VCAA Biology How are biochemical pathways regulated?
15 sample questions with marking guides and sample answers
Biomass can be used to produce bioethanol. The production of bioethanol is a process that has many steps.
In this process
bioethanol is the only product produced.
an environment free of microorganisms is required.
the same enzyme catalyses each step in the process.
the biomass could be plants containing high levels of cellulose.
Reveal Answer
bioethanol is the only product produced.
The fermentation process that produces bioethanol also produces carbon dioxide () as a byproduct, so bioethanol is not the only product.
an environment free of microorganisms is required.
Microorganisms, such as yeast or specific bacteria, are essential to carry out the fermentation step that converts sugars into bioethanol.
the same enzyme catalyses each step in the process.
The production of bioethanol involves multiple biochemical steps, each requiring specific, different enzymes (e.g., cellulase to break down cellulose, and various enzymes for fermentation).
the biomass could be plants containing high levels of cellulose.
Plants with high levels of cellulose are a common source of biomass, as the cellulose can be broken down into glucose sugars and subsequently fermented into bioethanol.
Where in a plant leaf cell does a light-dependent reaction occur?
stroma
mitochondrion
thylakoid membrane
chloroplast inner membrane
Reveal Answer
stroma
Incorrect. The stroma is the fluid-filled space inside the chloroplast where the light-independent reactions (Calvin cycle) take place, not the light-dependent ones.
mitochondrion
Incorrect. Mitochondria are the organelles responsible for cellular respiration and ATP production from glucose, not photosynthesis.
thylakoid membrane
Correct. The light-dependent reactions occur in the thylakoid membrane, which contains the chlorophyll and protein complexes (photosystems) needed to capture light energy.
chloroplast inner membrane
Incorrect. Although chloroplasts have an inner membrane, the light-dependent reactions specifically occur on the thylakoid membranes suspended within the chloroplast.
Both human and yeast cells use glucose as an energy source but variations in biochemical pathways produce different products.
One variation in a biochemical pathway that occurs in human cells is shown below.
glucose ⟶ pyruvate ⟶ lactic acid
Compare the biochemical pathway shown above to fermentation in yeast cells.
Reveal Answer
Similarities between the biochemical pathways of fermentation in human and yeast cells include:
- no oxygen is used
- both produce ATP/energy
- glycolysis occurs OR glucose is converted to pyruvate OR pyruvate is produced
- the same enzymes are used in glycolysis.
Differences between the biochemical pathways of fermentation in human and yeast cells include:
- different products are produced – lactic acid in humans compared to ethanol and carbon dioxide in yeast
- different enzymes are involved in the conversion of pyruvate to final products.
| Descriptor | Marks |
|---|---|
Any two of the following similarities or differences (1 mark for each correct point): Similarities:
Differences:
| 2 |
Coenzymes are used in fermentation.
Describe the general role of coenzymes in biochemical pathways.
Reveal Answer
The general role of coenzymes in biochemical pathways includes:
- assisting enzymes in catalysing reactions OR lowering activation energy
- cycling between loaded and unloaded forms
- carrying energy (e.g. ATP)
- carrying protons/hydrogen (ions) and/or electrons (e.g. NADH, FADH₂, NADPH).
| Descriptor | Marks |
|---|---|
Any three of the following points describing the general role of coenzymes (1 mark for each correct point):
| 3 |
A decrease in the rate of glycolysis within a healthy muscle cell is most likely caused by an increase in the
concentration of glucose.
number of NAD+ molecules.
concentration of oxygen.
number of ATP molecules.
Reveal Answer
concentration of glucose.
Glucose is the primary substrate for glycolysis. An increase in glucose concentration would typically increase or maintain the rate of glycolysis, not decrease it.
number of NAD+ molecules.
NAD+ is a necessary reactant for glycolysis to proceed. An increase in NAD+ availability would facilitate the pathway rather than inhibit it.
concentration of oxygen.
While oxygen allows for aerobic respiration, it does not directly inhibit the enzymes of glycolysis. The indirect slowing of glycolysis in the presence of oxygen is actually mediated by the resulting increase in ATP.
number of ATP molecules.
ATP acts as an allosteric inhibitor of phosphofructokinase, a key rate-limiting enzyme in glycolysis. High levels of ATP signal that the cell has sufficient energy, causing feedback inhibition that decreases the rate of glycolysis.
Three factors that affect the rate of photosynthesis in plants are stated below:
- Factor 1 – light intensity
- Factor 2 – carbon dioxide concentration
- Factor 3 – temperature
Scientific data consistently shows specific trends for each factor when other variables are controlled.
