1. A germinating seed has not yet begun significant photosynthesis, but its cells are actively dividing and synthesising new material. Its immediate need for respiration is best explained by the requirement for:
ⓐ. continuous formation of light energy inside non-green cells
ⓑ. usable energy for transport, biosynthesis and cellular activity
ⓒ. permanent storage of oxygen within developing tissues
ⓓ. conversion of all absorbed minerals into respiratory substrates
Correct Answer: usable energy for transport, biosynthesis and cellular activity
Explanation: Living cells require a continuously available energy supply even when they are not performing photosynthesis. A germinating seed carries out active transport, synthesis of proteins and other cell components, cell division and growth. These activities cannot be supported merely by the presence of stored food; the chemical energy in that food must be released in a controlled form that the cell can use. Cellular respiration performs this role and transfers part of the released energy to ATP. The example also shows that energy demand is a feature of living activity itself, not only of visible movement or green tissues exposed to light.
2. A leaf cell receives adequate water, minerals and respiratory substrate, but its ATP production falls sharply. The pair of processes most directly affected will be:
ⓐ. diffusion of oxygen and carbon dioxide along concentration gradients
ⓑ. osmotic movement of water across a selectively permeable membrane
ⓒ. passive movement of lipid-soluble molecules through the plasma membrane
ⓓ. active transport of ions and synthesis of cellular macromolecules
Correct Answer: active transport of ions and synthesis of cellular macromolecules
Explanation: Active transport moves substances against an electrochemical gradient and requires an energy input, commonly supplied through ATP-dependent membrane proteins. Biosynthesis also needs energy to form ordered, complex molecules from simpler precursors. Diffusion, osmosis and passive movement through a membrane proceed down an existing gradient and do not directly consume ATP for the movement itself. A fall in ATP production will affect energy-requiring transport and synthetic work before it stops purely passive processes. The decisive distinction is between processes driven by stored cellular energy and processes driven by concentration or water-potential differences already present across the membrane. Energy shortage can therefore impair maintenance of ion gradients and the construction of new protoplasm even while simple molecular movement continues.
3. Consider the following statements about the energy needs of living organisms.
I. Uptake of some mineral ions by roots may require metabolic energy.
II. Formation of complex cellular molecules requires an energy supply.
III. Diffusion of every gas molecule across a membrane directly consumes ATP.
IV. Cell division and reproductive activity depend on energy-releasing metabolism.
ⓐ. I, II and IV only
ⓑ. I and III only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Mineral ions are often absorbed against their concentration or electrochemical gradients, so active uptake can depend on metabolic energy. Biosynthesis joins smaller units into organised macromolecules, while cell division and reproduction involve synthesis, transport and structural rearrangement; all of these require energy. Gas diffusion differs fundamentally. A gas molecule moving down its concentration gradient does not need direct ATP expenditure by the cell, although living cells may use energy to maintain conditions that influence gradients. The valid set is I, II and IV. This combination separates continuously energy-dependent life activities from passive movement that occurs through the kinetic behaviour of molecules. Energy is required where the cell must create, maintain or reorganise biological order rather than simply permit spontaneous movement.
4. Assertion: Organic food respired by non-green root cells ultimately originates from photosynthesis.
Reason: Root cells manufacture all their respiratory carbohydrates directly from inorganic carbon in darkness.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Assertion is true, but Reason is false
Explanation: The assertion is true. Photosynthetic organisms capture light energy and store it in organic compounds, especially carbohydrates. In a green plant, sugars produced in photosynthetic tissues can be transported to non-green organs such as roots, or stored compounds can later be mobilised and respired there. The reason is false, since ordinary root cells in darkness do not fix inorganic carbon to manufacture all of their respiratory carbohydrate. They depend on organic food supplied by photosynthetic parts or by stored reserves originally formed from photosynthate. The origin of the substrate and the site at which it is later oxidised need not be the same tissue. A root may be a site of respiratory consumption without being the original site of photosynthetic food production.
