1. The central biological role of photosynthesis in a green plant is most accurately represented by the process of:
ⓐ. oxidation of stored food to release energy for cellular work
ⓑ. light-driven organic synthesis that stores chemical energy in products
ⓒ. absorption of ready-made organic food from the surrounding environment
ⓓ. breakdown of organic remains into mineral nutrients by decomposers
Correct Answer: light-driven organic synthesis that stores chemical energy in products
Explanation: Photosynthesis is an anabolic process in which green plants capture light energy and use it to build energy-rich organic compounds from simple inorganic raw materials, mainly carbon dioxide and water. The captured energy does not remain as light; it becomes chemical energy stored in the bonds of the products. This distinguishes photosynthesis from respiration, where organic compounds are oxidised to release usable energy, and from heterotrophic nutrition, where preformed organic food is obtained from another source. The process gives green plants their autotrophic role and creates organic matter that can later be used for growth, storage and respiration. New organic matter represents entry of chemical energy at the producer level of an ecosystem.
2. Consider the following statements about the autotrophic role of green plants.
I. They can synthesise organic compounds from inorganic raw materials.
II. They convert captured light energy into chemical energy.
III. They obtain all required organic food directly from other organisms.
IV. They can photosynthesise without carbon dioxide or water.
ⓐ. Statements I and III are correct; statements II and IV are incorrect
ⓑ. Statements II and IV are correct; statements I and III are incorrect
ⓒ. Statements I, II and III are correct; statement IV is incorrect
ⓓ. Statements I and II are correct; statements III and IV are incorrect
Correct Answer: Statements I and II are correct; statements III and IV are incorrect
Explanation: Autotrophic synthesis means that a green plant produces organic matter from inorganic substances rather than depending on preformed organic food as its carbon source. Carbon dioxide supplies carbon, water participates as a raw material, and light provides the energy that is converted into chemical form. Statements I and II express these two linked features. Obtaining all organic food from other organisms would describe heterotrophic dependence, not the usual nutritional role of a photosynthetic green plant. Photosynthesis also cannot proceed normally without essential raw materials such as carbon dioxide and water. Autotrophy concerns the source of organic carbon and energy capture; it does not mean independence from water, minerals or suitable environmental conditions.
3. A green plant is supplied with water, carbon dioxide and mineral nutrients, but it receives no usable light. The most immediate photosynthetic consequence is:
ⓐ. organic-compound synthesis declines as the energy input for the process is unavailable
ⓑ. organic-compound synthesis continues unchanged by using carbon dioxide as the energy source
ⓒ. oxygen uptake becomes the light-driven step that replaces photosynthetic energy capture
ⓓ. mineral absorption alone supplies the chemical energy required for carbohydrate formation
Correct Answer: organic-compound synthesis declines as the energy input for the process is unavailable
Explanation: Carbon dioxide and water provide matter for photosynthetic synthesis, but they do not supply the light energy that drives the process. Without usable light, the plant cannot maintain the photochemical production of energy-rich intermediates needed to support sustained formation of organic compounds. Carbon dioxide is a carbon source, not an energy source, and mineral uptake cannot replace the conversion of radiant energy into chemical energy. The plant may continue respiration by using previously stored food, but that is different from new photosynthetic production. The changed condition removes the energy input while leaving several raw materials present, so the direct effect is a decline in light-dependent organic synthesis. Supplying more carbon dioxide cannot compensate until a usable energy source is restored.
4. Evaluate the following statements about the biological significance of photosynthesis.
I. It forms the primary food base for most ecosystems.
II. It contributes oxygen to the atmosphere.
III. It converts solar energy into chemical energy stored in organic matter.
IV. It supplies organic food only to organisms that directly eat green leaves.
ⓐ. Statements I and IV are correct; statements II and III are incorrect
ⓑ. Statements II and III are correct; statements I and IV are incorrect
ⓒ. Statements I, II and III are correct; statement IV is incorrect
ⓓ. Statements I, II, III and IV are all correct
Correct Answer: Statements I, II and III are correct; statement IV is incorrect
Explanation: Photosynthetic organisms introduce newly synthesised organic matter into ecosystems, so their products support producers themselves and pass through feeding relationships to herbivores, carnivores, decomposers and many other organisms. The process also converts incoming solar energy into chemical energy that can move through these food relationships. Oxygen released by oxygenic photosynthesis contributes to the atmospheric supply used in aerobic respiration. The benefit is not restricted to organisms that eat green leaves directly; consumers may obtain photosynthetically derived matter through seeds, fruits, roots, aquatic producers or other consumers. The first three statements capture ecosystem-level food, energy and oxygen functions, whereas the last statement incorrectly narrows the dependency chain. Photosynthetic products can reach an organism through many intermediate feeding steps.
