Fig. 2.1 Fig. 2.1 shows a section through a leaf. The leaf is the main photosynthetic organ of a plant and its tissues are adapted to capture light and to e...

This question tests how the internal structure of a leaf is matched to its job of photosynthesis and gas exchange. Work through it layer by layer.

(a) Most photosynthesis takes place in the palisade mesophyll [1]. These are the tall, column-shaped cells packed just under the upper surface.

(b) The palisade layer sits near the upper surface because that is where it receives the most light [1]: light strikes the top of the leaf first and is not blocked or absorbed by the layers above it [1]. Placing the cells with the most chloroplasts where light is strongest maximises the light captured.

(c) The air spaces in the spongy mesophyll form connected channels that let gases diffuse freely through the inside of the leaf [1], bringing carbon dioxide to the photosynthesising cells and carrying oxygen away [1]. A large internal surface next to these spaces speeds up that diffusion.

(d) The green pigment that absorbs light is chlorophyll [1], held inside the chloroplasts.

(e) Trace the carbon dioxide molecule as a pathway of three linked steps [3]:

1. It enters the leaf through a stoma (a pore, mostly on the lower surface).
2. It passes into the air spaces of the spongy mesophyll.
3. It then diffuses through the air spaces, dissolves in the film of moisture on a cell wall and passes into a palisade cell.

(f) The leaf faces a conflict: it must let carbon dioxide in but not lose too much water. It solves this in three ways [3]: the waxy cuticle on the surface reduces evaporation [1]; most stomata are on the shaded, cooler lower surface, where less water evaporates [1]; and the stomata can close (using the guard cells) to cut water loss, opening again to admit gases when needed [1].

(g) The glucose made in photosynthesis is used in any two of these ways [2]: converted to starch for storage, broken down in respiration to release energy, or used to make cellulose for cell walls or sucrose for transport. Naming any two correct uses earns both marks.

Exam tip: when a part says "explain," a bare structure name is not enough - link each feature to how it helps light capture, gas exchange, or water conservation.