5. A variegated plant has green leaves, a non-green stem region and actively growing roots. The roots continue to respire in darkness. The most direct source of their respiratory organic substrate is:
ⓐ. photosynthate formed in green tissues and transported or stored
ⓑ. carbon dioxide absorbed by roots and reduced without light
ⓒ. oxygen converted into carbohydrate within non-green tissues
ⓓ. ATP transported from leaves as the principal carbon compound
Correct Answer: photosynthate formed in green tissues and transported or stored
Explanation: Green leaf regions capture light energy and synthesise organic food. Part of this food may be used locally, transported as soluble carbohydrate to other organs, or converted into storage material that can later be mobilised. Roots and non-green stem tissues can respire these compounds even though they do not carry out significant photosynthesis in darkness. Oxygen supports aerobic oxidation but does not supply the carbon skeleton of carbohydrate, and ATP is an energy-transfer molecule rather than the principal transported carbon food. The case distinguishes the production of organic substrate by photosynthesis from its use by respiration in another organ and at another time. Transport and storage connect spatially separated production and consumption within the same plant.
6. Arrange the following events in the order that links sunlight to energy use in a non-green plant cell.
P. Light energy is captured by photosynthetic tissue.
Q. Organic carbohydrate is synthesised.
R. Organic food reaches or is stored in the non-green cell.
S. Respiration transfers part of the food's chemical energy to ATP.
ⓐ. \(P \rightarrow R \rightarrow Q \rightarrow S\)
ⓑ. \(Q \rightarrow P \rightarrow S \rightarrow R\)
ⓒ. \(R \rightarrow Q \rightarrow P \rightarrow S\)
ⓓ. \(P \rightarrow Q \rightarrow R \rightarrow S\)
Correct Answer: \(P \rightarrow Q \rightarrow R \rightarrow S\)
Explanation: Photosynthetic tissue first captures light energy and uses it to synthesise carbohydrate from inorganic materials. The resulting organic food may then be transported to a non-green cell or stored and later made available there. Cellular respiration in that cell oxidises the substrate in controlled steps and captures part of its chemical energy in ATP. The order follows an energy-conversion pathway: radiant energy becomes chemical energy in organic food, the food reaches the consuming cell, and respiration converts part of that stored energy into a directly usable cellular form. Reversing carbohydrate formation and light capture breaks the dependency between the first two stages. Each stage depends on completion of the preceding one, so the order also explains how an autotrophic organ can support metabolism in a heterotrophic plant part. ATP is not transported as the original sunlight-derived food in this chain.
7. Evaluate the following statements.
I. Green plants store a portion of captured light energy in carbohydrates.
II. Heterotrophs obtain organic food directly or indirectly from photosynthetic organisms.
III. Non-green parts of a plant cannot perform cellular respiration.
IV. Photosynthetic cyanobacteria contribute to the biological origin of respiratory food.
ⓐ. I and III only
ⓑ. I, II and IV only
ⓒ. II and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Photosynthesis stores light-derived energy in organic compounds, and these compounds become respiratory food for the photosynthetic organism itself and for organisms that consume photosynthetic biomass directly or through food chains. Cyanobacteria are also photosynthetic and can produce organic matter that enters this broad biological supply. Non-green plant organs still contain living cells and carry out respiration using transported or stored substrates, so the third statement is invalid. The correct group links autotrophic synthesis with later heterotrophic or non-green-tissue use while avoiding the misconception that respiration is restricted to green cells or to organisms that cannot photosynthesise. Photosynthesis and respiration can also occur in the same green organism, so autotrophy does not remove the need for respiratory energy release.