5. In a grassland, prolonged failure of photosynthesis sharply reduces new organic-matter production by grasses. The most defensible ecosystem-level prediction is:
ⓐ. only carnivores are affected, since producers do not depend on photosynthesis for energy
ⓑ. reduced chemical-energy entry at producers, with effects across the food chain
ⓒ. herbivores remain unaffected, as atmospheric oxygen can replace organic food
ⓓ. decomposers become the new source of solar energy for the ecosystem
Correct Answer: reduced chemical-energy entry at producers, with effects across the food chain
Explanation: Grasses are primary producers that capture solar energy and store part of it in newly formed organic compounds. When this production falls, less food and chemical energy become available for herbivores. Carnivores are affected later through reduced herbivore biomass, and decomposers eventually receive less fresh organic input as well. Atmospheric oxygen cannot substitute for food, since oxygen supports respiration but does not provide the carbon skeletons or stored chemical energy needed for growth. Decomposers recycle matter from existing organic remains; they do not capture solar energy as a replacement for producers. The case traces a dependency chain from photosynthetic energy capture to successive trophic levels rather than limiting the effect to one consumer group. The decline may appear first in plant growth and herbivore food supply, then later in predator populations and detrital inputs. This time sequence follows the transfer of stored chemical energy through the ecosystem.
6. Assertion: Photosynthesis by green plants contributes to the replenishment of atmospheric oxygen.
Reason: Green tissues release oxygen during the light-driven photosynthetic process, and some of this oxygen enters the atmosphere.
ⓐ. Both the Assertion and the Reason are true, and the Reason correctly explains the Assertion
ⓑ. Both the Assertion and the Reason are true, but the Reason does not explain the Assertion
ⓒ. The Assertion is true, whereas the Reason is false and cannot explain the Assertion
ⓓ. The Assertion is false, whereas the Reason is true and cannot explain the Assertion
Correct Answer: Both the Assertion and the Reason are true, and the Reason correctly explains the Assertion
Explanation: The Assertion is true: oxygenic photosynthesis is a major biological process that adds oxygen to the surroundings and, on a large scale, contributes to atmospheric oxygen. The Reason is also true. When green tissues photosynthesise under suitable conditions, oxygen is produced during the light-dependent part of the process and can diffuse out of the tissue. That released oxygen provides the direct link between photosynthetic activity and atmospheric replenishment, so the Reason explains the Assertion. This relation should not be confused with the idea that every oxygen molecule remains permanently in the atmosphere; respiration, combustion and other oxidative processes continually consume oxygen, making its biological replenishment an ongoing balance. Atmospheric oxygen reflects a dynamic balance between photosynthetic release and consumption through respiration, combustion and other oxidative processes.
7. Equal-sized, destarched leaf regions are maintained under the conditions shown.
| Region | Tissue | Light | Carbon dioxide |
|---|
| P | Green | Available | Available |
| Q | Green | Absent | Available |
| R | Non-green | Available | Available |
| S | Green | Available | Removed |
After adequate treatment, the region most likely to become blue-black with iodine is:
ⓐ. Region Q
ⓑ. Region S
ⓒ. Region R
ⓓ. Region P
Correct Answer: Region P
Explanation: A positive iodine test after destarching requires newly formed starch. Region P alone combines chlorophyll-containing tissue, available light and available carbon dioxide. Green tissue supplies the photosynthetic apparatus, light provides energy, and carbon dioxide supplies carbon for organic-compound formation. Region Q lacks light, region R lacks chlorophyll-containing green tissue, and region S lacks carbon dioxide, so each misses a necessary condition represented in the setup. The table must be interpreted by combining all three variables rather than locating a single favourable entry. Since the regions began destarched, a blue-black result can be linked to starch produced during the treatment period, making P the region with the complete set of conditions. The design also separates the roles of the three requirements: changing any one of them while keeping the others favourable prevents the predicted positive result. This makes the table a combined requirement test rather than a simple colour identification.
