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301. Mitochondrial DNA in most eukaryotic cells is typically:
ⓐ. Linear and histone-rich
ⓑ. Circular and double-stranded
ⓒ. Single-stranded and branched
ⓓ. Protein-only with no nucleotides
Correct Answer: Circular and double-stranded
Explanation: Mitochondrial DNA is commonly described as a circular, double-stranded DNA molecule present within the mitochondrial matrix. This structural form supports replication and gene expression within the organelle and is a standard micro-point used to distinguish it from nuclear DNA organization. The DNA is packaged with proteins but not in the same histone-based chromatin form typical of the nucleus. Its circular nature is often linked with mitochondria’s evolutionary origin as once free-living bacteria-like entities. Because it is double-stranded, it can serve as a stable template for transcription and replication. Therefore, mitochondrial DNA is typically circular and double-stranded.
302. Mitochondrial DNA is located mainly in the:
ⓐ. Cytosol free fluid
ⓑ. Intermembrane space
ⓒ. Outer membrane channels
ⓓ. Mitochondrial matrix
Correct Answer: Mitochondrial matrix
Explanation: Mitochondrial DNA is found within the matrix, the innermost compartment enclosed by the inner membrane. This location keeps the genetic material close to mitochondrial ribosomes and enzymes needed for transcription and translation. It also places mtDNA near the inner membrane where many mitochondrial gene products function as components of oxidative phosphorylation complexes. The matrix environment provides the biochemical setting for organelle-specific gene expression and DNA maintenance. This is why mtDNA is not found in the intermembrane space or on membrane surfaces. Hence, mitochondrial DNA is located mainly in the mitochondrial matrix.
303. A key feature supporting the semi-autonomous nature of mitochondria is that they:
ⓐ. Have peptidoglycan cell wall
ⓑ. Have plasmodesmata connections
ⓒ. Contain DNA and ribosomes
ⓓ. Lack all enzymes for metabolism
Correct Answer: Contain DNA and ribosomes
Explanation: Mitochondria are considered semi-autonomous because they possess their own genetic material and translation machinery. With DNA in the matrix and ribosomes capable of synthesizing some proteins, mitochondria can express part of their genetic information independently of the nucleus. Many essential mitochondrial proteins are still encoded by nuclear genes, so mitochondria are not fully autonomous, but the presence of mtDNA and ribosomes shows partial independence. This feature also aligns with the evolutionary view that mitochondria originated from bacteria-like ancestors. The ability to produce some proteins internally supports efficient assembly of respiratory components near the inner membrane. Therefore, containing DNA and ribosomes is a key indicator of semi-autonomy.
304. In standard cell biology, the inheritance pattern of mitochondrial DNA is commonly described as:
ⓐ. Strictly paternal inheritance
ⓑ. Maternal inheritance
ⓒ. Equal inheritance from both parents
ⓓ. Inheritance only from siblings
Correct Answer: Maternal inheritance
Explanation: Mitochondrial DNA is commonly inherited maternally because the egg contributes most of the cytoplasm and organelles to the zygote. In contrast, the sperm typically contributes mainly nuclear genetic material and very little cytoplasm, so its mitochondria usually do not become the primary source for the embryo. This results in mtDNA being passed largely through the maternal line in many organisms, making it useful for tracing maternal ancestry patterns. The concept is a standard exam micro-point in inheritance and cell biology. It highlights that organelle inheritance can differ from nuclear gene inheritance. Hence, mitochondrial DNA is commonly described as maternally inherited.
305. Mitochondrial DNA is most closely associated with coding for:
ⓐ. Many cell wall enzymes
ⓑ. Some respiratory chain proteins
ⓒ. All cytosolic ribosomal proteins
ⓓ. All nuclear transcription factors
Correct Answer: Some respiratory chain proteins
Explanation: Mitochondrial DNA encodes a limited set of genes, many of which contribute to the oxidative phosphorylation system. This includes certain proteins that are part of respiratory chain complexes and ATP synthesis-related machinery in the inner membrane. While many mitochondrial proteins are encoded in the nucleus and imported, mtDNA provides a subset that is essential for mitochondrial energy function. This localized genetic contribution supports efficient production of some membrane-associated components. It also reinforces the semi-autonomous nature of mitochondria in maintaining key energy pathways. Therefore, mitochondrial DNA is most closely linked to coding for some respiratory chain proteins.
306. The organization of mitochondrial DNA in the matrix is commonly described as being in:
ⓐ. Nucleoids
ⓑ. Nucleosomes
ⓒ. Ribosome granules
ⓓ. Golgi stacks
Correct Answer: Nucleoids
Explanation: Mitochondrial DNA is organized into compact DNA–protein structures called nucleoids within the matrix. These nucleoids help maintain mtDNA stability and coordinate replication and transcription processes. The term is used to reflect a concentrated genetic region without a nuclear envelope, similar in naming to bacterial nucleoid organization. This differs from nuclear DNA, which is packaged into nucleosomes with histones. The nucleoid organization supports efficient expression of mitochondrial genes near relevant metabolic machinery. Hence, mitochondrial DNA is commonly organized in nucleoids.
307. A correct micro-point is that mitochondrial DNA replication is:
ⓐ. Always dependent on mitotic spindle action
ⓑ. Restricted strictly to S phase only
ⓒ. Not possible inside mitochondria
ⓓ. Independent of nuclear cell cycle timing
Correct Answer: Independent of nuclear cell cycle timing
Explanation: Mitochondria can replicate their DNA and divide without being strictly tied to the nuclear cell cycle phases. Because cells may need to adjust mitochondrial number and function based on energy demand, mtDNA replication can occur in response to metabolic requirements. This provides flexibility to increase mitochondrial content in actively respiring cells or maintain organelle populations during growth. The independence from strict nuclear cycle timing is a classic micro-point that separates mitochondrial dynamics from chromosome replication rules. It reflects the semi-autonomous behavior of mitochondria. Therefore, mitochondrial DNA replication is relatively independent of nuclear cell cycle timing.
308. Mitochondrial DNA differs from nuclear DNA mainly in that mitochondrial DNA is typically:
ⓐ. Located in nucleus only
ⓑ. Surrounded by a nuclear envelope
ⓒ. Circular, small genome
ⓓ. Bound to histone octamers
Correct Answer: Circular, small genome
Explanation: Mitochondrial DNA is generally a smaller genome compared with nuclear DNA and is typically circular in structure. It is housed in the mitochondrial matrix rather than within the nucleus and is not enclosed by a nuclear envelope. Its packaging is not the same as nuclear chromatin, and it encodes a limited number of genes mainly linked to mitochondrial function. This contrast helps students identify mtDNA as a distinct genetic system inside the cell. The small, circular genome feature is one of the easiest diagnostic micro-points. Hence, mitochondrial DNA is typically circular with a small genome.
309. A common consequence of mitochondrial DNA mutations is impaired:
ⓐ. Protein synthesis on cytosolic ribosomes
ⓑ. Oxidative phosphorylation efficiency
ⓒ. Cell wall cellulose deposition
ⓓ. Chromosome pairing in meiosis only
Correct Answer: Oxidative phosphorylation efficiency
Explanation: Because mitochondrial DNA encodes some components critical for the respiratory chain and ATP production, mutations can disrupt energy conversion. If these proteins are altered, electron transport and proton pumping can become less efficient, reducing the proton gradient used by ATP synthase. This leads to lower ATP output and can especially affect tissues with high energy demand. The link between mtDNA integrity and oxidative phosphorylation is therefore central to understanding why mtDNA changes can have strong physiological impacts. The outcome is directly tied to mitochondria’s primary function in aerobic ATP generation. Therefore, impaired oxidative phosphorylation efficiency is a common consequence of mtDNA mutations.
310. Mitochondrial DNA is best considered an example of:
ⓐ. Ribosomal genome only
ⓑ. Cell wall genome
ⓒ. Cytosolic genome only
ⓓ. Organelle genome
Correct Answer: Organelle genome
Explanation: Mitochondrial DNA represents genetic material housed within an organelle rather than in the nucleus. This makes it an organelle genome, distinct from the nuclear genome, and it contributes to the semi-autonomous behavior of mitochondria. The organelle genome encodes a limited set of genes important for mitochondrial structure and energy function. Its existence shows that genetic information in eukaryotic cells is not confined to the nucleus alone. This concept is important for understanding inheritance patterns and organelle biology. Hence, mitochondrial DNA is best described as an organelle genome.
311. The term “powerhouse of the cell” for mitochondria mainly reflects their role in:
ⓐ. Making glucose in cytosol pathways
ⓑ. Building ribosomes in nuclear regions
ⓒ. Producing ATP by respiration
ⓓ. Storing DNA as chromatin fibers
Correct Answer: Producing ATP by respiration
Explanation: Mitochondria are called the powerhouse because they generate most cellular ATP during aerobic respiration through oxidative phosphorylation. Electrons from reduced carriers move through inner-membrane complexes, releasing energy to pump protons and build an electrochemical gradient. ATP synthase then uses this gradient to convert ADP and inorganic phosphate into ATP. This links fuel oxidation to a large ATP output in a controlled, efficient way. The inner membrane’s organization into cristae increases the capacity of this process. Therefore, the “powerhouse” label primarily refers to ATP production by respiration.
312. The inner mitochondrial membrane is essential for ATP formation because it:
ⓐ. Maintains a proton gradient
ⓑ. Makes DNA into chromatin coils
ⓒ. Produces ribosomal RNA strands
ⓓ. Acts as a cell wall barrier
Correct Answer: Maintains a proton gradient
Explanation: Oxidative phosphorylation depends on a proton gradient across the inner mitochondrial membrane. Electron transport chain complexes embedded in this membrane pump protons from the matrix to the intermembrane space, storing energy as an electrochemical difference. The inner membrane’s high selectivity prevents rapid proton leakage, so the gradient remains strong enough to drive ATP synthase. ATP synthase then allows protons to return to the matrix in a controlled way, coupling this flow to ATP synthesis. Without a stable gradient, ATP synthase cannot operate efficiently. Hence, maintaining the proton gradient is the key inner-membrane requirement.
313. In aerobic respiration, oxygen is crucial because it:
ⓐ. Pumps protons into the matrix space
ⓑ. Forms ATP directly in the matrix
ⓒ. Builds cristae folds in the inner membrane
ⓓ. Accepts electrons at the end
Correct Answer: Accepts electrons at the end
Explanation: Oxygen serves as the terminal electron acceptor in the electron transport chain. By accepting electrons at the final step, oxygen allows continuous electron flow through the chain, which is necessary for sustained proton pumping across the inner membrane. This pumping establishes the proton gradient that powers ATP synthase, so oxygen indirectly supports ATP production by keeping the chain running. If oxygen is unavailable, electron flow backs up, proton pumping slows, and oxidative ATP synthesis drops sharply. This is why aerobic cells depend on oxygen for high ATP yield. Therefore, oxygen’s key role is accepting electrons at the end.
