Photosynthesis

Biology · Plant nutrition. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Photosynthesis is how plants make carbohydrates from carbon dioxide and water, using energy from light. It happens in chloroplasts, which contain the green pigment chlorophyll. Chlorophyll transfers light energy into chemical energy stored in glucose. Oxygen is given off.

The glucose is then used in several ways: for respiration, changed to starch for storage, to cellulose for cell walls, or to sucrose for transport around the plant.

The rate depends on light intensity, carbon dioxide concentration and temperature. The one that is in shortest supply holds the rate back and is called the limiting factor. Giving more of any other factor makes no difference.

Rules to remember

carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Uses of glucose: respiration (energy), starch (energy store), cellulose (cell walls), sucrose (transport in the phloem).
Starch test on a leaf: iodine solution turns blue-black where photosynthesis has taken place.
Destarch a plant first by keeping it in the dark for 48 hours, so any starch found afterwards was made during the experiment.
If the rate rises when a factor is increased, that factor was limiting. Where the graph is level, a different factor is limiting.
Temperature affects the rate because photosynthesis is controlled by enzymes.

Common mistake

On a graph of rate against light intensity, saying that light is still the limiting factor where the line is flat. On the flat part more light has no effect, so something else, such as carbon dioxide concentration or temperature, is limiting.

Worked example

Pondweed in bright light at 25 °C gives off 12 bubbles of gas per minute. Adding more light does not change this. When sodium hydrogencarbonate is dissolved in the water, the rate rises to 30 bubbles per minute. What was the limiting factor at first?

More light gave no increase, so light intensity was not limiting.
Sodium hydrogencarbonate supplies extra carbon dioxide to the water.
The rate rose from 12 to 30 bubbles per minute when carbon dioxide was added.
A factor that raises the rate when it is increased was the limiting factor.

Answer: Carbon dioxide concentration was the limiting factor.

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Nutrition in Plants

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