8. A destarched variegated leaf is exposed to light. After the iodine test, only its green regions become blue-black, while the non-green regions remain yellow-brown. The strongest conclusion is:
ⓐ. light alone is sufficient for starch formation in every leaf region
ⓑ. iodine turns blue-black only where chlorophyll was already present
ⓒ. chlorophyll-containing tissue is needed for starch formation in leaves
ⓓ. non-green tissue forms starch, but its colour prevents iodine detection
Correct Answer: chlorophyll-containing tissue is needed for starch formation in leaves
Explanation: Both green and non-green regions belong to the same leaf and receive the same illumination and iodine treatment. Their decisive difference is the presence of chlorophyll-containing tissue in the green areas. The blue-black iodine reaction indicates starch, so its restriction to green regions links new starch formation with chlorophyll-containing tissue. Light alone cannot explain the result because the illuminated non-green regions remain starch-negative. Iodine does not respond to chlorophyll itself; it reveals starch wherever starch is present. Destarching before illumination is essential because it removes previously stored carbohydrate and allows the final pattern to represent synthesis during the treatment. The evidence therefore supports chlorophyll-containing tissue as a requirement for photosynthetic starch formation under these conditions, while avoiding the stronger and unsupported claim that the experiment identifies every biochemical step between light absorption and carbohydrate storage.
9. A variegated leaf has a green centre and a white margin. An opaque strip covers a narrow band crossing both regions before the destarched leaf is illuminated. The iodine-positive area should be:
ⓐ. only the uncovered green region
ⓑ. both green regions, whether covered or uncovered
ⓒ. every uncovered region, green or white
ⓓ. only the covered white margin
Correct Answer: only the uncovered green region
Explanation: The setup combines two independent requirements within one leaf. Green tissue contains chlorophyll, while the opaque strip controls light availability. An area can form detectable starch during the treatment only when it is both green and illuminated. The uncovered green region satisfies both conditions and should become blue-black with iodine. A covered green band has chlorophyll but lacks light, whereas an uncovered white region receives light but lacks chlorophyll-containing tissue. The spatial description is enough to predict the result without seeing the leaf. This arrangement is more informative than testing colour or illumination alone, since it reveals that neither green colour nor light by itself is sufficient for starch formation in the stated experiment. The covered white region also remains negative, but it lacks both tested requirements and is less useful for separating their individual effects. The uncovered green area is the only location where the two necessary conditions coincide.
10. A variegated leaf is exposed to light without being destarched first. Why does this weaken the interpretation of the later iodine test?
ⓐ. It proves that light is unnecessary for starch formation
ⓑ. It prevents linking starch specifically to the light treatment
ⓒ. It removes chlorophyll from the green regions before the test
ⓓ. It makes iodine unable to react with newly formed starch
Correct Answer: It prevents linking starch specifically to the light treatment
Explanation: Destarching is a preparation step that reduces previously stored starch before the experimental treatment begins. If it is omitted, a blue-black region may contain starch formed before the leaf was placed under the stated light conditions. The investigator can then no longer decide whether the detected starch arose during the treatment or was already present. This is an evidence problem, not a change in iodine chemistry or chlorophyll content. The leaf may still photosynthesise, and iodine may still detect starch, but the time of starch formation is uncertain. A sound conclusion about the requirement being tested needs the final observation to reflect new production during the controlled exposure period. Without that time control, even a biologically real colour change cannot be assigned confidently to the experimental condition. Without destarching, a blue-black region could contain starch made before illumination, so the treatment cannot distinguish pre-existing reserves from new synthesis.
11. Assertion: In a destarched leaf, an opaque-covered region is expected to turn blue-black after the leaf is exposed to light.
Reason: The opaque cover prevents light from reaching that region.
