How is a leaf adapted for photosynthesis, and how do xylem and phloem transport substances in a plant?
The structure of a leaf and how it is adapted for photosynthesis and gas exchange, and the transport of water and food in the xylem and phloem.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on leaves and plant transport, covering how the leaf is adapted for photosynthesis and gas exchange, and the roles of the xylem and phloem in transporting water and dissolved food.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to describe the structure of a leaf and how it is adapted for photosynthesis and gas exchange, and explain how the xylem and phloem transport substances.
How the leaf is adapted
- Broad and flat: a large surface area to absorb light and carbon dioxide.
- Thin: gases only have to diffuse a short distance, and light reaches all the cells.
- Palisade layer: tall cells packed with chloroplasts near the upper surface, where the most light reaches.
- Spongy layer with air spaces: allows carbon dioxide and oxygen to diffuse easily to and from the cells.
- Stomata: tiny pores, mostly on the underside, that let carbon dioxide in and oxygen out; guard cells open and close them.
- Network of veins: contain xylem and phloem to supply water and remove the sugars made.
Gas exchange in the leaf
The stomata are the leaf's gas exchange surface. During the day, carbon dioxide diffuses in for photosynthesis and oxygen diffuses out. The guard cells swell to open the stomata when water is plentiful and close them when the plant is short of water, balancing gas exchange against water loss.
The transport tissues
Water enters the plant through the root hair cells, which have a large surface area, and travels up the xylem to replace water lost from the leaves. Sugars made in the leaves are loaded into the phloem and carried to growing parts and to storage organs such as roots.
The layers of a leaf
From top to bottom, a leaf is built in layers, each suited to its job. The waxy cuticle is a waterproof layer that reduces water loss. The upper epidermis is transparent so light can pass through to the cells below. The palisade mesophyll is packed with chloroplasts to do most of the photosynthesis. The spongy mesophyll has air spaces for gases to move around. The lower epidermis contains the stomata and guard cells for gas exchange. Knowing the order of the layers helps you explain why each adaptation is where it is.
Why minerals matter
The water travelling up the xylem also carries mineral ions the plant needs. Nitrate ions are needed to make amino acids and proteins for growth, and magnesium ions are needed to make chlorophyll. A plant short of nitrate grows poorly with yellowing older leaves, and a plant short of magnesium cannot make enough chlorophyll, so its leaves go yellow. These deficiency symptoms are a common exam point, linking mineral transport to healthy growth.
Try this
Q1. Name the cells that open and close the stomata. [1 mark]
- Cue. Guard cells.
Q2. State which tissue carries sugars made in the leaves to the rest of the plant. [1 mark]
- Cue. The phloem.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe two ways the structure of a leaf is adapted for efficient photosynthesis, explaining how each helps.Show worked answer →
A Unit 1 adaptation question worth 4 marks. Reward: the leaf is broad and flat to give a large surface area to absorb light (1); it is thin so gases and light reach the cells easily (1); the palisade layer near the top is packed with chloroplasts to absorb the most light (1); there are air spaces in the spongy layer to allow gases to diffuse (1); and stomata allow carbon dioxide in and oxygen out (1). Markers credit two adaptations each linked to photosynthesis or gas exchange. A common error is to name the feature without the reason.
WJEC style3 marksCompare the functions of the xylem and the phloem in a plant.Show worked answer →
A Unit 1 compare question. Reward: the xylem carries water and dissolved mineral ions from the roots up to the leaves (1); the phloem carries dissolved food (sugars) made in the leaves to all parts of the plant, including growing points and storage organs (1); xylem transport is one way (upwards) while phloem transport (translocation) is in both directions (1). Markers credit the substance carried and the direction for each. A common error is to swap xylem and phloem.
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