a traditional proposal rather than a confirmed mechanism. Still, the commonly stated nucleoid-related association is help in DNA segregation at division. Hence, helps DNA segregation at division is the best answer.
108. Which statement is accurate about mesosomes in many electron micrographs?
ⓐ. Always present in all living bacteria
ⓑ. Formed only from bacterial cell walls
ⓒ. Equivalent to mitochondria in structure
ⓓ. Often seen as fixation artifacts
Correct Answer: Often seen as fixation artifacts
Explanation: Many observations of mesosome-like structures became prominent in samples prepared using chemical fixation, leading to the view that these formations can arise as preparation artifacts rather than stable, universal cell structures. This is why some modern interpretations treat mesosomes cautiously, especially when comparing different preparation methods. The key accurate point is not that they are identical to mitochondria or that they originate from the cell wall, but that their appearance is strongly influenced by fixation conditions. Recognizing this helps students interpret micrographs critically and avoid overextending functional claims. Hence, “often seen as fixation artifacts” is accurate.
109. Mesosomes were linked to respiration mainly because:
ⓐ. They create a nuclear envelope around DNA
ⓑ. They replace ribosomes in protein synthesis
ⓒ. They increase membrane surface area
ⓓ. They store starch granules for energy reserve
Correct Answer: They increase membrane surface area
Explanation: Bacterial respiration occurs through enzyme complexes embedded in the plasma membrane rather than in mitochondria. Traditional descriptions linked mesosomes to respiration because membrane infoldings would appear to increase the membrane surface area available for respiratory enzymes. This “surface area” logic is the classic reason given in exam-style explanations for the respiration association. Modern interpretation emphasizes that mesosome appearance is often method-dependent during sample preparation, but the reasoning behind the older linkage remains based on increased surface area. Therefore, the best answer is that they increase membrane surface area.
110. A student says, “Mesosomes are bacterial mitochondria.” The best correction is:
ⓐ. Fixation artifact in micrographs
ⓑ. It is a chloroplast-like plastid
ⓒ. It is a nucleus-bound organelle only
ⓓ. It is a Golgi-derived vesicle stack
Correct Answer: Fixation artifact in micrographs
Explanation: Bacteria do not have mitochondria; their respiratory enzymes are located on the plasma membrane. The structures called mesosomes were once described as membrane infoldings and were compared functionally to mitochondria in older summaries, but strong evidence shows many mesosome-like forms arise during chemical fixation for electron microscopy. Therefore, calling them “bacterial mitochondria” is misleading because it suggests a true organelle equivalent. The most accurate correction is that mesosomes are commonly fixation-related artifacts seen in micrographs rather than mitochondria-like organelles. Hence, fixation artifact in micrographs is the best correction.
111. In photosynthetic bacteria, chromatophores are best described as:
ⓐ. A pigment layer outside cell wall
ⓑ. Chloroplast-like organelles with DNA
ⓒ. Membrane folds with pigments
ⓓ. A nucleus-like region for rRNA
Correct Answer: Membrane folds with pigments
Explanation: Chromatophores are specialized intracytoplasmic membrane structures in photosynthetic bacteria that contain photosynthetic pigments and the associated electron transport components. They are formed as infoldings or vesicular extensions of the plasma membrane, increasing the membrane surface available for light-driven reactions. These membranes house reaction centers and carrier molecules that capture light energy and convert it into chemical energy through electron flow. Because bacteria lack chloroplasts, chromatophores function as the primary site for bacterial photosynthetic processes. Their membrane nature is central, since photophosphorylation depends on membrane-associated electron transport and proton gradients. Therefore, chromatophores are correctly identified as membrane folds containing pigments.
112. The main photosynthetic pigments commonly associated with chromatophores in many photosynthetic bacteria are:
ⓐ. Bacteriochlorophyll and carotenoids
ⓑ. Chlorophyll a and phycobilins only
ⓒ. Xanthophylls with anthocyanins only
ⓓ. Hemoglobin with cytochrome c only
Correct Answer: Bacteriochlorophyll and carotenoids
Explanation: Many photosynthetic bacteria contain bacteriochlorophyll as the principal light-harvesting pigment, often accompanied by carotenoids that broaden light absorption and provide photoprotection. These pigments are embedded in chromatophore membranes, allowing efficient capture of light energy for the bacterial photosystems. Carotenoids help prevent photooxidative damage by quenching reactive excited states and can also contribute to characteristic bacterial coloration. This pigment set differs from plant chlorophyll systems that rely heavily on chlorophyll a and accessory pigments in chloroplasts. The pairing of bacteriochlorophyll with carotenoids is therefore a standard hallmark of chromatophore-based bacterial photosynthesis. Hence, bacteriochlorophyll and carotenoids is the correct combination.
113. Chromatophores are commonly found in:
ⓐ. Typical non-photosynthetic bacteria only
ⓑ. Most animal cells as surface coats
ⓒ. Yeast cells as respiratory membranes
ⓓ. Purple photosynthetic bacteria
Correct Answer: Purple photosynthetic bacteria
Explanation: Chromatophores are classically described in purple photosynthetic bacteria, where they appear as vesicles or membrane infoldings containing the photosynthetic machinery. These bacteria use light energy with bacteriochlorophyll-containing systems embedded in such membranes. The chromatophore structures provide large membrane area needed for light-driven electron transport and ATP generation. This arrangement substitutes for chloroplasts, which prokaryotes do not possess. Because purple bacteria are a standard example of photosynthetic prokaryotes, chromatophores are strongly associated with them in basic cell biology. Therefore, purple photosynthetic bacteria is the correct association.
114. A key functional role of chromatophores in photosynthetic bacteria is:
ⓐ. Protein splicing within a nuclear envelope
ⓑ. Light-driven ATP formation
ⓒ. Packaging secretory proteins into vesicles
ⓓ. Building a cellulose wall around cytoplasm
Correct Answer: Light-driven ATP formation
Explanation: Chromatophores host the light-harvesting complexes, reaction centers, and electron carriers required for photophosphorylation in photosynthetic bacteria. When light excites the pigments, electrons flow through membrane-bound carriers, generating a proton gradient across the chromatophore membrane. ATP synthase uses this gradient to produce ATP, making the chromatophore the major energy-conversion platform in these cells. This role is conceptually similar to thylakoid membranes in chloroplasts, but chromatophores occur in prokaryotes without true organelles. The critical point is that the process depends on a membrane system capable of electron transport and gradient formation. Hence, chromatophores are central to light-driven ATP formation.
115. Compared with chloroplasts, chromatophores are best characterized as:
ⓐ. Membrane systems without organelle envelope
ⓑ. Double-membraned organelles with stroma
ⓒ. Nuclear compartments with DNA pores
ⓓ. Cell wall layers rich in cellulose fibers
Correct Answer: Membrane systems without organelle envelope
Explanation: Chromatophores are not independent organelles; they are internal membrane systems derived from the plasma membrane in photosynthetic bacteria. They lack a surrounding organelle envelope and do not have the compartment organization typical of chloroplasts in eukaryotes. Their function is achieved by embedding pigments and electron transport components directly in these membranes, enabling light capture and ATP generation. This reflects a prokaryotic strategy where key bioenergetic processes occur on membrane surfaces rather than within membrane-bound organelles. The distinction is important for classifying cells and understanding why bacteria do not have plastids. Therefore, chromatophores are best described as membrane systems lacking an organelle envelope.
116. Chromatophores are most accurately considered an example of:
ⓐ. Nucleus-derived membrane folding
ⓑ. Cell wall thickening for rigidity
ⓒ. Glycocalyx specialization for adhesion
ⓓ. Plasma membrane infoldings
Correct Answer: Plasma membrane infoldings
Explanation: In photosynthetic bacteria, chromatophores are described as infoldings or internal extensions of the plasma membrane that increase internal membrane surface area. This expansion supports the dense arrangement of pigments, reaction centers, and electron carriers needed for photosynthesis. Because the plasma membrane is the main site for many metabolic pathways in prokaryotes, modifying it into internal photosynthetic membranes is a logical structural adaptation. These structures remain fundamentally membrane-based rather than being separate compartments like eukaryotic organelles. Their formation directly illustrates how prokaryotes achieve functional specialization without membrane-bound organelles. Hence, chromatophores are best viewed as plasma membrane infoldings.
117. In cyanobacteria, the photosynthetic membranes are typically organized as:
ⓐ. Chromatophores as vesicles only
ⓑ. Cristae-like folds inside mitochondria
ⓒ. Thylakoid membranes in cytoplasm
ⓓ. Golgi stacks with pigment granules
Correct Answer: Thylakoid membranes in cytoplasm
Explanation: Cyanobacteria are photosynthetic prokaryotes, but their photosynthetic apparatus is typically arranged on thylakoid-like internal membranes rather than chromatophore vesicles. These membranes carry pigments and electron transport components necessary for light reactions, enabling ATP and reducing power generation. This organization supports efficient photosynthesis while still lacking a true chloroplast organelle. The internal thylakoid system is a key identifying feature of cyanobacteria in standard biology descriptions. It also helps distinguish cyanobacteria from purple bacteria where chromatophores are commonly emphasized. Therefore, thylakoid membranes in the cytoplasm best describes cyanobacterial photosynthetic membranes.