8. Cellular respiration is most accurately described as:
ⓐ. exchange of respiratory gases without intracellular oxidation
ⓑ. rapid burning of food outside cells with heat release
ⓒ. controlled intracellular oxidation of organic substrates
ⓓ. synthesis of carbohydrate from inorganic carbon using light
Correct Answer: controlled intracellular oxidation of organic substrates
Explanation: Cellular respiration occurs within living cells and involves the controlled oxidation of organic substrates through a sequence of reactions. Breaking and rearranging carbon-containing compounds releases chemical energy, part of which is conserved in ATP rather than appearing only as heat. Gas exchange may supply oxygen and remove carbon dioxide, but it is not the complete intracellular pathway. Combustion also oxidises material, yet it releases energy rapidly and lacks the organised enzyme-coupled capture characteristic of respiration. Photosynthesis performs the opposite broad energy-storage role by building organic compounds. The definition must include both intracellular oxidation and controlled energy capture to distinguish respiration from these related processes.
9. A leaf admits oxygen through stomata, after which its cells oxidise organic molecules and form ATP. The relation between the two events is best stated as:
ⓐ. stomatal gas exchange and cellular respiration are identical processes
ⓑ. gas exchange releases ATP, while respiration only transports oxygen
ⓒ. respiration occurs at the surface, while gas exchange occurs inside cells
ⓓ. gas exchange supplies oxygen; respiration releases energy inside cells
Correct Answer: gas exchange supplies oxygen; respiration releases energy inside cells
Explanation: Entry of oxygen through stomata is part of gaseous exchange. It changes the availability of a respiratory gas but does not itself oxidise the organic substrate or synthesise ATP. Once oxygen reaches living cells, the intracellular respiratory pathway can use it in aerobic metabolism and capture energy from organic compounds. The two processes are linked, yet they occur at different organisational levels and perform different functions. Gas exchange provides and removes gases across surfaces and internal spaces; cellular respiration is the chemical pathway inside cells. This boundary prevents the common error of using breathing, gaseous exchange and cellular respiration as interchangeable terms. Oxygen entry is a physical transport event, whereas ATP formation depends on a biochemical sequence using respiratory substrate.
10. In cultured plant cells, oxygen enters normally and carbon dioxide can leave, but an intracellular enzyme defect prevents controlled oxidation of organic substrate. ATP production declines. The observation supports the conclusion that:
ⓐ. gaseous exchange alone completes the release of usable cellular energy
ⓑ. normal gas movement cannot replace the intracellular respiratory pathway
ⓒ. carbon dioxide removal is the only energy-yielding step of respiration
ⓓ. oxygen entry directly converts ADP into ATP at the cell surface
Correct Answer: normal gas movement cannot replace the intracellular respiratory pathway
Explanation: The case separates gas movement from the biochemical reactions of respiration. Oxygen can reach the cells and carbon dioxide can leave, so the exchange pathway is available. ATP still declines when intracellular oxidation is blocked, showing that gas availability alone does not release and conserve the substrate's chemical energy. Enzyme-controlled reactions inside the cell must process the organic molecule and couple portions of the released energy to ATP formation. The evidence does not identify a single respiratory enzyme or stage, but it clearly establishes the level at which the failure occurs. External or surface exchange supports respiration; it cannot substitute for the intracellular sequence. Adequate gas exchange is therefore necessary for aerobic respiration but is not sufficient when the internal catalytic machinery is defective.
11. A defining feature of cellular respiration is the controlled oxidation of complex organic compounds, including the breaking of [blank] that releases considerable energy.
ⓐ. carbon-carbon bonds in substrates
ⓑ. peptide bonds in all cellular enzymes
ⓒ. phosphodiester bonds in chromosomes
ⓓ. hydrogen bonds between water molecules
Correct Answer: carbon-carbon bonds in substrates
Explanation: Respiratory substrates contain carbon skeletons in which carbon atoms are joined through carbon-carbon bonds. During controlled intracellular oxidation, these complex compounds are transformed through a sequence of reactions, and considerable chemical energy is released as the carbon framework is progressively oxidised. The process does not depend on indiscriminate destruction of enzymes, chromosomes or water structure. Peptide and phosphodiester bonds have important biological roles, but their general breakdown is not the defining basis of cellular respiration. The blank identifies the bond relation emphasised in the definition while keeping the essential qualifier: oxidation occurs through organised cellular reactions that can conserve part of the released energy.