314. A direct way mitochondrial matrix reactions support the “powerhouse” function is by producing:
ⓐ. Starch granules for long storage
ⓑ. NADH for the electron chain
ⓒ. Cellulose units for wall formation
ⓓ. rRNA for ribosome assembly
Correct Answer: NADH for the electron chain
Explanation: The mitochondrial matrix contains pathways such as the Krebs cycle that generate reduced electron carriers, especially NADH. NADH donates high-energy electrons to the electron transport chain in the inner membrane, providing the energy source for proton pumping. Proton pumping builds the gradient that ATP synthase uses to form ATP, so matrix NADH production is a direct input to oxidative phosphorylation. This connection explains why matrix metabolism and inner-membrane respiration are tightly coupled. When NADH supply is reduced, electron flow and ATP output typically decline. Hence, producing NADH for the electron chain is a key matrix contribution to the powerhouse role.
315. Cristae make mitochondria more efficient “powerhouses” mainly because they:
ⓐ. Increase inner membrane area
ⓑ. Store water in the matrix fluid
ⓒ. Protect DNA with a nuclear envelope
ⓓ. Form cell wall support layers
Correct Answer: Increase inner membrane area
Explanation: Cristae are folds of the inner mitochondrial membrane that greatly increase the membrane surface available for respiration. The electron transport chain complexes and ATP synthase are embedded in this inner membrane, so more surface area allows more of these protein machines to operate simultaneously. This raises the capacity for electron transfer, proton pumping, and ATP synthesis within the same organelle volume. Cells with high energy demand often show more developed cristae because they need greater oxidative phosphorylation throughput. The structural design therefore directly supports higher ATP production. Thus, increasing inner membrane area is the key efficiency advantage of cristae.
316. Mitochondrial DNA supports the “powerhouse” function mainly because it encodes:
ⓐ. All enzymes of glycolysis
ⓑ. All proteins of the cell membrane
ⓒ. Some respiration complex proteins
ⓓ. All histones for chromosome packing
Correct Answer: Some respiration complex proteins
Explanation: Mitochondrial DNA encodes a limited set of genes, and several of them contribute to oxidative phosphorylation by producing key protein components of respiration complexes. These gene products are closely tied to electron transport and ATP synthesis on the inner membrane, so defects in mtDNA can reduce ATP output. Although many mitochondrial proteins are nuclear-encoded and imported, mtDNA provides essential parts that help the organelle maintain its energy-conversion machinery. This is why mtDNA integrity is strongly linked to cellular energy performance, especially in high-demand tissues. The presence of mtDNA also reflects the semi-autonomous nature of mitochondria. Therefore, mtDNA supports the powerhouse role by encoding some respiration complex proteins.
317. Mature mammalian red blood cells do not act as strong “powerhouses” mainly because they:
ⓐ. Have extra mitochondria in cytosol
ⓑ. Lack mitochondria and use glycolysis
ⓒ. Store oxygen only in mitochondria cristae
ⓓ. Perform oxidative phosphorylation in nucleus
Correct Answer: Lack mitochondria and use glycolysis
Explanation: Mature mammalian red blood cells lack mitochondria, so they cannot carry out oxidative phosphorylation. Without mitochondria, they cannot build a proton gradient across an inner mitochondrial membrane or run an electron transport chain to produce large amounts of ATP. As a result, their ATP production relies mainly on glycolysis in the cytosol, which yields much less ATP per glucose than aerobic respiration. This also helps red blood cells conserve the oxygen they transport, rather than consuming it for their own respiration. The absence of mitochondria therefore explains both their energy strategy and their specialized function. Hence, they lack mitochondria and use glycolysis.
318. An uncoupler that collapses the proton gradient would most directly cause:
ⓐ. Increased ATP synthesis with low oxygen use
ⓑ. Increased ATP synthesis with high gradient
ⓒ. No electron flow through membrane complexes
ⓓ. Low ATP output despite respiration
Correct Answer: Low ATP output despite respiration
Explanation: Uncouplers reduce or eliminate the proton gradient across the inner mitochondrial membrane by allowing protons to return without passing through ATP synthase in a controlled way. Electron transport can continue and may even accelerate because the chain is no longer limited by gradient buildup, so oxygen consumption can persist. However, ATP synthase loses the driving force required to convert ADP and inorganic phosphate into ATP efficiently. The result is that respiration-related electron flow is not effectively converted into ATP, reducing the cell’s usable energy output. This demonstrates that the gradient is the key link between electron transport and ATP formation. Therefore, ATP output becomes low despite ongoing respiration.
319. Cells such as cardiac muscle have many mitochondria mainly because they need:
ⓐ. High continuous ATP supply
ⓑ. Large stores of nuclear DNA
ⓒ. High rates of cell wall synthesis
ⓓ. Extra ribosome assembly centers
Correct Answer: High continuous ATP supply
Explanation: Cardiac muscle contracts continuously and therefore requires a steady, high-rate supply of ATP. Mitochondria provide the most efficient large-scale ATP production through aerobic respiration, so cells with persistent energy demand increase mitochondrial number and capacity. More mitochondria allow greater total oxidative phosphorylation output, supported by extensive cristae and abundant respiratory proteins. This structural abundance is a functional adaptation that maintains contractile performance without rapid energy depletion. It also ensures ATP availability for ion pumps and other processes essential for rhythmic contraction. Thus, many mitochondria reflect the need for high continuous ATP supply.
320. A precise statement linking mitochondria to ATP output is that ATP is produced mainly when protons:
ⓐ. Move from matrix to intermembrane space
ⓑ. Exit the cell through plasma membrane pores
ⓒ. Flow into matrix through ATP synthase
ⓓ. Enter nucleus through nuclear pore complexes
Correct Answer: Flow into matrix through ATP synthase
Explanation: Electron transport pumps protons out of the matrix into the intermembrane space, building a gradient across the inner membrane. ATP synthase then provides a route for protons to flow back into the matrix, and the energy released during this controlled return drives ATP formation from ADP and inorganic phosphate. This chemiosmotic coupling is the central logic behind mitochondria’s high ATP yield and explains why inner membrane integrity is essential. If proton return occurs without ATP synthase, the energy is not captured as ATP. Therefore, ATP production is tied to protons flowing into the matrix through ATP synthase.
321. In a chloroplast, grana are best described as:
ⓐ. Stacks of thylakoid membranes
ⓑ. DNA coils inside nucleus
ⓒ. Cristae folds in mitochondria
ⓓ. Ribosome clusters on rough ER
Correct Answer: Stacks of thylakoid membranes
Explanation: Grana are the stack-like arrangements of thylakoid membranes inside chloroplasts. These stacked membranes provide a large surface area to house pigments and protein complexes involved in the light reactions of photosynthesis. The organized stacking helps efficiently capture light energy and support electron transport processes that generate chemical energy. Because thylakoids are the functional membrane system for light-driven reactions, their stacking into grana is a key structural micro-point. This feature distinguishes chloroplast internal organization from other organelles. Therefore, grana are stacks of thylakoid membranes.
322. The stroma of a chloroplast is best described as the:
ⓐ. Membrane folds carrying ATP synthase only
ⓑ. Space between two mitochondrial membranes
ⓒ. Outer chloroplast wall made of cellulose
ⓓ. Fluid matrix surrounding thylakoids
Correct Answer: Fluid matrix surrounding thylakoids
Explanation: The stroma is the fluid-filled internal matrix of the chloroplast that surrounds the grana and thylakoid system. It contains enzymes, metabolites, and other components needed for the carbon-fixation phase of photosynthesis. The stroma also contains chloroplast DNA and ribosomes, supporting partial independence in protein synthesis. Its role is distinct from the thylakoid membranes where light reactions occur; the stroma supports the biochemical steps that use products of light reactions to build carbohydrates. This compartmental separation improves efficiency by placing related reactions in the same region. Hence, the stroma is the fluid matrix surrounding thylakoids.
323. A correct location match is that light reactions occur mainly on:
ⓐ. Outer chloroplast membrane surface
ⓑ. Stroma fluid outside membranes
ⓒ. Thylakoid membranes in grana
ⓓ. Nuclear envelope inner layer
Correct Answer: Thylakoid membranes in grana
Explanation: The light reactions of photosynthesis take place on thylakoid membranes because these membranes contain chlorophyll and associated protein complexes for light capture and electron transport. Grana are stacks of thylakoids, so they provide extensive membrane area for these reactions. Light energy drives electron movement through membrane complexes, leading to formation of energy-rich molecules needed for later steps. This location is a key micro-point because it contrasts with carbon-fixation reactions that occur in the stroma. The membrane-based organization also supports formation of a proton gradient across the thylakoid membrane. Therefore, light reactions occur mainly on thylakoid membranes in grana.
324. A correct location match is that carbon-fixation reactions occur mainly in:
ⓐ. Stroma
ⓑ. Thylakoid lumen
ⓒ. Outer membrane space
ⓓ. Nuclear nucleoplasm
Correct Answer: Stroma
Explanation: Carbon-fixation reactions occur in the chloroplast stroma where the required enzymes and substrates are present. The stroma provides the biochemical environment for using energy-rich products generated by light reactions to build carbohydrate-related intermediates. This compartment contains soluble enzymes and supports stepwise synthesis processes that are not membrane-bound like the light reactions. The separation of stroma-based carbon fixation from thylakoid membrane light reactions helps coordinate energy capture with carbon assimilation. It also allows regulated exchange of molecules between the two compartments. Hence, carbon-fixation reactions occur mainly in the stroma.
325. Thylakoids within a granum are interconnected with other grana mainly by:
ⓐ. Centrioles
ⓑ. Lamellae
ⓒ. Nuclear pores
ⓓ. Cell wall pits
Correct Answer: Lamellae
Explanation: Within chloroplasts, thylakoid stacks (grana) are connected by unstacked thylakoid membranes called stroma lamellae. These lamellae link the internal membrane system into a continuous network, enabling coordinated flow of electrons and distribution of photosynthetic components. The connectivity ensures that different regions of the thylakoid system function together rather than as isolated stacks. This structural arrangement is a standard micro-point describing chloroplast internal architecture. It helps explain how photosynthetic reactions can be integrated across the entire chloroplast. Therefore, the connecting structures are lamellae.