118. Which statement best links chromatophores to prokaryotic cell organization?
ⓐ. They prove prokaryotes have chloroplast organelles
ⓑ. They enable photosynthesis without chloroplasts
ⓒ. They replace the nucleoid in DNA functions
ⓓ. They form nuclear pores for RNA traffic
Correct Answer: They enable photosynthesis without chloroplasts
Explanation: Prokaryotes do not possess chloroplasts, yet some perform photosynthesis using specialized internal membrane systems. Chromatophores provide a membrane platform where pigments, reaction centers, and electron transport chains can be assembled to capture light and generate ATP. This reflects a core prokaryotic theme: metabolic specialization is achieved through membrane modifications rather than discrete membrane-bound organelles. The presence of chromatophores therefore explains how photosynthetic bacteria can conduct light-driven energy conversion while still being prokaryotic. It also reinforces the idea that internal compartment-like function can exist without true organelles. Hence, chromatophores enable photosynthesis without chloroplasts.
119. The reaction centers and electron carriers in many photosynthetic bacteria are mainly located on:
ⓐ. Nuclear envelope membranes
ⓑ. Outer cell wall surface layers
ⓒ. Cytosolic protein granules only
ⓓ. Chromatophore membranes
Correct Answer: Chromatophore membranes
Explanation: Photosynthetic electron transport requires an organized membrane to hold reaction centers, carrier molecules, and ATP synthase in a functional sequence. In photosynthetic bacteria, chromatophore membranes provide this organization, enabling light excitation to drive electron flow through membrane-bound carriers. The resulting proton gradient forms across the chromatophore membrane and powers ATP synthesis, making membrane localization essential. This arrangement is efficient because it concentrates key components and maintains spatial separation of charges across the membrane. Without such membrane placement, photophosphorylation would not proceed effectively. Therefore, reaction centers and electron carriers are mainly located on chromatophore membranes.
120. A bacterium contains bacteriochlorophyll-bearing internal membranes but has no nucleus. The best identification is:
ⓐ. Photosynthetic bacterium with chromatophores
ⓑ. Plant cell with a true chloroplast and nucleus
ⓒ. Animal cell with mitochondria and ER
ⓓ. Fungal cell with chitin wall and nucleus
Correct Answer: Photosynthetic bacterium with chromatophores
Explanation: The absence of a nucleus indicates a prokaryotic cell, and the presence of bacteriochlorophyll-bearing internal membranes strongly suggests a photosynthetic bacterium rather than a plant or fungus. In such bacteria, photosynthetic pigments and electron transport components are embedded in internal membrane systems commonly described as chromatophores. These membranes allow light-driven energy conversion without chloroplast organelles, which are eukaryotic structures. The combination of prokaryotic organization (no true nucleus) and bacterial photosynthetic pigments points directly to chromatophore-based photosynthesis. This diagnosis fits standard cell biology classification criteria used in exams. Therefore, the best identification is a photosynthetic bacterium with chromatophores.
121. The “mosaic” part of the fluid mosaic model mainly refers to:
ⓐ. Patchwork of proteins
ⓑ. Uniform lipid layer only
ⓒ. Identical pores in rows
ⓓ. Cell wall plus membrane layers
Correct Answer: Patchwork of proteins
Explanation: The membrane is called a “mosaic” because different proteins are irregularly arranged within or on the lipid bilayer, creating a patchwork-like pattern rather than a repeating rigid design. These proteins include channels, carriers, receptors, and enzymes, each contributing specific functions at different membrane locations. Their uneven distribution helps cells localize transport and signaling to particular regions of the surface. The mosaic idea also implies diversity in protein types and orientations, not a single repeated unit. This explains how the same lipid framework can support many different cellular roles. Therefore, the “mosaic” primarily highlights the patchwork of proteins.
122. The “fluid” part of the fluid mosaic model mainly emphasizes:
ⓐ. Proteins moving across membranes freely
ⓑ. Membrane dissolves in water easily
ⓒ. Cell wall fluidity in bacteria
ⓓ. Lateral movement in bilayer
Correct Answer: Lateral movement in bilayer
Explanation: “Fluid” refers to the ability of membrane lipids (and many proteins) to move sideways within the plane of the bilayer, making the membrane dynamic rather than rigid. This lateral mobility supports processes such as membrane fusion, endocytosis, and redistribution of receptors during signaling. Fluidity also helps membranes self-seal after small disruptions and allows flexible shape changes. It depends on lipid composition, temperature, and cholesterol content, so it can be regulated by the cell. The membrane is not freely soluble in water; it remains a stable bilayer while still allowing movement within it. Hence, fluidity mainly means lateral movement within the bilayer.
123. Which observation best supports the lipid bilayer arrangement predicted by the fluid mosaic model?
ⓐ. Nucleus is separated by a membrane
ⓑ. Cell wall prevents osmotic lysis
ⓒ. Polar heads face water
ⓓ. Ribosomes attach to rough ER
Correct Answer: Polar heads face water
Explanation: Amphipathic phospholipids have hydrophilic (polar) heads and hydrophobic (nonpolar) tails, so they arrange themselves to minimize exposure of tails to water. In a bilayer, polar heads face the aqueous cytosol and extracellular fluid, while tails pack inward away from water, creating a stable barrier. This arrangement directly explains why membranes act as selective boundaries and why nonpolar molecules cross more easily than polar ones. It also supports the idea that the membrane is self-assembling due to lipid properties. The head-facing-water observation is a core physical basis of bilayer structure. Therefore, polar heads facing water strongly supports the bilayer arrangement.
124. In the fluid mosaic model, proteins that span the bilayer are called:
ⓐ. Peripheral proteins
ⓑ. Extracellular matrix proteins
ⓒ. Fibrous wall proteins
ⓓ. Integral proteins
Correct Answer: Integral proteins
Explanation: Integral proteins are embedded in the lipid bilayer, and many extend across it, forming transmembrane proteins. Their hydrophobic regions interact with the lipid tails, anchoring them firmly within the membrane. Because they span the bilayer, these proteins commonly function as channels, carriers, or receptors that communicate across the membrane barrier. This placement fits the model’s view that proteins are not merely attached on the surface but are part of the membrane’s functional architecture. Their distribution contributes to the “mosaic” pattern and supports selective transport and signaling. Hence, proteins that span the bilayer are termed integral proteins.
125. Which point best reflects membrane asymmetry in the fluid mosaic model?
ⓐ. Carbohydrates face outside
ⓑ. Same lipids on both sides
ⓒ. Proteins are absent in lipids
ⓓ. Nucleic acids coat membrane surface
Correct Answer: Carbohydrates face outside
Explanation: Membrane asymmetry means the two sides of the membrane are not identical in composition and orientation. Carbohydrate chains, attached to proteins and lipids, are typically exposed on the outer surface, forming part of the glycocalyx involved in recognition and adhesion. This outward orientation supports cell–cell interactions and immune recognition, since these sugar patterns are “read” externally. Asymmetry also applies to protein orientation and lipid distribution, but the outward-facing carbohydrate coat is a classic, testable feature. This arrangement is maintained by membrane trafficking and enzymes that preserve sidedness. Therefore, carbohydrates facing outside is a key expression of membrane asymmetry.
126. Cholesterol mainly helps animal plasma membranes by:
ⓐ. Making membranes fully rigid
ⓑ. Creating new pores in bilayers
ⓒ. Buffering fluidity changes
ⓓ. Replacing proteins in the mosaic
Correct Answer: Buffering fluidity changes
Explanation: Cholesterol acts as a fluidity regulator by preventing extremes in membrane behavior under different temperatures. At higher temperatures, it restrains excessive movement of lipid tails, reducing over-fluidity and helping the membrane maintain barrier integrity. At lower temperatures, it disrupts tight packing of phospholipids, preventing the membrane from becoming too rigid. This stabilizing effect supports proper function of membrane proteins, transport, and signaling, which depend on a suitable membrane environment. Cholesterol does not replace proteins or create new transport pores by itself; its main role is modulation of bilayer properties. Hence, cholesterol buffers membrane fluidity changes.
127. A hallmark of the fluid mosaic model is that many membrane proteins:
ⓐ. Move laterally
ⓑ. Form cellulose microfibrils
ⓒ. Stay fixed in straight lines
ⓓ. Always face the same direction
Correct Answer: Move laterally
Explanation: The model proposes that membranes are dynamic, allowing many proteins to drift sideways within the bilayer rather than being locked in one position. This lateral movement helps cells cluster receptors, reorganize signaling complexes, and adjust transport capacity where needed. It also supports membrane repair and remodeling because components can redistribute to stabilize disturbed regions. While some proteins are anchored by cytoskeletal or extracellular attachments, many retain significant mobility, consistent with a fluid environment. This mobility is essential for flexible cell responses and is a core distinction from older rigid membrane ideas. Therefore, lateral movement of many membrane proteins is a hallmark of the fluid mosaic model.
128. In freeze-fracture studies, the split often occurs through the:
ⓐ. Cell wall outer coat
ⓑ. Protein–carbohydrate layer
ⓒ. Nuclear envelope pores
ⓓ. Hydrophobic core region
Correct Answer: Hydrophobic core region
Explanation: Freeze-fracture commonly splits the membrane along the plane of the bilayer where hydrophobic lipid tails meet, because this interior region is a weak point compared with the strongly hydrated surfaces. When fractured this way, embedded proteins appear as particles on the exposed faces, supporting the idea that proteins are inserted within the bilayer rather than forming a continuous external coat. The technique therefore provided strong structural support for the mosaic concept, showing a patchy distribution of proteins. It also aligns with the bilayer model, since the fracture plane corresponds to the bilayer’s hydrophobic interior. Hence, the split often occurs through the hydrophobic core region.
129. Which statement best matches selective permeability under the fluid mosaic model?
ⓐ. Ions cross easily through lipids
ⓑ. Sugars diffuse rapidly without help
ⓒ. Nonpolar molecules diffuse faster
ⓓ. All molecules cross at equal rates
Correct Answer: Nonpolar molecules diffuse faster
Explanation: The lipid bilayer’s interior is hydrophobic, so small nonpolar molecules can dissolve into it and diffuse across more readily than polar or charged substances. This property explains why gases and many lipid-soluble molecules cross membranes without specialized transport. In contrast, ions and many polar solutes require specific proteins, such as channels or carriers, because the hydrophobic core resists their passage. The model links permeability to the bilayer’s chemical nature while also assigning transport control to embedded proteins. This dual structure-function view is central to understanding membrane transport. Therefore, nonpolar molecules diffuse faster across the membrane.
