A
Explain why the spongy mesophyll layer contains large air spaces.
Large air spaces allow rapid diffusion of carbon dioxide and oxygen within the leaf.
Describe one structural adaptation of the leaf epidermis and explain its function.
The epidermis is thin and transparent, allowing light to pass through to the photosynthesizing cells.
A plant is placed in bright light but the rate of photosynthesis does not increase.
State two possible limiting factors other than light.
Low carbon dioxide concentration and unsuitable temperature.
Describe the pathway taken by water from the soil to a leaf mesophyll cell.
Soil → root hair cell → cortex → xylem → leaf xylem → mesophyll cell.
Explain why transpiration rate increases on a windy day.
Wind removes water vapour from around the leaf, maintaining a steep diffusion gradient.
Explain how the position of stomata on a leaf reduces excessive water loss.
Most stomata are on the lower surface, which is shaded and cooler, reducing evaporation.
Explain why destarched plants are used in photosynthesis experiments.
To ensure that any starch formed during the experiment is newly produced by photosynthesis.
Describe two structural differences between the palisade mesophyll and spongy mesophyll layers.
Palisade mesophyll cells are elongated and closely packed, while spongy mesophyll cells are irregular in shape and loosely packed with air spaces.
Explain how carbon dioxide enters the leaf and reaches the palisade cells.
Carbon dioxide diffuses through stomata and moves through air spaces to the palisade cells.
Describe how water moves across the root cortex to the xylem.
By osmosis through cells or through cell walls.
A plant is grown in magnesium-deficient soil.
a) State one visible symptom.
b) Explain why this symptom occurs.
a) Yellowing of leaves (chlorosis).
b) Magnesium is needed to make chlorophyll; without it, chlorophyll production decreases.
Explain why ringing (removal of bark) causes swelling above the cut.
Phloem is removed, so sugars accumulate above the cut and cause swelling.
Describe how the structure of a leaf helps to reduce water loss while allowing gas exchange.
A waxy cuticle reduces water loss, while stomata allow controlled gas exchange.
Explain the importance of stomatal opening during daylight hours.
Stomata open to allow carbon dioxide to enter for photosynthesis and oxygen to exit.
Explain why phloem transport can occur in both directions.
Sugars move from sources to sinks, which can be in different parts of the plant.
Explain how active transport is involved in mineral ion uptake by roots.
Mineral ions are absorbed against their concentration gradient using energy from respiration.
Describe how palisade mesophyll cells are adapted to maximize photosynthesis.
They contain many chloroplasts, are elongated to pack closely together, and are located near the upper leaf surface to receive maximum light.
Suggest two environmental conditions that would reduce transpiration rate and explain why.
High humidity reduces diffusion gradient; low temperature reduces evaporation rate.
A plant has its stomata closed during the day.
Suggest one advantage and one disadvantage.
Advantage: reduces water loss.
Disadvantage: limits carbon dioxide uptake for photosynthesis.
Define transpiration and name the tissue involved.
Transpiration is the loss of water vapour from plant leaves through stomata.
State the balanced chemical equation for photosynthesis and name the source of each reactant.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Carbon dioxide comes from the air; water comes from the soil via roots.
Explain why transpiration helps in cooling the plant.
Evaporation of water uses heat energy from the leaf, reducing temperature.
Describe the role of nitrate ions in plant growth.
Nitrate ions are used to make amino acids and proteins needed for growth.
Describe the role of lignin in xylem vessels.
Lignin strengthens the vessel walls and prevents collapse.
Explain how transpiration creates a pull that moves water up the stem.
Water loss from leaves creates tension, pulling a continuous column of water up the xylem.