By the end of this lesson you should be able to
- Write the word and chemical equations for photosynthesis
- Explain how the leaf is adapted for photosynthesis
- Describe the factors that limit the rate of photosynthesis
- Describe experiments on photosynthesis, including the starch test
Plants make their own food
Animals get energy by eating other organisms. Green plants make their own food, using light energy from the Sun, in a process called photosynthesis. This process is the source of food and oxygen for nearly all living things.
Key ideas
Photosynthesis is the process by which green plants make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll, and release oxygen.
Word equation:
Chemical equation:
- Chlorophyll is the green pigment that absorbs light. It is found in chloroplasts.
- Carbon dioxide enters the leaf through the stomata by diffusion.
- Water is absorbed by the roots and carried to the leaf in the xylem.
- Oxygen is released through the stomata as a by-product.
How the leaf is adapted
| Feature | How it helps photosynthesis |
|---|---|
| Broad, flat shape | Large surface area to absorb light |
| Thin | Short distance for gases to diffuse, and light reaches all cells |
| Transparent upper epidermis | Lets light pass through to the palisade cells |
| Palisade cells with many chloroplasts, near the top | Absorb most of the light |
| Air spaces in the spongy layer | Let carbon dioxide diffuse to the cells |
| Stomata, opened and closed by guard cells | Let gases in and out while controlling water loss |
| Xylem and phloem in the veins | Xylem brings water, and phloem carries sugars away |
What happens to the glucose
Plants use the glucose they make in several ways:
- Respiration: to release energy for life processes.
- Storage: changed into starch, which is insoluble and stored in leaves, stems, roots and seeds.
- Cell walls: changed into cellulose.
- Transport: changed into sucrose and carried in the phloem.
- Making other substances: with nitrate ions from the soil, glucose is used to make amino acids and then proteins. Magnesium ions are needed to make chlorophyll. Glucose is also used to make fats and oils.
Limiting factors
The rate of photosynthesis depends on several factors. The factor in shortest supply is the limiting factor, and increasing it raises the rate.
| Factor | Effect |
|---|---|
| Light intensity | Rate rises as light increases, until another factor becomes limiting |
| Carbon dioxide concentration | Rate rises as concentration increases, until another factor becomes limiting |
| Temperature | Rate rises up to an optimum (often around 25 to 35 °C). High temperatures denature the enzymes involved and the rate falls |
When a graph of rate against light intensity levels off, light is no longer the limiting factor, and another factor, such as carbon dioxide or temperature, is now limiting.
Growers use this idea in greenhouses by adding light, warmth or extra carbon dioxide to increase the crop.
Experiments
The starch test on a leaf. Starch is made from the glucose produced in photosynthesis, so a leaf that has photosynthesised contains starch.
- Dip the leaf in boiling water for about a minute. This kills the cells and softens the leaf.
- Turn off the flame. Place the leaf in a test tube of ethanol standing in hot water. The ethanol removes the chlorophyll, so the leaf becomes pale.
- Rinse the leaf in warm water to soften it.
- Add iodine solution. A blue-black colour shows starch is present. If there is no starch, it stays orange-brown.
Ethanol is flammable, so it must never be heated directly over a flame.
Investigating the factors needed.
- Light: cover part of a leaf with black paper or foil. Only the uncovered part turns blue-black.
- Chlorophyll: use a variegated leaf, which has green and white parts. Only the green parts turn blue-black.
- Carbon dioxide: put soda lime, which absorbs carbon dioxide, in a sealed container with the plant. The leaf gives a negative starch test.
Before these tests the plant is kept in the dark for 24 to 48 hours to destarch it, so that any starch found was made during the experiment.
Measuring the rate with pondweed. Water plants release bubbles of oxygen. A lamp is placed at different distances from the plant, and the bubbles are counted for one minute.
| Distance of lamp (cm) | 10 | 20 | 30 | 40 |
|---|---|---|---|---|
| Bubbles per minute | 36 | 16 | 8 | 5 |
The rate falls as the lamp moves away, because the light intensity falls, so light is the limiting factor.
Percentage decrease from 10 cm to 20 cm:
Common mistakes
Saying plants “breathe in” carbon dioxide only. Plants photosynthesise in light, but they respire all the time, day and night, so they also use oxygen and give out carbon dioxide.
Saying photosynthesis “makes energy”. Light energy is changed into chemical energy stored in glucose.
Heating ethanol over a flame. Always use a water bath, because ethanol is flammable.
Forgetting to say “limiting factor”. A graph that levels off means another factor is now limiting. It does not mean that photosynthesis has stopped.
Practice questions
- Write the word equation for photosynthesis.
- Name the pigment that absorbs light, and say where in the cell it is found.
- State two adaptations of a leaf for photosynthesis.
- Why is a plant kept in the dark for 24 to 48 hours before a starch test?
- In the starch test, why is the leaf placed in hot ethanol?
- The bubble count of a pondweed plant is 36 per minute with the lamp at 10 cm and 16 per minute at 20 cm. Explain the change and calculate the percentage decrease.
- On a graph, the rate of photosynthesis rises with light intensity and then levels off. Explain why.
- Name the mineral ions needed to make (a) amino acids and (b) chlorophyll.
Show answers
- Carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
- Chlorophyll, found in the chloroplasts.
- Any two of: broad, flat shape for a large surface area, thin for a short diffusion distance, many chloroplasts in the palisade cells, stomata for gas exchange, air spaces, veins to supply water and remove sugars.
- To destarch it, so that any starch found in the leaf was made during the experiment.
- Ethanol removes the green chlorophyll, so the colour change with iodine can be seen clearly.
- As the lamp moves further away, the light intensity falls, so the rate of photosynthesis falls, because light is the limiting factor. Percentage decrease .
- Another factor, such as carbon dioxide concentration or temperature, has become the limiting factor, so more light does not increase the rate.
- (a) Nitrate ions. (b) Magnesium ions.
Summary
- Photosynthesis makes glucose and oxygen from carbon dioxide and water, using light absorbed by chlorophyll.
- The leaf is adapted to absorb light, let gases in, and carry water and sugars.
- The rate depends on light intensity, carbon dioxide concentration and temperature. The factor in shortest supply is the limiting factor.
- Glucose is used for respiration, stored as starch, and used to make cellulose and other substances.
Now practise this topic
Real questions show you what still needs work. Try a past paper under timed conditions, or take a short quiz.