130. In the fluid mosaic model, peripheral proteins are typically:
ⓐ. Fully embedded across bilayer
ⓑ. Loosely attached on surface
ⓒ. Covalently linked to DNA strands
ⓓ. Built from cellulose units only
Correct Answer: Loosely attached on surface
Explanation: Peripheral proteins are not deeply embedded in the hydrophobic core; instead, they associate with the membrane surface through interactions with integral proteins or polar head groups. Because they are surface-associated, they often function in signaling, enzymatic activity, or structural support by linking the membrane to cytoskeletal elements. Their attachment is generally weaker than that of integral proteins, so they can be removed more easily by changing ionic conditions. This placement supports the “mosaic” concept by adding different protein classes with distinct membrane relationships. Peripheral proteins contribute to membrane function without spanning the bilayer. Hence, they are typically loosely attached on the surface.
131. Which keyword most strongly indicates passive transport across a membrane?
ⓐ. Down concentration gradient
ⓑ. Against concentration gradient always
ⓒ. Uses ATP-driven pump action directly
ⓓ. Coupled to energy-rich phosphate bonds
Correct Answer: Down concentration gradient
Explanation: Passive transport is defined by movement of substances from a region of higher concentration (or higher electrochemical potential) to a region of lower concentration without direct energy input from the cell. The driving force is the existing gradient itself, so the process proceeds “downhill” until equilibrium is approached. This is why passive movement can occur even in cells with limited energy availability, as long as the gradient exists. It also explains why passive transport cannot accumulate a solute above its equilibrium level. The phrase “down concentration gradient” captures the core idea that the gradient supplies the driving force.
132. Which feature best identifies primary active transport?
ⓐ. Movement through lipid bilayer by diffusion
ⓑ. Movement of water across a semipermeable layer
ⓒ. Solute moves down its gradient without energy
ⓓ. ATP hydrolysis drives a pump
Correct Answer: ATP hydrolysis drives a pump
Explanation: Primary active transport uses energy directly from ATP breakdown to move substances against their concentration or electrochemical gradients. The transporter is typically a pump that changes shape during the cycle, using ATP hydrolysis to power uphill movement. This allows cells to build and maintain steep gradients, which are essential for membrane potential, nutrient uptake, and volume regulation. The key “keyword” is direct ATP use by the transporter itself, rather than using an existing ion gradient. Without ATP hydrolysis, the pump cannot sustain transport against the gradient. Therefore, “ATP hydrolysis drives a pump” is the defining feature.
133. Which keyword set best matches facilitated diffusion?
ⓐ. Requires ATP to move solute uphill
ⓑ. Uses carrier or channel
ⓒ. Vesicles move cargo across the membrane
ⓓ. Splits ATP and exports phosphate group
Correct Answer: Uses carrier or channel
Explanation: Facilitated diffusion is passive transport that still requires a specific membrane protein to help a solute cross the hydrophobic bilayer. The solute moves down its concentration or electrochemical gradient, but the pathway is provided by a channel or carrier protein. This explains why the process is selective and can be regulated by gating or conformational changes in the protein. Because it depends on transporter availability, it can show saturation at high solute levels, unlike simple diffusion through lipids. The key identifier is the presence of a carrier or channel without direct ATP use. Hence, “uses carrier or channel” best matches facilitated diffusion.
134. Which phrase most accurately captures osmosis as a passive process?
ⓐ. Water moves out using ATP energy
ⓑ. Solute pumps create water flow only
ⓒ. Water moves to lower potential
ⓓ. Large particles cross by vesicle movement
Correct Answer: Water moves to lower potential
Explanation: Osmosis is the net movement of water across a selectively permeable membrane driven by differences in water potential (or effective free water concentration). Water moves from the side with higher water potential to the side with lower water potential until equilibrium is approached. This is a passive process because the driving force is the gradient in water potential, not ATP hydrolysis by the cell. In biological membranes, water often crosses through specific channels, but the direction is still determined by the gradient. The concept explains swelling, shrinking, and turgor-related effects in cells. Therefore, “water moves to lower potential” captures osmosis as a passive process.
135. Which keyword most strongly indicates active transport rather than passive transport?
ⓐ. Moves against gradient
ⓑ. Moves down gradient naturally
ⓒ. No energy input is required
ⓓ. Random molecular motion dominates
Correct Answer: Moves against gradient
Explanation: Active transport is distinguished by its ability to move substances “uphill,” from lower concentration to higher concentration, or against an electrochemical gradient. This cannot occur by diffusion alone because diffusion naturally proceeds down a gradient. To push molecules against the gradient, the cell must supply energy either directly from ATP (primary active transport) or indirectly from another gradient (secondary active transport). This feature allows cells to concentrate nutrients, remove wastes, and maintain ionic differences critical for cell function. The phrase “moves against gradient” is therefore the most direct keyword indicator of active transport. Hence, it identifies active transport clearly.
136. Which property is most characteristic of carrier-mediated transport (facilitated or active)?
ⓐ. Rate increases forever with concentration
ⓑ. No specificity for the transported solute
ⓒ. Shows saturation at high solute
ⓓ. Membrane becomes freely permeable to ions
Correct Answer: Shows saturation at high solute
Explanation: Carrier-mediated transport relies on a limited number of transporter proteins, each with binding sites and a finite turnover rate. As solute concentration rises, transport rate increases initially but eventually reaches a maximum because all carriers become occupied and cycle at their top speed. This saturation behavior is a key diagnostic difference from simple diffusion through the lipid bilayer, where rate keeps rising proportionally with concentration difference. Saturation also implies specificity, since carriers bind particular substrates. The concept is important for understanding why adding more substrate does not always increase uptake. Therefore, “shows saturation at high solute” is the characteristic property.
137. Which example is a clear case of primary active transport in animal cells?
ⓐ. Diffusion of oxygen across a membrane
ⓑ. Osmosis of water through a membrane layer
ⓒ. Glucose movement via a passive carrier
ⓓ. Na+/K+ ATPase pump
Correct Answer: Na+/K+ ATPase pump
Explanation: The Na+/K+ ATPase is a classic primary active transport system that uses ATP hydrolysis directly to move ions against their gradients. By exporting sodium ions and importing potassium ions, it maintains ionic gradients that support membrane potential and many secondary transport processes. The defining keyword is “ATPase,” indicating that ATP is split to power transport. This pump does not rely on diffusion; it actively maintains differences that diffusion would otherwise eliminate. Its continuous activity is essential for excitability and overall ionic homeostasis in many cells. Hence, the Na+/K+ ATPase pump is a clear example of primary active transport.
138. Which phrase best defines secondary active transport?
ⓐ. ATP splits at the transporter site
ⓑ. Uses ion gradient energy
ⓒ. Solute crosses without a protein pathway
ⓓ. Water crosses by random diffusion only
Correct Answer: Uses ion gradient energy
Explanation: Secondary active transport moves one solute against its gradient by coupling it to the downhill movement of another solute, typically an ion such as Na⁺ or H⁺. The transporter itself does not directly hydrolyze ATP; instead, it uses the potential energy stored in the ion gradient that was usually created by primary active transport. This is why the process is called “secondary” even though it can still drive uphill movement. The keyword is “coupled” transport driven by an existing gradient. This explains symport and antiport mechanisms seen in nutrient uptake. Therefore, “uses ion gradient energy” best defines secondary active transport.
139. Which factor most directly increases the rate of simple diffusion across a membrane?
ⓐ. Steeper concentration gradient
ⓑ. Greater ATP availability in cytosol
ⓒ. More vesicle formation at the surface
ⓓ. More carrier proteins with ATPase activity
Correct Answer: Steeper concentration gradient
Explanation: Simple diffusion is a passive process driven by the concentration difference between two sides of a membrane. Increasing the gradient increases the net movement because more particles move from high to low concentration per unit time. This does not require ATP, carriers, or vesicles; it depends mainly on the driving force and permeability for that substance. The same principle explains why gases like O₂ and CO₂ move quickly when their gradients are large. As equilibrium is approached, the net rate decreases because the gradient weakens. Hence, a steeper concentration gradient most directly increases simple diffusion.
140. If ATP production suddenly drops in a cell, which change is most expected?
ⓐ. Simple diffusion increases sharply
ⓑ. Osmosis stops in all conditions
ⓒ. Active transport decreases
ⓓ. Lipid bilayer becomes permanently rigid
Correct Answer: Active transport decreases
Explanation: Active transport depends on cellular energy, either directly through ATP hydrolysis (primary active transport) or indirectly through gradients maintained by ATP-driven pumps (secondary active transport). When ATP availability falls, pumps slow down, and uphill movement becomes less effective, causing gradients to weaken over time. Passive processes like diffusion and osmosis can still occur because they are driven by existing gradients, not ATP. However, as active pumping declines, the gradients that drive many physiological processes may collapse, disrupting homeostasis. This is why energy failure leads to ion imbalance and impaired nutrient handling. Therefore, the expected immediate trend is that active transport decreases.
