201. Fish deaths during some red tides are most directly associated with:
ⓐ. toxins released by dense dinoflagellate populations
ⓑ. silica released from large populations of dead diatoms
ⓒ. methane released by dense archaebacterial populations
ⓓ. predation by rapidly multiplying ciliated protozoans
Correct Answer: toxins released by dense dinoflagellate populations
Explanation: During certain red tides, rapid multiplication creates a very large population of toxin-producing dinoflagellates. The released toxins may kill fish and other marine animals. Water discolouration identifies the population event, while toxicity explains the biological damage. Silica deposits, methane formation, and ciliary feeding belong to unrelated organisms or processes. The causal sequence is population increase, toxin accumulation, exposure of marine animals, and mortality. Population expansion raises toxin concentration, and affected fish may die despite the protists remaining photosynthetic. The causal chain is multiplication, toxin accumulation, and injury to other marine organisms.
202. Most euglenoids are commonly found in:
ⓐ. dry terrestrial litter
ⓑ. stagnant freshwater
ⓒ. hot acidic springs
ⓓ. highly saline lakes
Correct Answer: stagnant freshwater
Explanation: Euglenoids commonly inhabit stagnant freshwater environments. This habitat supports their identification when combined with a flexible pellicle, unequal flagella, and condition-dependent nutrition. Stagnant water alone is not sufficient for final placement because many microorganisms can occur there, but it provides a useful ecological clue. Hot acidic springs and highly saline lakes are associated more closely with specialised archaebacteria, while dry litter supports different saprophytic organisms. This habitat commonly contains the organic material and light conditions that support their flexible nutrition. The habitat description is common rather than an absolute requirement for every euglenoid.
203. The flexibility of a euglenoid cell is mainly due to the presence of:
ⓐ. overlapping silica shells
ⓑ. a rigid cellulose wall
ⓒ. a protein-rich pellicle
ⓓ. a chitinous sporangium
Correct Answer: a protein-rich pellicle
Explanation: Euglenoids lack a rigid cell wall and instead possess a protein-rich pellicle. This covering permits the cell to remain flexible while maintaining its form. Dinoflagellates have stiff cellulose plates, diatoms possess silica-containing wall halves, and chitinous structures are associated with fungi. The covering provides both a structural and functional clue. Recognising flexibility as the outcome of the pellicle helps distinguish euglenoids from other externally protected protists. The pellicle supports the cell yet bends with it, unlike a rigid cellulose or silica wall. Flexibility allows characteristic changes in body shape during movement.
204. A euglenoid is observed with two flagella of different lengths. The most accurate description is:
ⓐ. two equal transverse flagella
ⓑ. one flagellum and one pseudopodium
ⓒ. one long and one short flagellum
ⓓ. numerous equal cilia surrounding the cell
Correct Answer: one long and one short flagellum
Explanation: Euglenoids possess two unequal flagella, commonly described as one long and one short. This arrangement differs from dinoflagellates, whose flagella are distinguished mainly by longitudinal and transverse orientation. Pseudopodia characterise amoeboid protozoans, while numerous cilia occur in ciliated protozoans such as Paramoecium. Flagellar length provides an identifying structural clue, especially when combined with the pellicle and stagnant freshwater habitat. The unequal lengths are a standard structural clue for euglenoids and do not imply two different cell types. The longer flagellum contributes more visibly to locomotion, while the shorter one remains part of the same cell.
205. Sunlight is removed from a culture of Euglena, but suitable small organisms remain available as food. The most likely response is that Euglena:
ⓐ. deposits silica walls while becoming photosynthetic
ⓑ. switches to heterotrophic feeding on smaller organisms
ⓒ. permanently loses its nucleus and eukaryotic organelles
ⓓ. develops cellulose plates and produces a red tide
Correct Answer: switches to heterotrophic feeding on smaller organisms
Explanation: Euglena photosynthesises when sunlight is available but can behave heterotrophically when light is absent. The presence of smaller organisms supplies a possible food source under darkness. This nutritional shift does not change its kingdom, cell type, pigments, or pellicle. It remains a unicellular eukaryotic protist while altering the way it obtains organic matter. The prediction depends on identifying the changed environmental condition and applying the conditional nutritional response characteristic of euglenoids. Darkness removes photosynthetic energy, but available prey permits the same cell to switch to heterotrophic feeding. Returning light would permit photosynthesis again, showing that the shift is reversible.
