Bioenergetics

Biology, HSSC-Punjab First Year. This chapter is split into 25 topics and has 609 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. Pyruvic acid oxidation and the Krebs cycle 25 questions
2. Photosynthetic pigments and absorption spectra 25 questions
3. Glycolysis and anaerobic respiration (fermentation) 25 questions
4. Photosynthesis: role of light, carbon dioxide and water 25 questions
5. Photosystems I and II and light-dependent reactions 25 questions
6. Calvin cycle (light-independent reactions) 25 questions
7. ATP: the energy currency of cells 25 questions
8. Photorespiration 25 questions
9. Chapter review: bioenergetics (end-of-chapter exercise focus) 25 questions
10. CAM metabolism and adaptations to hot climates (C-3, C-4 and CAM compared) 25 questions
11. Electron transport chain, chemiosmosis and ATP yields 25 questions
12. C-4 photosynthesis: the Hatch-Slack pathway 25 questions
13. Photosynthesis overview: energy conversion, equation and compensation point 20 questions
14. Chlorophyll structure and kinds (Figure 6.4) 21 questions
15. Absorption and action spectra, accessory pigments and chloroplast structure 28 questions
16. Non-cyclic photophosphorylation: steps and Figure 6.9 20 questions
17. Cyclic photophosphorylation and chemiosmosis in the chloroplast (Figures 6.10-6.11) 21 questions
18. Calvin cycle: carbon, ATP and NADPH bookkeeping (Figure 6.12) 23 questions
19. Glycolysis step by step (Figure 6.15) with counting 25 questions
20. Fermentation: equations and Figure 6.13 20 questions
21. ATP and the aerobic respiration balance sheet (Figures 6.1, 6.2, 6.14, 6.20) 30 questions
22. Pyruvic acid oxidation, Krebs cycle and electron transport: figures and tallies 27 questions
23. Other organic molecules as fuel for cellular respiration 20 questions
24. Chapter 6 exercise items, photorespiration figures and photosynthesis-respiration comparison 29 questions
25. Redox roles and cellular locations of the reactions of bioenergetics 25 questions

Sample questions: Bioenergetics

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. Which statement best summarizes the flow of carbon in a C-4 plant?

• A. CO₂ is fixed onto PEP in mesophyll cells; the resulting 4-carbon acid passes to bundle-sheath cells, where CO₂ is released for the Calvin cycle, and the remaining acid returns to the mesophyll to pick up more CO₂.(correct answer)
• B. CO₂ is attached to pyruvic acid in mesophyll cells to form a 5-carbon compound that enters the Calvin cycle directly.
• C. CO₂ attaches to RuBP in the mesophyll cells, and the Calvin cycle then finishes in the bundle-sheath cells.
• D. Malic acid is formed in bundle-sheath cells and is transported to the mesophyll cells, where it is broken down into pyruvic acid and CO₂.

Answer: A.In C-4 photosynthesis, carbon flows from CO₂ to PEP to a 4-carbon acid in the mesophyll, then to the bundle-sheath cell where CO₂ is released for the Calvin cycle, while the leftover acid returns to the mesophyll.

Q2. Which statement about C-3, C-4 and CAM plants is correct?

• A. CAM plants use only the C-4 pathway and never run the Calvin cycle.
• B. At high temperatures, photorespiration benefits C-3 plants by increasing their yields.
• C. C-4 plants open their stomata at night, whereas CAM plants fix CO₂ during the day.
• D. C-3 plants can lose 25% to 50% of their fixed carbon to photorespiration, while C-4 and CAM plants reduce photorespiration by keeping the CO₂ concentration high.(correct answer)

Answer: D.C-3 plants may lose 25-50% of their fixed carbon to photorespiration; C-4 and CAM plants avoid this loss by maintaining a high internal CO₂ concentration, and CAM plants, like C-4 plants, use both the C-4 and C-3 pathways.

Q3. Why is there a net release of oxygen coupled with the uptake of carbon dioxide at noon?

• A. Respiration alone can supply all the carbon dioxide needed by the leaf
• B. Photosynthesis stops and only respiration continues
• C. The oxygen produced by photosynthesis is more than the need of respiring cells, and respiration cannot supply all the required carbon dioxide(correct answer)
• D. Stomata close at noon to trap oxygen inside the leaf

Answer: C.At high light intensity at noon, the oxygen produced during photosynthesis exceeds the needs of respiring cells, while respiration cannot supply all the required CO₂, so there is net O₂ release with CO₂ uptake.

Q4. Which statement about the absorption of light by photosynthetic pigments is correct?

