A Level H2 Biology Plant Biology Quiz

A-Level Biology H2 Quiz - Plant Biology (Answer Key)

Total Marks: 40

Section A: Photosynthesis Mechanisms

1.
(a) Water is split (photolysis) [1] by light energy/photons at PSII, providing electrons to replace those lost by chlorophyll and producing protons (H⁺) and oxygen [1].
(b) Electrons move down the electron transport chain (ETC) from PSII to PSI [1]. Energy released from electron movement is used to pump protons (H⁺) from the stroma into the thylakoid lumen [1]. This creates a proton gradient/proton motive force, which drives ATP synthase to produce ATP via chemiosmosis [1].

2.
(a) High temperatures increase the rate of photorespiration [1]. RuBisCO acts as an oxygenase, binding O₂ instead of CO₂ to RuBP [1]. This produces phosphoglycolate which cannot enter the Calvin cycle directly, reducing the efficiency of carbon fixation/photosynthesis [1].
(b) C4 plants have Kranz anatomy with bundle sheath cells surrounding vascular bundles [1]. Mesophyll cells fix CO₂ into a 4-carbon compound (oxaloacetate/malate) using PEP carboxylase, which has a high affinity for CO₂ and no oxygenase activity [1]. This 4-carbon compound is transported to bundle sheath cells where CO₂ is released [1]. This maintains a high concentration of CO₂ around RuBisCO in bundle sheath cells, suppressing photorespiration even when stomata are partially closed to save water [1].

3.
(a) Oxygen is a product of the light-dependent reactions (photolysis of water) [1]. The rate of oxygen production is proportional to the rate of the light-dependent reactions, which limits the overall rate of photosynthesis [1].
(b) At low light intensities, light is the limiting factor [1]. Increasing temperature increases the kinetic energy of enzymes involved in the light-independent reactions (Calvin cycle) [1]. However, since light is limiting, the rate of ATP/NADPH production does not increase significantly, so the overall rate of photosynthesis will not increase significantly or may only increase slightly due to enzyme kinetics until light becomes non-limiting [1].

4.
(a) Any two from: Thick cuticle [1], Sunken stomata [1], Rolled leaf / hairs on epidermis [1].
(b) If thick cuticle chosen: Reduces evaporation of water from the leaf surface / provides a waterproof barrier [1].
If sunken stomata chosen: Traps moist air in the pit / reduces the water potential gradient between the leaf interior and the atmosphere / reduces air movement over the stomata [1].
If rolled leaf/hairs chosen: Traps moist air / reduces air movement / reduces water potential gradient [1].

5.
(a) Nitrate ions are used to synthesize amino acids [1], which are then used to make proteins / nucleotides / DNA / RNA / chlorophyll [1].
(b) Waterlogged soil has low oxygen concentration / is anaerobic [1]. Root cells cannot perform aerobic respiration [1]. Lack of ATP means active transport of nitrate ions into root hairs cannot occur [1]. Additionally, denitrifying bacteria thrive in anaerobic conditions, converting nitrates into nitrogen gas, reducing nitrate availability [1].

Section B: Data Interpretation and Transport

6.
(a) A graph showing the rate of photosynthesis [1] at different wavelengths of light [1].
(b) The action spectrum closely matches the absorption spectrum of chlorophylls a and b [1]. Peaks in the action spectrum (blue-violet and red regions) correspond to the wavelengths where chlorophyll absorbs light most strongly [1]. This indicates that chlorophylls are the primary pigments driving photosynthesis [1].

7.
(a) As CO₂ concentration increases from 200 to 800 ppm, the dry mass of Type A plants increases significantly [1]. Above 800 ppm, the increase levels off / becomes less significant [1].
(b) Type A (C3) plants are limited by photorespiration at normal CO₂ levels [1]. Increasing CO₂ increases the ratio of CO₂ to O₂ at the active site of RuBisCO, reducing photorespiration and increasing carbon fixation [1]. Type B (C4) plants already concentrate CO₂ around RuBisCO, so they are saturated at lower CO₂ concentrations [1]. Therefore, increasing CO₂ further has little effect on their rate of photosynthesis/growth [1].