141. Endocytosis is best defined as:
ⓐ. Vesicular intake
ⓑ. Ion flow by simple diffusion
ⓒ. Water movement by osmosis
ⓓ. Solute pump using ATPase
Correct Answer: Vesicular intake
Explanation: Endocytosis is a process in which a eukaryotic cell takes materials into the cell by forming membrane-bound vesicles from the plasma membrane. The membrane invaginates, encloses extracellular material, and pinches off to create an internal vesicle. This mechanism is useful for taking in large particles, macromolecules, or bulk fluid that cannot pass through transport proteins. It also allows selective uptake when specific surface receptors are involved. Because vesicle formation and trafficking require cellular energy and cytoskeletal support, it is considered an energy-dependent bulk transport method. Hence, endocytosis is correctly defined as vesicular intake.
142. Exocytosis refers to:
ⓐ. Passive carrier flow
ⓑ. Inward diffusion
ⓒ. Osmotic influx
ⓓ. Vesicular release
Correct Answer: Vesicular release
Explanation: Exocytosis is the process by which intracellular vesicles fuse with the plasma membrane to release their contents outside the cell. This is a major route for secretion of hormones, enzymes, and neurotransmitters, as well as for exporting waste materials. The vesicle membrane becomes part of the plasma membrane during fusion, which also helps maintain membrane balance when endocytosis removes membrane. Exocytosis requires coordinated membrane fusion machinery and energy-dependent steps for vesicle transport and docking. It is a defining feature of eukaryotic cells because it relies on an internal endomembrane system. Therefore, exocytosis is correctly described as vesicular release.
143. Phagocytosis is a type of endocytosis primarily used for:
ⓐ. Water movement only
ⓑ. Small ions only
ⓒ. Large solid particles
ⓓ. Lipid tail transport
Correct Answer: Large solid particles
Explanation: Phagocytosis involves engulfing large solid materials such as bacteria, cell debris, or food particles by extending the cell membrane around them to form a vesicle. This process typically requires significant cytoskeletal rearrangement and is common in specialized cells like macrophages. The internalized vesicle can fuse with digestive compartments where enzymes break down the particle. It is distinct from simple diffusion or carrier transport because it handles items far larger than typical solutes. The key identifying feature is uptake of large solids rather than fluids or dissolved molecules. Hence, phagocytosis is mainly for large solid particles.
144. Pinocytosis is best described as:
ⓐ. Fluid uptake
ⓑ. Solid particle ingestion
ⓒ. Protein secretion
ⓓ. Ion pumping
Correct Answer: Fluid uptake
Explanation: Pinocytosis is often described as “cell drinking” because it involves the uptake of extracellular fluid along with dissolved solutes into small vesicles. The cell membrane invaginates and pinches off, forming vesicles that carry the fluid inward. This process is less selective than receptor-mediated uptake, though it can still be regulated by cell type and conditions. Pinocytosis helps cells sample their environment and internalize nutrients or signaling molecules present in solution. Like other forms of endocytosis, it depends on vesicle formation and cellular energy. Therefore, pinocytosis is best described as fluid uptake.
145. Receptor-mediated endocytosis is best identified by:
ⓐ. Random fluid uptake
ⓑ. Specific ligand binding
ⓒ. Osmosis through aquaporins
ⓓ. Ion leakage through lipids
Correct Answer: Specific ligand binding
Explanation: Receptor-mediated endocytosis involves selective internalization of molecules that bind to specific receptors on the cell surface. The binding concentrates the target molecules at defined membrane regions, which then invaginate and form vesicles carrying the receptor–ligand complexes. This selectivity allows efficient uptake even when the target is present at low external concentrations. It is crucial for processes like uptake of certain nutrients and regulation of receptor numbers at the surface. The defining feature is the requirement for receptor–ligand recognition rather than random engulfment. Hence, it is best identified by specific ligand binding.
146. During exocytosis, the vesicle membrane:
ⓐ. Dissolves into cytosol
ⓑ. Becomes part of nucleus membrane
ⓒ. Converts into cell wall layer
ⓓ. Fuses with plasma membrane
Correct Answer: Fuses with plasma membrane
Explanation: In exocytosis, a vesicle moves to the cell surface and merges its membrane with the plasma membrane. This fusion creates a continuous membrane and opens a path for the vesicle contents to be released outside. The integration of vesicle membrane into the plasma membrane is important for maintaining membrane composition and surface area, especially when paired with endocytosis that removes membrane portions. Fusion is mediated by specialized proteins that ensure correct targeting and timing. The process is therefore not a dissolution but a controlled membrane-merging event. Hence, the vesicle membrane fuses with the plasma membrane.
147. Endocytosis and exocytosis are considered active processes mainly because they:
ⓐ. Use only lipid diffusion
ⓑ. Move down gradients
ⓒ. Use vesicles and energy
ⓓ. Occur in all bacteria
Correct Answer: Use vesicles and energy
Explanation: Both endocytosis and exocytosis require formation, movement, and fusion of membrane-bound vesicles, which involves ATP-dependent steps and cytoskeletal participation. Vesicle budding, transport along cytoskeletal tracks, docking, and membrane fusion are coordinated events that do not occur spontaneously without cellular machinery. These processes can move large amounts of material independent of concentration gradients, which is another hallmark of active transport. They are characteristic of eukaryotes because they depend on an internal membrane trafficking system. The energy requirement and complex vesicle machinery distinguish them from passive diffusion mechanisms. Therefore, they are active mainly because they use vesicles and energy.
148. A key cellular system that supports endocytosis and exocytosis in eukaryotes is the:
ⓐ. Endomembrane system
ⓑ. Nucleoid region
ⓒ. Peptidoglycan wall
ⓓ. Chromatophore layer
Correct Answer: Endomembrane system
Explanation: Endocytosis and exocytosis depend on internal membrane compartments and vesicle trafficking pathways, which are features of the eukaryotic endomembrane system. Organelles such as the ER and Golgi help form, modify, sort, and route vesicles to specific destinations, including the plasma membrane. This system also ensures that membrane proteins and lipids are delivered correctly and that secretory materials are packaged for release. Without these internal compartments and trafficking routes, regulated secretion and bulk uptake would be inefficient or impossible. Prokaryotes lack this kind of complex endomembrane organization. Hence, the endomembrane system is key.
149. If a cell increases secretion of proteins, a related expected change is:
ⓐ. Increased simple diffusion only
ⓑ. Increased exocytosis
ⓒ. Increased osmosis only
ⓓ. Increased nucleoid formation
Correct Answer: Increased exocytosis
Explanation: Protein secretion commonly occurs through vesicles that carry proteins from internal processing compartments to the plasma membrane and then release them outside via exocytosis. When secretion demand rises, cells typically increase vesicle formation, trafficking, docking, and fusion events at the surface. This directly increases exocytosis frequency to export the packaged proteins. Simple diffusion and osmosis cannot account for export of large proteins because they cannot cross the lipid bilayer freely. The process also reflects eukaryotic reliance on the secretory pathway. Therefore, increased protein secretion is associated with increased exocytosis.
150. Endocytosis is generally absent in most prokaryotes primarily because:
ⓐ. Lack endomembrane system
ⓑ. Have no ribosomes
ⓒ. Have no cell wall
ⓓ. Have no cytoplasm
Correct Answer: Lack endomembrane system
Explanation: Endocytosis requires membrane bending, vesicle formation, and regulated trafficking within an internal membrane network. Most prokaryotes lack an endomembrane system with organelle-based vesicle transport machinery, making classic endocytosis uncommon. Their transport is mainly handled by membrane proteins and other non-vesicular mechanisms rather than bulk vesicle uptake. This structural limitation is tied to their simpler internal organization and absence of compartmentalized trafficking routes. The absence of endocytosis is therefore not due to missing cytoplasm or ribosomes, which prokaryotes do have. Hence, the primary reason is the lack of an endomembrane system.
151. In receptor-mediated endocytosis, the most common protein coat for coated pits is:
ⓐ. COPII coat
ⓑ. Clathrin
ⓒ. COPI coat
ⓓ. Caveolin
Correct Answer: Clathrin
Explanation: Receptor-mediated endocytosis typically begins at specialized coated pits where specific receptors bind their ligands and then cluster together. Clathrin assembles on the cytosolic side as a coat that helps bend the plasma membrane into an invaginating bud. This coating provides structural support for vesicle formation and helps concentrate the selected cargo efficiently. The clathrin-coated bud then pinches off to form an internal vesicle for further sorting. This mechanism enables highly selective uptake even when the ligand is present at low concentration outside. Therefore, clathrin is the characteristic coat protein for this pathway.
152. The key protein machinery that directly mediates vesicle–plasma membrane fusion during exocytosis is:
ⓐ. Integrins
ⓑ. Clathrin
ⓒ. Tubulins
ⓓ. SNAREs
Correct Answer: SNAREs
Explanation: Vesicle fusion with the plasma membrane requires precise docking and membrane-merging steps to release cargo outside the cell. SNARE proteins on the vesicle and target membrane interact in a highly specific manner to bring the two lipid bilayers into close contact. This close apposition overcomes the natural repulsion between membranes and promotes fusion, allowing the vesicle contents to exit. The process is tightly regulated to ensure cargo is released at the correct site and time. SNARE-driven fusion is central to secretion, neurotransmitter release, and membrane recycling in eukaryotic cells. Hence, SNAREs are the core fusion machinery.
153. The secretion pathway that occurs continuously in most cells without a special trigger is:
ⓐ. Regulated release
ⓑ. Receptor-mediated uptake
ⓒ. Vesicular uptake
ⓓ. Constitutive
Correct Answer: Constitutive
Explanation: Constitutive secretion is the default exocytic route in which vesicles routinely deliver membrane components and soluble proteins to the cell surface. It operates continuously to maintain plasma membrane composition, supply membrane lipids, and export certain proteins without requiring a specific external signal. This pathway supports steady turnover of membrane proteins and contributes to cell growth and surface renewal. Because it is always active, it provides a baseline level of secretion and membrane addition in many cell types. This is distinct from stimulus-dependent pathways that release cargo only upon a signal. Therefore, the continuous default pathway is constitutive secretion.
