Photosynthesis and Plant Nutrition: Question 9
Syllabus 6.2
The diagram below describes, in words, a cross-section through part of a leaf, from the upper surface down to the lower surface:
- a thin, transparent, waxy layer covers the very top of the leaf;
- beneath this is a layer of tall, closely packed cells full of chloroplasts;
- below that is a layer of loosely packed, irregularly shaped cells, with large, interconnected air spaces between them;
- at the very bottom of the leaf, the lower surface contains many small pores, each surrounded by a pair of guard cells.
(a) Name the small pores found in the lower surface of the leaf, surrounded by pairs of guard cells, and state one gas that moves into the leaf and one gas that moves out of the leaf through these pores during daylight. [2]
(b) Name the loosely packed layer of cells with large air spaces between them, and explain how the large air spaces in this layer help photosynthesis to happen efficiently. [2]
(c) The leaf as a whole is broad and thin. Explain one way in which being broad, and one way in which being thin, each help the leaf carry out photosynthesis efficiently. [2]
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Worked solution
Part (a): The stomata
The small pores in the lower surface, each surrounded by a pair of guard cells, are called stomata (singular: stoma). During daylight, when the leaf is photosynthesising faster than it is respiring, carbon dioxide diffuses into the leaf through the stomata (a raw material for photosynthesis), while oxygen diffuses out of the leaf through the stomata (a product of photosynthesis).
Part (b): The spongy mesophyll
The loosely packed, irregularly shaped layer with large air spaces between its cells is the spongy mesophyll. This is different from the tightly packed, chloroplast-rich layer above it (the palisade mesophyll). The large, interconnected air spaces in the spongy mesophyll greatly increase the surface area available for gas exchange inside the leaf, and let carbon dioxide and oxygen diffuse quickly between the stomata and the mesophyll cells that need them, so gas exchange for photosynthesis can happen efficiently throughout the leaf.
Part (c): The broad, thin shape of the leaf
- Broad: a broad leaf has a large surface area, so more of its chlorophyll-containing cells are exposed to light, allowing the leaf to absorb more light energy for photosynthesis.
- Thin: a thin leaf means gases and light only have to travel a short distance to reach the cells inside, carbon dioxide entering through the stomata quickly reaches the mesophyll cells, and light quickly reaches the chloroplasts in the palisade layer, without being absorbed or scattered by too many cell layers first.
Final answers
- (a) Stomata; carbon dioxide moves in, oxygen moves out (during daylight).
- (b) Spongy mesophyll; its large air spaces increase surface area and allow fast diffusion of gases.
- (c) Broad shape increases the surface area for light absorption; thin shape shortens the diffusion distance for gases and light.