A Level H1 Biology Plant Biology Quiz

A-Level Biology H1 Quiz - Plant Biology (Answer Key)

Total Marks: 40

Section A: Structure and Function

1.
(a) Palisade mesophyll [1]
(b) Any two of:

• Contains many chloroplasts [1]
• Arranged vertically/columnar to maximize light absorption/minimize shading [1]
• Located near the upper surface to receive maximum light [1]
  (Max 2)
  (c) - Loosely packed cells create large air spaces [1]
• Allows for rapid diffusion of CO₂ and O₂ to/from mesophyll cells [1]

2.
(a) Granum (or Thylakoids) [1]
(b) Chlorophyll [1]
(c) - The stroma is the site of the light-independent reactions (Calvin Cycle) [1]

• These reactions are enzyme-controlled (e.g., Rubisco), requiring a high concentration of enzymes for efficiency [1]

3.

• Role: Waterproof layer to prevent excessive water loss (transpiration) [1]
• Humid environment: Thinner cuticle [1] because water conservation is less critical/transpiration rate is lower due to high humidity. [1]
• Arid environment: Thicker cuticle [1] to minimize water loss. (Note: Question asks for variation explanation, award marks for comparative logic).
  (Max 3: 1 for role, 2 for comparison logic)

4.
(a) - Increases surface area [1]

• Increases the rate of osmosis/water uptake [1]
  (b) Active transport [1]

5.
(a) Xylem: Unidirectional (upwards only / root to leaf) [1]
Phloem: Bidirectional (source to sink / can be up or down) [1]
(b) Xylem: Water and mineral ions [1]
Phloem: Organic solutes / Sucrose / Amino acids [1]

Section B: Photosynthesis and Energy

6.
(a) $2H_2O \rightarrow 4H^+ + 4e^- + O_2$ [2] (1 for correct reactants/products, 1 for balancing)
(b) Any two:

• Electrons replace those lost by chlorophyll (in PSII) [1]
• Protons ($H^+$) contribute to the proton gradient for chemiosmosis/ATP synthesis [1]
• Oxygen is released as a by-product [1]
  (Max 2)

7.

• Chlorophyll absorbs light energy [1]
• Electrons are excited/raised to a higher energy level [1]

8.

• CO₂ combines with Ribulose Bisphosphate (RuBP) [1]
• Catalyzed by Rubisco [1]
• Forms two molecules of Glycerate-3-phosphate (GP) [1]
• GP is reduced to Triose Phosphate (TP) [1]
• Using ATP (for energy) and Reduced NADP (for hydrogen/electrons) [1]
• Some TP is converted to glucose/organic molecules, most is regenerated to RuBP [1]
  (Max 6)

9.
(a) Light intensity [1]
(b) - At high light, light is no longer the limiting factor [1]

• Temperature affects the rate of enzyme-controlled reactions (Calvin cycle) [1]
• 25°C is closer to the optimum temperature for these enzymes than 15°C, so kinetic energy/collision rate is higher [1]

10.
(a) Cyclic: Involves only Photosystem I [1]
Non-cyclic: Involves Photosystem II and Photosystem I [1]
(b) Cyclic: Produces ATP only [1]
Non-cyclic: Produces ATP, Reduced NADP, and Oxygen [1]

11.

• Electron flow from PSII to PSI is stopped [1]
• Non-cyclic photophosphorylation cannot occur [1]
• No proton gradient generated across thylakoid membrane (from ETC) [1]
• Therefore, no ATP is produced via chemiosmosis, and no Reduced NADP is formed (as electrons don't reach NADP) [1]

12.

• Mitochondrial matrix [1]
• Plant cells need mitochondria to produce ATP via aerobic respiration [1]
• Chloroplasts only produce ATP during the day/light; mitochondria provide ATP for cellular processes at night and in non-photosynthetic tissues [1]

Section C: Transport and Growth

13.

• Sucrose is actively loaded into phloem sieve tubes at the source (leaf) [1]
• This lowers water potential in the phloem [1]
• Water enters phloem from xylem by osmosis [1]
• Creates high hydrostatic pressure at the source [1]
• Mass flow of sucrose solution occurs towards the sink (low pressure) where sucrose is unloaded [1]

14.
(a) Any two:

• Rate of water uptake equals rate of transpiration [1]
• No water is used for photosynthesis/turgor/growth [1]
• The apparatus is watertight [1]
  (Max 2)
  (b) - Wind removes water vapor from around the leaf [1]
• Maintains a steep water potential/concentration gradient between leaf and air [1]
• Increases the rate of diffusion/transpiration, thus increasing water uptake [1]

15.

• Water stress causes ABA production [1]
• ABA binds to receptors on guard cell membrane [1]
• Causes $K^+$ channels to open, $K^+$ leaves guard cells [1]
• Water potential increases (becomes less negative), water leaves by osmosis, guard cells become flaccid, stomata close [1]

16.

• Apical dominance: Auxin produced in apical bud inhibits lateral bud growth [1]
• Seed germination: Gibberellin stimulates the production of amylase [1]
• Amylase breaks down starch to maltose/glucose for respiration/growth [1]
• Auxin maintains dominance; Gibberellin breaks dormancy [1]

17.

• Transpiration pull is the upward pull of water in the xylem due to transpiration [1]
• Water molecules are cohesive (hydrogen bonds between water molecules) [1]
• This allows water to be pulled up as a continuous column without breaking [1]

18.

• C4 plants use PEP carboxylase which has a higher affinity for CO₂ than Rubisco [1]
• PEP carboxylase fixes CO₂ into a 4-carbon compound in mesophyll cells [1]
• This is transported to bundle sheath cells where CO₂ is released at high concentration [1]
• High CO₂ concentration minimizes photorespiration (competition with O₂ at Rubisco active site) [1]

19.
(a) The tendency of water molecules to move from one region to another (or chemical potential of water) [1]
(b) The water potential of the potato cells is lower (more negative) than the solution [1]

20.

• One male gamete fuses with the egg cell to form the zygote (2n) [1]
• The other male gamete fuses with the two polar nuclei [1]
• To form the triploid endosperm nucleus (3n) [1]
• This ensures food supply for the developing embryo [1]
  (Note: 1 mark for describing the two fusions, 1 mark for correct ploidy of zygote, 1 mark for correct ploidy of endosperm, 1 mark for clarity/completeness)