12. Match each potential respiratory substrate with the description that fits it best. A Column II entry is used once.
| Column I | Column II |
|---|
| P. Carbohydrate | 1. Commonly favoured respiratory substrate |
| Q. Fat | 2. Energy-rich storage reserve that may be mobilised |
| R. Protein | 3. Nitrogen-containing reserve that may supply respiratory carbon |
| S. Organic acid | 4. Acidic non-carbohydrate compound that may also be oxidised |
ⓐ. P-2, Q-1, R-4, S-3
ⓑ. P-1, Q-3, R-2, S-4
ⓒ. P-1, Q-2, R-3, S-4
ⓓ. P-4, Q-2, R-1, S-3
Correct Answer: P-1, Q-2, R-3, S-4
Explanation: Carbohydrates are commonly the favoured respiratory substrates and feed readily into the central respiratory pathway. Fats can also supply respiratory material and commonly occur as energy-rich storage reserves that can be mobilised. Proteins are nitrogen-containing reserves whose carbon component may contribute to respiration under suitable conditions. Organic acids form another class of non-carbohydrate compounds that may be oxidised. The matching task establishes a hierarchy without treating carbohydrate as the only possible substrate. It also keeps the alternatives biologically distinct through their storage or chemical character. The decisive property is not simply whether a substance contains energy, but whether its carbon can contribute to respiratory metabolism under suitable physiological conditions in the tissue. The mapping distinguishes the commonly favoured carbohydrate substrate from alternative reserves or compounds that can supply respiratory carbon under suitable conditions.
13. Consider the following statements about respiratory substrates.
I. Carbohydrates are commonly preferred as respiratory substrates.
II. Fats, proteins and organic acids may also be oxidised under suitable conditions.
III. ATP is the main carbon-containing substrate from which respiration begins.
IV. The substrate used by a tissue can depend on availability and physiological condition.
ⓐ. I and III only
ⓑ. II and IV only
ⓒ. I, II and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Carbohydrates commonly provide the immediate substrate for respiration, but the pathway is not restricted to them. Stored fats, protein-derived carbon skeletons and organic acids can contribute under appropriate physiological conditions. Substrate use may shift with developmental state, storage reserves and availability. ATP has a different role: it is an energy-transfer product used to drive cellular work, not the principal carbon-rich food molecule entering respiration. The accepted statements identify both the usual preference and the qualified alternatives. This distinction is important in plants, where a germinating seed or storage organ may mobilise whichever reserve class it contains rather than relying on newly synthesised glucose alone. This flexibility links respiration to the changing metabolic state of a tissue rather than to one compulsory substrate.
14. During germination, a seed's readily available carbohydrate falls while stored lipid is mobilised and oxygen consumption continues. The strongest conclusion is that:
ⓐ. fat can serve as a respiratory substrate when carbohydrate is scarce
ⓑ. oxygen consumption proves that only carbohydrate is being oxidised
ⓒ. lipid mobilisation stops respiration until photosynthesis begins
ⓓ. ATP becomes the carbon substrate replacing both fat and carbohydrate
Correct Answer: fat can serve as a respiratory substrate when carbohydrate is scarce
Explanation: Continued oxygen consumption indicates that aerobic oxidation is still occurring, while the rise in lipid mobilisation identifies a likely alternative source of respiratory carbon and energy. Fats can be mobilised, and their carbon can then be oxidised through respiratory metabolism. The observation does not imply that oxygen uniquely identifies carbohydrate oxidation; several organic substrates can consume oxygen during aerobic respiration. ATP cannot replace stored food as a carbon substrate, since it transfers captured energy rather than serving as the main reserve being oxidised. The changed condition demonstrates the qualifier attached to respiratory substrates: carbohydrate is commonly favoured, but reserve composition and availability can shift the material being used. The interpretation remains limited to the supplied evidence: it supports use of lipid-derived material, not exclusive oxidation of one named molecule throughout the tissue.