ⓐ. Both the Assertion and the Reason are true, and the Reason correctly explains the Assertion
ⓑ. Both the Assertion and the Reason are true, but the Reason does not explain the Assertion
ⓒ. The Assertion is true, whereas the Reason is false and cannot explain the Assertion
ⓓ. The Assertion is false, whereas the Reason is true and cannot explain the Assertion
Correct Answer: The Assertion is false, whereas the Reason is true and cannot explain the Assertion
Explanation: The Assertion is false. A properly destarched region covered by an opaque material does not receive the light needed for photosynthetic starch formation, so it is expected to remain yellow-brown after iodine rather than becoming blue-black. The Reason is true, since excluding light is the purpose of the opaque cover. This true statement also explains why the expected observation in the Assertion is wrong. Destarching ensures that the covered region does not retain enough pre-existing starch to confuse the result. The uncovered part of the same leaf acts as the comparison and can become starch-positive when the other necessary conditions remain available. The truth values are different: the proposed blue-black outcome is wrong, while the statement about light exclusion is valid.
12. A destarched leaf is partly covered with opaque paper and then illuminated. The uncovered region becomes blue-black with iodine, while the covered region remains yellow-brown. This result most directly supports the inference that:
ⓐ. light is needed for starch formation in comparable leaf regions
ⓑ. opaque paper chemically destroys starch only in the covered region
ⓒ. iodine can enter illuminated tissue but cannot enter covered tissue
ⓓ. carbon dioxide is absent only from the region beneath the paper
Correct Answer: light is needed for starch formation in comparable leaf regions
Explanation: The two regions are parts of the same destarched leaf, so they share leaf age, internal organisation, chlorophyll status and much of their surrounding environment. The planned difference is light exposure: one region receives light while the opaque-covered region does not. Blue-black colour in the illuminated portion indicates newly formed starch, whereas the yellow-brown covered portion lacks detectable starch. The observation supports light as a required condition for starch formation under the setup. It does not establish that the paper destroys starch or blocks iodine entry, since the cover is removed before testing. Nor is carbon dioxide specifically removed by the paper. The inference rests on controlled comparison of light availability within one leaf. An internal comparison is especially useful here, as both regions were destarched together and remain connected to the same plant. The result establishes a requirement for light in starch formation; it does not by itself describe the later biochemical reactions that produce carbohydrate.
13. In a partially covered-leaf experiment, a transparent sheet is mistakenly used instead of opaque paper. Both regions later become blue-black with iodine. The main procedural defect is that:
ⓐ. carbon dioxide became available only to the covered region
ⓑ. chlorophyll was removed only from the uncovered region
ⓒ. light was not excluded from the intended treatment region
ⓓ. iodine was prevented from entering the exposed region
Correct Answer: light was not excluded from the intended treatment region
Explanation: The purpose of covering part of the leaf is to create a region that differs from the exposed region in light availability. A transparent sheet fails to produce that difference, so both regions may receive enough light for photosynthesis and starch formation. The blue-black result in both regions then cannot test whether light is required, since the supposed no-light treatment was never established. This is a failure of variable control rather than an effect on chlorophyll, carbon dioxide or iodine penetration. A valid comparison requires an opaque cover, careful destarching and otherwise similar conditions. The result illustrates that an experiment can produce clear observations yet still fail to answer the intended question when the treatment does not alter the target variable. Replacing the transparent sheet with an opaque material would restore the intended comparison while keeping the remaining procedure unchanged.
14. A researcher uses an illuminated and an opaque-covered region of the same destarched leaf rather than two unrelated leaves. The chief advantage of this design is:
ⓐ. the covered region receives more carbon dioxide than the exposed region
ⓑ. the uncovered region loses chlorophyll during the treatment
ⓒ. starch can move only between regions of the same leaf
ⓓ. shared leaf variables with a local difference in light availability
Correct Answer: shared leaf variables with a local difference in light availability
Explanation: Using two regions of one leaf creates an internal comparison. Both regions have the same developmental history, similar tissue organisation, the same plant water status and broadly similar access to carbon dioxide. The experiment deliberately changes light availability by placing an opaque cover over one portion. Sharing many other features reduces the chance that a difference in starch response is caused by leaf age, health or initial condition rather than illumination. This does not mean every variable is perfectly identical, but the design is stronger than comparing unrelated leaves. The biological inference becomes more focused: when the uncovered area forms starch and the covered area does not, the local difference in light has greater explanatory value.