154. After endocytosis, the first major sorting compartment that receives many internalized vesicles is:
ⓐ. Nuclear matrix
ⓑ. Secretory granule
ⓒ. Early endosome
ⓓ. Golgi cisterna
Correct Answer: Early endosome
Explanation: Endocytosed vesicles commonly fuse with the early endosome, which functions as the primary sorting station inside the cell. Here, internalized cargo can be separated into different routes, such as recycling back to the membrane or transport toward degradative compartments. The early endosome environment supports ligand–receptor handling and selective recycling, which helps regulate surface receptor levels. This sorting step is essential for controlling nutrient uptake, signaling duration, and membrane turnover. Efficient sorting prevents unnecessary degradation of receptors that need to be reused. Hence, the early endosome is the first key sorting compartment after endocytosis.
155. A membrane transporter that moves two different solutes in the same direction is called:
ⓐ. Uniport
ⓑ. Symport
ⓒ. Antiport
ⓓ. Exocytosis
Correct Answer: Symport
Explanation: Symporters couple the movement of two different substances through the same transport protein in the same direction across the membrane. This coupling is especially important when one solute’s downhill movement can help drive the other solute’s movement in a coordinated manner. Such transporters are widely used to bring nutrients into cells efficiently when direct diffusion is insufficient. The “same direction” keyword is the defining clue, separating symport from antiport where directions are opposite. This concept is central in understanding coupled transport mechanisms in membranes. Therefore, the correct term for same-direction co-transport is symport.
156. A membrane pump that produces a net charge difference across the membrane is termed:
ⓐ. Voltage-fixed
ⓑ. Osmotic-neutral
ⓒ. Isotonic-only
ⓓ. Electrogenic
Correct Answer: Electrogenic
Explanation: An electrogenic pump creates a net movement of charge across the plasma membrane, contributing to membrane potential and electrical gradients. This happens when the number of positive and negative charges moved in opposite directions is not equal, leading to a net charge transfer. Such pumps are physiologically important because they help establish electrical conditions that influence transport, excitability, and ion distribution. The concept links active transport with electrical properties of membranes, which is commonly tested in competitive-style questions. By maintaining charge separation, electrogenic pumps support downstream transport processes that depend on electrochemical gradients. Hence, the correct keyword is electrogenic.
157. The most common membrane proteins that allow rapid passive water movement are:
ⓐ. Aquaporins
ⓑ. Porins only
ⓒ. Clathrin coat
ⓓ. SNARE proteins
Correct Answer: Aquaporins
Explanation: Aquaporins are specialized channel proteins that greatly increase membrane permeability to water while still allowing selective control of flow. Water moves through them passively, driven by differences in water potential, so the channels speed up osmosis without requiring ATP. Their presence explains why certain cells can rapidly adjust volume or water balance in response to osmotic changes. Aquaporins are highly selective for water, helping maintain ionic gradients by limiting ion passage. This protein-based pathway is much faster than water crossing directly through the lipid bilayer alone. Therefore, aquaporins are the key facilitators of rapid passive water movement.
158. In a plant cell placed in a hypertonic solution, the shrinkage of the protoplast away from the cell wall is called:
ⓐ. Endocytosis
ⓑ. Cytokinesis
ⓒ. Plasmolysis
ⓓ. Phagocytosis
Correct Answer: Plasmolysis
Explanation: In a hypertonic medium, water leaves the plant cell due to a water potential gradient, reducing the volume of the protoplast. As water loss continues, the plasma membrane and cytoplasm pull away from the rigid cell wall, producing a clear separation. This phenomenon is a direct outcome of osmotic water movement across the selectively permeable plasma membrane. The cell wall remains in place, but the living contents contract, which is a distinctive diagnostic feature in plant cells. Plasmolysis is therefore used to demonstrate osmosis and the role of the membrane in controlling water movement. Hence, the correct term is plasmolysis.
159. A transport process that reaches a maximum rate (Vmax) as solute concentration increases is best indicated by the keyword:
ⓐ. Proportional
ⓑ. Saturation
ⓒ. Osmoregulation
ⓓ. Vesiculation
Correct Answer: Saturation
Explanation: Saturation occurs when increasing solute concentration no longer increases transport rate because the available transport proteins are fully occupied. This behavior is characteristic of carrier-mediated transport, where a finite number of binding sites and turnover limits create a maximum rate. The idea is crucial for interpreting uptake curves and for distinguishing protein-mediated transport from simple diffusion, which does not show a fixed maximum in the same way. Saturation also implies specificity, since transport depends on particular carrier binding interactions. Once all carriers are engaged, the system cannot accelerate further without adding more carriers. Therefore, the keyword that signals Vmax behavior is saturation.
160. The Na⁺–glucose cotransporter is best categorized as:
ⓐ. Secondary active
ⓑ. Facilitated diffusion
ⓒ. Primary active pump
ⓓ. Vesicular transport
Correct Answer: Secondary active
Explanation: The Na⁺–glucose cotransporter uses the energy stored in a sodium gradient to bring glucose into the cell along with sodium. The transporter itself does not directly split ATP; instead, it depends on the sodium gradient that is typically maintained by ATP-driven pumps elsewhere in the membrane. This is why the mechanism is termed secondary active transport: it can move glucose uphill by coupling it to sodium’s downhill movement. The concept is a classic example of coupled transport and is frequently tested using the “gradient energy” keyword. Because glucose uptake is linked to Na⁺ flow, the process is driven indirectly by cellular energy. Hence, it is best categorized as secondary active transport.
161. The nuclear envelope is best described as:
ⓐ. A single membrane without pores
ⓑ. A rigid wall made of cellulose
ⓒ. A double membrane with pores
ⓓ. A protein coat with no lipid layer
Correct Answer: A double membrane with pores
Explanation: The nuclear envelope surrounds the nucleus in eukaryotic cells and consists of two lipid bilayers—an outer and an inner membrane—separated by a narrow space. Nuclear pore complexes are embedded in this envelope and act as regulated gateways for movement between nucleus and cytoplasm. This arrangement keeps nuclear contents separated while still allowing controlled exchange of RNAs, proteins, and small molecules. The double membrane design also supports structural stability and organization of genetic material inside the nucleus. Because transport is selective, the envelope helps regulate gene expression by controlling what enters and exits. Hence, the nuclear envelope is correctly described as a double membrane with pores.
162. The space between the inner and outer nuclear membranes is called:
ⓐ. Perinuclear space
ⓑ. Nuclear matrix space
ⓒ. Nucleoplasm gap
ⓓ. Chromatin space region
Correct Answer: Perinuclear space
Explanation: The inner and outer membranes of the nuclear envelope are separated by a narrow region known as the perinuclear space. This space is continuous with the lumen of the endoplasmic reticulum, reflecting the structural link between the nuclear envelope and the ER system. The term helps students map nuclear envelope architecture correctly as a double-membrane boundary rather than a single sheet. Recognizing this space clarifies that the outer membrane behaves like an ER membrane in continuity and composition. It is therefore a standard micro-point used to describe nuclear envelope structure. Hence, the correct term is perinuclear space.
163. The outer nuclear membrane is continuous with the:
ⓐ. Golgi cisternae network
ⓑ. Lysosomal membrane layer
ⓒ. Plasma membrane boundary
ⓓ. Endoplasmic reticulum membrane
Correct Answer: Endoplasmic reticulum membrane
Explanation: The outer nuclear membrane is structurally continuous with the membrane of the endoplasmic reticulum, meaning they form a connected membrane system. This continuity explains why the perinuclear space is continuous with the ER lumen and why ER-associated features can appear on the outer nuclear membrane. It also helps connect nuclear organization with the broader endomembrane system of eukaryotic cells. Because membranes and proteins can be shared or routed through this system, the nucleus is integrated into cellular trafficking architecture. This is a key concept for understanding how the cell coordinates nuclear boundary function with internal membrane networks. Therefore, the outer nuclear membrane is continuous with the endoplasmic reticulum membrane.
164. Nuclear pores mainly function in:
ⓐ. DNA replication by making new strands
ⓑ. Controlled nucleo-cytoplasmic transport
ⓒ. ATP synthesis by electron transport chains
ⓓ. Making cell wall microfibrils for shape
Correct Answer: Controlled nucleo-cytoplasmic transport
Explanation: Nuclear pores are specialized openings in the nuclear envelope that allow regulated exchange between the nucleus and cytoplasm. They permit selected proteins to enter the nucleus and allow RNAs, including mRNA and rRNA components, to exit for use in protein synthesis and ribosome assembly. This transport is not random leakage; it is selective and often signal-dependent, ensuring nuclear processes remain properly controlled. By regulating molecular traffic, pores help coordinate transcription in the nucleus with translation in the cytoplasm. This control is essential for accurate gene expression and cellular function. Hence, nuclear pores mainly enable controlled nucleo-cytoplasmic transport.
165. A common feature of the outer nuclear membrane is that it may bear:
ⓐ. Ribosomes on its surface
ⓑ. Centrioles attached permanently
ⓒ. Starch granules embedded inside
ⓓ. Cellulose microfibrils outside
Correct Answer: Ribosomes on its surface
Explanation: The outer nuclear membrane is continuous with the endoplasmic reticulum and can resemble rough ER in having ribosomes attached on its cytosolic surface. This explains why proteins destined for membranes or secretion can be synthesized near the nuclear boundary and routed through the connected ER network. The presence of ribosomes is therefore a structural clue linking the nuclear envelope to the endomembrane system. It also helps students differentiate the nuclear envelope from structures like cell walls or cytoskeletal components. While the inner membrane has distinct associations with nuclear organization, ribosome attachment is a known feature of the outer membrane side. Thus, ribosomes on its surface is the correct feature.