326. The chloroplast stroma typically contains:
ⓐ. 80S ribosomes only
ⓑ. DNA and 70S ribosomes
ⓒ. Peptidoglycan wall sheets
ⓓ. Spindle fibers for mitosis
Correct Answer: DNA and 70S ribosomes
Explanation: Chloroplasts contain their own genetic material and ribosomes, and these components are located in the stroma. The ribosomes in chloroplasts are typically described as 70S type, reflecting a prokaryote-like feature of these organelles. This supports the idea that chloroplasts are semi-autonomous, able to synthesize some of their proteins internally. The stroma therefore houses not only enzymes for carbon-fixation reactions but also the genetic and translational machinery for organelle-specific protein production. This micro-point is frequently tested to link chloroplast structure with functional independence. Hence, the stroma contains DNA and 70S ribosomes.
327. The thylakoid lumen refers to:
ⓐ. Fluid space inside thylakoid sacs
ⓑ. Fluid space outside chloroplast
ⓒ. Matrix inside mitochondria only
ⓓ. Space inside nucleus envelope
Correct Answer: Fluid space inside thylakoid sacs
Explanation: Thylakoid lumen is the internal space enclosed by the thylakoid membrane in chloroplasts. During light reactions, protons accumulate in this lumen, creating a gradient across the thylakoid membrane that helps drive ATP formation. The lumen is therefore a key compartment in photosynthetic energy conversion, not merely an empty cavity. Its existence allows separation of proton-rich and proton-poor regions, enabling chemiosmotic coupling similar in principle to mitochondria but with different compartments. This micro-point helps students distinguish between thylakoid membrane, lumen, and stroma. Thus, the thylakoid lumen is the fluid space inside thylakoid sacs.
328. A correct micro-point is that chlorophyll pigments are mainly located in:
ⓐ. Nucleus chromatin region
ⓑ. Stroma fluid only
ⓒ. Outer chloroplast envelope
ⓓ. Thylakoid membranes
Correct Answer: Thylakoid membranes
Explanation: Chlorophyll and accessory pigments are embedded in protein complexes within thylakoid membranes. This placement allows pigments to capture light and transfer excitation energy to reaction centers that initiate electron transport. The membrane environment is essential because the resulting electron flow occurs through membrane-bound carriers and supports proton movement across the thylakoid membrane. Pigments therefore must be positioned where light capture can directly connect to electron transport and gradient formation. This is why pigment localization is a key structural micro-point in chloroplast function. Hence, chlorophyll pigments are mainly located in thylakoid membranes.
329. If a chloroplast shows highly developed grana, it most directly suggests high capacity for:
ⓐ. Ribosome assembly in nucleolus
ⓑ. DNA replication in nucleus
ⓒ. Light energy capture reactions
ⓓ. Lipid detox in smooth ER
Correct Answer: Light energy capture reactions
Explanation: Grana represent stacks of thylakoid membranes, and the thylakoid membranes are the main site for the light reactions of photosynthesis. More or more developed grana usually indicate expanded thylakoid membrane area and thus greater capacity for pigment-based light capture and electron transport. This supports higher potential for generating energy-rich products used in subsequent carbon assimilation steps. The structural increase is therefore a functional adaptation to maximize light-driven processes. This relationship between membrane stacking and photosynthetic capacity is a common exam logic point. Therefore, highly developed grana suggest high capacity for light energy capture reactions.
330. A correct chloroplast-part comparison is that grana are mainly:
ⓐ. Membrane stacks, stroma is fluid matrix
ⓑ. Fluid matrix, stroma is membrane stacks
ⓒ. Nuclear pores, stroma is chromosomes
ⓓ. Cristae folds, stroma is intermembrane space
Correct Answer: Membrane stacks, stroma is fluid matrix
Explanation: Grana are the stacked thylakoid membranes that form the membrane-rich zones of the chloroplast, while the stroma is the surrounding fluid matrix. The thylakoid membranes support light reactions, whereas the stroma contains enzymes and components for carbon-fixation processes and also contains chloroplast DNA and ribosomes. This division of labor depends on clear structural separation: membranes for light-driven electron transport and soluble matrix for enzyme-driven synthesis steps. The terminology is therefore a direct map of chloroplast architecture and function. Remembering this comparison helps avoid confusion between thylakoid-related parts and the surrounding stroma. Hence, grana are membrane stacks and stroma is the fluid matrix.
331. Chromoplasts are plastids mainly specialized for:
ⓐ. Protein export toward Golgi vesicles
ⓑ. Chlorophyll synthesis in grana stacks
ⓒ. Starch storage for seed nutrition
ⓓ. Carotenoid pigment storage
Correct Answer: Carotenoid pigment storage
Explanation: Chromoplasts are plastids whose primary specialization is the accumulation of coloured pigments, mainly carotenoids, giving yellow, orange, or red colour to plant parts. These pigments are not just “colour” but are stored in a plastid system designed for pigment deposition and stability. Chromoplasts commonly develop in petals and ripening fruits where visible colour has ecological value. Carotenoid storage in chromoplasts supports attraction of pollinators and animals involved in seed dispersal. Unlike chloroplasts, the focus is not efficient light-energy conversion but pigment-based coloration. Hence, chromoplasts are mainly specialized for carotenoid pigment storage.
332. A plant tissue most typically rich in chromoplasts is:
ⓐ. Root meristem near the tip zone
ⓑ. Young leaf mesophyll in early growth
ⓒ. Ripe tomato pericarp tissue
ⓓ. Xylem vessel wall thickening region
Correct Answer: Ripe tomato pericarp tissue
Explanation: Chromoplasts are especially abundant in coloured non-photosynthetic tissues, and ripening fruits are classic examples where pigmentation increases as the fruit matures. In ripe tomato pericarp, chloroplasts in earlier stages commonly shift toward chromoplast-like pigment storage, producing characteristic red coloration. This reflects active carotenoid accumulation rather than chlorophyll-based photosynthesis. Such colour development is a functional signal for animals that helps in dispersal when the fruit is mature. The tissue-level abundance of chromoplasts therefore correlates with visible colour intensity in ripe fruit parts. Hence, ripe tomato pericarp tissue is typically rich in chromoplasts.
333. The main pigment group responsible for most chromoplast colours is:
ⓐ. Chlorophyll a and chlorophyll b
ⓑ. Phycobilins in cyanobacterial cells
ⓒ. Anthocyanins in cell vacuoles
ⓓ. Carotenoids as plastid pigments
Correct Answer: Carotenoids as plastid pigments
Explanation: Chromoplast coloration is primarily due to carotenoids, which are plastid-localized pigments producing yellow, orange, and red shades. These pigments accumulate within chromoplast structures and give stable coloration to petals and fruits. Carotenoids are chemically suited for storage in plastid membranes or associated bodies, so chromoplasts specialize in their deposition. This pigment accumulation often rises during ripening or floral development, making colours more intense at functional stages. The link between carotenoids and chromoplasts is a high-frequency exam micro-point in plastid differentiation. Therefore, carotenoids as plastid pigments are responsible for most chromoplast colours.
334. During fruit ripening, the plastid change most directly linked with increased red/orange colour is:
ⓐ. Chromoplast converts to chloroplast
ⓑ. Chloroplast converts to chromoplast
ⓒ. Leucoplast converts to mitochondrion
ⓓ. Nucleus converts to pigment vesicle
Correct Answer: Chloroplast converts to chromoplast
Explanation: In many ripening fruits, plastids that were earlier chloroplasts lose photosynthetic structures and shift toward pigment accumulation, forming chromoplasts. This transition accompanies chlorophyll reduction and carotenoid buildup, producing the visible colour change typical of ripening. The change is not a random loss of organelles; it is a differentiated shift in plastid function from photosynthesis to pigmentation. Chromoplast development supports ecological signalling that the fruit is ready, improving the chance of seed dispersal. This transformation is therefore a direct structural-functional basis for ripening colour changes. Hence, chloroplast converts to chromoplast is the correct plastid change.
335. The most direct ecological role of chromoplast-based pigmentation in flowers is:
ⓐ. Attract pollinators and dispersers
ⓑ. Improve water conduction in xylem
ⓒ. Increase nitrogen fixation in roots
ⓓ. Strengthen cellulose microfibril layers
Correct Answer: Attract pollinators and dispersers
Explanation: Chromoplast pigments create bright colours in petals and fruits that serve as visual signals to animals. In flowers, this colour display helps attract pollinators to the reproductive structures, improving the chance of successful pollination. In fruits, strong coloration often signals ripeness and encourages animals to eat the fruit and disperse seeds. This role is functional even though chromoplasts do not primarily produce ATP through photosynthesis. The pigment-based attraction strategy is a key example of how cell organelles support plant reproductive success. Therefore, the direct ecological role is to attract pollinators and dispersers.
336. A correct functional distinction is that chromoplasts generally:
ⓐ. Perform high-rate photolysis of water
ⓑ. Synthesize ATP mainly by respiration
ⓒ. Lack major photosynthetic activity
ⓓ. Build ribosomes for cytosolic translation
Correct Answer: Lack major photosynthetic activity
Explanation: Chromoplasts are plastids specialized for pigment accumulation rather than for efficient photosynthesis. As they develop, the internal organization shifts away from photosynthetic membrane systems and toward structures that store carotenoid pigments. This means their primary contribution is colour formation in petals, fruits, and some roots, not sustained light-energy conversion. Although they remain plastids, their functional emphasis is different from chloroplasts, which are optimized for light reactions and carbon assimilation support. This distinction explains why chromoplast-rich tissues are often coloured but not major sites of photosynthetic output. Hence, chromoplasts generally lack major photosynthetic activity.
337. The pigment category commonly accumulated in chromoplasts includes:
ⓐ. Phycocyanin-type pigments
ⓑ. Xanthophyll pigments group
ⓒ. Hemoglobin-like oxygen pigments
ⓓ. Histone-based chromatin pigments
Correct Answer: Xanthophyll pigments group
Explanation: Chromoplasts store carotenoid pigments, and xanthophylls are a major carotenoid subgroup commonly associated with yellow coloration. These pigments accumulate within plastid structures designed for stable storage and visible colour expression. Xanthophyll presence in chromoplasts is consistent with their role in petals and ripening fruits where colour intensity is biologically meaningful. The storage of such pigments supports signalling rather than direct nuclear functions or oxygen transport. This pigment-based specialization is a standard point when differentiating plastid types by their dominant contents. Therefore, xanthophyll pigments group is commonly accumulated in chromoplasts.
338. Chromoplasts are correctly classified as a type of:
ⓐ. Endoplasmic reticulum network region
ⓑ. Mitochondrial inner membrane fold
ⓒ. Lysosomal enzyme storage vesicle
ⓓ. Plastid organelle category
Correct Answer: Plastid organelle category
Explanation: Chromoplasts belong to the plastid family of organelles found in plant cells and some algae. Plastids include chloroplasts for photosynthesis, leucoplasts for storage, and chromoplasts for pigmentation, showing functional specialization within a related organelle group. Chromoplasts share the basic plastid identity but differ in internal organization and dominant contents, especially carotenoid pigments. Their classification as plastids explains their presence mainly in plant tissues and their ability to arise by differentiation from other plastid forms. This is a foundational micro-point used to organize organelles by lineage and function. Hence, chromoplasts are a plastid organelle category.