15. In an experiment testing the need for carbon dioxide in starch formation, potassium hydroxide is placed inside a sealed container mainly to:
ⓐ. supply water vapour to the enclosed leaf
ⓑ. absorb carbon dioxide from the enclosed air
ⓒ. remove chlorophyll from the enclosed tissue
ⓓ. act as the indicator for newly formed starch
Correct Answer: absorb carbon dioxide from the enclosed air
Explanation: Potassium hydroxide is used as a carbon-dioxide absorbent. In a sealed setup, it lowers the availability of carbon dioxide around the enclosed leaf region while light, green tissue and other relevant conditions are maintained as far as possible. After the exposure period, iodine is used separately to test for starch. Potassium hydroxide is not the starch indicator, does not supply the carbon source and is not intended to remove chlorophyll. Its experimental value lies in selectively changing carbon-dioxide availability. When a comparable region with access to ordinary air forms starch but the potassium-hydroxide-treated region does not, the contrast can be used to assess the requirement for carbon dioxide. A positive starch test indicates accumulated carbohydrate after treatment, not the immediate product of the light reaction. The strongest diagnosis comes from locating the first failed step while upstream functions remain intact in the stated starch-test experiment.
16. Two sealed bell jars each contain a destarched green leaf in light. One jar contains potassium hydroxide and the other contains water. Only the leaf in the water jar becomes blue-black with iodine. The strongest conclusion is:
ⓐ. potassium hydroxide directly destroys any starch formed in light
ⓑ. water prevents the leaf from using previously stored carbohydrates
ⓒ. carbon dioxide is required for starch formation under these conditions
ⓓ. absence of oxygen is the only cause of the negative iodine test
Correct Answer: carbon dioxide is required for starch formation under these conditions
Explanation: The decisive difference between the jars is the substance placed inside them. Potassium hydroxide removes carbon dioxide from the enclosed air, whereas water serves as a comparison that does not perform that function. Both leaves are green, destarched, illuminated and enclosed. The starch-positive result in the water jar shows that the general apparatus and light conditions can support starch formation. The negative result in the potassium hydroxide jar is linked to carbon-dioxide removal, provided the setup prevents direct chemical contact with the leaf and controls other conditions. The experiment supports carbon dioxide as a necessary raw material for photosynthetic starch formation; it does not show that potassium hydroxide destroys starch within the tissue. The water jar is important as a procedural control: its positive result shows that sealing a leaf in a jar does not automatically prevent starch formation. Interpreting the contrast requires the potassium hydroxide to remain physically separated from the leaf.
17. A destarched leaf remains attached to a plant. One portion is enclosed in an airtight flask containing potassium hydroxide without touching it, while the rest stays exposed to air. Both portions receive light. The comparison primarily tests:
ⓐ. carbon-dioxide supply to enclosed and exposed leaf regions
ⓑ. chlorophyll distribution between the upper and lower epidermis
ⓒ. iodine movement through attached and detached leaf tissues
ⓓ. the effect of leaf age on long-term starch storage
Correct Answer: carbon-dioxide supply to enclosed and exposed leaf regions
Explanation: The enclosed portion is surrounded by air from which potassium hydroxide removes carbon dioxide, while the exposed portion continues to receive carbon dioxide from the atmosphere. Keeping the leaf attached helps both regions share the same plant, water supply and general physiological condition. Both are illuminated, so light is not the intended variable. The leaf must also be destarched before treatment so that any later iodine response reflects new starch formation. The apparatus is designed around a local difference in carbon-dioxide availability, not around chlorophyll distribution, iodine movement or leaf age. Comparing the enclosed and exposed regions can reveal whether access to carbon dioxide is necessary for starch production under otherwise similar conditions. The expected pattern is a starch-negative enclosed region and a starch-positive air-exposed region. That prediction depends on an airtight seal, effective carbon-dioxide absorption and sufficient illumination, so each structural feature of the apparatus contributes to the inference.
