How do plants make food by photosynthesis, and how do they transport water and food?
Photosynthesis as an endothermic reaction, the limiting factors of photosynthesis, the structure of a leaf, and transport in plants by the xylem and phloem including transpiration.
A focused answer to Edexcel GCSE Combined Science Topic 6 (CB6), covering photosynthesis as an endothermic reaction, the limiting factors, the structure of a leaf, and transport in plants through the xylem and phloem including transpiration.
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What this dot point is asking
Edexcel wants you to describe photosynthesis as an endothermic reaction with its equation, explain the limiting factors of photosynthesis, describe how a leaf is adapted for photosynthesis, and describe transport in plants through the xylem and phloem including transpiration.
Photosynthesis
The balanced symbol equation is:
The glucose made is used in respiration, converted to starch for storage, used to make cellulose for cell walls, and used with nitrate ions to make proteins.
Limiting factors
The rate of photosynthesis is controlled by limiting factors: the factor in shortest supply limits the rate.
- Light intensity - more light gives more energy, up to a point.
- Carbon dioxide concentration - more carbon dioxide gives a faster rate, up to a point.
- Temperature - rate rises with temperature because the enzymes work faster, until it gets so hot that the enzymes denature.
Leaf structure
A leaf is adapted for photosynthesis. It is broad with a large surface area to capture light, thin so gases diffuse quickly, and has many chloroplasts in the palisade layer near the top. The stomata (pores, mainly on the underside) let carbon dioxide in and oxygen out, and are opened and closed by guard cells.
Transport in plants
Plants have two transport tissues:
- Xylem carries water and dissolved mineral ions from the roots up to the leaves. It is a one-way flow.
- Phloem carries dissolved sugars (food) made in the leaves to the rest of the plant, in a process called translocation, in both directions.
Transpiration is the loss of water vapour from the leaves, mostly through the stomata. It creates the transpiration stream that pulls water up the xylem. Its rate increases with higher temperature, more wind, lower humidity and brighter light, because each of these speeds up the evaporation and diffusion of water from the leaf. Water is also needed inside the plant to keep cells turgid, which supports the leaves and stems, and as a raw material for photosynthesis, so a plant that loses water faster than it can take it up will wilt.
Try this
Q1. State the word equation for photosynthesis. [2 marks]
- Cue. Carbon dioxide + water glucose + oxygen.
Q2. Name the tissue that transports sugars in a plant. [1 mark]
- Cue. The phloem.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksA student investigates the effect of light intensity on the rate of photosynthesis of pondweed by counting bubbles of gas produced per minute. Explain why the rate increases as light intensity increases, and why it eventually levels off.Show worked answer →
A 4-mark explain question on limiting factors.
As light intensity increases, more light energy is available for photosynthesis, so the rate increases and more oxygen bubbles are produced (2 marks). The rate eventually levels off because another factor has become limiting (1 mark), for example carbon dioxide concentration or temperature, so increasing the light no longer increases the rate (1 mark).
Markers reward light as the limiting factor for the rise, and the idea that a different factor (carbon dioxide or temperature) limits the rate when it levels off.
Edexcel 20213 marksDescribe the function of the xylem and the phloem in a plant, and state what is meant by transpiration.Show worked answer →
A 3-mark describe question on plant transport.
The xylem transports water and dissolved mineral ions from the roots up to the leaves; it is a one-way flow (1 mark). The phloem transports dissolved sugars (food) made in the leaves to all other parts of the plant, in a process called translocation, in both directions (1 mark). Transpiration is the loss of water vapour from the leaves (mainly through the stomata), which pulls water up the xylem (1 mark).
Markers reward xylem for water and minerals (up), phloem for sugars (both ways), and transpiration as water loss from the leaves.
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Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)