A Level H1 Biology Plant Biology Quiz

A-Level Biology H1 Quiz – Plant Biology – Answer Key

Section A: Short Answer and Structured Questions

1. State the overall equation for photosynthesis. [2 marks]
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(Accept: 1 mark for correct reactants; 1 mark for correct products. Light and chlorophyll not required for 2 marks but expected.)

2. Describe the arrangement of thylakoid membranes within a chloroplast. [3 marks]

• Thylakoids are flattened membrane-bound sacs (1)
• Thylakoids stack to form grana (1)
• Grana are interconnected by stroma lamellae (1)

3. Explain why the light-dependent reactions of photosynthesis cannot occur in the dark. [3 marks]

• Light energy is essential for exciting electrons in chlorophyll (1)
• Without light, photolysis of water cannot occur (electrons not replaced) (1)
• Electron transport chain and ATP synthesis require light-driven electron flow (1)

4. Identify two products of the light-dependent reactions that are used in the light-independent reactions. [2 marks]

• ATP (1)
• Reduced NADP (NADPH) (1)

5. With reference to the absorption spectrum of chlorophyll, explain why plants appear green. [4 marks]

• Chlorophyll absorbs light mostly in the blue and red regions of the spectrum (1)
• Chlorophyll absorbs green light poorly (reflects/transmits green light) (1)
• The green light reaches the eye (1)
• Therefore, plants appear green (1)

6. Define photolysis and state its significance in photosynthesis. [3 marks]

• Photolysis is the splitting of water using light energy (1)
• Significance: replaces electrons lost by chlorophyll (1), produces oxygen as a by-product (1)

7. Explain how temperature affects the rate of photosynthesis. [3 marks]

• Photosynthesis is enzyme-controlled; temperature influences enzyme activity (1)
• As temperature increases, enzyme activity and kinetic energy increase → higher rate (1)
• Beyond optimum temperature, enzymes denature → rate decreases (1)

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Section B: Diagram and Data Interpretation

8.
(a) Label the parts A, B, and C. [3 marks]
A: Thylakoid membrane (1)
B: Granum (accept: stack of thylakoids) (1)
C: Stroma (1)

(b) State the location of the light-dependent reactions. [1 mark]
Thylakoid membrane (1)

(c) Explain the role of part C in the light-independent reactions. [2 marks]

• The stroma contains the enzymes for the Calvin cycle (1)
• It is the site where CO₂ is fixed and sugars are synthesised (1)

9.
(a) With reference to Figure 2, state the wavelengths of light absorbed most strongly by chlorophyll a. [2 marks]

• Blue light (around 430 nm) (1)
• Red light (around 662 nm) (1)
  (Accept stated wavelength ranges.)

(b) Explain why chlorophyll appears green based on the absorption spectrum. [3 marks]

• Chlorophyll strongly absorbs blue and red light (1)
• Green light (500–550 nm) is absorbed very weakly (reflected or transmitted) (1)
• The green light reflected reaches the observer’s eye, so chlorophyll appears green (1)

10.
(a) Using the data, describe the relationship between light intensity and the rate of photosynthesis. [2 marks]

• As light intensity increases from 0 to 800 lux, the rate increases (1)
• The rate plateaus at higher intensities (800–1200 lux) (1)

(b) Explain the shape of the curve with reference to limiting factors. [3 marks]

• Initially, light is the limiting factor (1)
• Increasing light increases rate until another factor becomes limiting (1)
• At the plateau, factors like CO₂ concentration or temperature become limiting (1)

11.
(a) With reference to Figure 3, describe the flow of electrons from photosystem II to photosystem I. [4 marks]

• Light energy excites electrons in photosystem II (1)
• Excited electrons are passed to an electron transport chain (1)
• Electrons travel through cytochrome complex and are transferred to plastocyanin (1)
• Plastocyanin carries electrons to photosystem I (1)

(b) Explain how the electron flow leads to ATP synthesis. [4 marks]

• Energy from electrons is used to pump protons (H⁺) into the thylakoid space (1)
• This establishes a proton gradient across the thylakoid membrane (1)
• Protons flow back through ATP synthase (1)
• The flow drives the synthesis of ATP (chemiosmosis) (1)

12.
(a) Compare the response of Species X and Species Y to increasing CO₂ concentration. [2 marks]

• Both species show an increased rate with higher CO₂ (1)
• Species X plateaus at 400 ppm, while Species Y continues to show a steady increase (1)

(b) Suggest a reason for the difference in the response of Species Y at high CO₂ concentration. [2 marks]

• Species Y may be a C3 plant with a lower affinity for CO₂ than Species X (1)
• Species X could be a C4 plant that is CO₂‑saturated at a lower concentration (1)
  (Also accept: differences in RuBisCO kinetics; stomatal differences; CO₂‑compensation point.)