339. A plant part where chromoplasts are commonly abundant is:
ⓐ. Petal epidermal cells
ⓑ. Guard cells of young leaves
ⓒ. Root meristem dividing cells
ⓓ. Phloem sieve tube lumen
Correct Answer: Petal epidermal cells
Explanation: Petals are often brightly coloured, and that colour is typically produced by pigment-containing structures within their cells. Chromoplasts accumulate carotenoids in many petals, creating yellow, orange, or red shades that enhance visibility to pollinators. Petal epidermal cells are therefore a common site where chromoplast abundance is high, aligning structure with the flower’s attraction function. This localization supports the broader biological role of colour signals in reproduction. Because meristematic and transport tissues focus on division or conduction, they are less characteristically pigment-rich. Thus, petal epidermal cells commonly contain abundant chromoplasts.
340. A leucoplast is most likely to differentiate toward a chromoplast when:
ⓐ. Starch hydrolysis stops completely
ⓑ. Chlorophyll content sharply rises
ⓒ. Carotenoids accumulate strongly
ⓓ. Nuclear DNA becomes circular
Correct Answer: Carotenoids accumulate strongly
Explanation: Plastids can interconvert based on developmental cues and the biochemical demands of the tissue. When a tissue begins to accumulate carotenoid pigments for visible coloration, plastids shift toward chromoplast characteristics to support pigment deposition and storage. This differentiation reflects a functional change: from storage-oriented plastids toward pigment-specialized plastids. The trigger is closely tied to carotenoid synthesis and accumulation, which produces the visible colour typical of chromoplast-rich tissues. This concept explains colour development in specific plant organs and is frequently tested as plastid differentiation logic. Therefore, leucoplasts can differentiate toward chromoplasts when carotenoids accumulate strongly.
341. Leucoplasts are best described as plastids that are typically:
ⓐ. Green and photosynthetic in leaves
ⓑ. Pigmented red/orange in ripe fruits
ⓒ. Colourless and storage-oriented
ⓓ. Enzyme sacs for intracellular digestion
Correct Answer: Colourless and storage-oriented
Explanation: Leucoplasts are plastids that generally lack visible pigments, so they appear colourless in most tissues. Their primary role is storage and related metabolism rather than light capture, which is why they are common in non-green plant parts. They can store carbohydrates, oils, or proteins depending on the specific leucoplast type and the tissue’s needs. This specialization supports energy reserve formation in seeds, tubers, and other storage organs. Because they are plastids, they share a basic organelle identity with chloroplasts and chromoplasts, but their dominant function is storage. Hence, leucoplasts are colourless and storage-oriented.
342. The leucoplast type mainly involved in starch storage is:
ⓐ. Amyloplast
ⓑ. Elaioplast
ⓒ. Chromoplast
ⓓ. Protein body plastid
Correct Answer: Amyloplast
Explanation: Amyloplasts are leucoplasts specialized for synthesizing and storing starch as a reserve carbohydrate. They are prominent in storage tissues such as tubers, seeds, and some roots where long-term energy storage is required. Their internal organization supports accumulation of starch grains that can later be mobilized during germination or growth. This function distinguishes them from leucoplasts that store oils or proteins. Because starch is a major plant reserve, amyloplasts play a key role in plant nutrition and development. Therefore, the leucoplast for starch storage is the amyloplast.
343. A tissue most likely to contain many leucoplasts is:
ⓐ. Palisade mesophyll of a young leaf
ⓑ. Epidermis of a brightly coloured petal
ⓒ. Spongy mesophyll of a green leaf
ⓓ. Potato tuber storage tissue
Correct Answer: Potato tuber storage tissue
Explanation: Leucoplasts are common in non-green storage organs where photosynthesis is not the primary activity. Potato tubers are classic storage tissues that accumulate reserve food, especially starch, and therefore contain many amyloplasts. These plastids support deposition of starch grains that act as energy reserves for sprouting and growth when conditions become favourable. The lack of chlorophyll in tuber storage tissue matches the colourless nature of leucoplasts. This organelle distribution reflects functional adaptation to storage rather than light harvesting. Hence, potato tuber storage tissue is rich in leucoplasts.
344. The leucoplast type mainly associated with oil or fat storage is:
ⓐ. Amyloplast
ⓑ. Proteinoplast
ⓒ. Elaioplast
ⓓ. Chloroplast
Correct Answer: Elaioplast
Explanation: Elaioplasts are leucoplasts specialized for storing lipids, commonly seen in tissues where oils are accumulated as reserve materials. Their internal structure supports deposition of lipid droplets or oil-rich components that can be mobilized when needed. This is distinct from amyloplasts, which store starch, and proteinoplasts, which store proteins. Lipid reserves are especially important in certain seeds and plant tissues that require dense energy storage. Because leucoplasts are non-pigmented, this storage occurs without contributing green colour to the tissue. Therefore, the leucoplast linked with oil storage is the elaioplast.
345. A protein-storing leucoplast is commonly termed:
ⓐ. Elaioplast
ⓑ. Proteinoplast
ⓒ. Chromoplast
ⓓ. Thylakoid plastid
Correct Answer: Proteinoplast
Explanation: Proteinoplasts are leucoplasts specialized for storage of proteins, particularly in tissues such as certain seeds where protein reserves support early growth after germination. Their function is to accumulate and maintain protein materials in a form that can later be broken down into amino acids for new synthesis. This role differs from lipid storage in elaioplasts and starch storage in amyloplasts, showing that leucoplasts can specialize based on reserve type. Protein storage contributes to seed nutritional quality and early developmental needs. The organelle’s lack of photosynthetic pigment is consistent with storage roles in non-green tissues. Hence, a protein-storing leucoplast is termed a proteinoplast.
346. A correct comparison is that leucoplasts differ from chloroplasts mainly because leucoplasts:
ⓐ. Contain chlorophyll and grana stacks
ⓑ. Are digestive vesicles with hydrolases
ⓒ. Have only one surrounding membrane
ⓓ. Are not specialized for photosynthesis
Correct Answer: Are not specialized for photosynthesis
Explanation: Chloroplasts are specialized for photosynthesis with pigment systems and thylakoid membranes for light reactions, while leucoplasts are mainly storage-oriented plastids without such photosynthetic specialization. Leucoplasts typically lack chlorophyll, so they do not contribute to light capture and appear colourless. Their internal components support synthesis and accumulation of reserves such as starch, oils, or proteins, depending on the leucoplast subtype. This functional difference explains why leucoplasts are abundant in storage organs and seeds, whereas chloroplasts are abundant in green, light-exposed tissues. The contrast is therefore a functional adaptation to tissue role rather than a random structural variation. Hence, leucoplasts are not specialized for photosynthesis.
347. A leucoplast most directly linked with carbohydrate reserve accumulation is a:
ⓐ. Elaioplast
ⓑ. Proteinoplast
ⓒ. Amyloplast
ⓓ. Chromoplast
Correct Answer: Amyloplast
Explanation: Carbohydrate reserves in many plant storage tissues are largely stored as starch, and amyloplasts are the plastids that specialize in starch synthesis and accumulation. Their structure supports formation of starch grains that can be stored for long periods and mobilized when energy is needed. This is especially important in organs like tubers and seeds that act as reserve reservoirs. The link between amyloplasts and starch provides a clear organelle-to-function mapping commonly tested in exams. Because starch is a carbohydrate polymer, identifying its storage plastid helps connect cell biology to plant physiology. Therefore, the leucoplast tied to carbohydrate reserve accumulation is the amyloplast.
348. During seed development, leucoplasts are especially important because they:
ⓐ. Replace mitochondria in respiration fully
ⓑ. Store reserve food for germination
ⓒ. Split water to release oxygen directly
ⓓ. Assemble spindle fibers for mitosis
Correct Answer: Store reserve food for germination
Explanation: Seeds need stored reserves to support early growth when the seedling cannot yet photosynthesize effectively. Leucoplasts contribute by accumulating storage materials such as starch, oils, and proteins, depending on the seed type and tissue. These reserves are later mobilized during germination to fuel cell division, growth, and synthesis of new structures. The storage function is central to seed viability and successful establishment of the young plant. Because leucoplasts are colourless and storage-focused, they fit the role of reserve accumulation rather than pigment-based functions. Hence, leucoplasts are important in seeds because they store reserve food for germination.
349. In many plant cells, plastid interconversion can occur; a common change during greening is:
ⓐ. Leucoplast to chloroplast
ⓑ. Chromoplast to lysosome
ⓒ. Mitochondrion to leucoplast
ⓓ. Nucleus to plastid vesicle
Correct Answer: Leucoplast to chloroplast
Explanation: Plastids can differentiate based on environmental and developmental cues, and exposure to light can trigger development of photosynthetic structures. In tissues that begin to receive light, colourless plastids can develop chlorophyll and thylakoid membrane systems, becoming functional chloroplasts. This change supports the plant’s shift from dependence on stored reserves toward active photosynthesis. The interconversion highlights that plastids are a related organelle family capable of functional specialization changes. This concept is commonly tested to connect plastid biology with plant development and light response. Therefore, a common greening change is leucoplast to chloroplast.
350. A correct match of leucoplast subtype to stored material is:
ⓐ. Elaioplast—starch storage mainly
ⓑ. Amyloplast—protein storage mainly
ⓒ. Proteinoplast—oil storage mainly
ⓓ. Amyloplast—starch storage mainly
Correct Answer: Amyloplast—starch storage mainly
Explanation: Amyloplasts are leucoplasts that specialize in synthesizing and storing starch, making this match a standard reference point for plastid subtypes. Their presence is strongly associated with storage organs like tubers and with tissues that accumulate carbohydrate reserves. The stored starch grains serve as a long-term energy reservoir that can be broken down when needed for growth or germination. This subtype-based storage logic helps distinguish leucoplast forms by function rather than by appearance alone. Remembering this correct pairing also helps avoid common confusion between starch, oil, and protein storage plastids. Hence, the correct match is amyloplast—starch storage mainly.
351. A secreted protein in a eukaryotic cell is most likely synthesized on:
ⓐ. RER-bound ribosomes
ⓑ. Free ribosomes in cytosol
ⓒ. Ribosomes inside mitochondria
ⓓ. Ribosomes inside chloroplasts
Correct Answer: RER-bound ribosomes
Explanation: Proteins destined for secretion enter the endomembrane pathway, and their translation is directed to ribosomes attached to the rough endoplasmic reticulum. This targeting happens because the growing polypeptide carries an ER-targeting signal that routes the ribosome–mRNA complex to the ER membrane. As translation continues, the polypeptide is inserted into or translocated across the ER membrane, which is essential for later packaging and export. This ER entry step is a defining feature of the secretory route in eukaryotic cells. The “bound ribosome” location therefore matches the secretion destination logic. Hence, RER-bound ribosomes are most likely involved.