Graphs were plotted showing the rate of photosynthesis against an increasing change in each factor.
If plotted over a large range, which of the following would show graphs with the same trend?
factors 1 and 2
factors 1 and 3
factors 2 and 3
factors 1, 2 and 3
Reveal Answer
factors 1 and 2
Both light intensity and carbon dioxide concentration show the same trend: the rate of photosynthesis increases initially and then plateaus when another factor becomes limiting.
factors 1 and 3
Temperature shows a different trend than light intensity; it increases to an optimum point and then rapidly decreases as enzymes denature, whereas light intensity plateaus.
factors 2 and 3
Temperature increases to an optimum and then decreases due to enzyme denaturation, which is a different trend from carbon dioxide concentration, which simply plateaus.
factors 1, 2 and 3
Temperature has a distinct peak and decline curve due to enzyme denaturation, which differs from the plateauing curves of light intensity and carbon dioxide concentration.
In cellular respiration, the net ATP yield from a single glucose molecule is affected by various factors.
Which one of the following factors immediately influences the yield of ATP molecules produced during the electron transport chain?
the availability of glucose in the cristae
the number of oxygen molecules available in the mitochondria
the concentration of pyruvate in the mitochondrial matrix
the rate of glycolysis in the cytosol
Reveal Answer
the availability of glucose in the cristae
Glucose is broken down in the cytosol during glycolysis and does not directly enter the cristae, which is the site of the electron transport chain.
the number of oxygen molecules available in the mitochondria
Oxygen acts as the final electron acceptor in the electron transport chain. Its availability directly and immediately determines whether electron flow and subsequent ATP synthesis can continue.
the concentration of pyruvate in the mitochondrial matrix
While pyruvate enters the mitochondrial matrix to be oxidized and fuel the Krebs cycle, this is an upstream process and does not immediately influence the electron transport chain.
the rate of glycolysis in the cytosol
Glycolysis occurs in the cytosol and provides precursors for later stages of cellular respiration, making it an upstream process rather than an immediate influence on the electron transport chain.
ATP synthase is an enzyme that is used in the formation of ATP.
A competitive inhibitor to ATP synthase is introduced into a cell.
The competitive inhibitor would initially
bind to the active site of ATP, blocking it from binding to ATP synthase.
reduce the number of ADP molecules available for moving cellular energy.
bind to the allosteric site of ATP synthase, preventing ADP from binding.
cause more ADP molecules to be present in the cell.
Reveal Answer
bind to the active site of ATP, blocking it from binding to ATP synthase.
A competitive inhibitor binds to the active site of the enzyme (ATP synthase), not to the product (ATP).
reduce the number of ADP molecules available for moving cellular energy.
Inhibiting ATP synthase prevents the conversion of ADP to ATP, which would actually increase the amount of ADP present, not reduce it.
bind to the allosteric site of ATP synthase, preventing ADP from binding.
Competitive inhibitors bind directly to the active site of an enzyme. Inhibitors that bind to an allosteric site are known as non-competitive inhibitors.
cause more ADP molecules to be present in the cell.
By inhibiting ATP synthase, the enzyme can no longer convert ADP and inorganic phosphate into ATP, leading to an initial accumulation of ADP molecules in the cell.
The two reactions shown below are both part of a biochemical pathway occurring in the same stage of photosynthesis in a plant cell.
| Inputs | Outputs | |
|---|---|---|
| Reaction 1 | water | hydrogen ions and oxygen gas |
| Reaction 2 | hydrogen ions | NADPH |
A limiting factor for this stage of photosynthesis could be
light intensity.
glucose concentration.
carbon dioxide concentration.
the amount of ATP molecules.
Reveal Answer
light intensity.
Light intensity directly affects the rate of the light-dependent reactions, as light energy is required to excite electrons and drive the production of ATP and NADPH.
glucose concentration.
Glucose is a product of the overall photosynthetic process, not a reactant, so its concentration does not act as a limiting factor for photosynthesis.
carbon dioxide concentration.
Carbon dioxide concentration is a limiting factor for the light-independent reactions (Calvin cycle), not the light-dependent stage.
the amount of ATP molecules.
ATP is a product of the light-dependent reactions rather than a reactant, so its initial amount does not limit this stage.
Which one of the following statements about the electron transport chain is correct?
The outputs of the electron transport chain are water, ATP and NADH. The role of water is to donate energy.
The outputs of the electron transport chain are oxygen, ATP and NAD. The role of oxygen is to accept energy from ATP molecules.
The outputs of the electron transport chain are water, ATP and NAD. The role of NADH is to donate electrons and hydrogen ions.
