Photosynthesis and respiration

Question 1

1. In which structures are the chloroplast adapted for its function? (3 marks)

Answer 1

• Chlorophyll are closely packed in thylakoids which is arranged and stacked in granum, to maximize the surface area for absorption of light, which increases the rate of photochemical reactions (1)
• Enzymes and intermediates for Calvin Cycle are enclosed within the stroma in the chloroplast envelope, which increases the efficiency of the chemical reaction (1)
• There are interconnected thylakoid membranes, which facilitate the transportation of products of photochemical reaction to stroma, where Calvin Cycle takes place (1)

Question 2

2. What is the significance of the products in photochemical reactions to the whole photosynthesis process? (4 marks)

Answer 2

• ATP provides energy for converting the 3-C compound to triose phosphate, as well as for the regeneration of 5-C compound from triose phosphate (1)
• NADPH is a reducing agent, which provides the H to reduce 3-C compound to triose phosphate (1)
• Thus, ATP and NADPH are essential to sustain the continuation of Calvin Cycle (1)
• And also for the production of glucose, which is the main product of photosynthesis, as two triose phosphate molecules combine to become one glucose molecule (1)

Question 3

3. What is the advantage of having a larger blade area? (2 marks)

Answer 3

• A larger blade area increases the surface area of blade that are in contact with sunlight (1)
• This increases the chance of capturing light, hence increases the photochemical rate and allows the plant to produce more food (1)

Question 4

4. State the relationship between photochemical reaction and Calvin Cycle (3 marks)

Answer 4

• Photochemical reaction produces NADPH, which can be supplied to Calvin Cycle to reduce 3-C compound to triose phosphate then to glucose, as NADPH provides reducing power (1)
• Photochemical reaction also produces ATP, which provides energy to convert 3-C compound to triose phosphate, and also regenerates 5-C compound from triose phosphate to act as a carbon dioxide acceptor(1)
• Calvin Cycle oxidizes NADPH to regenerate NADP, and also regenerates ADP, which can be reused in photochemical reactions to continue the cycle (1)

Question 5

5. What are the factors that may affect the rate of photosynthesis? (6 marks)

Answer 5

• Light intensity (1) The higher the light intensity, the higher the number of electrons get excited. More ATP and NADPH are produced in photochemical reactions, and this leads to a higher rate of photosynthesis (1)
• Temperature (1) A higher temperature allows enzymes related to photosynthesis more active as they possess a higher kinetic energy. Thus, the enzymatic activity is higher, which increases the rate of photosynthesis (1)
• Carbon dioxide concentration (1) The higher the concentration of CO2, the higher the rate of Calvin Cycle, as it accepts more CO2 per unit time. Thus this leads to higher rate of photosynthesis (1)

Question 6

6. Refer to the graph below. Explain the drop of 5C compound (4 marks)

Answer 6

• In dark, photochemical reactions cannot be carried out. Hence there will be no production if ATP and NADPH for Calvin Cycle (1)
• Without them, 3C compound cannot be reduced to triose phosphate, and without ATP, triose phosphate cannot be converted to 5C compound. (1)
• The 5-C compound continuously accepts carbon dioxide to form 3C compounds (1)
• There is a net loss of 5C compound, leading it to decrease in concentration (1)

Question 7

7. Refer to the graph, account for the rise of 3C compound (4 marks)

Answer 7

• In dark, photochemical reactions cannot be carried out. Hence there will be no production if ATP and NADPH for Calvin Cycle (1)
• Without them, 3C compound cannot be reduced to triose phosphate, and without ATP, triose phosphate cannot be converted to 5C compound. (1)
• Hence 3C compound cannot be converted, but constantly increases due to the conversion of 5C compound accepts carbon dioxide (1)
• There is a net gain of 3C compound, which it accumulates and thus the concentration of it increases (1)

Question 8

1. Explain why the electron transport chain as well as the Krebs Cycle cannot operate without oxygen (3 marks)

Answer 8

• As oxygen acts as the final electron and hydrogen ions acceptor from the electron transport chain, and if there is no oxygen (1)
• The electron carriers in the electron transport chain cannot be oxidized, and will remain in a reduced state, so the electron transport chain stops. (1)
• The electron carriers fails to oxidise NADH and FADH, which NAD and FAD fails to regenerate respectively, thus stopping the Krebs Cycle (1)

Question 9

2. It is given that mature red blood cells lack mitochondria. Describe how pyruvate produced in glycolysis is further metabolized in red blood cells, and how this helps sustaining glycolysis in these cells (2 marks)

Answer 9

• Pyruvate is reduced to lactic acid by NADH, in the process of anaerobic respiration (1)
• This helps the regeneration of NAD, which can sustain the process of glycolysis (1)

Question 10

3. Explain the significance of lack of mitochondria in red blood cells (3 marks)

Answer 10

• As red blood cells are oxygen carriers and are responsible for oxygen transport (1)
• A lack of mitochondria means that there will be no Krebs Cycle and also oxidative phosphorylation occurring (1)
• Thus, aerobic respiration will not be carried out, and oxygen can be preserved in red blood cells and therefore more oxygen can be transported to target body cells (1)

Question 11

4. Explain the change in lactic acid concentration in blood during vigorous exercise (3 marks)

Answer 11

• As the energy released from aerobic respiration is not sufficient to compensate for the contraction of muscles, anaerobic respiration of muscles are carried out to provide additional energy (1)
• Anaerobic respiration produces lactic acid as a by-product, which the formation of lactic acid rate is higher than that of breakdown of lactic acid rate by liver (1)
• There is a net increase in lactic acid in blood, which increases the concentration (1)

Question 12

5. Explain why it is harmful to body cells if the body contains a high level of lactic acid (2 mark)

Answer 12

• The high level of lactic acid lowers the pH of blood and tissue fluid
• Which may alter the conformation of metabolic enzymes in our body, causing them to denature (1) And adversely affect the cellular activities

Question 13

6. Explain whether sitting down or jogging is more effective in removing lactic acid (4 marks)

Answer 13

• Slow jogging allows a relatively higher rate of heartbeat, which enhances the blood circulation to transport lactic acid away from muscle cells into blood more quickly (1)
• Slow jogging also allows a relatively higher rate of breathing, which increases the rate of oxygen supply to the body (1)
• This enhances the rate of breakdown of lactic acid to pyruvate in the liver, which will re-enter the Krebs Cycle (1)
• While some will be converted to glycogen and is stored in the liver (1), therefore the lactic acid concentration in blood decreases more than just sitting down

Question 14

7. Explain the significance of products of oxidative phosphorylation (4 marks)

Answer 14

• During this process, ATP is produced (1)
• It is important to provide energy for the usual cellular activities in our body (1)
• NAD and FAD are also regenerated (1)
• So that they can re-enter the Krebs Cycle and act as hydrogen carriers again to continue the cycle (1)

Question 15

8. State the main steps and where it takes place of Respiration (1 mark)

Answer 15

• Glycolysis: Glucose converted to pyruvate  in cytoplasm
• Link reaction: Pyruvate converted to Acetyl-CoA  in mitochondrial matrix
• Krebs Cycle: Acetyl-CoA + 4C compound to 6C to 4C  in mitochondrial matrix
• Oxidative phosphorylation: FADH and NADH to FAD and NAD, ADP to ATP, O2 + H + e- to water  on inner mitochondrial membrane