352. A protein that stays and functions in the cytosol is typically made on:
ⓐ. RER-bound ribosomes
ⓑ. Free ribosomes in cytosol
ⓒ. Ribosomes in Golgi cisternae
ⓓ. Ribosomes in lysosomal lumen
Correct Answer: Free ribosomes in cytosol
Explanation: Cytosolic proteins are generally synthesized on free ribosomes because they do not need to enter the ER-based trafficking system. Without an ER-targeting signal, translation proceeds in the cytosol and the completed protein remains available for cytosolic functions. This matches the basic sorting rule that ribosome attachment to ER is mainly for proteins entering the secretory and membrane pathways. Free ribosomes therefore supply most enzymes and structural proteins that operate in the cytoplasmic matrix. The location of synthesis aligns directly with the final destination for many proteins. Thus, free ribosomes in the cytosol typically make cytosolic proteins.
353. Ribosomes attached to RER differ from free ribosomes mainly in:
ⓐ. Ribosome size and subunit count
ⓑ. rRNA chemical composition
ⓒ. Protein destination being targeted
ⓓ. Having an extra membrane around them
Correct Answer: Protein destination being targeted
Explanation: Free and bound ribosomes are structurally the same type in the cytoplasm; what differs is which proteins they are translating and where those proteins are directed. When a translated polypeptide carries an ER-targeting signal, the translating ribosome becomes associated with the ER membrane and the protein enters the endomembrane pathway. If no such signal is present, the ribosome remains free and the protein stays in the cytosol or is targeted to other organelles after synthesis. This is a sorting-by-signal concept rather than a ribosome-structure difference. Therefore, the key difference is the destination being targeted.
354. A major reason RER appears “rough” under a microscope is:
ⓐ. Lipid droplets embedded in ER
ⓑ. Dense glycocalyx on ER surface
ⓒ. Chlorophyll pigment granules
ⓓ. Ribosomes on cytosolic ER face
Correct Answer: Ribosomes on cytosolic ER face
Explanation: Rough ER looks rough because many ribosomes are attached to its cytosolic surface, creating a granular appearance. These ribosomes are actively translating proteins that are destined for secretion, membranes, or certain internal compartments, so the ER becomes heavily ribosome-studded in high protein-output cells. The ribosomes themselves are the visible particles that create the “rough” texture in electron micrographs. This is a structural marker of intense protein synthesis linked to the endomembrane system. The association is functional and dynamic, but it produces a consistent rough appearance when many ribosomes are engaged. Hence, ribosomes on the cytosolic ER face cause the roughness.
355. A membrane protein of the plasma membrane is most likely synthesized on:
ⓐ. Free ribosomes in cytosol
ⓑ. Ribosomes inside the nucleus
ⓒ. Ribosomes attached to RER
ⓓ. Ribosomes inside lysosomes
Correct Answer: Ribosomes attached to RER
Explanation: Plasma membrane proteins are inserted into membranes during synthesis, and this co-translational insertion occurs at the rough ER. The ER membrane provides the machinery to position hydrophobic segments into the lipid bilayer with correct orientation, which is essential for receptors and channels. After insertion into the ER membrane, these proteins travel by vesicular transport through the Golgi and then to the plasma membrane. This pathway ensures membrane proteins are not exposed as free hydrophobic chains in the cytosol. The site of synthesis therefore matches the need for early membrane insertion. Thus, ribosomes attached to RER synthesize plasma membrane proteins.
356. Which statement correctly describes free vs bound ribosomes in eukaryotic cells?
ⓐ. Free ribosomes cannot form polyribosomes
ⓑ. Bound ribosomes are permanently fixed
ⓒ. Free ribosomes only make lysosomal enzymes
ⓓ. Same ribosomes, different targeting
Correct Answer: Same ribosomes, different targeting
Explanation: Free and bound ribosomes are not two different ribosome types; they are the same cytoplasmic ribosomes that shift location based on the protein being translated. When translation begins for a protein with an ER-targeting signal, the ribosome becomes associated with the ER membrane and continues translation there. When translation ends, the ribosome can detach and return to the cytosolic pool, showing the association is not permanent. The key determinant is the sorting signal on the nascent polypeptide, not a structural difference in the ribosome. This is why cells can rapidly adjust the fraction of bound ribosomes as secretion demand changes. Therefore, the correct description is same ribosomes, different targeting.
357. A lysosomal hydrolytic enzyme is typically synthesized on:
ⓐ. Free ribosomes in cytosol
ⓑ. Ribosomes attached to RER
ⓒ. Ribosomes inside mitochondria
ⓓ. Ribosomes inside chloroplasts
Correct Answer: Ribosomes attached to RER
Explanation: Lysosomal enzymes enter the endomembrane system because they must be packaged and delivered to lysosome-related compartments rather than staying in the cytosol. Their synthesis begins on ribosomes that become associated with the rough ER, allowing the enzyme polypeptide to enter the ER lumen during translation. From there, the protein is processed and sorted through Golgi-dependent trafficking steps before reaching lysosomal pathways. This routing protects the cell by keeping potent hydrolytic enzymes enclosed within membrane-bound compartments. The synthesis site therefore aligns with the requirement for ER entry and vesicular delivery. Hence, ribosomes attached to RER typically synthesize lysosomal hydrolytic enzymes.
358. A nuclear protein (e.g., a transcription factor) is generally synthesized on:
ⓐ. Ribosomes attached to RER
ⓑ. Ribosomes inside Golgi stacks
ⓒ. Free ribosomes in cytosol
ⓓ. Ribosomes inside lysosomes
Correct Answer: Free ribosomes in cytosol
Explanation: Most nuclear proteins are synthesized on free ribosomes in the cytosol and then imported into the nucleus after translation. This occurs because these proteins do not need to enter the ER lumen or the secretory pathway; instead, they carry nuclear-targeting information that directs post-translational transport through nuclear pores. The cytosolic synthesis step keeps translation separate from membrane trafficking and fits the nucleus’s selective import mechanism. This destination logic is a standard micro-point: nuclear localization is typically achieved by import signals, not by ER routing. Therefore, nuclear proteins are generally made on free cytosolic ribosomes.
359. In a cell with very high secretion of proteins, you most expect:
ⓐ. More free ribosomes and less ER
ⓑ. Reduced Golgi packaging activity
ⓒ. Fewer ribosomes overall in cytosol
ⓓ. More RER-bound ribosomes
Correct Answer: More RER-bound ribosomes
Explanation: High protein secretion requires heavy use of the ER–Golgi pathway, so the cell increases translation on ER-associated ribosomes to feed proteins into this system. More ribosomes attach to the rough ER, making the ER extensive and visibly rough, and this supports high-volume synthesis and early processing of secretory proteins. The increased bound-ribosome fraction is a functional response to the need for efficient folding, quality control, and vesicular export. This structural shift is commonly seen in secretory cells that release enzymes or peptide signals. The logic is that secretion demand drives ER entry demand, and ER entry drives ribosome attachment. Hence, more RER-bound ribosomes are expected.
360. A protein destined for the mitochondrial matrix is typically synthesized on:
ⓐ. Free ribosomes in cytosol
ⓑ. Ribosomes attached to RER
ⓒ. Ribosomes inside Golgi lumen
ⓓ. Ribosomes inside lysosomes
Correct Answer: Free ribosomes in cytosol
Explanation: Most mitochondrial proteins are encoded by nuclear genes and are translated on free ribosomes in the cytosol before being imported into mitochondria. After synthesis, targeting information on the protein guides it to mitochondrial import machinery, allowing entry across mitochondrial membranes to the correct compartment. This route differs from the ER pathway because mitochondrial proteins generally do not enter the secretory system. The cytosolic synthesis step also allows wide distribution of newly made mitochondrial proteins to multiple mitochondria in the cell. This import-based targeting is a core concept linking free ribosomes to non-endomembrane destinations. Therefore, mitochondrial matrix-destined proteins are typically synthesized on free ribosomes in the cytosol.
361. The “70S” ribosome of prokaryotes is composed of:
ⓐ. 50S and 30S subunits
ⓑ. 60S and 40S subunits
ⓒ. 45S and 25S subunits
ⓓ. 55S and 15S subunits
Correct Answer: 50S and 30S subunits
Explanation: Prokaryotic ribosomes are termed 70S based on their sedimentation behavior, and they consist of a large 50S subunit and a small 30S subunit. The “S” unit reflects sedimentation rate rather than simple additive mass, so 50S + 30S gives 70S as a characteristic property. These ribosomes carry out protein synthesis in bacteria and are a standard marker used to distinguish prokaryotic translation machinery from eukaryotic cytosolic ribosomes. This subunit composition is a high-frequency exam point for identifying ribosome types in cells. Hence, 70S ribosomes are composed of 50S and 30S subunits.
362. In a typical bacterial cell, 70S ribosomes are found mainly in the:
ⓐ. Nucleolus inside nucleus
ⓑ. Golgi cisternae
ⓒ. Lysosomal lumen
ⓓ. Cytoplasm
Correct Answer: Cytoplasm
Explanation: Bacteria lack membrane-bound organelles such as nucleus, Golgi, and lysosomes, so their translation machinery is located in the cytoplasm. 70S ribosomes are dispersed throughout the bacterial cytoplasm where they translate mRNA into proteins, often forming polyribosomes. Because there is no nuclear envelope, transcription and translation can occur in close proximity, enabling rapid protein production. The cytoplasmic location is therefore fundamental to prokaryotic cell organization and gene expression. This placement is a key micro-point for comparing prokaryotic and eukaryotic cells. Hence, in bacteria, 70S ribosomes are mainly found in the cytoplasm.
363. A correct statement about sedimentation “S” units is that:
ⓐ. S values are strictly additive
ⓑ. S values depend on shape and size
ⓒ. S equals molecular mass directly
ⓓ. S measures pH of ribosomes
Correct Answer: S values depend on shape and size
Explanation: The Svedberg (S) unit reflects how particles sediment in a centrifugal field, which depends on their size, shape, and density rather than just mass. This is why ribosomal subunits do not add arithmetically to the whole ribosome in a simple mass sense, even though they are called 50S and 30S forming a 70S particle. The sedimentation property captures physical behavior of the complex as a whole. Understanding this prevents the common misconception that the numbers represent direct weights. Therefore, S values depend on shape and size.
