A Level H2 Biology Plant Biology Quiz

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

Total Marks: 40

Section A: Photosynthesis and Chloroplast Structure

1.
(a) A: Grana (or Thylakoid stack) [1]; B: Stroma [1]
(b) The discs are thylakoids [1]. Stacking increases the surface area for the attachment of chlorophyll, electron carriers, and ATP synthase enzymes [1], maximizing light absorption and ATP production.
(c) Stroma [1]

2.
(a) $2H_2O \rightarrow 4H^+ + 4e^- + O_2$ [1] (Accept correct stoichiometry)
(b) Protons accumulate in the thylakoid lumen [1], creating a proton gradient (chemiosmotic gradient) across the thylakoid membrane [1]. This gradient drives protons through ATP synthase to generate ATP.

3.
(a) P680 (Chlorophyll a) [1]
(b) Plastoquinone accepts electrons from Photosystem II [1] and transports them to the Cytochrome b6f complex [1]. It also carries protons from the stroma into the thylakoid lumen.

4.
(a) Chlorophyll absorbs mainly blue and red light [1]. Green light (500 nm) is reflected or transmitted rather than absorbed [1], so less energy is available for photochemistry.
(b) Although energy per photon is lower, Photosystem I (P700) absorbs efficiently at this wavelength [1]. The Z-scheme requires both PSII and PSI to function; if PSI is excited, electron flow can continue (cyclic or non-cyclic) provided PSII is also active, but red light is highly effective for driving the overall process due to absorption peaks of chlorophyll a [1].

5.
(a) NADPH production will stop/decrease significantly [1].
(b) DCMU blocks electron flow, stopping the proton gradient and ATP synthesis [1]. Without ATP and NADPH, the Calvin cycle cannot run to produce organic molecules for growth/respiration [1]. Even with external glucose, the plant cannot sustain long-term growth or repair without photosynthetic autonomy and may suffer from oxidative damage due to blocked electron transport.

Section B: The Calvin Cycle and Limiting Factors

6.
(a) Rubisco (Ribulose bisphosphate carboxylase/oxygenase) [1]
(b) RuBP: 5 carbons [0.5]; Initial Product (GP): 3 carbons [0.5] (Note: The unstable 6C intermediate splits immediately into two 3C molecules).

7.
(a) Light off $\rightarrow$ No light-dependent reactions $\rightarrow$ No ATP and reduced NADP (NADPH) produced [1]. GP cannot be converted to TP because this step requires ATP and NADPH [1]. However, $CO_2$ fixation continues for a short time, converting RuBP to GP [1]. Thus, GP accumulates.
(b) TP decreases because it is still being used to regenerate RuBP and synthesize glucose/starch [1], but it is not being replenished from GP due to the lack of ATP/NADPH [1].

8.
(a) $CO_2$ concentration [1]
(b) At low $CO_2$, $CO_2$ is the limiting factor [1]. The rate of reaction is determined by substrate availability, not enzyme activity (temperature) [1].
(c) At high $CO_2$, $CO_2$ is no longer limiting. Temperature becomes the limiting factor [1]. Higher temperature increases the kinetic energy of enzymes (Rubisco) and substrates, increasing the rate of collision and enzyme-substrate complex formation [1].

9.
(a) High temperature, high light intensity, low $CO_2$ concentration (closed stomata) [1] (Any one).
(b) RuBP is combined with $O_2$ instead of $CO_2$ [1]. This produces phosphoglycolate, which is toxic and must be recycled via a process that consumes ATP and releases $CO_2$, resulting in a net loss of carbon and energy [1].

10.
(a) Mesophyll cells are arranged in a ring around the bundle sheath cells [1]. Bundle sheath cells contain large chloroplasts and are located deep in the leaf [1].
(b) $CO_2$ is fixed in mesophyll cells into a 4C compound (malate) [1]. This is transported to bundle sheath cells where $CO_2$ is released at high concentration [1]. This high local $CO_2$ concentration suppresses the oxygenase activity of Rubisco, minimizing photorespiration.

Section C: Plant Transport and Adaptations

11.
(a) Palisade mesophyll [1]
(b) Spongy mesophyll cells are loosely packed with large air spaces between them [1]. This allows for rapid diffusion of $CO_2$ and $O_2$ to and from the photosynthetic cells [1].

12.
(a) Cohesion-Tension Theory [1]
(b) Water molecules are polar and form hydrogen bonds with each other (cohesion) [1]. This creates a continuous column of water in the xylem that does not break under tension [1].

13.
(a) High humidity reduces the water vapour potential gradient between the leaf air spaces and the atmosphere [1]. This reduces the rate of diffusion of water vapour out of the stomata [1].
(b) $K^+$ ions are actively pumped into guard cells [1]. This lowers the water potential in guard cells [1]. Water enters by osmosis, increasing turgor pressure, causing the thin outer wall to stretch and the thick inner wall to curve, opening the stomata [1].

14.
(a) A region where sugars are produced (e.g., leaf) or released from storage [1].
(b) $H^+$ ions are actively pumped out of companion cells into the apoplast using ATP [1]. This creates a proton gradient. $H^+$ ions diffuse back into the companion cell through a co-transporter protein, bringing sucrose with it against its concentration gradient [1]. Sucrose then diffuses into sieve tube elements via plasmodesmata [1].

15.
(a) Adaptation: Thick cuticle / Sunken stomata / Reduced leaf area (spines) / Rolled leaves [1]. Explanation: Increases diffusion path length / reduces surface area for evaporation / traps moist air near stomata [1].
(b) Stomata open at night to take in $CO_2$ when temperatures are lower and humidity is higher [1]. $CO_2$ is stored as malic acid. During the day, stomata close to prevent water loss, and $CO_2$ is released for the Calvin cycle [1].

Section D: Plant Growth and Responses

16.
(a) Shoot apex / Tip [1]
(b) Auxin is produced in the tip and moves down the shoot [1]. In unilateral light, auxin moves to the shaded side [1]. Higher auxin concentration on the shaded side stimulates cell elongation [1]. The shaded side grows faster, causing the shoot to bend towards the light.

17.
(a) Water imbibition triggers the embryo to produce gibberellin [1]. Gibberellin diffuses to the aleurone layer and stimulates the synthesis of amylase [1]. Amylase hydrolyses starch in the endosperm to maltose/glucose, which provides energy for the growing embryo [1].

18.
(a) It is a gas and acts locally or at a distance, but unlike typical hormones, it is not produced in a specific gland and acts via simple diffusion [1]. (Accept: It is a simple hydrocarbon gas).
(b) Ripening fruit for transport / Promoting fruit drop (abscission) / Inducing flowering in pineapples [1].

19.
(a) ABA promotes the formation of the abscission layer at the base of the leaf petiole [1]. It stimulates the production of enzymes (cellulase/pectinase) that break down cell walls, causing the leaf to fall [1]. This reduces water loss via transpiration.

20.
(a) Gibberellins (or Auxins/Cytokinins depending on context, but Gibberellins promote stem elongation and strength in some contexts, or Brassinosteroids. For A-Level, Gibberellins or Auxins are acceptable for growth, but Lignin deposition is key. If asking for hormone promoting secondary thickening/strength: Auxins stimulate cambium activity). Let's accept Auxins [1].
(b) Auxins stimulate the division of cambium cells [1]. This leads to the production of secondary xylem (wood) which contains lignin, providing structural support and strength to the stem [1].