18. Suppose the potassium hydroxide in a sealed leaf experiment has already absorbed as much carbon dioxide as it can and no longer removes the gas efficiently. The most likely consequence is:
ⓐ. both leaf regions may lose chlorophyll and remain permanently starch-negative
ⓑ. the exposed region may stop receiving usable light during the treatment
ⓒ. iodine may fail to detect starch in either illuminated leaf region
ⓓ. carbon dioxide may remain available, allowing enclosed tissue to form starch
Correct Answer: carbon dioxide may remain available, allowing enclosed tissue to form starch
Explanation: The experiment depends on potassium hydroxide maintaining a carbon-dioxide-poor environment around the enclosed leaf region. If the absorbent is ineffective, residual or newly available carbon dioxide may remain in the flask. A green illuminated region could then photosynthesise and accumulate starch, giving a blue-black iodine response even though the investigator intended it to be carbon-dioxide deprived. Such a result would reduce the contrast with the air-exposed region and weaken the test of carbon-dioxide requirement. The failure would not directly remove chlorophyll, block light or alter iodine chemistry. It would instead collapse the intended treatment by allowing the supposedly excluded raw material to remain available. A positive result inside the flask would then be ambiguous: it could indicate failure of carbon-dioxide removal rather than independence of photosynthesis from carbon dioxide. The investigator must first verify the treatment condition before drawing a biological conclusion.
19. Four treatments are arranged as shown.
| Treatment | Enclosure | Leaf condition | Light | Material in jar |
|---|
| P | Sealed | Green and destarched | Available | Potassium hydroxide |
| Q | Sealed | Green and destarched | Available | Water |
| R | Open | Green and destarched | Available | Potassium hydroxide |
| S | Sealed | Green and destarched | Absent | Water |
The most suitable pair for isolating the effect of carbon-dioxide availability while keeping enclosure and light conditions comparable is:
ⓐ. P and R
ⓑ. Q and S
ⓒ. P and Q
ⓓ. R and S
Correct Answer: P and Q
Explanation: Treatments P and Q are both sealed, contain green destarched leaves and receive light. Their planned difference is the material inside the jar. Potassium hydroxide in P absorbs carbon dioxide, while water in Q does not remove it and serves as the comparison. A difference in starch formation between these treatments can be linked most directly to carbon-dioxide availability. Comparing P with R would also change whether the jar is sealed, and comparing Q with S would change light availability. The table must be read by identifying the pair in which non-target variables remain matched. P and Q create the cleanest controlled contrast for testing carbon dioxide as a requirement for photosynthetic starch formation. The expected evidence would be a negative iodine test in P and a positive test in Q. That contrast is meaningful only after confirming that the leaves began destarched and that potassium hydroxide did not contact or damage the leaf tissue.
20. Assertion: Destarching is required before a potassium-hydroxide leaf experiment if the iodine result is to be linked to the treatment period.
Reason: Potassium hydroxide absorbs carbon dioxide from the enclosed air.
ⓐ. Both the Assertion and the Reason are true, and the Reason correctly explains the Assertion
ⓑ. Both the Assertion and the Reason are true, but the Reason does not explain the Assertion
ⓒ. The Assertion is true, whereas the Reason is false and cannot explain the Assertion
ⓓ. The Assertion is false, whereas the Reason is true and cannot explain the Assertion
Correct Answer: Both the Assertion and the Reason are true, but the Reason does not explain the Assertion
Explanation: The Assertion is true. Destarching removes or greatly reduces starch formed before the experiment, so a later iodine-positive result can be associated with photosynthesis during the controlled treatment. The Reason is also true: potassium hydroxide is used to absorb carbon dioxide from the enclosed air. However, that property does not explain why destarching is necessary. Destarching addresses the timing and origin of detected starch, whereas potassium hydroxide controls carbon-dioxide availability. The two procedures support different parts of the experimental logic. A valid interpretation needs both: one prevents pre-existing starch from confusing the observation, and the other establishes the carbon-dioxide-deprived condition being tested. Since the Reason is true but addresses a different control, it does not provide the causal explanation for the Assertion. The relation is decided by whether the reason accounts for the assertion rather than merely mentioning the same topic in the stated starch-test experiment.