364. Compared with eukaryotic cytosolic ribosomes, prokaryotic ribosomes are:
ⓐ. Larger and called 80S
ⓑ. Smaller and called 60S
ⓒ. Identical and called 90S
ⓓ. Smaller and called 70S
Correct Answer: Smaller and called 70S
Explanation: Prokaryotic ribosomes are typically 70S, while eukaryotic cytosolic ribosomes are typically 80S. The difference reflects distinct subunit compositions and structural features of translation machinery between these cell types. This size distinction is widely used in cell biology to differentiate prokaryotic cells from eukaryotic cells at the level of protein synthesis systems. The smaller prokaryotic ribosome supports bacterial translation and is also a frequent focus in competitive-style questions. Remembering 70S vs 80S helps connect ribosome type to cell organization. Thus, prokaryotic ribosomes are smaller and called 70S.
365. A correct location match is that 70S-type ribosomes are present in:
ⓐ. Eukaryotic nucleus and nucleolus
ⓑ. Mitochondria and chloroplasts
ⓒ. Golgi stacks and lysosomes only
ⓓ. Rough ER surface and Golgi only
Correct Answer: Mitochondria and chloroplasts
Explanation: Eukaryotic cytosol contains 80S ribosomes, but mitochondria and chloroplasts contain their own ribosomes for internal protein synthesis. These organelle ribosomes are prokaryote-like and commonly referred to as 70S-type in standard biology because of the evolutionary origin of these organelles. Chloroplast ribosomes are characteristically 70S, and mitochondrial ribosomes are also prokaryote-like in their translation features. Golgi stacks and lysosomes do not contain ribosomes for translation, and rough ER–bound ribosomes are still 80S ribosomes that are temporarily attached. Therefore, the correct location match is mitochondria and chloroplasts.
366. The primary role of 70S ribosomes in prokaryotes is to:
ⓐ. Store lipids inside cytoplasm
ⓑ. Generate ATP by respiration
ⓒ. Synthesize proteins from mRNA
ⓓ. Package enzymes into vesicles
Correct Answer: Synthesize proteins from mRNA
Explanation: Ribosomes are the cellular machines that translate genetic information carried by mRNA into polypeptide chains, and 70S ribosomes perform this function in prokaryotes. They bind mRNA, align tRNA molecules, and catalyze peptide bond formation through rRNA and associated proteins. This translation process is central to gene expression and determines how bacterial cells produce enzymes, structural proteins, and regulatory factors. Because prokaryotes rely on cytoplasmic translation without compartmental separation, 70S ribosomes are critical for rapid response to environmental changes. This is the fundamental functional identity of ribosomes. Hence, 70S ribosomes synthesize proteins from mRNA.
367. A correct subunit comparison is that a bacterial large ribosomal subunit is:
ⓐ. 60S
ⓑ. 50S
ⓒ. 40S
ⓓ. 30S
Correct Answer: 50S
Explanation: In bacteria, the ribosome is 70S and consists of two subunits: a large 50S subunit and a small 30S subunit. The large subunit contributes key catalytic functions for peptide bond formation and works with the small subunit to ensure correct decoding and assembly of polypeptides. This subunit identity is a core micro-point used to test understanding of prokaryotic translation machinery. It also helps distinguish bacterial ribosomes from eukaryotic cytosolic ribosomes, which use different subunit sizes. Remembering “50S large, 30S small” prevents confusion with 60S/40S values. Therefore, the bacterial large ribosomal subunit is 50S.
368. A correct subunit comparison is that a bacterial small ribosomal subunit is:
ⓐ. 60S
ⓑ. 50S
ⓒ. 40S
ⓓ. 30S
Correct Answer: 30S
Explanation: The bacterial ribosome’s small subunit is 30S, which combines with the 50S large subunit to form the complete 70S ribosome. The small subunit plays a central role in binding mRNA and ensuring correct pairing between codons and tRNA anticodons during translation. This decoding function is essential for accuracy in protein synthesis, preventing incorrect amino acid incorporation. The 30S identity is a classic exam point that pairs with the 50S large subunit fact. Correctly identifying 30S helps students differentiate prokaryotic ribosomes from eukaryotic 40S small subunits. Hence, the bacterial small subunit is 30S.
369. The presence of 70S ribosomes in bacteria most directly supports which cellular capability?
ⓐ. Vesicle-based secretion through Golgi
ⓑ. Photosynthesis in thylakoids always
ⓒ. Protein synthesis in cytoplasm
ⓓ. Nuclear mitosis organization
Correct Answer: Protein synthesis in cytoplasm
Explanation: 70S ribosomes are the translation machinery of bacteria, enabling them to synthesize proteins directly in the cytoplasm. This capability is essential because bacteria lack membrane-bound organelles and rely on cytoplasmic processes for gene expression. By translating mRNA into proteins, 70S ribosomes produce enzymes for metabolism, transport proteins for membranes, and structural proteins for cell maintenance. The direct cytoplasmic location also allows quick coupling of gene expression to environmental changes. This is why ribosome type is a foundational marker in prokaryotic cell biology. Therefore, 70S ribosomes most directly support protein synthesis in the cytoplasm.
370. A correct statement about eukaryotic cytosolic ribosomes compared to prokaryotic ribosomes is:
ⓐ. Eukaryotic cytosolic ribosomes are 80S
ⓑ. Eukaryotic cytosolic ribosomes are 70S
ⓒ. Eukaryotic cytosolic ribosomes are 50S
ⓓ. Eukaryotic cytosolic ribosomes are 30S
Correct Answer: Eukaryotic cytosolic ribosomes are 80S
Explanation: Eukaryotic cytosolic ribosomes are typically described as 80S, distinguishing them from the 70S ribosomes of prokaryotes. This difference reflects distinct ribosomal subunit compositions and structural features of translation systems between eukaryotic and prokaryotic cells. The 80S ribosome supports cytosolic protein synthesis in eukaryotes and is a standard micro-point used in questions comparing cell types. Remembering 80S for eukaryotic cytosol also helps reconcile why mitochondria and chloroplasts in eukaryotes can have 70S ribosomes while the cytosol has 80S. Hence, eukaryotic cytosolic ribosomes are 80S.
371. In a typical eukaryotic cell, mitochondrial ribosomes are mainly located in the:
ⓐ. Outer membrane surface layer
ⓑ. Intermembrane space region
ⓒ. Mitochondrial matrix
ⓓ. Golgi cisternae lumen
Correct Answer: Mitochondrial matrix
Explanation: Mitochondrial ribosomes are found chiefly in the matrix, the internal compartment enclosed by the inner membrane. This location places them close to mitochondrial DNA and mRNAs so translation can occur efficiently within the organelle. Many proteins made by mitochondrial ribosomes are destined for the inner membrane, so matrix-side synthesis supports rapid delivery and assembly near their functional sites. The matrix environment also contains factors required for mitochondrial translation and processing. This arrangement reflects the semi-autonomous nature of mitochondria in producing a subset of their own proteins. Therefore, the mitochondrial matrix is the main location of mitochondrial ribosomes.
372. Mitochondrial ribosomes are most commonly described as being similar to:
ⓐ. Prokaryote-like ribosomes
ⓑ. 80S cytosolic ribosomes only
ⓒ. 90S nucleolar particles mainly
ⓓ. Only 60S large subunits here
Correct Answer: Prokaryote-like ribosomes
Explanation: Mitochondria synthesize a subset of proteins inside the organelle, so they contain their own ribosomes distinct from cytosolic 80S ribosomes. These mitochondrial ribosomes are described as prokaryote-like because mitochondria originated from prokaryotic ancestors and retained key translation-related characteristics. While the exact sedimentation value can vary among organisms, the standard conceptual point is that mitochondrial ribosomes are closer to prokaryotic ribosomes than to cytosolic ribosomes in origin and translation style. Nucleolar particles relate to ribosome assembly in the nucleus and are not mitochondrial. Hence, mitochondrial ribosomes are most commonly described as prokaryote-like ribosomes.
373. A protein most likely synthesized by mitochondrial ribosomes is a component of:
ⓐ. Cell wall cellulose framework
ⓑ. Nuclear chromatin packaging system
ⓒ. Cytosolic microtubule spindle unit
ⓓ. Inner membrane respiratory complexes
Correct Answer: Inner membrane respiratory complexes
Explanation: Mitochondrial DNA encodes a limited set of proteins, many of which are core subunits of oxidative phosphorylation complexes located in the inner mitochondrial membrane. Mitochondrial ribosomes translate these mtDNA-derived mRNAs, producing proteins that are inserted into or assembled with inner-membrane complexes. This direct link supports efficient energy conversion because the products are made close to where they function. It also explains why defects in mitochondrial translation can reduce ATP output by disrupting respiratory chain assembly. The focus is not general cytosolic structures but energy-related inner membrane machinery. Therefore, mitochondrial ribosomes most likely synthesize components of inner membrane respiratory complexes.
374. A correct “semi-autonomous” reason mitochondria retain ribosomes is that mitochondria can:
ⓐ. Replace the nucleus for all genes
ⓑ. Make some proteins internally
ⓒ. Perform photosynthesis in grana
ⓓ. Build peptidoglycan wall layers
Correct Answer: Make some proteins internally
Explanation: Mitochondria are termed semi-autonomous because they possess DNA and ribosomes that enable them to synthesize a subset of proteins within the organelle. These internally produced proteins are closely tied to mitochondrial energy functions, particularly oxidative phosphorylation. Although many mitochondrial proteins are encoded by nuclear genes and imported after cytosolic synthesis, the retained translation system provides partial independence and supports local assembly of key complexes. This arrangement improves functional coordination between mitochondrial gene expression and energy demand. It also helps explain why mitochondria are not fully dependent on the cytosol for every protein component. Hence, mitochondria retain ribosomes because they can make some proteins internally.
375. The majority of mitochondrial proteins in eukaryotic cells are synthesized on:
ⓐ. Cytosolic free ribosomes
ⓑ. Ribosomes bound to Golgi stacks
ⓒ. Ribosomes inside lysosomes
ⓓ. Ribosomes inside peroxisomes
Correct Answer: Cytosolic free ribosomes
Explanation: Most mitochondrial proteins are encoded by nuclear genes and are translated on free ribosomes in the cytosol. After synthesis, these proteins carry targeting information that directs them to mitochondrial import machinery, allowing them to enter the organelle and reach the correct compartment. This explains why mitochondrial ribosomes are essential but not sufficient for producing the entire mitochondrial proteome. The cell therefore uses a dual-source strategy: a small set made inside mitochondria and a much larger set imported from the cytosol. This division of labor is a core concept linking organelle function with nuclear control. Therefore, the majority of mitochondrial proteins are synthesized on cytosolic free ribosomes.