206. Pigments in euglenoids are described as being identical to those in:
ⓐ. archaebacteria
ⓑ. slime moulds
ⓒ. fungi
ⓓ. higher plants
Correct Answer: higher plants
Explanation: Euglenoids possess photosynthetic pigments like those found in higher plants. This similarity enables photosynthesis under suitable illumination but does not place euglenoids in Plantae. Their unicellular eukaryotic organisation, pellicle, unequal flagella, and ability to become heterotrophic in darkness support protistan placement. Pigment similarity is thus one shared physiological character within a broader profile containing important differences. Classification requires the complete character combination rather than pigment identity alone. The pigment similarity explains plant-like photosynthesis, while the pellicle and nutritional flexibility support protistan placement. Similar pigments reveal functional resemblance but not identical kingdom placement.
207. Euglena cultures are maintained under two conditions.
| Culture | Light | Available food organisms | Observed nutrition | Pellicle and nucleus |
|---|
| P | Present | Absent | Photosynthetic | Present |
| Q | Absent | Present | Heterotrophic | Present |
The strongest conclusion from the changing and unchanged columns is that:
ⓐ. light permanently converts a protist into a plant
ⓑ. heterotrophic feeding removes the eukaryotic nucleus
ⓒ. nutritional mode can change while cellular identity remains stable
ⓓ. the pellicle is produced only during photosynthesis
Correct Answer: nutritional mode can change while cellular identity remains stable
Explanation: Culture P uses light for photosynthesis, whereas Culture Q lacks light and uses available food organisms heterotrophically. In both conditions, the pellicle and membrane-bound nucleus remain present. The table therefore separates a reversible physiological response from stable cellular characters. Nutrition changes with the environment, but the organism does not change kingdoms or lose its eukaryotic organisation. Reading only the nutrition column would miss this distinction; the unchanged structural column shows why Euglena remains a protist while expressing different feeding modes.
208. Assertion: Euglena is not placed in Plantae merely because it contains plant-like photosynthetic pigments.
Reason: It is a unicellular eukaryote with a pellicle and can become heterotrophic when sunlight is absent.
ⓐ. Both statements are true, and the Reason correctly explains the Assertion
ⓑ. Both statements are true, but the Reason does not explain the Assertion
ⓒ. The Assertion statement is true, whereas the Reason statement is false
ⓓ. The Assertion statement is false, whereas the Reason statement is true
Correct Answer: Both statements are true, and the Reason correctly explains the Assertion
Explanation: Euglena possesses pigments comparable to those of higher plants and can photosynthesise in light. Plant-like pigmentation alone is insufficient for kingdom placement because Euglena has a unicellular eukaryotic body, lacks a rigid plant cell wall, possesses a pellicle, and can feed heterotrophically in darkness. These combined characters support placement in Protista. The reason identifies the biological evidence that prevents photosynthetic ability from being treated as a universally sufficient plant criterion. This pattern reflects the broader principle that classification uses an integrated character profile rather than one conspicuous similarity. The reason identifies the complete mixed profile and directly explains why pigment alone cannot justify plant placement.
209. A unicellular protist has a flexible pellicle and two unequal flagella rather than stiff wall plates with transverse and longitudinal flagella. This profile identifies:
ⓐ. a diatom rather than a euglenoid
ⓑ. a cellulose-plated dinoflagellate rather than a diatom
ⓒ. a euglenoid rather than a dinoflagellate
ⓓ. a slime mould rather than a euglenoid
Correct Answer: a euglenoid rather than a dinoflagellate
Explanation: Euglenoids possess a protein-rich pellicle that maintains cell form while permitting flexibility. They also have two unequal flagella, commonly described as one long and one short. Dinoflagellates differ in both characters: their exterior bears stiff cellulose plates, and their two flagella occupy distinct longitudinal and transverse orientations, with the transverse flagellum lying in a furrow. The comparison must use covering and flagellar arrangement together, since the mere presence of two flagella occurs in both groups. Diatoms have overlapping silica-rich wall halves, while slime moulds lack this flagellated pellicle-based profile. The combined structural evidence places the organism among euglenoids.
210. Euglena is transferred from illuminated medium to darkness containing small food organisms and later returned to light without those organisms. The expected nutritional sequence is:
ⓐ. photosynthetic → heterotrophic → photosynthetic
ⓑ. heterotrophic → photosynthetic → heterotrophic
ⓒ. photosynthetic → photosynthetic → heterotrophic
ⓓ. heterotrophic → heterotrophic → photosynthetic
Correct Answer: photosynthetic → heterotrophic → photosynthetic
Explanation: In light, Euglena uses its plant-like pigments to synthesise food photosynthetically. Darkness removes the energy source required for that process, while the availability of smaller organisms permits heterotrophic feeding. Returning the culture to light restores conditions for photosynthesis, and removal of the available food organisms further favours that mode. This sequence represents a reversible response to environmental conditions rather than a change of kingdom or permanent loss of pigments. Euglena remains a unicellular eukaryotic protist throughout. The case adds a second condition change and shows that nutritional expression follows the available light and food resources.