166. The nuclear envelope is absent in:
ⓐ. Typical animal cells
ⓑ. Typical plant cells
ⓒ. Typical fungal cells
ⓓ. Typical prokaryotic cells
Correct Answer: Typical prokaryotic cells
Explanation: Prokaryotic cells do not have a true nucleus, so they lack a nuclear envelope and nuclear pores. Their DNA lies in the nucleoid region directly within the cytoplasm, without a membrane boundary separating it from ribosomes. This structural difference is one of the most reliable criteria for distinguishing prokaryotic cells from eukaryotic cells. Because there is no nuclear compartment, transcription and translation can occur in the same general space in prokaryotes. The absence of a nuclear envelope is therefore central to prokaryotic organization and gene-expression dynamics. Hence, the nuclear envelope is absent in typical prokaryotic cells.
167. During cell division in many eukaryotic cells, the nuclear envelope typically:
ⓐ. Becomes a thick cell wall layer
ⓑ. Breaks down and later reforms
ⓒ. Converts into mitochondria membranes
ⓓ. Permanently disappears after division
Correct Answer: Breaks down and later reforms
Explanation: In many eukaryotic cells, the nuclear envelope disassembles during the early stages of division and is reassembled around the separated sets of chromosomes later. This breakdown allows spindle structures to access chromosomes for orderly separation. After chromosome segregation, the envelope reforms to re-establish a distinct nuclear compartment in each daughter cell. The cycle highlights that the envelope is a dynamic structure, not a fixed permanent barrier. Understanding this helps link nuclear envelope structure with the process of cell division and nuclear compartment restoration. Therefore, the nuclear envelope typically breaks down and later reforms.
168. The inner nuclear membrane is most closely associated with:
ⓐ. Photosynthesis pigment membranes
ⓑ. Cytosolic ribosome binding sites
ⓒ. Nuclear lamina support layer
ⓓ. Outer cell wall strengthening fibers
Correct Answer: Nuclear lamina support layer
Explanation: The inner nuclear membrane is supported by the nuclear lamina, a protein network that provides structural stability and helps maintain nuclear shape. This association also contributes to organizing chromatin and anchoring specific regions of the genome near the nuclear periphery. By strengthening the nuclear boundary from the inside, the lamina helps the nucleus withstand mechanical stresses and maintain compartment integrity. This feature distinguishes the inner membrane’s specialized support roles from the outer membrane’s ER-like associations. The concept is frequently tested as a micro-point linking nuclear envelope structure to nuclear organization. Hence, the inner nuclear membrane is closely associated with the nuclear lamina support layer.
169. A correct statement about nuclear pores is:
ⓐ. They are sealed and stop all exchange
ⓑ. They are holes only in the outer membrane
ⓒ. They allow only water by diffusion
ⓓ. They span both membranes
Correct Answer: They span both membranes
Explanation: Nuclear pores are complexes that form channels through the nuclear envelope, connecting the nucleus and cytoplasm across the double-membrane barrier. They are positioned where inner and outer membranes come together, creating a continuous passage that spans both membranes rather than being limited to one layer. This architecture is essential because transport must cross the entire envelope to move materials in or out. The pores support selective movement of macromolecules and help maintain nuclear compartmentalization while enabling communication. This explains how large RNAs and proteins can be exchanged without losing the boundary function of the envelope. Therefore, nuclear pores span both membranes.
170. Export of mRNA from nucleus to cytoplasm mainly occurs through:
ⓐ. Lipid diffusion across the inner membrane
ⓑ. Nuclear pore complexes
ⓒ. Cell wall channels called pits
ⓓ. Chloroplast membrane pores only
Correct Answer: Nuclear pore complexes
Explanation: mRNA is synthesized in the nucleus but must reach cytosolic ribosomes for translation, and this transfer occurs through nuclear pore complexes. These pores act as regulated gates that allow properly processed RNAs to exit while maintaining the separation between nuclear and cytoplasmic environments. The process supports correct gene expression by ensuring transcripts are exported only after key processing steps are completed. Because the lipid bilayer is not freely permeable to large RNA molecules, pores provide the essential pathway for macromolecular movement. This is a defining eukaryotic transport feature linked to the presence of a true nuclear envelope. Hence, export of mRNA mainly occurs through nuclear pore complexes.
171. Nuclear pores are built from a large protein assembly called:
ⓐ. Nuclear lamina scaffold proteins
ⓑ. Membrane lipid bilayer patches
ⓒ. Nuclear pore complex
ⓓ. Chromatin packing protein units
Correct Answer: Nuclear pore complex
Explanation: Nuclear pores are not simple holes in the membrane; they are formed by a massive, organized protein structure called the nuclear pore complex. This complex spans the double nuclear envelope and creates a selective gateway between nucleus and cytoplasm. Its architecture allows small molecules to pass more freely while controlling the movement of large RNAs and proteins. The pore complex also maintains compartment identity by preventing uncontrolled mixing of nuclear and cytosolic contents. Because transport selectivity is a core nuclear feature, the complex is essential for regulated gene expression. Therefore, the correct name for the pore assembly is the nuclear pore complex.
172. Through nuclear pores, small ions and many small metabolites mainly cross by:
ⓐ. Passive diffusion
ⓑ. ATP-powered pumping only
ⓒ. Vesicle-mediated secretion route
ⓓ. Nuclear membrane rupture events
Correct Answer: Passive diffusion
Explanation: Small ions and many small metabolites can pass through nuclear pores without being carried by transport receptors because their size allows movement through the pore’s permeability pathway. The direction of this movement depends on concentration differences, so it is fundamentally diffusion-based rather than ATP-driven pumping. This helps the nucleus rapidly equilibrate many small solutes with the cytoplasm while still keeping macromolecules under tight control. The selective barrier is therefore size- and signal-dependent, not an absolute seal. This arrangement supports efficient cellular chemistry while preserving nuclear compartmentalization. Hence, passive diffusion is the main mode for many small solutes.
173. The most accurate statement about nuclear pore selectivity is:
ⓐ. All proteins pass freely at equal rates
ⓑ. Only lipids can cross the pore channel
ⓒ. Only DNA can exit to cytoplasm freely
ⓓ. Large cargo needs transport signals
Correct Answer: Large cargo needs transport signals
Explanation: Nuclear pores allow regulated exchange, and large macromolecules typically require specific transport signals to move efficiently across the nuclear envelope. Proteins often carry localization or export signals that are recognized by transport receptors, enabling controlled passage through the pore complex. This prevents random leakage of nuclear proteins and ensures that RNAs and ribosomal subunits exit in a coordinated manner. The selectivity is therefore based on size plus signal-directed transport rather than equal free passage for all macromolecules. This control is essential for maintaining nuclear identity and correct gene regulation. Therefore, large cargo generally needs transport signals for effective movement.
174. The short amino acid “address tag” that helps many proteins enter the nucleus is:
ⓐ. Ribosome-binding sequence
ⓑ. Nuclear localization signal
ⓒ. Mitochondrial targeting peptide
ⓓ. Secretory signal peptide chain
Correct Answer: Nuclear localization signal
Explanation: Many nuclear proteins contain a nuclear localization signal that acts like an address label directing them to the nucleus. This signal is recognized by import receptors that guide the cargo to the nuclear pore complex and facilitate entry. The mechanism ensures that transcription factors, DNA-binding proteins, and repair enzymes accumulate in the nucleus where they function. Without such targeting information, large proteins would not be efficiently enriched inside the nucleus. This selective import supports regulated gene expression and nuclear maintenance. Hence, the correct “address tag” for nuclear import is the nuclear localization signal.
175. Export of many proteins from nucleus to cytoplasm commonly depends on:
ⓐ. Random drift through pores only
ⓑ. DNA replication forks at envelope
ⓒ. Nuclear export signal
ⓓ. Cell wall channels in plants
Correct Answer: Nuclear export signal
Explanation: Many proteins that must leave the nucleus contain a nuclear export signal that is recognized by export receptors. This signal-directed pathway ensures proteins move out in a regulated manner rather than relying on random leakage. Export control is essential for timing of cell-cycle regulators and for recycling transport factors back to the cytoplasm. The process uses the nuclear pore complex as the physical gateway while signal recognition provides directionality and selectivity. This coordination prevents improper retention or loss of key nuclear proteins. Therefore, a nuclear export signal is a common requirement for protein export.
176. Directionality of many nuclear import–export cycles is strongly linked to the:
ⓐ. Ran GTP gradient
ⓑ. Ribosomal subunit size rule
ⓒ. Peptidoglycan wall thickness
ⓓ. Chloroplast membrane pores
Correct Answer: Ran GTP gradient
Explanation: Directional nuclear transport is driven by a gradient of Ran in its GTP-bound and GDP-bound forms across the nuclear envelope. This gradient acts as a molecular “switch” that controls when cargo binds or releases from transport receptors on each side of the pore. As a result, import complexes assemble and disassemble in the correct compartments, and export complexes do the same in the opposite direction. This prevents futile back-and-forth movement and ensures net transport toward the appropriate side. The gradient is therefore central to making transport selective and directional rather than purely diffusive. Hence, the Ran GTP gradient is the key driver of directionality.
177. The receptors that commonly bind nuclear localization signals and carry cargo inward are:
ⓐ. Cyclins that trigger division timing
ⓑ. Importins as transport receptors
ⓒ. Actins that form cell cortex
ⓓ. Histones that pack chromatin
Correct Answer: Importins as transport receptors
Explanation: Importins are transport receptors that recognize nuclear localization signals on cargo proteins and guide them through the nuclear pore complex. They form a cargo–receptor complex that can interact with pore components to pass the selective barrier. Once inside the nucleus, regulatory factors promote cargo release so the protein can perform its nuclear function. This system allows the cell to target large proteins to the nucleus efficiently and selectively. It also supports rapid changes in gene expression by controlling when regulatory proteins enter the nucleus. Therefore, importins are the key receptors for signal-dependent nuclear import.
