Feature Palisade mesophyll Spongy mesophyll shape of cells column-shaped irregular number of chloroplasts many few air spaces between cells few many A stude...

This question compares the two mesophyll layers of a leaf and links their structure to photosynthesis and water movement.

(a) The layer that contains more chloroplasts is the palisade mesophyll [1].

(b) Having many chloroplasts adapts the palisade cells because each chloroplast contains chlorophyll, so more chloroplasts means more chlorophyll [1]; this allows the cell to absorb more light for photosynthesis [1].

(c) The palisade cells are column-shaped and packed closely together near the top of the leaf so that a large number of chloroplasts are placed in the upper region of the leaf [1], which is where the most light reaches; this maximises the amount of light absorbed for photosynthesis [1].

(d) The large air spaces in the spongy mesophyll allow gases to diffuse freely through the leaf [1]; this lets carbon dioxide reach the photosynthesising cells [1]; and allows oxygen and water vapour to move away from the cells and leave the leaf [1].

(e) The tissue that transports water to the mesophyll cells is the xylem [1].

(f) Water reaching the mesophyll is used and lost in these ways: some is used as a raw material in photosynthesis [1]; some keeps the cells turgid, which supports the leaf and keeps it firm [1]; and excess water evaporates from the cell surfaces and is lost as water vapour through the stomata [1].

(g) Transpiration is the loss of water vapour from a plant [1], by evaporation from the surfaces of the mesophyll cells and diffusion out through the stomata of the leaves [1].

Exam tip: keep "used" (photosynthesis and support) separate from "lost" (evaporation through stomata); each is a distinct marking point in part (f).