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A Level H2 Biology Plant Biology Quiz

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A-Level Biology H2 Quiz - Plant Biology

Name: __________________________
Class: __________________________
Date: __________________________
Score: ________ / 40

Duration: 45 minutes
Total Marks: 40
Instructions:

Answer all questions in the spaces provided.
The number of marks is given in brackets [ ] at the end of each question or part question.
Use clear scientific terminology.
Diagrams are not drawn to scale unless stated.

Section A: Photosynthesis and Chloroplast Structure (Questions 1–5)

1. Fig. 1.1 shows a transmission electron micrograph of a chloroplast.

(Note: Imagine Fig 1.1 showing a chloroplast with labelled regions A (grana), B (stroma), and C (outer membrane).)

(a) Identify the structures labelled A and B. [2]
A: _______________________________________________________
B: _______________________________________________________

(b) Explain why the structure labelled A appears as a stack of discs. [2]

2. During the light-dependent reactions, water is photolysed.

(a) Write the balanced chemical equation for the photolysis of water. [1]

(b) Explain the significance of the protons ( $H^+$ ) released during photolysis in the context of ATP synthesis. [2]

3. Fig. 3.1 illustrates the Z-scheme of electron flow in the thylakoid membrane.

(Note: Imagine Fig 3.1 showing PSII, ETC, PSI, and NADP reductase.)

(a) Name the primary pigment found in the reaction centre of Photosystem II. [1]

(b) Describe the role of plastoquinone in the electron transport chain. [2]

4. An experiment was conducted to investigate the effect of light wavelength on the rate of photosynthesis in Elodea. The results are shown in Table 4.1.

Table 4.1

Wavelength (nm)	Colour	Rate of $O_2$ production ( $cm^3 h^{-1}$ )
400	Violet	12.5
500	Green	2.1
600	Orange	8.4
700	Red	11.8

(a) Explain why the rate of photosynthesis is lowest at 500 nm. [2]

(b) Suggest why the rate at 700 nm is high, despite the lower energy per photon compared to 400 nm. [2]

5. DCMU is a herbicide that blocks electron transfer from Photosystem II to plastoquinone.

(a) Predict the effect of DCMU on the production of NADPH. [1]

(b) Explain why the plant eventually dies if treated with DCMU, even if provided with glucose externally. [2]

Section B: The Calvin Cycle and Limiting Factors (Questions 6–10)

6. The Calvin cycle involves the fixation of carbon dioxide.

(a) Name the enzyme responsible for fixing $CO_2$ to ribulose bisphosphate (RuBP). [1]

(b) State the number of carbon atoms in one molecule of RuBP and one molecule of the initial product formed after fixation. [1]
RuBP: ______ carbons
Initial Product: ______ carbons

7. Fig. 7.1 shows the changes in the concentration of glycerate-3-phosphate (GP) and triose phosphate (TP) in a chloroplast when light is suddenly switched off.

(Note: Imagine Fig 7.1 showing GP rising and TP falling after time t=0.)

(a) Explain why the concentration of GP increases immediately after the light is switched off. [3]

(b) Explain why the concentration of TP decreases. [2]

8. A student investigated the effect of carbon dioxide concentration on the rate of photosynthesis in tomato plants at two different temperatures ( $20^\circ C$ and $30^\circ C$ ).

(a) Identify the independent variable in this investigation. [1]

(b) At low $CO_2$ concentrations, the rate of photosynthesis is the same at both temperatures. Explain why. [2]

(c) At high $CO_2$ concentrations, the rate is higher at $30^\circ C$ than at $20^\circ C$ . Explain this difference. [2]

9. Photorespiration occurs when Rubisco acts as an oxygenase rather than a carboxylase.

(a) Under what environmental conditions is photorespiration most likely to occur? [1]

(b) Explain why photorespiration reduces the yield of photosynthesis. [2]

10. C4 plants, such as maize, have a different anatomical structure compared to C3 plants.

(a) Describe the arrangement of mesophyll and bundle sheath cells in a C4 leaf (Kranz anatomy). [2]

(b) Explain how this anatomy helps C4 plants minimize photorespiration. [2]

Section C: Plant Transport and Adaptations (Questions 11–15)

11. Fig. 11.1 shows a cross-section of a leaf.

(Note: Imagine Fig 11.1 showing upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis with stomata.)

(a) Identify the tissue layer responsible for the majority of photosynthesis. [1]

(b) Explain how the arrangement of cells in the spongy mesophyll facilitates gas exchange. [2]

12. Water moves up the xylem in plants.

(a) Name the theory that explains the movement of water in the xylem. [1]

(b) Explain the role of cohesion in this process. [2]

13. Transpiration rate is affected by environmental factors.

(a) Explain how high humidity affects the rate of transpiration. [2]

(b) Describe the mechanism by which guard cells open stomata. [3]

14. Phloem transports assimilates from source to sink.

(a) Define the term "source" in the context of translocation. [1]

(b) Describe the process of active loading of sucrose into the phloem at the source. [3]

15. Xerophytes are plants adapted to dry environments.

(a) Describe one structural adaptation of xerophytic leaves and explain how it reduces water loss. [2]
Adaptation: ______________________________________________________________
Explanation: _____________________________________________________________

(b) Explain how CAM (Crassulacean Acid Metabolism) plants conserve water. [2]

Section D: Plant Growth and Responses (Questions 16–20)

16. Auxins are plant growth regulators.

(a) State the site of auxin production in a shoot tip. [1]

(b) Explain how auxin causes phototropism in shoots. [3]

17. Gibberellins play a role in seed germination.

(a) Describe the sequence of events involving gibberellin, amylase, and starch during germination. [3]

18. Ethene (ethylene) is involved in fruit ripening.

(a) Explain why ethene is classified as a plant hormone rather than a growth regulator. [1]

(b) Suggest one commercial application of ethene in agriculture. [1]

19. Abscisic acid (ABA) is known as the "stress hormone".

(a) Explain the role of ABA in leaf abscission. [2]

20. A farmer notices that his crop plants are lodging (falling over) due to weak stems.

(a) Suggest which plant hormone might be involved in stem strengthening and secondary growth. [1]

(b) Explain how this hormone promotes stem strength. [2]

End of Quiz

Answers

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].