178. Many nuclear pore proteins are rich in repeats that create a selective barrier known as:
ⓐ. Collagen triple-helix fibers
ⓑ. Tubulin lattice microfilaments
ⓒ. Cellulose microfibril bundles
ⓓ. FG-repeat nucleoporins
Correct Answer: FG-repeat nucleoporins
Explanation: The nuclear pore’s selective barrier is largely formed by nucleoporins that contain FG repeats, creating a dynamic meshwork within the pore channel. Transport receptors interact with these repeats to move cargo through while non-specific large macromolecules are excluded. This design allows both selectivity and flexibility, enabling high traffic without losing compartment control. The FG-repeat network is therefore central to how pores can be both fast and selective at the same time. It provides a molecular filter rather than a rigid gate. Hence, FG-repeat nucleoporins are the key components of the selective barrier.
179. During active gene expression, a major transport task of nuclear pores is:
ⓐ. Export of mRNA to cytosol
ⓑ. Import of oxygen by diffusion
ⓒ. Packing DNA into nucleosomes
ⓓ. Building a cell wall framework
Correct Answer: Export of mRNA to cytosol
Explanation: mRNA is produced in the nucleus but must reach cytosolic ribosomes for translation, so efficient export through nuclear pores is essential. Nuclear pores provide the controlled pathway for mRNA to leave while preserving the nuclear environment. This export links transcription to protein synthesis and allows cells to regulate gene expression by controlling which transcripts are released. Because mRNA is large and charged, it cannot cross the lipid bilayer directly and therefore depends on pore-mediated transport. Accurate export ensures that cytosolic translation reflects properly processed nuclear transcripts. Thus, export of mRNA to the cytosol is a major pore function during active gene expression.
180. A correct structural feature of nuclear pores is that they:
ⓐ. Occur where both membranes join
ⓑ. Exist only in the outer membrane
ⓒ. Form only during cytokinesis
ⓓ. Replace ribosomes in translation
Correct Answer: Occur where both membranes join
Explanation: Nuclear pores are positioned at sites where the inner and outer nuclear membranes come together, creating a continuous passage across the double envelope. This placement is necessary because transport must cross the entire nuclear boundary, not just one membrane layer. The pore complex anchors at these junctions and maintains a stable gateway for controlled exchange. This arrangement supports selective transport while keeping the rest of the envelope as an effective barrier. It also explains why pores are described as spanning the nuclear envelope rather than being simple openings in one sheet. Therefore, nuclear pores occur where both nuclear membranes join.
181. The nucleolus is best described as:
ⓐ. Site of rRNA synthesis
ⓑ. Site of glycolysis enzymes
ⓒ. Site of lipid oxidation only
ⓓ. Site of chromosome splitting
Correct Answer: Site of rRNA synthesis
Explanation: The nucleolus is a dense region inside the nucleus where ribosomal RNA is transcribed and processed. It also serves as a major assembly center where rRNA combines with ribosomal proteins to form ribosomal subunits. This function links the nucleolus directly to the cell’s capacity for protein synthesis, since ribosomes are required for translation. The nucleolus is therefore prominent in cells actively making proteins because rRNA production and subunit assembly are high. It is not a membrane-bound organelle, but a functional nuclear region organized around rRNA gene activity. Hence, the nucleolus is best described as the site of rRNA synthesis.
182. The nucleolus is present because eukaryotic cells have:
ⓐ. A true nucleus with envelope
ⓑ. A nucleoid without boundary
ⓒ. A peptidoglycan cell wall
ⓓ. A chromatophore membrane system
Correct Answer: A true nucleus with envelope
Explanation: The nucleolus is a specialized region within the nucleus, so it depends on the presence of a true nucleus bounded by a nuclear envelope. Eukaryotic cells compartmentalize transcription and RNA processing in the nucleus, allowing nucleolar organization of rRNA synthesis and ribosome subunit assembly. Prokaryotic cells lack a nuclear envelope and therefore do not form a nucleolus as a distinct nuclear structure. The nucleolus exists as an internal nuclear domain organized around active rRNA genes. This compartment-based organization is a major hallmark of eukaryotic cell structure. Therefore, a true nucleus with an envelope is the condition that allows a nucleolus.
183. Ribosomal subunits assembled in the nucleolus are exported mainly through:
ⓐ. Mitochondrial crista pores
ⓑ. Plasma membrane channels
ⓒ. Cell wall pits
ⓓ. Nuclear pore complexes
Correct Answer: Nuclear pore complexes
Explanation: Ribosomal subunits formed in the nucleolus must reach the cytoplasm to participate in translation, and this movement occurs through nuclear pore complexes. These pores provide the regulated gateway for large ribonucleoprotein particles to exit the nucleus while maintaining nuclear compartment integrity. Because ribosomal subunits are large and charged, they cannot diffuse through the lipid bilayer of the nuclear envelope. The pore system ensures selective export of properly assembled subunits, linking nucleolar activity to cytosolic protein synthesis. Efficient subunit export is therefore essential for maintaining translation capacity. Hence, nuclear pore complexes are the primary route for ribosomal subunit export.
184. A cell with an unusually prominent nucleolus is most likely actively:
ⓐ. Performing photosynthesis rapidly
ⓑ. Making ribosomes rapidly
ⓒ. Synthesizing cell wall cellulose
ⓓ. Digesting fats in lysosomes only
Correct Answer: Making ribosomes rapidly
Explanation: The nucleolus enlarges and becomes more prominent when rRNA transcription and ribosomal subunit assembly are high. Cells engaged in rapid growth or high protein production require many ribosomes, so they increase nucleolar activity to meet demand. A prominent nucleolus therefore indicates strong ribosome biogenesis rather than photosynthesis or cell wall synthesis. This relationship is widely used as a cytological clue to cellular metabolic and growth activity. The nucleolus reflects the cell’s investment in protein synthesis capacity by producing ribosomal components. Therefore, an unusually prominent nucleolus suggests the cell is making ribosomes rapidly.
185. The nucleolus is best categorized structurally as:
ⓐ. A membrane-bound organelle
ⓑ. A nuclear region without membrane
ⓒ. A cytoplasmic vesicle structure
ⓓ. A cell wall thickening layer
Correct Answer: A nuclear region without membrane
Explanation: The nucleolus is not surrounded by a membrane; it is a dense, organized region within the nucleus formed around active rRNA gene clusters. Its structure is maintained by ongoing rRNA transcription, processing, and assembly interactions rather than by a lipid boundary. This non-membranous organization allows dynamic changes in size and activity depending on the cell’s needs for ribosome production. It is therefore described as a nuclear substructure or domain rather than an independent organelle. This distinction is important in exams to avoid confusing it with membrane-bound compartments like mitochondria. Hence, it is a nuclear region without a membrane.
186. The most direct product synthesized in the nucleolus is:
ⓐ. rRNA molecules
ⓑ. ATP molecules
ⓒ. Glucose molecules
ⓓ. DNA nucleotides
Correct Answer: rRNA molecules
Explanation: The nucleolus is the primary site for synthesis and early processing of ribosomal RNA, which forms the structural and catalytic core of ribosomes. rRNA production is essential because ribosomal subunits cannot assemble without it, even if ribosomal proteins are available. The nucleolus organizes transcription of rRNA genes and processing steps that convert precursor rRNA into mature rRNA species. This activity is directly tied to ribosome formation and thus to overall protein synthesis capacity in the cell. The nucleolus does not produce ATP or glucose, and it is not the main site of DNA nucleotide synthesis. Therefore, rRNA molecules are the most direct product of the nucleolus.
187. During cell division in many eukaryotic cells, the nucleolus typically:
ⓐ. Moves into the cytosol permanently
ⓑ. Becomes a mitochondrion
ⓒ. Disappears and later reappears
ⓓ. Forms a new cell wall layer
Correct Answer: Disappears and later reappears
Explanation: The nucleolus is closely linked to active rRNA transcription and ribosome assembly, processes that are reduced or reorganized during division in many cells. As the nucleus reorganizes for chromosome separation, the nucleolus commonly disassembles and becomes less visible. After division, when nuclei reform and transcription resumes, the nucleolus reappears around rRNA gene regions. This behavior shows that the nucleolus is a functional assembly region rather than a rigid organelle. Its presence depends on ongoing activity and nuclear organization state. Hence, it typically disappears during division and later reappears.
188. The nucleolus is formed around chromosomal regions called:
ⓐ. Spindle attachment fibers
ⓑ. Centromere plates
ⓒ. Telomeric caps
ⓓ. Nucleolar organizer regions
Correct Answer: Nucleolar organizer regions
Explanation: The nucleolus forms around specific chromosomal sites that contain rRNA gene clusters, known as nucleolar organizer regions. These regions drive high levels of rRNA transcription, and the concentration of rRNA processing and assembly factors around them creates the nucleolus. This explains why nucleolar formation is tied to particular chromosomes and why nucleolar size changes with rRNA gene activity. The organizer regions act as the functional nucleation points for nucleolar structure. This concept connects chromosome organization to nuclear substructure formation. Therefore, nucleolar organizer regions are the chromosomal basis of nucleolus formation.