211. A mass of protistan cells moves over decaying twigs and leaves while engulfing organic material. The organisms are best identified as:
ⓐ. photosynthetic chrysophytes
ⓑ. ciliated protozoans
ⓒ. chemosynthetic bacteria
ⓓ. saprophytic slime moulds
Correct Answer: saprophytic slime moulds
Explanation: Slime moulds are saprophytic protists associated with decaying plant material such as twigs and leaves. They move over the substrate and engulf organic matter, combining a fungus-like ecological role with a protistan feeding process. The use of dead material distinguishes saprophytic feeding from parasitism on living tissue. Chrysophytes are mainly photosynthetic aquatic organisms, while chemosynthetic bacteria obtain energy through oxidation of inorganic substances. Ciliated protozoans use cilia for movement and feeding but do not form the characteristic condition-dependent plasmodial organisation of slime moulds. The substrate, feeding behaviour, and protistan identity together support the classification.
212. Under suitable environmental conditions, separate slime-mould cells may aggregate and form:
ⓐ. an ascocarp
ⓑ. a spreading plasmodium
ⓒ. a bacterial endospore
ⓓ. a cyanobacterial heterocyst
Correct Answer: a spreading plasmodium
Explanation: Suitable conditions favour aggregation of slime-mould cells into a spreading mass called a plasmodium. This stage moves over the substrate and continues feeding on organic material. The plasmodium is a vegetative feeding condition, not the resistant stage produced during adversity. An ascocarp is a fungal fruiting body, a bacterial spore is associated with bacterial survival, and a heterocyst is a specialised nitrogen-fixing cyanobacterial cell. Environmental context is decisive: favourable conditions support growth, movement, and feeding, while unfavourable conditions later trigger differentiation into spore-bearing fruiting bodies. The plasmodium is an active multinucleate-looking mass formed for feeding and movement across the substrate.
213. The plasmodium of a slime mould should not be confused with Plasmodium, since the former is:
ⓐ. a feeding plasmodial mass in a slime-mould life cycle
ⓑ. a malarial sporozoan transmitted between animal hosts
ⓒ. a methane-producing prokaryotic colony in a marsh habitat
ⓓ. an ascus containing sexually formed ascospores
Correct Answer: a feeding plasmodial mass in a slime-mould life cycle
Explanation: In slime moulds, plasmodium is the name of a spreading feeding mass produced by aggregation under suitable conditions. Plasmodium, written as a scientific genus name, refers to the sporozoan parasite associated with malaria. The similarity in spelling does not indicate identical organisms, structures, or life cycles. The slime-mould plasmodium moves over decaying material and engulfs food, whereas the malarial parasite is classified through an infectious spore-like stage in its life cycle. Careful interpretation of context prevents a stage name in one protistan group from being mistaken for the genus name of another.
214. Arrange the events of a slime-mould response in biological order.
P. A spreading plasmodium forms under suitable conditions.
Q. Conditions become unfavourable.
R. Fruiting bodies bearing spores develop.
S. Resistant spores disperse through air currents.
ⓐ. Q → P → S → R
ⓑ. P → R → Q → S
ⓒ. P → Q → R → S
ⓓ. R → P → Q → S
Correct Answer: P → Q → R → S
Explanation: Suitable conditions first support formation and growth of the feeding plasmodium. When the environment becomes unfavourable, continued spreading and feeding are replaced by a survival-oriented reproductive response. The plasmodium differentiates into fruiting bodies, and spores are produced at their tips. These resistant spores can then be carried away by air currents. The order follows dependence between stages: the plasmodium must exist before it can differentiate, adversity provides the trigger for fruiting-body formation, and mature spores must be produced before dispersal. Reversing these dependencies would place a response before its environmental signal or dispersal before spore formation.
215. A slime-mould structure rises above the substrate and bears small resistant bodies at its tip. The raised structure is functioning as:
ⓐ. a photosynthetic filament bearing vegetative cells
ⓑ. a reproductive body bearing resistant spores
ⓒ. a nitrogen-fixing heterocyst within a filament
ⓓ. a food-conducting gullet lined with cilia
Correct Answer: a reproductive body bearing resistant spores
Explanation: During unfavourable conditions, the slime-mould plasmodium differentiates into fruiting bodies. Spores develop at the tips of these raised structures, where their position favours later release and dispersal. The description does not represent a photosynthetic filament or a feeding structure, since the organism has shifted from active saprophytic feeding to production of resistant propagules. Heterocysts occur in certain cyanobacterial filaments, and a gullet is associated with feeding in ciliated protozoans. The spatial clue at the tip and the environmental survival role identify the structure as a slime-mould fruiting body. Its elevated position aids release and dispersal of the resistant spores produced under unfavourable conditions.