The outputs of the electron transport chain are oxygen, ATP and NADP. The role of NADP is to accept electrons from the electron transport chain.
Reveal Answer
The outputs of the electron transport chain are water, ATP and NADH. The role of water is to donate energy.
NADH is an input to the electron transport chain, not an output, as it gets oxidized to NAD. Additionally, water is a byproduct formed when oxygen accepts electrons, not an energy donor.
The outputs of the electron transport chain are oxygen, ATP and NAD. The role of oxygen is to accept energy from ATP molecules.
Oxygen is an input (the final electron acceptor), not an output. Its role is to pull electrons down the chain and combine with hydrogen ions to form water, not to accept energy from ATP.
The outputs of the electron transport chain are water, ATP and NAD. The role of NADH is to donate electrons and hydrogen ions.
This is correct because NADH donates electrons and protons to the chain, becoming oxidized to NAD. The process drives ATP synthesis and reduces oxygen to form water as final outputs.
The outputs of the electron transport chain are oxygen, ATP and NADP. The role of NADP is to accept electrons from the electron transport chain.
NADP is an electron carrier involved in photosynthesis, not the cellular respiration electron transport chain. Furthermore, oxygen is an input to this process, not an output.
During photosynthesis
ATP and NADH created in the light-independent stage are transported to the chloroplasts’ thylakoid membranes.
ADP and NADH are used in the electron transport chain after being created in the light-dependent stage.
ATP and NADPH are created in the grana of the chloroplasts and are used in the light-independent stage.
ADP and NADPH are created during the Krebs cycle and carried to the stroma of the chloroplasts.
Reveal Answer
ATP and NADH created in the light-independent stage are transported to the chloroplasts’ thylakoid membranes.
ATP and NADPH (not NADH) are produced during the light-dependent stage in the thylakoid membranes, and they are transported to the stroma for the light-independent stage.
ADP and NADH are used in the electron transport chain after being created in the light-dependent stage.
Photosynthesis utilizes NADPH, not NADH. Furthermore, the light-dependent stage produces ATP and NADPH, rather than ADP.
ATP and NADPH are created in the grana of the chloroplasts and are used in the light-independent stage.
The light-dependent reactions occur in the grana (stacks of thylakoids) to produce ATP and NADPH, which are then used to power the light-independent reactions (Calvin cycle) in the stroma.
ADP and NADPH are created during the Krebs cycle and carried to the stroma of the chloroplasts.
The Krebs cycle is a stage of cellular respiration that occurs in the mitochondria, not a part of photosynthesis.
A student notices that many plants grow in a way that reduces the overlap of their leaves.
This adaptation allows a faster rate of photosynthesis in the plants as the plants can absorb more
carbon dioxide into the bundle sheath cells.
glucose through the mesophyll cell wall.
oxygen from the air.
light on the grana.
Reveal Answer
carbon dioxide into the bundle sheath cells.
While carbon dioxide is required for photosynthesis, reducing leaf overlap primarily serves to maximize light exposure rather than directly increasing gas exchange.
glucose through the mesophyll cell wall.
Glucose is a product synthesized during photosynthesis, not a reactant that the plant absorbs from its environment.
oxygen from the air.
Oxygen is a byproduct of photosynthesis, not a reactant that needs to be absorbed to increase the rate of the process.
light on the grana.
Reducing leaf overlap maximizes the surface area exposed to sunlight, allowing more light to be absorbed by the chlorophyll located in the grana of the chloroplasts.
Scientists measured the metabolic activity of mammalian cells by measuring the uptake of glucose into the cells. The cells were maintained at 37 °C with a pH of 7.4 and suspended in a nutrient solution containing glucose. The uptake of glucose into the cells was recorded for the next 30 minutes.
Explain why the uptake of glucose into the cells could be used to measure the metabolic activity of the cells.
Reveal Answer
Glucose is used in aerobic respiration and ATP is produced from glucose for metabolism.
| Descriptor | Marks |
|---|---|
States that glucose is used in aerobic respiration | 1 |
States that ATP is produced from glucose for metabolism | 1 |
The scientists repeated the experiment. They kept all conditions the same as for the first experiment, except that the cells were kept in low-oxygen conditions.
Would the uptake of glucose into the cells be expected to be higher, lower or the same as for the first experiment? Justify your response.
Reveal Answer
Higher uptake
- With less oxygen the cell would not respire aerobically producing less ATP for each glucose molecule.
