Photosynthesis and aerobic respiration

Photosynthesis: making food from light

Photosynthesis is the process by which green plants, algae and some bacteria make glucose using light energy. Plants are producers: they make their own food, and nearly every food chain starts with them.

The word equation is:

Why is each part needed?

• Carbon dioxide enters the leaf from the air through tiny pores called stomata. It supplies the carbon for glucose.

• Water is absorbed from the soil by the roots and carried up to the leaves.

• Light supplies the energy. Light energy is changed into chemical energy stored in glucose.

• Chlorophyll, the green pigment, absorbs the light. It is found in the chloroplasts, which are where photosynthesis happens.

The balanced equation and energy

In symbols, photosynthesis is:

An equation is balanced when each element has the same number of atoms on both sides. Count them:

The energy is not an atom, so it does not appear in the count. It is written above the arrow.

Testing a leaf for starch

Plants store extra glucose as starch. If a leaf contains starch, it has been photosynthesising. The standard investigation is:

1. Destarch the plant by keeping it in darkness for 48 hours, so any starch already present is used up. Then any starch found later was made during the experiment.

2. Cover part of a leaf with black card and place the plant in bright light for several hours.

3. Dip the leaf in boiling water to kill it and soften the cell walls.

4. Soak it in hot ethanol to remove the green chlorophyll. (Ethanol is flammable, so heat it in a hot water bath, never over a flame.) Without this step the green colour would hide any colour change.

5. Rinse in water and add iodine solution. Starch turns it from brown/orange to blue-black.

To show that carbon dioxide is needed, keep one plant with its leaf in a sealed container with a chemical that absorbs CO₂ (such as soda lime), and a control plant with normal air. Test both leaves for starch: only the control should turn blue-black. Everything else, such as light and water, must be kept the same.

Factors that limit photosynthesis

The rate of photosynthesis depends on the light intensity, the carbon dioxide concentration and the temperature. Water is also needed, but it is rarely short in Ireland. The factor in shortest supply is the limiting factor: increasing it speeds up the reaction, while increasing the others does not help.

A common investigation counts bubbles of oxygen given off by pondweed at different distances from a lamp. The bubbles come from oxygen made by photosynthesis. Light intensity falls as the lamp moves away.

Each time the distance doubles, the number of bubbles halves. The farther the lamp, the lower the light intensity and the slower the photosynthesis. In this data, the rate falls by half each time the distance doubles.

Other factors that limit the rate are carbon dioxide concentration and temperature. Very high temperatures damage the enzymes in the plant and the rate falls.

Aerobic respiration

Plants make glucose, which stores chemical energy. Animals get glucose by eating plants or other animals, so energy flows along every food chain, with some lost as heat at each step. To use the energy in glucose, a cell must release it by respiration. This energy is used for movement, growth, keeping warm and making new substances. Do not confuse it with breathing, which is only how gases get to the cells.

Most of the energy is released in the mitochondria. Some of the early steps happen in the cytoplasm. Plants have mitochondria too, so plant cells also respire.

Compare the two equations. Respiration is almost the reverse of photosynthesis: the products of one are the raw materials of the other. Photosynthesis stores energy, and respiration releases it.

Anaerobic respiration and exercise

When oxygen is in short supply, some cells can release a smaller amount of energy from glucose without it. This is anaerobic respiration. It releases much less energy per glucose molecule than aerobic respiration.

In yeast (used in baking and brewing):

In dough, the CO₂ forms bubbles that make it rise. The ethanol evaporates during baking.

In muscle cells during hard exercise:

During exercise, the breathing rate and heart rate increase to supply more oxygen and glucose to the muscles and to remove CO₂ faster. A runner's oxygen use might rise from $3.0\ \text{dm}^3$ to $45\ \text{dm}^3$ per minute.

If the oxygen supply cannot keep up, lactic acid builds up in the muscles and causes fatigue and cramps. Afterwards the person keeps breathing heavily to take in extra oxygen. This oxygen is used to break down the lactic acid, mainly in the liver. This is called repaying the oxygen debt.

A linked cycle

The products of one process are the raw materials of the other:

Photosynthesis removes CO₂ from the air and adds O₂. Respiration does the opposite. Together they help keep the levels of these gases fairly steady. This is part of the carbon cycle: carbon moves from CO₂ in the air into glucose in plants, then to animals that eat them, and returns to the air as CO₂ through respiration.

Now think about a sealed container with a plant in a well-lit room. In daylight, photosynthesis is faster than respiration, so CO₂ falls. At night there is no photosynthesis, but respiration continues, so CO₂ rises. At the compensation point the rate of photosynthesis equals the rate of respiration, so there is no net change in CO₂ concentration: the level stays constant. If you cover the container with black cloth during the day, photosynthesis stops and only respiration remains, so CO₂ rises as at night.

Back to the start: the pondweed gives off bubbles of oxygen in the light because it is photosynthesising. In the dark there is no light energy, so photosynthesis stops and no oxygen bubbles are made (the plant still respires).

Summary