216. The ability of slime-mould spores to survive adverse conditions for several years is most directly related to their:
ⓐ. requirement for continuous environmental moisture
ⓑ. possession of two unequal locomotory flagella
ⓒ. absence of any protective external covering
ⓓ. presence of thick and resistant spore walls
Correct Answer: presence of thick and resistant spore walls
Explanation: Slime-mould spores possess true walls that provide resistance against unfavourable environmental conditions. This protection allows the spores to remain viable for long periods, including several years, until circumstances become suitable for renewed activity. The spore stage is specialised for persistence rather than active feeding or locomotion. Unequal flagella characterise euglenoids, while continuous moisture dependence would reduce rather than explain long-term survival during adversity. The wall-survival relation also separates slime-mould spores from the unprotected feeding plasmodium. Structural resistance preserves the living potential of the organism through conditions that the active stage may not tolerate.
217. Air currents increase around mature slime-mould fruiting bodies without changing spore production. The most direct predicted effect is:
ⓐ. stronger airflow reduces dispersal from the fruiting bodies
ⓑ. airflow triggers immediate return to the feeding plasmodium
ⓒ. increased dispersal of resistant spores by air currents
ⓓ. spore walls become less resistant during transport
Correct Answer: increased dispersal of resistant spores by air currents
Explanation: Mature slime-mould spores are carried by air currents, so stronger air movement can transport them farther from the parent fruiting bodies. The changed condition affects dispersal rather than the earlier processes of spore formation or wall development. Spore resistance allows survival during transport, while the elevated position on fruiting bodies helps expose the spores to moving air. Germination and return to an active stage would require suitable environmental conditions after dispersal and cannot be inferred immediately from increased airflow. The prediction is limited to the process directly influenced by the changed physical factor.
218. A mutation prevents a slime mould from forming true walls around its spores, while fruiting bodies still develop normally. The most likely consequence is:
ⓐ. lower spore survival during prolonged adverse periods
ⓑ. thinner spore walls increase long-term resistance
ⓒ. spores retain full resistance despite lacking true walls
ⓓ. wall loss improves survival by preventing dehydration
Correct Answer: lower spore survival during prolonged adverse periods
Explanation: The true wall is the major protective feature that makes slime-mould spores highly resistant. If wall formation fails, fruiting bodies may still position and release the propagules, but the spores would be more vulnerable to drying and other adverse conditions. Their ability to remain viable for several years would consequently decline. Dispersal and survival are related but distinct outcomes: air currents may still carry the wall-defective spores, yet successful persistence during and after transport would be reduced. The mutation interrupts the structural basis of resistance without necessarily preventing formation of the spore-bearing structure.
219. Assertion: Unfavourable conditions promote formation of fruiting bodies in slime moulds.
Reason: Fruiting bodies produce resistant spores that can survive adversity and disperse through air.
ⓐ. Both statements are true, and the Reason correctly explains the Assertion
ⓑ. Both statements are true, but the Reason does not explain the Assertion
ⓒ. The Assertion statement is true, whereas the Reason statement is false
ⓓ. The Assertion statement is false, whereas the Reason statement is true
Correct Answer: Both statements are true, and the Reason correctly explains the Assertion
Explanation: The assertion is true: environmental adversity causes the feeding plasmodium to differentiate into fruiting bodies. The reason is also true, as these structures bear resistant spores capable of surviving harsh conditions and travelling through air currents. The reason explains the biological value of the response described in the assertion. Remaining indefinitely as an exposed feeding plasmodium would provide less protection during prolonged adversity. Production of resistant, dispersible spores preserves continuity and also increases the chance that some propagules reach a more favourable location. The environmental trigger and survival outcome are linked through fruiting-body formation.
220. Slime moulds are observed under four conditions.
| Condition | Dominant observation |
|---|
| P | Suitable conditions; active spreading mass |
| Q | Unfavourable conditions; raised spore-bearing structures |
| R | Dry air; resistant spores remain viable |
| S | Air currents; spores move away from the source |
The sequence of biological functions represented by P, Q, R and S is:
ⓐ. dispersal → feeding → photosynthesis → fusion
ⓑ. survival → feeding → fusion → dispersal
ⓒ. feeding → nitrogen fixation → survival → ingestion
ⓓ. feeding → spore production → survival → dispersal
Correct Answer: feeding → spore production → survival → dispersal
Explanation: Under suitable conditions, the spreading plasmodium moves across decaying material and feeds, so P represents feeding. Unfavourable conditions trigger development of raised fruiting bodies bearing spores, making Q the spore-production stage. The resistant walls of those spores account for continued viability in dry conditions at R. Air currents then carry mature spores away from the original site, giving dispersal at S. The table presents four successive biological functions rather than four unrelated observations. Reading the environmental condition together with the structure or behaviour reveals the transition from active growth to reproduction, persistence, and distribution.