376. The most direct evidence that mitochondria have their own protein-synthesis system is the presence of:
ⓐ. Thylakoid stacks and stroma
ⓑ. Cell wall middle lamella
ⓒ. Nuclear envelope pores
ⓓ. DNA and ribosomes in matrix
Correct Answer: DNA and ribosomes in matrix
Explanation: Mitochondria contain their own DNA and ribosomes, and both are mainly located in the matrix, supporting local transcription and translation. This combination provides the essential machinery to express organelle-encoded genes without immediate dependence on nuclear translation systems. The internally synthesized proteins are important for mitochondrial respiratory function and assembly of inner-membrane complexes. This is a standard micro-point used to justify the semi-autonomous nature of mitochondria in cell biology. While mitochondria still rely heavily on nuclear-encoded proteins, the presence of DNA plus ribosomes confirms an intrinsic protein-synthesis capability. Hence, DNA and ribosomes in the matrix provide direct evidence of a mitochondrial protein-synthesis system.
377. If a drug selectively inhibits 70S-type ribosomes, a likely direct cellular effect is reduced:
ⓐ. Golgi glycosylation reactions only
ⓑ. Cytosolic 80S translation output
ⓒ. Mitochondrial protein synthesis
ⓓ. Nuclear DNA replication speed only
Correct Answer: Mitochondrial protein synthesis
Explanation: In eukaryotic cells, the main translation in the cytosol uses 80S ribosomes, so a 70S-type inhibitor is less likely to directly block cytosolic protein synthesis. Instead, 70S-type (prokaryote-like) ribosomes are associated with organelles of prokaryotic origin, especially mitochondria (and chloroplasts in plants). Inhibiting these ribosomes can directly reduce translation of organelle-encoded proteins, many of which are essential components of energy-generating pathways. Therefore, the most direct cellular impact is reduced mitochondrial protein synthesis, which can subsequently affect ATP production and overall cellular metabolism. Hence, mitochondrial protein synthesis is the correct effect.
378. A correct contrast is that cytosolic ribosomes in eukaryotes are typically:
ⓐ. 80S in sedimentation
ⓑ. 70S in sedimentation
ⓒ. 50S as a whole ribosome
ⓓ. 30S as a whole ribosome
Correct Answer: 80S in sedimentation
Explanation: Eukaryotic cytosolic ribosomes are typically described as 80S, distinguishing them from 70S-like ribosomes associated with mitochondria and from bacterial 70S ribosomes. This difference reflects distinct translation machinery and helps explain why some inhibitors affect bacterial and mitochondrial translation more than cytosolic translation. The 80S designation is a standard classification micro-point used to identify where protein synthesis occurs and to compare cell types and organelles. Recognizing 80S as cytosolic also supports correct reasoning about free vs bound ribosomes, since both are cytosolic 80S ribosomes differing by targeting. Thus, cytosolic ribosomes in eukaryotes are typically 80S in sedimentation.
379. Mitochondrial ribosomes primarily translate mRNA that is derived from:
ⓐ. Only Golgi vesicle signals
ⓑ. Only lysosomal enzyme tags
ⓒ. Only plasma membrane channels
ⓓ. Mitochondrial DNA genes
Correct Answer: Mitochondrial DNA genes
Explanation: Mitochondrial ribosomes translate mitochondrial mRNAs that originate from genes located on mitochondrial DNA. This supports the organelle’s ability to produce a subset of proteins internally, especially those linked to respiration. The translation occurs largely in the matrix, where mtDNA is present and where mRNAs can be processed and made available for ribosomal decoding. This internal gene-expression loop is a key element of mitochondrial semi-autonomy and is frequently used to connect organelle genetics with function. While many mitochondrial proteins are nuclear-encoded, mitochondrial ribosomes specifically serve the mtDNA-coded portion. Therefore, mitochondrial ribosomes primarily translate mRNA derived from mitochondrial DNA genes.
380. A high-energy-demand cell is more sensitive to defects in mitochondrial ribosomes mainly because such defects reduce:
ⓐ. Cell wall thickening capacity
ⓑ. Oxidative ATP production
ⓒ. Chloroplast pigment formation
ⓓ. Nucleolar rRNA processing
Correct Answer: Oxidative ATP production
Explanation: Mitochondrial ribosomes produce proteins that are essential for forming and maintaining oxidative phosphorylation complexes. If mitochondrial translation is impaired, assembly of respiratory chain components can fail, lowering electron transport efficiency and weakening the proton gradient needed for ATP synthase. This directly reduces ATP output from aerobic respiration, which is critical in cells with constant high energy demand. Such cells rely heavily on mitochondrial ATP to power processes like ion pumping, contraction, and sustained biosynthesis. The effect is therefore functional and immediate at the level of energy production rather than unrelated cellular activities. Hence, defects in mitochondrial ribosomes mainly reduce oxidative ATP production.
381. In plant cells, chloroplast ribosomes are mainly located in the:
ⓐ. Thylakoid lumen space
ⓑ. Nuclear nucleoplasm
ⓒ. Intermembrane space of mitochondria
ⓓ. Chloroplast stroma
Correct Answer: Chloroplast stroma
Explanation: Chloroplast ribosomes are primarily found in the stroma, the fluid matrix that surrounds the thylakoid system. This location places them near chloroplast DNA and chloroplast mRNAs, enabling efficient translation of organelle-encoded genes. Many proteins synthesized in chloroplasts function within the thylakoid membranes, so stroma-side translation supports timely delivery and assembly of these components. The stroma also contains enzymes and factors needed for chloroplast gene expression and protein synthesis. This arrangement reflects the semi-autonomous nature of chloroplasts in producing part of their own protein machinery. Therefore, chloroplast ribosomes are mainly located in the chloroplast stroma.
382. Chloroplast ribosomes are most commonly described as:
ⓐ. 70S type ribosomes
ⓑ. 80S type ribosomes
ⓒ. 90S particles only
ⓓ. 60S subunits only
Correct Answer: 70S type ribosomes
Explanation: Chloroplast ribosomes are commonly described as 70S type, similar to prokaryotic ribosomes in sedimentation behavior. This feature supports the idea that chloroplasts have a partial genetic and translational system that is distinct from the eukaryotic cytosolic 80S system. The 70S label is widely used in exams to connect chloroplast semi-autonomy with organelle-specific protein synthesis. It also helps explain why certain translation inhibitors can affect chloroplast function more than cytosolic protein synthesis. While chloroplast ribosomes have their own specific features, the key comparative micro-point is their 70S type classification. Hence, chloroplast ribosomes are most commonly described as 70S type ribosomes.
383. Chloroplast ribosomes mainly synthesize proteins that are most directly required for:
ⓐ. Cell wall lignin cross-linking
ⓑ. Nuclear chromosome separation
ⓒ. Thylakoid membrane photosynthetic machinery
ⓓ. Lysosomal hydrolase packaging
Correct Answer: Thylakoid membrane photosynthetic machinery
Explanation: Chloroplast DNA encodes a limited set of proteins, many of which contribute to photosynthetic processes located in thylakoid membranes. Chloroplast ribosomes translate these organelle-encoded mRNAs in the stroma, producing proteins that are inserted into or assembled with thylakoid membrane complexes. This supports efficient construction and maintenance of photosynthetic electron transport components. Because photosynthesis depends on specialized membrane-bound complexes, local synthesis helps coordinate assembly within the chloroplast. Defects in chloroplast translation can therefore impair light reactions and overall photosynthetic performance. Thus, chloroplast ribosomes mainly support thylakoid membrane photosynthetic machinery.
384. A correct evidence that chloroplasts are semi-autonomous is the presence of:
ⓐ. A nuclear envelope around chloroplasts
ⓑ. Cell wall layers around chloroplasts
ⓒ. DNA and ribosomes inside chloroplasts
ⓓ. Golgi stacks inside chloroplasts
Correct Answer: DNA and ribosomes inside chloroplasts
Explanation: Chloroplasts contain their own DNA and ribosomes, allowing them to express a subset of genes and synthesize some proteins internally. This is a hallmark of semi-autonomy because chloroplasts do not rely entirely on nuclear-cytosolic translation for all protein components. The organelle’s translation machinery in the stroma works with chloroplast genetic material to produce proteins important for photosynthesis and chloroplast maintenance. Although many chloroplast proteins are nuclear-encoded and imported, the internal DNA plus ribosomes provide partial independence. This combination is a standard micro-point used to justify chloroplast semi-autonomous behavior. Therefore, DNA and ribosomes inside chloroplasts support semi-autonomy.
385. The majority of chloroplast proteins are synthesized on:
ⓐ. Cytosolic 80S ribosomes and imported
ⓑ. Chloroplast 70S ribosomes only
ⓒ. Ribosomes inside lysosomes
ⓓ. Ribosomes bound to Golgi membranes
Correct Answer: Cytosolic 80S ribosomes and imported
Explanation: Most chloroplast proteins are encoded by nuclear genes and are translated on cytosolic 80S ribosomes. After synthesis, these proteins carry targeting information that directs them into chloroplasts, where they are delivered to the stroma, thylakoid membranes, or other compartments. Chloroplast 70S ribosomes synthesize only a limited subset of proteins encoded by chloroplast DNA. This dual-source arrangement explains why chloroplast function depends on both nuclear control and internal chloroplast gene expression. It also supports the concept that chloroplasts are semi-autonomous rather than fully independent. Hence, the majority of chloroplast proteins are made on cytosolic 80S ribosomes and imported.
386. A chloroplast translation inhibitor targeting 70S ribosomes would most directly reduce:
ⓐ. Cytosolic protein synthesis output
ⓑ. Photosynthetic complex protein synthesis
ⓒ. Nuclear DNA replication accuracy
ⓓ. Golgi vesicle formation rate
Correct Answer: Photosynthetic complex protein synthesis
Explanation: Chloroplast ribosomes are commonly described as 70S type, so inhibitors aimed at 70S translation can directly interfere with chloroplast protein synthesis. Because chloroplast-encoded proteins often contribute to photosynthetic machinery, blocking chloroplast translation reduces the production of key components required for thylakoid-based light reactions. This can weaken electron transport efficiency, pigment-protein complex maintenance, and overall photosynthetic output. The effect is more direct for chloroplast function than for cytosolic translation, which uses 80S ribosomes. This application is a classic reasoning-based question around ribosome type and organelle function. Therefore, such inhibition most directly reduces photosynthetic complex protein synthesis.
387. A correct location match is that chloroplast DNA and ribosomes are both mainly found in:
ⓐ. Cytosol near cell wall
ⓑ. Thylakoid lumen region
ⓒ. Outer envelope surface region
ⓓ. Stroma region
Correct Answer: Stroma region
Explanation: Chloroplast DNA is located in the stroma, and chloroplast ribosomes are also present in the stroma, enabling organelle gene expression and protein synthesis within the same compartment. This co-location supports efficient transcription and translation of chloroplast genes and rapid use of the protein products in chloroplast structures. The stroma also contains enzymes and factors needed for these processes, reinforcing its role as a functional matrix. Many translated proteins are destined for thylakoid membranes, and stroma-side synthesis supports their delivery and assembly. This arrangement is a standard structural micro-point describing chloroplast internal organization. Hence, both chloroplast DNA and ribosomes are mainly found in the stroma region.