15. A tissue is examined under four nutritional conditions.
| Condition | Available carbohydrate | Mobilisation of another reserve | Respiratory gas exchange |
|---|
| P | High | Low | Continues |
| Q | Low | High fat mobilisation | Continues |
| R | Low | High protein breakdown | Continues |
| S | Low | None detected | Falls sharply |
The combined pattern in Q, R and S most strongly supports which inference?
ⓐ. Gas exchange continues only when carbohydrate remains abundant
ⓑ. Protein and fat must first be converted completely into glucose
ⓒ. Reserve mobilisation has no relation to continued respiration
ⓓ. alternative reserves sustain respiration when carbohydrate is scarce
Correct Answer: alternative reserves sustain respiration when carbohydrate is scarce
Explanation: Conditions Q and R show continued respiratory gas exchange despite low carbohydrate, provided that another organic reserve is being mobilised. Condition S provides the useful comparison: when carbohydrate is low and no alternative reserve is detected, gas exchange falls sharply. Taken together, the rows support the inference that fat- or protein-derived material can feed respiratory metabolism under suitable conditions. The table does not establish that either reserve must be converted completely into glucose, nor does it show that every tissue always uses alternatives. Its strongest justified conclusion is conditional substrate flexibility, with reserve availability influencing whether respiration can be maintained during carbohydrate shortage. Q and R represent two different reserve classes, which strengthens the conclusion beyond a single exceptional case. The fall in S also acts as a comparison showing that low carbohydrate alone need not stop respiration when another reserve is available.
16. Assertion: Cellular respiration is gradual and enzyme-controlled rather than a rapid uncontrolled oxidation.
Reason: ATP formed during respiration can later supply energy to cellular activities.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, but Reason does not explain Assertion
Explanation: The assertion is true: respiration proceeds through a series of enzyme-catalysed reactions, allowing energy to be released in manageable portions. The reason is also true, since ATP produced through respiratory energy capture can drive transport, synthesis and other cellular work. However, the stated use of ATP does not by itself explain why substrate oxidation occurs gradually. The direct explanation is that an ordered enzyme-controlled pathway divides oxidation into successive steps and provides opportunities to couple released energy to energy-conserving reactions. ATP's later use describes the value of the captured energy, while the stepwise organisation explains the controlled nature of its release. The explanatory link would require mention of staged catalysis and coupling, not merely the later destination of ATP.
17. Two oxidation processes are compared.
| Feature | Process P | Process Q |
|---|
| Rate of energy release | Gradual | Rapid |
| Control | Many enzyme-catalysed steps | No cellular enzyme sequence |
| Energy outcome | Part captured in ATP | Large fraction dispersed as heat |
Process P and Process Q are, respectively:
ⓐ. cellular respiration and direct combustion
ⓑ. direct combustion and cellular respiration
ⓒ. aerobic respiration and anaerobic respiration
ⓓ. photosynthesis and cellular respiration
Correct Answer: cellular respiration and direct combustion
Explanation: Process P combines gradual oxidation, enzyme control and ATP coupling, which are defining features of cellular respiration. Process Q releases energy rapidly without the organised cellular sequence needed to conserve useful portions in ATP, matching direct combustion. Both processes may involve oxidation, so the distinction is not simply whether oxygen participates. Instead, the table requires synthesis of rate, control and energy outcome. Anaerobic respiration or fermentation also consists of enzyme-mediated cellular reactions and cannot be identified with uncontrolled burning. Photosynthesis stores light energy in organic compounds and does not fit the oxidation pattern shown for either column. The comparison highlights how pathway organisation changes the biological usefulness of released energy. The rapid release in Q gives fewer opportunities for intermediate energy conservation, whereas P distributes release across reactions that can be linked to ATP synthesis. The table therefore tests mechanism and outcome together.
