Leaf Structure and Function, practice questions

B, The palisade mesophyll cells are packed with chloroplasts and positioned near the upper surface where light intensity is greatest.

C, When guard cells are turgid, their unevenly thickened walls bow apart, opening the pore between them.

B, Phloem transports the sugars produced in photosynthesis to other parts of the plant.

C, The compensation point is defined as the light intensity at which the rate of photosynthesis exactly equals the rate of respiration.

B, The lower surface is cooler and less exposed to direct sunlight, so placing most stomata there reduces the rate of water loss.

C, A potometer measures how fast a cut shoot takes up water, which is used to estimate the rate of transpiration.

B, The spongy mesophyll has large air spaces between its irregularly shaped cells, allowing gases to diffuse quickly.

C, Wind disturbs the thin, humid layer of air next to the leaf surface, steepening the concentration gradient for water vapour and increasing the transpiration rate.

B, The waxy cuticle covering the epidermis reduces water loss through the leaf surface.

B, Once light is no longer limiting, a different factor takes over as the one restricting the rate of photosynthesis.

• (a) Spongy mesophyll
• (b) Provides air spaces that allow gases to diffuse to and from the mesophyll cells and stomata
• (c) Placing the palisade mesophyll on top means the cells with the most chloroplasts receive light first, before it is absorbed or scattered by other tissue, maximising the rate of photosynthesis

• Highest rate: moving air from a fan, because it removes the humid boundary layer around the stomata and maintains a steep water vapour concentration gradient
• Lowest rate: high humidity, because the water vapour concentration outside the leaf is close to that inside, reducing the rate of diffusion out of the stomata
• Still air gives an intermediate rate between the two

• (a) In darkness only respiration occurs, so the plant is a net absorber of oxygen and net releaser of carbon dioxide; as light intensity increases, the rate of photosynthesis increases and net oxygen release increases, first slowly and then more steeply, until a further increase in light intensity no longer increases net oxygen release because another factor becomes limiting
• (b) The compensation point: the light intensity at which the rate of photosynthesis exactly equals the rate of respiration, giving zero net gas exchange

From top to bottom: waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis with stomata, and veins.

Stomata let carbon dioxide in and oxygen out; the spongy mesophyll has air spaces for diffusion.

Water evaporates from the mesophyll and diffuses out through the stomata.

The palisade mesophyll near the top is packed with chloroplasts to trap light.

A broad, thin blade, many chloroplasts near the top, and stomata mainly on the lower surface make the leaf efficient.

The light intensity at which the rate of photosynthesis exactly equals the rate of respiration, so there is no net gas exchange.

A pair of guard cells surrounds each stoma; when turgid, their unevenly thickened walls curve the pair apart to open the pore, and when flaccid, the pore closes, regulating both gas exchange and water loss.

Leaf veins contain xylem, which transports water and mineral ions from the roots to the mesophyll, and phloem, which transports the sugars produced by photosynthesis to other parts of the plant.

A waxy, waterproof layer covering the upper, and to a lesser extent lower, epidermis that reduces uncontrolled water loss through the leaf surface itself, forcing most water loss to occur through the stomata instead.

As light intensity increases from zero, the rate of photosynthesis increases, first limited by light itself, then eventually limited by another factor such as carbon dioxide concentration or temperature, at which point further increases in light no longer raise the rate.

A leaf's broad, flat shape gives it a large surface area relative to its volume, which increases the area available for light absorption, gas exchange and water loss, all of which occur across the leaf's surfaces rather than through its bulk.

A thin, still layer of humid air can build up just outside the stomata; wind or air movement disturbs this layer and increases the rate of transpiration by maintaining a steeper water vapour concentration gradient between the leaf and the surrounding air.

The midrib is the thickened central vein running the length of the leaf, and the petiole is the stalk joining the leaf to the stem; both contain vascular tissue and give the thin lamina mechanical support so it can be held at an angle to the light.