388. A correct contrast is that eukaryotic cytosolic ribosomes are:
ⓐ. 70S type
ⓑ. 80S type
ⓒ. 50S type
ⓓ. 30S type
Correct Answer: 80S type
Explanation: Eukaryotic cytosolic ribosomes are typically described as 80S, which differs from the 70S ribosomes of prokaryotes and from the 70S-type ribosomes commonly associated with chloroplasts. This difference in ribosome type is a key comparative point used to distinguish cytosolic translation from organelle translation in plant cells. It also supports conceptual questions about why certain inhibitors affect bacterial or chloroplast translation more than cytosolic protein synthesis. Recognizing 80S as the cytosolic standard helps organize cell biology facts into a coherent targeting framework. Thus, eukaryotic cytosolic ribosomes are 80S type.
389. Chloroplast ribosomes primarily translate mRNA that is derived from:
ⓐ. Golgi-sorted secretory genes
ⓑ. Cell wall synthesis genes only
ⓒ. Lysosomal enzyme genes only
ⓓ. Chloroplast genome genes
Correct Answer: Chloroplast genome genes
Explanation: Chloroplast ribosomes translate chloroplast mRNAs that originate from genes present in the chloroplast genome. This allows chloroplasts to produce certain proteins internally, especially those linked to photosynthetic functions and chloroplast maintenance. The mRNAs are transcribed within the chloroplast and translated in the stroma, supporting localized assembly of chloroplast complexes. This internal gene-expression loop is part of the semi-autonomous character of chloroplasts. While many chloroplast proteins come from nuclear genes, chloroplast ribosomes specifically serve the chloroplast genome-derived portion. Therefore, chloroplast ribosomes primarily translate mRNA derived from chloroplast genome genes.
390. A plant cell type with very high photosynthetic activity would typically show:
ⓐ. Reduced chloroplast number and few ribosomes
ⓑ. Many chloroplasts with active ribosomes
ⓒ. No thylakoids and no grana stacks
ⓓ. Only leucoplasts and no chloroplast DNA
Correct Answer: Many chloroplasts with active ribosomes
Explanation: High photosynthetic activity requires abundant chloroplasts with well-developed thylakoid systems and strong capacity to maintain photosynthetic complexes. Active chloroplast ribosomes support ongoing synthesis of organelle-encoded proteins needed for thylakoid function and repair of photosynthetic machinery. This helps sustain efficient light reactions and maintain electron transport components under high light and metabolic demand. Cells in photosynthetically active tissues therefore typically contain numerous chloroplasts and robust internal protein-synthesis capacity within those chloroplasts. The combination of many chloroplasts and active ribosomes reflects functional adaptation for sustained photosynthesis. Hence, such cells show many chloroplasts with active ribosomes.
391. The basic building unit of a microtubule is:
ⓐ. G-actin monomer units
ⓑ. Collagen triple-helix units
ⓒ. Keratin filament tetramers
ⓓ. α/β tubulin dimer
Correct Answer: α/β tubulin dimer
Explanation: Microtubules are polymers assembled from repeating α- and β-tubulin heterodimers. These dimers line up head-to-tail to form protofilaments, and multiple protofilaments associate side-by-side to create the hollow tubular microtubule. Because the dimers are oriented in one direction, the microtubule becomes structurally polar with distinct plus and minus ends. This polarity is crucial for directional growth and for motor-protein movement along the microtubule surface. The α/β tubulin dimer is therefore the fundamental subunit that determines microtubule architecture. Hence, α/β tubulin dimer is correct.
392. The typical diameter of a microtubule is:
ⓐ. About 25 nm wide
ⓑ. Roughly seven nanometres wide
ⓒ. Roughly ten nanometres wide
ⓓ. Roughly two nanometres wide
Correct Answer: About 25 nm wide
Explanation: Microtubules are the thickest of the major cytoskeletal filaments and form hollow cylinders with a characteristic outer diameter close to 25 nm. This size reflects their construction from multiple protofilaments arranged in a tube, giving greater rigidity compared with thinner filaments. The larger diameter supports functions requiring stiffness, such as maintaining cell shape, forming the mitotic spindle, and providing stable tracks for long-range transport. Because the dimension is a standard identification feature, it is frequently used to distinguish microtubules from other cytoskeletal elements. Therefore, about 25 nm wide is the correct value.
393. The feature that stabilizes a growing microtubule end during polymerization is the:
ⓐ. ATP-bound actin cap
ⓑ. GTP-tubulin cap
ⓒ. Calcium ion shell layer
ⓓ. A thick phosphate coat
Correct Answer: GTP-tubulin cap
Explanation: Microtubule growth is promoted when tubulin dimers add in a GTP-bound state at the growing end, creating a stabilizing “GTP cap.” This cap helps keep the end in a straight, assembly-favored conformation, allowing continued addition of more tubulin dimers. As GTP is hydrolyzed after incorporation, the lattice becomes less stable, making loss of the cap a key trigger for rapid depolymerization. The presence of a GTP-tubulin cap is therefore central to dynamic instability and controlled microtubule length changes. Hence, GTP-tubulin cap is correct.
394. The internal microtubule arrangement of a typical motile cilium/flagellum is:
ⓐ. Nine plus zero pattern only
ⓑ. Thirteen-protofilament ring only
ⓒ. 9+2 axoneme pattern
ⓓ. Eight plus one arrangement
Correct Answer: 9+2 axoneme pattern
Explanation: Motile cilia and flagella contain an axoneme organized as nine outer doublet microtubules surrounding two central singlet microtubules, called the 9+2 pattern. This arrangement provides a stable scaffold that supports bending and coordinated movement. The geometry allows accessory structures to interact with the doublets in a repeated, symmetric manner along the length of the axoneme. Because this pattern is a defining structural signature of motile cilia/flagella, it is widely tested in organelle ultrastructure questions. Therefore, 9+2 axoneme pattern is correct.
395. A centriole (and basal body) is characterized by:
ⓐ. 9 triplet microtubules
ⓑ. Nine doublet microtubules only
ⓒ. Nine plus two microtubules set
ⓓ. Thirteen singlet protofilaments ring
Correct Answer: 9 triplet microtubules
Explanation: Centrioles and basal bodies have a distinctive “9 triplet” arrangement, where nine sets of microtubule triplets form a cylindrical structure. This triplet organization provides a robust template for organizing microtubules and is linked to the formation of basal bodies that anchor cilia and flagella. The structural strength of triplets supports their role as organizing platforms rather than as flexible, beating axonemes. This is a classic micro-point used to differentiate centrioles/basal bodies from the 9+2 axoneme pattern of motile cilia. Hence, 9 triplet microtubules is correct.
396. A compound that directly inhibits microtubule polymerization by binding tubulin is:
ⓐ. Taxol stabilizing polymers
ⓑ. Phalloidin stabilizing actin
ⓒ. Cytochalasin blocking actin
ⓓ. Colchicine binding tubulin
Correct Answer: Colchicine binding tubulin
Explanation: Colchicine binds to tubulin and prevents proper addition of tubulin dimers into growing microtubules, thereby inhibiting polymerization. Because microtubules are essential for mitotic spindle formation, disrupting their assembly can block cell division and alter intracellular transport processes. The key concept is that colchicine acts at the tubulin level, reducing the pool of assembly-competent subunits. This leads to destabilization of microtubule-dependent structures and is a standard example used in questions on cytoskeleton inhibitors. Therefore, colchicine binding tubulin is correct.
397. The motor protein that typically moves cargo toward the plus end of microtubules is:
ⓐ. Dynein heavy-chain motor
ⓑ. Kinesin motor protein
ⓒ. Myosin head ATPase
ⓓ. Actin-binding tropomyosin
Correct Answer: Kinesin motor protein
Explanation: Kinesin is a microtubule-based motor protein that generally transports cargo toward the plus end of microtubules. This directionality is critical for organized intracellular trafficking, especially in long cells where materials must move efficiently over large distances. Kinesin “walks” along microtubule tracks using ATP-driven conformational changes, enabling targeted delivery of vesicles and organelles. The plus-end-directed nature of kinesin is a high-yield concept that links microtubule polarity to directional transport. Hence, kinesin motor protein is correct.
398. In animal cells, the major microtubule-organizing center (MTOC) is the:
ⓐ. Nucleolus structure region
ⓑ. Golgi cis-face region
ⓒ. Centrosome region
ⓓ. Plasma membrane region
Correct Answer: Centrosome region
Explanation: The centrosome functions as the primary microtubule-organizing center in many animal cells by nucleating and anchoring microtubules. From this site, microtubules radiate outward to help organize cell shape, polarity, and intracellular transport routes. During cell division, centrosomes help establish the spindle poles needed for accurate chromosome separation. The organizing role is based on concentrated nucleation machinery located at the centrosome. This makes the centrosome a central control point for microtubule network architecture. Therefore, centrosome region is correct.
399. The cytoskeletal elements that form the core framework of the mitotic spindle are:
ⓐ. Microtubule spindle fibers
ⓑ. Actin stress fiber bundles
ⓒ. Keratin filament networks
ⓓ. Collagen fibril assemblies
Correct Answer: Microtubule spindle fibers
Explanation: The mitotic spindle is primarily built from microtubules that assemble into spindle fibers connecting spindle poles and chromosomes. These microtubules capture chromosomes, align them, and then help separate sister chromatids to opposite poles during division. Their dynamic growth and shrinkage allow rapid remodeling of spindle length and attachment states. The spindle’s ability to reorganize efficiently depends on microtubule polarity, dynamic instability, and coordinated interactions at the poles. This microtubule-based framework is fundamental for accurate distribution of genetic material into daughter cells. Hence, microtubule spindle fibers is correct.
400. A defining property of microtubules that supports rapid reorganization in cells is:
ⓐ. Permanent covalent crosslinking only
ⓑ. Complete rigidity with no turnover
ⓒ. Lack of polarity in filament ends
ⓓ. Dynamic instability at ends
Correct Answer: Dynamic instability at ends
Explanation: Microtubules exhibit dynamic instability, meaning they can switch between phases of growth and rapid shrinkage, especially at their ends. This behavior allows cells to quickly remodel microtubule arrays in response to changing needs such as division, migration, and vesicle trafficking. Dynamic instability depends on the state of tubulin at the end, with stability linked to a protective cap during growth and destabilization when that cap is lost. The result is a highly adaptable cytoskeletal system that can search cellular space and reorganize efficiently. Therefore, dynamic instability at ends is the defining property supporting rapid reorganization.