Leaves are the main organs of photosynthesis and their internal tissues are arranged to capture light and exchange gases efficiently. The diagram shows the ...
Question 1 Report
Leaves are the main organs of photosynthesis and their internal tissues are arranged to capture light and exchange gases efficiently. The diagram shows the internal structure of a spinach leaf, seen in cross-section under a high-power microscope. The section reveals the waxy cuticle and epidermis covering the outer surfaces, a densely packed upper layer of elongated cells, a looser middle layer containing large air spaces, and the vein tissues that carry water and sugars. A student was asked to relate each of these features to the leaf's function in trapping light and obtaining the raw materials needed for photosynthesis. Use the diagram together with your own knowledge to answer the questions that follow.
(a) Name the layer of cells in which most photosynthesis takes place. [1]
(b) Explain two ways in which the cells of this layer are adapted for photosynthesis. [3]
(c) Describe how carbon dioxide in the air outside the leaf reaches the mesophyll cells. [3]
(d) Explain the function of the waxy cuticle on the upper surface. [2]
(e) Explain why a broad, thin, flat leaf is efficient at photosynthesis. [2]
Answer Details
(a) Layer where most photosynthesis occurs [1]
The palisade mesophyll (the densely packed, elongated cells just below the upper epidermis).
(b) Two adaptations of these cells for photosynthesis [3, max 3]
The palisade cells are column-shaped and packed close to the upper surface [1]; they contain many chloroplasts to trap light [1]; and their position near the top of the leaf lets them receive the most light [1].
(c) How carbon dioxide reaches the mesophyll cells [3]
Carbon dioxide diffuses in from the outside air through the stomata [1]; it moves into the air spaces of the spongy mesophyll [1]; and then diffuses into the mesophyll cells down a concentration gradient (its concentration inside the cells is kept low by photosynthesis) [1].
(d) Function of the waxy cuticle on the upper surface [2]
The cuticle is waterproof [1]; so it reduces water loss by evaporation from the upper leaf surface, which is most exposed to sunlight [1].
(e) Why a broad, thin, flat leaf is efficient [2]
Being thin gives a short diffusion distance for gases moving in and out [1]; being broad and flat gives a large surface area to absorb sunlight [1].
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