• A. Pigments absorb different wavelengths at different rates, and the wavelengths are also differently effective in photosynthesis(correct answer)
• B. The wavelength of light does not affect the rate of photosynthesis
• C. All wavelengths of light are absorbed at equal rates
• D. Only green light is absorbed effectively

Answer: A.Photosynthetic pigments absorb different wavelengths of light at different rates, and different wavelengths are also differently effective in photosynthesis; this effectiveness is determined by the action spectrum.

Q5. Which of the following correctly pairs a pigment with its characteristic?

• A. Chlorophyll b - contains a methyl group on the second pyrrole ring
• B. Phycobilins - found in green plants and algae
• C. P700 - chlorophyll a of the PS-I reaction centre, absorbing maximum light at 700 nm(correct answer)
• D. Carotenoids - absorb maximally in the green and red regions of the spectrum

Answer: C.P700 is the chlorophyll a molecule of the PS-I reaction centre that absorbs maximum light at 700 nm; chlorophyll b has an aldehyde group, phycobilins occur in red algae and cyanobacteria, and carotenoids capture blue and green light.

Q6. During carbon fixation (Phase I of the Calvin cycle), the enzyme Rubisco combines three molecules of CO₂ with three molecules of which substance, and what is formed?

• A. RuBP (ribulose biphosphate); forming six molecules of 3-PGA (3-phosphoglyceric acid)(correct answer)
• B. RuBP; forming six molecules of G₃P (glyceraldehyde 3-phosphate)
• C. 3-PGA; forming six molecules of RuBP
• D. RuP; forming three molecules of glucose

Answer: A.Rubisco (probably the most abundant protein on Earth) combines 3 CO₂ with 3 RuBP to form 6 molecules of the three-carbon compound 3-PGA; because the first product is a three-carbon compound, the Calvin cycle is also called the C-3 pathway.

Q7. Why can the light-independent reactions (dark reactions) occur in the absence as well as in the presence of light?

• A. Because Rubisco is active only in darkness
• B. Because they occur inside the thylakoid lumen, where light cannot reach
• C. Because they do not use light directly; they proceed as long as ATP and NADPH from the light-dependent reactions are available(correct answer)
• D. Because they use energy stored in glucose instead of light energy

Answer: C.They are called dark reactions because light is not needed directly; they can occur in the absence or presence of light as long as ATP and NADPH are available, and these are supplied by the light-dependent reactions.

Q8. For every three molecules of CO₂ fixed in one turn of the Calvin cycle, how many molecules of ATP and NADPH are consumed in total by the reduction and regeneration phases?

• A. 3 ATP and 6 NADPH
• B. 9 ATP and 6 NADPH(correct answer)
• C. 6 ATP and 6 NADPH
• D. 9 ATP and 9 NADPH

Answer: B.The reduction phase uses 6 ATP (six phosphate groups from six ATPs) and 6 NADPH (hydrogen for the reduction), and regeneration of three RuBP uses a further 3 ATP (Fig. 6.12) - a total of 9 ATP and 6 NADPH per three CO₂ fixed.

Q9. In the reduction phase of the Calvin cycle, six molecules of G₃P are produced. Which statement correctly describes what happens to these G₃P molecules?

• A. Five molecules leave the cycle to form glucose, while one molecule is used to regenerate RuBP
• B. One molecule leaves the cycle and combines with another G₃P to form glucose (which may be converted into other carbohydrates), while the remaining five are used to regenerate RuBP(correct answer)
• C. All six molecules are converted into RuBP using three ATPs, and none leaves the cycle
• D. All six leave the cycle and are converted directly into three molecules of glucose

Answer: B.One of the six G₃P molecules leaves the cycle and combines with another G₃P to make glucose, which may then be converted to other carbohydrates; the remaining five are converted into RuP and, using three ATPs, back into three molecules of RuBP so the cycle can continue.

Q10. Which statement correctly contrasts anaerobic and aerobic respiration?

• A. Anaerobic respiration completely oxidizes glucose, while aerobic respiration does not
• B. Aerobic respiration takes place only in the cytosol, while anaerobic respiration occurs only in mitochondria
• C. Both release the same amount of energy per glucose molecule
• D. Anaerobic respiration does not completely oxidize glucose and releases only a small amount of energy, while aerobic respiration completely oxidizes glucose to CO₂ and water(correct answer)

Answer: D.In anaerobic respiration glucose is incompletely oxidized and only about 2% of its energy is released, whereas in aerobic respiration glucose is completely oxidized to CO₂ and water and all the energy of its C-H bonds is released.