8.
(a) Sucrose is actively transported into companion cells/sieve tube elements using H⁺/sucrose co-transporters [1]. This requires ATP to pump H⁺ out of the companion cell, creating a gradient [1]. Sucrose moves down its concentration gradient into the phloem [1].
(b) Loading sucrose lowers the water potential in the phloem [1]. Water moves from the xylem into the phloem by osmosis [1].

9.
(a) Long projection/hair increases surface area [1] for faster absorption of water by osmosis [1].
(b) Apoplast pathway: Water moves through cell walls and intercellular spaces [1]. It does not enter the cytoplasm [1].
Symplast pathway: Water moves through the cytoplasm of cells, connected by plasmodesmata [1]. It moves from cell to cell via osmosis/diffusion [1].

10.
(a) Potassium ions (K⁺) enter guard cells [1]. This lowers the water potential in guard cells [1]. Water enters by osmosis, making guard cells turgid [1]. The uneven thickening of guard cell walls causes them to bend outward, opening the stomatal pore [1].
(b) K⁺ ions are actively pumped into guard cells [1]. This accumulation of solutes lowers the water potential, driving water influx [1].

Section C: Advanced Concepts in Plant Physiology

11.
(a) Ribulose bisphosphate / RuBP [1].
(b) ATP provides energy [1] for the reduction of GP to TP and the regeneration of RuBP [1]. Reduced NADP provides hydrogen/electrons [1] for the reduction of GP to TP.

12.
(a) The loss of water vapor from the aerial parts of the plant, mainly through stomata [1].
(b) Wind removes water vapor from around the leaf surface [1]. This maintains a steep water potential gradient between the leaf interior and the atmosphere, increasing the rate of diffusion/transpiration [1].

13.
(a) Magnesium is a central component of the chlorophyll molecule [1].
(b) Chlorosis / yellowing of leaves [1]. This occurs because magnesium is needed for chlorophyll synthesis, and without it, chlorophyll breaks down or cannot be made, revealing other pigments [1].

14.
(a) Stroma [1].
(b) It provides a large surface area for the attachment of photosystems, electron carriers, and ATP synthase [1], maximizing the rate of light-dependent reactions [1].

15.
(a) C4 plants minimize photorespiration / can keep stomata partially closed to reduce water loss while maintaining photosynthesis [1].
(b) C4 plants use ATP to convert pyruvate back to PEP in mesophyll cells [1]. This additional step requires energy, whereas C3 plants do not have this regeneration cost for the initial CO₂ acceptor [1].

Section D: Experimental Analysis and Application

16.
(a) The rate of water uptake is equal to the rate of transpiration / water loss [1].
(b) Cut the shoot under water to prevent air bubbles entering the xylem [1]; Ensure the apparatus is airtight / use vaseline at joints [1].

17.
(a) A factor that is in shortest supply and thus limits the rate of a physiological process [1].
(b) Another factor (e.g., CO₂ concentration or temperature) becomes limiting [1]. The enzymes or substrates for the light-independent reactions are working at maximum capacity [1].

18.
(a) Sucrose is actively transported out of sieve tubes into sink cells [1]. This may involve co-transport with H⁺ or diffusion if concentration is high [1].
(b) Water potential in the phloem increases, so water leaves the phloem by osmosis and returns to the xylem [1].

19.
(a) Large air spaces / aerenchyma [1] allowing diffusion of gases to submerged parts.
(b) Water loss is not a problem in aquatic environments [1]. A cuticle would impede the direct absorption of water and dissolved minerals/gases through the epidermis [1].

20.
(a) C4 plants may expand their range into hotter/drier areas as they are more efficient under these conditions [1]. C3 plants may struggle in hotter regions due to increased photorespiration unless CO₂ levels rise significantly [1].
(b) Development of drought-resistant crop varieties / irrigation systems / mulching to reduce soil evaporation [1].