18. Equal samples of a cell extract containing glucose are prepared. In Treatment P, respiratory enzymes remain active; in Treatment Q, the enzymes are denatured before incubation. Only P shows gradual oxygen uptake and a rise in ATP. The strongest supported inference is:
ⓐ. glucose releases ATP spontaneously whenever oxygen is present
ⓑ. oxygen uptake alone forms ATP without substrate oxidation
ⓒ. enzyme-controlled reactions are needed for coupled respiration
ⓓ. denaturation increases the efficiency of energy capture from glucose
Correct Answer: enzyme-controlled reactions are needed for coupled respiration
Explanation: The treatments differ in the functional state of the enzymes, while substrate availability is retained. Gradual oxygen uptake and ATP accumulation occur only when those enzymes remain active. This supports the inference that cellular respiration depends on an organised sequence of catalysed reactions and that ATP formation is coupled to that functioning pathway. The experiment does not prove the role of every individual enzyme, and it does not show that oxygen directly converts glucose into ATP. Denaturation disrupts protein function and removes the controlled route through which oxidation and energy conservation occur. The comparison uses a clear treatment difference to connect enzyme activity with both substrate oxidation and usable energy capture. A proper control would also confirm that heating itself did not change glucose availability, but the supplied contrast already supports the stated enzyme-dependent inference. The result is about coupling under these conditions, not about spontaneous combustion.
19. Arrange the stages that connect controlled substrate oxidation with cellular work.
P. An organic substrate undergoes an enzyme-catalysed oxidation step.
Q. Part of the released energy is conserved through an energy-coupling process.
R. ATP is formed from ADP and inorganic phosphate.
S. ATP is used during an energy-requiring cellular activity.
ⓐ. \(Q \rightarrow P \rightarrow R \rightarrow S\)
ⓑ. \(P \rightarrow Q \rightarrow R \rightarrow S\)
ⓒ. \(P \rightarrow R \rightarrow S \rightarrow Q\)
ⓓ. \(R \rightarrow P \rightarrow Q \rightarrow S\)
Correct Answer: \(P \rightarrow Q \rightarrow R \rightarrow S\)
Explanation: The sequence begins with an enzyme-catalysed change in the respiratory substrate. Energy released during that controlled step is not useful merely as heat; part is conserved through a coupling mechanism. This conserved energy supports phosphorylation of ADP to form ATP. ATP can then be hydrolysed or otherwise used to drive an energy-requiring process such as active transport or biosynthesis. The dependency fixes the order: ATP cannot perform work before it is formed, and efficient ATP formation depends on conserving energy released from substrate oxidation. The sequence expresses the functional advantage of stepwise respiration rather than treating ATP as energy that appears directly from glucose without intermediate coupling. Q represents conservation of released energy before ATP formation, making it distinct from the later use of ATP. This dependency-based reasoning is more informative than memorising the four labels as an isolated list.
20. Imagine a hypothetical cell that releases the same total chemical energy from glucose in one rapid, uncontrolled reaction instead of through a staged respiratory pathway. The most likely consequence would be:
ⓐ. more energy stored in ATP with less heat production
ⓑ. unchanged ATP capture since total released energy is unchanged
ⓒ. complete prevention of heat release during substrate oxidation
ⓓ. greater heat loss with less energy conserved in usable form
Correct Answer: greater heat loss with less energy conserved in usable form
Explanation: The amount of chemical energy present in the substrate does not by itself determine how much becomes biologically usable. A staged enzyme-controlled pathway releases energy in smaller portions and links suitable portions to ATP-forming processes. If the same oxidation occurred rapidly and without that organisation, a larger share would be dispersed as heat before the cell could conserve it. ATP capture would fall even though the total energy released from glucose might remain similar. The prediction explains why respiration cannot be modelled as combustion occurring inside a cell. Biological efficiency depends on controlled coupling and timing, not simply on the total energy difference between reactants and products. This also predicts reduced capacity for active transport and biosynthesis, even if temperature rises from the liberated heat. The cellular consequence follows from loss of energy conservation, not from failure to oxidise the substrate.