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Section C: Extended Response

13. Describe how light energy is converted into chemical energy in the form of ATP during the light-dependent reactions of photosynthesis. [8 marks]

Marking points (any 8 of the following):

• Light energy is absorbed by chlorophyll molecules in the thylakoid membrane
• Electrons in chlorophyll become excited (raised to a higher energy level)
• Excited electrons are passed to an electron transport chain
• Energy from electrons is used to pump protons (H⁺) into the thylakoid interior
• This creates a proton gradient (difference in proton concentration across the membrane)
• Protons flow back into the stroma through ATP synthase
• This flow (chemiosmosis) drives the synthesis of ATP from ADP and inorganic phosphate
• Photolysis of water provides replacement electrons for chlorophyll
• The process occurs in photosystem II and photosystem I (non‑cyclic photophosphorylation)
  (8 marks maximum.)

14. Explain the role of NADP in photosynthesis and describe how it is reduced. [4 marks]

Marking points (any 4):

• NADP is a coenzyme that acts as a hydrogen/electron carrier
• It accepts electrons and H⁺ ions to form reduced NADP (NADPH)
• Reduction occurs in the stroma, catalysed by NADP reductase
• The electrons come from photosystem I
• NADPH carries reducing power to the Calvin cycle for carbon fixation

15. Compare cyclic and non-cyclic photophosphorylation. [6 marks]

Similarities (2 marks):

• Both occur in the thylakoid membrane (1)
• Both produce ATP via chemiosmosis (1)

Differences (4 marks):

• Cyclic involves only photosystem I; non-cyclic involves both photosystems I and II (1)
• Cyclic produces ATP only; non-cyclic produces ATP, NADPH, and O₂ (1)
• In cyclic, electrons return to photosystem I; in non-cyclic, electrons are replaced by photolysis of water (1)
• Cyclic does not involve photolysis or NADP reduction; non-cyclic does (1)

(Total 6 marks.)

16. Discuss how plants adapted to arid conditions carry out photosynthesis efficiently. [6 marks]

Marking points (any 6):

• Stomata open at night (reducing daytime water loss) – CAM photosynthesis
• CO₂ is fixed into organic acids at night and released during the day for the Calvin cycle
• Thick waxy cuticle reduces water loss by transpiration
• Reduced leaf surface area (spines, needles) minimises water loss
• Deep root systems to obtain water from deep soil layers
• Succulent tissues store water for dry periods
• C4 pathway: spatial separation of initial carbon fixation (mesophyll cells) and Calvin cycle (bundle sheath cells), increasing CO₂ concentration at RuBisCO and reducing photorespiration

17. With reference to the structure of a leaf, explain how it is adapted for photosynthesis. [6 marks]

Marking points (any 6):

• Large, flat surface area for maximum light interception
• Thin structure for short diffusion distance for gases
• Palisade mesophyll cells are tightly packed and contain many chloroplasts for light absorption
• Spongy mesophyll contains air spaces for rapid gas exchange
• Stomata allow CO₂ entry and O₂ exit; guard cells regulate opening
• Vascular bundles (xylem and phloem) supply water and remove sugars

18.
(a) Calculate the percentage increase in the rate of photosynthesis from low light to high light at 25°C. [2 marks]
Rate at low light = 5 bubbles/min; rate at high light = 25 bubbles/min
Increase = 25 – 5 = 20 bubbles/min (1)
Percentage increase = (20 ÷ 5) × 100 = 400% (1)

(b) Explain the effect of increasing temperature from 25°C to 35°C on the rate of photosynthesis. [3 marks]

• Rate increased from 25 to 30 bubbles/min (1)
• Higher temperature increases the kinetic energy of enzyme and substrate molecules, increasing enzyme activity (1)
• At 35°C, temperature is closer to or at the optimum for the Calvin‑cycle enzymes (1)

(c) Suggest why adding extra CO₂ increased the rate of photosynthesis. [2 marks]

• CO₂ is a substrate for the Calvin cycle (1)
• At high light and temperature, CO₂ may have been the limiting factor; extra CO₂ allows faster carbon fixation (1)

19. Discuss the significance of photosynthesis in maintaining the balance of atmospheric oxygen and carbon dioxide. [5 marks]

Marking points (any 5):

• Photosynthesis removes CO₂ from the atmosphere and releases O₂
• It counterbalances respiration and combustion processes that consume O₂ and release CO₂
• Photosynthesis is the primary source of atmospheric O₂
• It plays a key role in the carbon cycle
• By absorbing CO₂ it mitigates the greenhouse effect
• It supports the survival of aerobic organisms

20.
(a) With reference to the absorption spectrum of chlorophyll, explain the results for blue and green light. [4 marks]

• Chlorophyll absorbs blue light very strongly (1) → high rate of photosynthesis under blue light (1)
• Chlorophyll absorbs green light poorly (it is reflected/transmitted) (1) → low rate of photosynthesis under green light (1)

(b) Predict the rate of photosynthesis under white light and explain your reasoning. [2 marks]

• Rate would be higher than under any single coloured light (1)
• White light contains all wavelengths, allowing chlorophyll to absorb both blue and red light simultaneously (1)
  (Accept a reasonable estimate, e.g., 5.0 cm³ O₂ m⁻² s⁻¹.)

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End of Answer Key