Plant nutrition and transport
How plants make food by photosynthesis, the factors that limit it, and how water, ions and sugars move through a plant.
📌 Key points
- Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light: carbon dioxide + water → glucose + oxygen (light, chlorophyll); 6CO2 + 6H2O → C6H12O6 + 6O2.
- Chlorophyll is a green pigment in chloroplasts that transfers light energy into chemical energy in molecules.
- Glucose is used for respiration and converted to starch (storage), cellulose (cell walls), sucrose (transport in phloem), amino acids and fats and oils.
- Plants need nitrate ions to make amino acids and magnesium ions to make chlorophyll; magnesium deficiency causes yellowing of leaves.
- The rate of photosynthesis is limited by light intensity, carbon dioxide concentration and temperature; the factor in shortest supply is the limiting factor.
- Leaves are adapted: large surface area, thin; palisade mesophyll cells near the top with many chloroplasts; spongy mesophyll with air spaces; stomata opened and closed by guard cells; xylem and phloem in veins.
- Xylem transports water and mineral ions from roots to leaves and gives support; phloem transports sucrose and amino acids from sources (e.g. leaves) to sinks (e.g. roots, fruits) – this is translocation.
- Water enters root hair cells by osmosis (large surface area), passes through the root cortex into xylem, up the stem to the leaf, into mesophyll cells, evaporates from their surfaces and diffuses out through stomata as water vapour.
- Transpiration is the loss of water vapour from leaves. Its rate increases with higher temperature, lower humidity and higher wind speed.
- If water loss is greater than water uptake, cells lose turgor and the plant wilts.
📖 Key terms
- Photosynthesis
- Making carbohydrates from raw materials using energy from light.
- Chlorophyll
- A green pigment that transfers light energy into chemical energy.
- Limiting factor
- Something present in the environment in such short supply that it restricts a process.
- Transpiration
- Loss of water vapour from leaves by evaporation and diffusion through stomata.
- Translocation
- Movement of sucrose and amino acids in phloem from sources to sinks.
- Xylem
- Tissue of dead, hollow, lignified cells that carries water and mineral ions.
- Stomata
- Pores, mainly on the underside of leaves, controlled by guard cells.
✏️ Worked example
A student puts pondweed at different distances from a lamp and counts the bubbles of gas released per minute. Results: 10 cm – 40 bubbles; 20 cm – 18 bubbles; 40 cm – 6 bubbles. Explain the results.
- The bubbles are oxygen, a product of photosynthesis, so the bubble count measures the rate.
- As the lamp moves further away, the light intensity falls.
- Fewer bubbles are produced as light intensity falls, so the rate of photosynthesis decreases.
- Light intensity is the limiting factor in this experiment.
- Answer: the rate of photosynthesis increases as light intensity increases.
On a rate graph that rises then levels off, light (or CO2) is limiting where the line rises; where it is flat, another factor (CO2, temperature) is now limiting.
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