189. Which statement correctly links nucleolus to cytoplasmic translation?
ⓐ. Nucleolus makes ribosomal subunits
ⓑ. Nucleolus makes glucose for ATP
ⓒ. Nucleolus makes membrane lipids
ⓓ. Nucleolus pumps ions across membrane
Correct Answer: Nucleolus makes ribosomal subunits
Explanation: Cytoplasmic translation requires ribosomes, and ribosomes are built from ribosomal subunits assembled in the nucleus with a major role for the nucleolus. The nucleolus synthesizes rRNA and organizes assembly of ribosomal proteins with rRNA to form the large and small subunits. These subunits are then exported to the cytoplasm, where they combine on mRNA to carry out protein synthesis. Thus, nucleolar activity directly determines the cell’s capacity to produce functional ribosomes for translation. This link is fundamental for understanding how nuclear processes support cytoplasmic protein production. Hence, the correct link is that the nucleolus makes ribosomal subunits.
190. A good micro-point difference between nucleolus and nucleoplasm is:
ⓐ. Nucleoplasm is outside nuclear envelope
ⓑ. Nucleolus is double-membrane bound
ⓒ. Nucleolus is rRNA assembly hub
ⓓ. Nucleoplasm lacks all enzymes and ions
Correct Answer: Nucleolus is rRNA assembly hub
Explanation: The nucleolus is a specialized nuclear domain dedicated mainly to rRNA synthesis and ribosomal subunit assembly, making it an rRNA and ribosome-biogenesis hub. Nucleoplasm, in contrast, is the general internal matrix of the nucleus where chromatin is suspended and many nuclear activities occur. The nucleolus is not membrane-bound, so describing it as double-membrane bound would be incorrect, and nucleoplasm is not outside the nucleus. Nucleoplasm contains ions, nucleotides, enzymes, and regulatory proteins necessary for nuclear functions. This micro-point helps students distinguish a functional subregion (nucleolus) from the overall nuclear interior (nucleoplasm). Therefore, “nucleolus is rRNA assembly hub” is the correct distinguishing statement.
191. In a typical eukaryotic interphase nucleus, chromatin is best defined as:
ⓐ. DNA with proteins complex
ⓑ. DNA with lipids and pigments
ⓒ. Only RNA with ribosomal enzymes
ⓓ. Only proteins with no nucleic acid
Correct Answer: DNA with proteins complex
Explanation: Chromatin is the material of the nucleus made of DNA associated with proteins, mainly histones, and also other non-histone proteins. This DNA–protein organization allows a very long DNA molecule to fit inside the nucleus while still remaining accessible for replication and gene expression. During interphase, chromatin is generally less condensed than visible chromosomes, which helps regulatory proteins access specific DNA regions. The protein components also help fold DNA into higher-order structures and control when genes can be transcribed. Thus, chromatin is correctly defined as a DNA with proteins complex.
192. Euchromatin is generally characterized by:
ⓐ. Very dense packing at all times
ⓑ. No DNA and only histone proteins
ⓒ. Permanently silent gene regions only
ⓓ. Looser packing, active genes
Correct Answer: Looser packing, active genes
Explanation: Euchromatin is the less condensed form of chromatin that is typically richer in actively expressed genes. Because it is loosely packed, transcription factors and RNA polymerase can more easily access DNA sequences required for transcription. This open organization supports higher levels of gene activity compared with tightly packed regions. Euchromatin commonly replicates earlier in the cell cycle and is associated with a more “accessible” chromatin environment. This structural state is therefore linked to functional gene expression. Hence, euchromatin is best described as looser packing with active genes.
193. Heterochromatin is best described as chromatin that is:
ⓐ. Mostly lipid-coated and non-nuclear
ⓑ. Loosely packed for rapid transcription
ⓒ. Tightly packed, less active
ⓓ. Always made of rRNA and proteins
Correct Answer: Tightly packed, less active
Explanation: Heterochromatin refers to chromatin that is highly condensed and generally shows lower levels of transcriptional activity. Its tight packing reduces accessibility of DNA to transcription machinery, so genes in these regions are commonly less expressed. Heterochromatin often contains repetitive DNA and helps maintain chromosome stability and nuclear organization. It can also contribute to silencing of specific genomic regions when required for cellular control. This compact structure is therefore linked to reduced gene activity and structural roles. Thus, heterochromatin is correctly identified as tightly packed and less active.
194. The basic repeating unit of chromatin is the:
ⓐ. Nucleosome unit
ⓑ. Centrosome organizer unit
ⓒ. Ribosome 80S unit
ⓓ. Golgi cisterna unit
Correct Answer: Nucleosome unit
Explanation: The nucleosome is the fundamental structural unit of chromatin, consisting of DNA wrapped around a core of histone proteins. This arrangement compacts DNA while still permitting regulated access for replication and transcription. Nucleosomes create a “beads-on-a-string” appearance at the early level of chromatin packing, forming the first step in higher-order folding. The histone core stabilizes DNA bending and helps organize the genome within the nucleus. Because chromatin structure is built from these repeating units, nucleosomes are central to chromatin organization. Therefore, the basic repeating unit is the nucleosome unit.
195. Which histone is most closely associated with linker DNA and nucleosome stabilization?
ⓐ. Histone H4 core protein
ⓑ. Histone H1 linker protein
ⓒ. Histone H3 core protein
ⓓ. Histone H2A core protein
Correct Answer: Histone H1 linker protein
Explanation: Histone H1 is commonly called the linker histone because it binds to the DNA segment between nucleosomes and helps stabilize the entry and exit points of DNA on the nucleosome. By securing linker DNA, H1 supports tighter packing of nucleosomes into higher-order chromatin fibers. This contributes to overall chromatin compaction and influences accessibility of DNA for transcription. The core nucleosome is formed mainly by histones H2A, H2B, H3, and H4, while H1 plays a stabilizing and organizing role outside the core. Hence, the histone linked to linker DNA is Histone H1 linker protein.
196. During the cell cycle, chromatin most clearly condenses into visible chromosomes in:
ⓐ. S phase of interphase stage
ⓑ. G1 phase before DNA synthesis
ⓒ. G2 phase after DNA synthesis
ⓓ. Prophase of mitosis
Correct Answer: Prophase of mitosis
Explanation: Chromatin condensation becomes most pronounced at the start of mitosis, especially during prophase, when chromosomes become visibly distinct under a microscope. This condensation compacts DNA into manageable units so that duplicated genetic material can be accurately separated. The structural tightening reduces tangling and helps ensure efficient attachment and movement during division. This is why chromosome observation and counting become practical as condensation progresses into mitotic stages. The key idea is that interphase chromatin is generally less condensed, whereas mitosis begins with strong compaction. Therefore, visible chromosome formation is most clearly associated with prophase of mitosis.
197. A Barr body is best described as:
ⓐ. Condensed inactive X chromosome
ⓑ. Active X chromosome during S phase
ⓒ. Mitochondrial DNA cluster region
ⓓ. Nucleolus core rRNA region
Correct Answer: Condensed inactive X chromosome
Explanation: A Barr body is a condensed, transcriptionally inactive X chromosome found in the nuclei of many female somatic cells. It represents a form of heterochromatin created to balance gene expression between individuals with different numbers of X chromosomes. Because the inactivated X is highly compact, it appears as a dense chromatin mass at the nuclear periphery in many cells. This condensation reflects reduced gene activity on that chromosome and is a classic cytological marker of X-chromosome inactivation. The concept links chromatin state with long-term gene regulation. Hence, a Barr body is correctly described as a condensed inactive X chromosome.
198. Histone acetylation is most commonly associated with:
ⓐ. More open chromatin state
ⓑ. Tighter chromatin state always
ⓒ. Removal of DNA from nucleus
ⓓ. Formation of peptidoglycan wall
Correct Answer: More open chromatin state
Explanation: Histone acetylation typically reduces the positive charge on histone tails, weakening their interaction with negatively charged DNA. This loosens chromatin packing and increases accessibility of DNA to transcription factors and RNA polymerase. As a result, acetylation is commonly linked with gene activation and an open chromatin configuration. This modification provides a reversible way to regulate gene expression without changing the DNA sequence itself. Cells use such chromatin modifications to control which genes are turned on or off in different conditions. Therefore, histone acetylation is most associated with a more open chromatin state.
199. For preparing a karyotype, chromosomes are most commonly examined at:
ⓐ. Prophase when condensation begins
ⓑ. Metaphase when aligned
ⓒ. Telophase when nuclei reform
ⓓ. G1 phase before replication starts
Correct Answer: Metaphase when aligned
Explanation: Metaphase is commonly chosen for karyotype analysis because chromosomes are maximally condensed and well separated, making them easier to observe and distinguish. At this stage, chromosomes align at the equatorial plane, which helps in capturing clear images for counting and identifying structural features. High condensation reduces overlap and improves the visibility of chromosome length, centromere position, and banding patterns used in classification. These properties support accurate arrangement into homologous pairs for a karyotype. The key micro-point is that maximal condensation and clear individualization occur around metaphase. Hence, chromosomes are most commonly examined at metaphase when aligned.
200. Nucleolar organizer regions are chromosomal sites mainly containing:
ⓐ. Genes for membrane receptors
ⓑ. Genes for glycolysis enzymes
ⓒ. Genes for histone proteins only
ⓓ. Genes for rRNA
Correct Answer: Genes for rRNA
Explanation: Nucleolar organizer regions are specific chromosomal segments that carry clusters of rRNA genes. These regions are transcriptionally active for rRNA production and act as the organizing sites around which the nucleolus forms. Because rRNA synthesis is essential for ribosome biogenesis, these chromosomal sites strongly influence nucleolar size and activity in cells making many proteins. The association of rRNA gene clusters with nucleolar formation is a standard micro-point linking chromatin organization to nuclear substructures. This explains why nucleolus formation depends on particular chromosomal loci rather than being randomly placed. Therefore, nucleolar organizer regions mainly contain genes for rRNA.