- The cell would take up more glucose to get the same amount of energy.
| Descriptor | Marks |
|---|---|
Identifies higher uptake | 1 |
States that with less oxygen the cell would not respire aerobically | 1 |
States this produces less ATP for each glucose molecule | 1 |
Concludes the cell would take up more glucose to get the same amount of energy | 1 |
During cellular respiration
cells will consistently yield 36 or 38 molecules of ATP from each molecule of glucose.
there is a difference between the theoretical and actual ATP yields when cells break down glucose.
ATP is not produced in the cells in the roots of plants as these cells have no glucose source.
C4 plants consistently produce more ATP from each glucose molecule compared to C3 plants.
Reveal Answer
cells will consistently yield 36 or 38 molecules of ATP from each molecule of glucose.
While 36 to 38 ATP is often taught as the theoretical maximum yield per glucose molecule, cells rarely achieve this consistently due to proton leakage and the energy costs of transporting molecules into the mitochondria.
there is a difference between the theoretical and actual ATP yields when cells break down glucose.
The actual ATP yield (typically around 30-32 ATP) is lower than the theoretical maximum (36-38 ATP) because the proton gradient is also used to drive other transport processes, and some protons leak across the inner mitochondrial membrane.
ATP is not produced in the cells in the roots of plants as these cells have no glucose source.
Plant root cells actively perform cellular respiration to produce ATP. They receive the necessary glucose (transported as sucrose) from the photosynthetic leaves via the plant's phloem.
C4 plants consistently produce more ATP from each glucose molecule compared to C3 plants.
C3 and C4 designations refer to different photosynthetic pathways for carbon fixation, not cellular respiration. Both types of plants use the same general cellular respiration pathways to break down glucose.
In photosynthesis, which one of the following correctly describes the relationship between the light-dependent and the light-independent reactions?
The light-dependent reactions produce ATP, which is used in the light-independent reactions to help fix carbon into organic molecules.
The light-dependent reactions produce ATP from carbon dioxide and water, which is used in the light-dependent reactions.
The light-dependent reactions use ATP to produce glucose, while the light-independent reactions produce ATP from glucose.
The light-independent reactions convert ATP into carbon dioxide and water, which are then used in the light-dependent reactions to form glucose.
Reveal Answer
The light-dependent reactions produce ATP, which is used in the light-independent reactions to help fix carbon into organic molecules.
This is correct because the light-dependent reactions harness solar energy to generate ATP and NADPH. These energy carriers are then utilized by the Calvin cycle (light-independent reactions) to synthesize organic molecules from .
The light-dependent reactions produce ATP from carbon dioxide and water, which is used in the light-dependent reactions.
This is incorrect because the light-dependent reactions produce ATP using light energy, not carbon dioxide. Furthermore, this ATP is consumed in the light-independent reactions, not recycled in the light-dependent ones.
The light-dependent reactions use ATP to produce glucose, while the light-independent reactions produce ATP from glucose.
This is incorrect because it reverses the actual process. The light-dependent reactions produce ATP, while the light-independent reactions use that ATP to synthesize glucose from .
The light-independent reactions convert ATP into carbon dioxide and water, which are then used in the light-dependent reactions to form glucose.
This is incorrect because the light-independent reactions consume ATP and to build glucose. They do not break ATP down into and water.
What could explain the differences in the ATP yield between aerobic and anaerobic respiration in a human cell?
The electron transport chain is not used in anaerobic respiration, leading to lower ATP production.
Anaerobic respiration produces more NADH compared to aerobic respiration, which decreases ATP yield.
The Krebs cycle is more efficient in anaerobic respiration, resulting in higher ATP yield.
Aerobic respiration produces fewer ATP molecules due to the incomplete breakdown of glucose.
Reveal Answer
The electron transport chain is not used in anaerobic respiration, leading to lower ATP production.
This is correct because anaerobic respiration in human cells relies solely on glycolysis and bypasses the electron transport chain, which is responsible for producing the vast majority of ATP during aerobic respiration.
Anaerobic respiration produces more NADH compared to aerobic respiration, which decreases ATP yield.
This is incorrect because anaerobic respiration actually produces less NADH than aerobic respiration. Furthermore, NADH is consumed during fermentation to regenerate NAD+, not to decrease ATP yield.
The Krebs cycle is more efficient in anaerobic respiration, resulting in higher ATP yield.
This is incorrect because the Krebs cycle requires oxygen and does not occur during anaerobic respiration. Additionally, anaerobic respiration results in a much lower ATP yield, not a higher one.
Aerobic respiration produces fewer ATP molecules due to the incomplete breakdown of glucose.
This is incorrect because aerobic respiration involves the complete breakdown of glucose, which produces significantly more ATP molecules (typically 36-38 ATP) compared to the incomplete breakdown in anaerobic respiration (2 ATP).