Multiple Choice
1. B The thick, inelastic inner wall resists expansion while the thinner outer wall stretches when turgid. This forces the cells to bow outward, pulling the inner walls apart and opening the pore. The cells do not expand uniformly or contract.
2. D At the compensation point, both processes occur simultaneously at equal rates. CO₂ produced by respiration is immediately consumed by photosynthesis, and O₂ produced by photosynthesis is immediately consumed by respiration. No net gas exchange occurs with the atmosphere, but both processes are fully active.
3. A Lenticels are permanently open pores in woody bark with loosely packed parenchyma cells creating air spaces for passive gas diffusion. They have no guard cells and are not actively regulated. They are found in stems, not leaves.
4. C Submerged leaves are surrounded by water, so there is no evaporative water loss, the cuticle's waterproofing function is unnecessary. A thin or absent cuticle allows dissolved gases (CO₂ and O₂) to diffuse more readily directly through the leaf surface from the surrounding water.
5. B ABA triggers K⁺ efflux (potassium ions leave guard cells), raising water potential inside the guard cells above that of surrounding cells. Water leaves by osmosis, reducing turgor pressure. The flaccid guard cells lose their bowed shape and the stoma closes.
Q6, Model Answer
• Chloroplasts: Guard cells are the only epidermal cells that contain chloroplasts. In light, chloroplasts perform photosynthesis, producing ATP.
• Ion pumping: ATP powers H⁺-ATPase pumps in the guard cell membrane, which actively pump H⁺ ions out of the guard cells. This creates a charge gradient that drives K⁺ ions into the guard cells through specific ion channels.
• Water potential: The accumulation of K⁺ ions inside the guard cells lowers their water potential below that of surrounding epidermal cells.
• Osmosis: Water moves into the guard cells by osmosis (from higher water potential in surrounding cells to lower water potential in guard cells), increasing turgor pressure and causing the cells to swell.
• Cell wall structure: The inner wall of each guard cell (facing the pore) is thicker and less elastic than the outer wall. When turgid, the outer wall stretches while the inner wall resists, causing the cells to bow outward and pulling the pore open.
Q7, Model Answer
The conclusion is incorrect. The plant is producing CO₂ continuously during the day via cellular respiration, which occurs in all living cells at all times regardless of light availability.
The net uptake of CO₂ observed during the day does not mean CO₂ production has stopped, it means the rate of photosynthesis exceeds the rate of cellular respiration. Photosynthesis consumes CO₂ faster than respiration produces it, resulting in a net decrease in CO₂ from the leaf's perspective.
The correct interpretation is that both photosynthesis and respiration are occurring simultaneously, with photosynthesis dominant during bright daylight. Some of the CO₂ produced by respiration is immediately consumed by photosynthesis, never leaving the cell, while the remainder of the photosynthesis CO₂ demand is met by uptake from the atmosphere through stomata.
Q8, Model Answer
Difference 1, Cuticle: Terrestrial plants have a thick, waxy cuticle covering the leaf surface, whereas submerged aquatic plants have a thin or absent cuticle. Terrestrial plants risk significant water loss by evaporation from leaf surfaces, the waxy cuticle is hydrophobic and reduces this evaporative loss. Submerged aquatic plants are surrounded by water and face no evaporative water loss; a thin cuticle instead maximises diffusion of dissolved CO₂ and O₂ directly through the leaf surface from the surrounding water.
Difference 2, Stomata position: Terrestrial plants have stomata predominantly on the lower (abaxial) leaf surface, whereas floating aquatic leaves (e.g. water lily) have stomata only on the upper surface, and fully submerged leaves may lack functional stomata entirely. In terrestrial plants, lower surface stomata are shaded from direct sunlight, reducing leaf temperature and evaporative water loss through the open pores. In floating leaves, the lower surface is in contact with water, placing stomata there would block gas exchange with the atmosphere entirely, so stomata are on the upper (air-exposed) surface. Fully submerged leaves exchange gases directly through the leaf surface from dissolved gases in the water, making stomata non-functional or unnecessary.