Do plants breathe

A potato in a dark cupboard

A potato forgotten in a dark cupboard sprouts. The sprouts have no light, so they cannot make food. Yet they grow. Where does the energy for the growth come from?

It comes from the food stored in the potato. The cells break that food down and use the energy that it holds. This breaking down of food inside a cell is called cellular respiration.

What is cellular respiration?

Cellular respiration is the breakdown of food materials inside the cell, in steps, to release energy and to trap that energy in the molecule ATP. ATP is short for adenosine triphosphate. It is the energy currency of the cell: the cell earns ATP in respiration and spends it on growth, on taking in minerals and on every other work.

The food that is broken down is called the respiratory substrate. These are the usual substrates.

• Carbohydrates. This is the most common substrate, and glucose is the favourite one.
• Fats and proteins. They are used when carbohydrates are short.
• Organic acids. They are used by some tissues.

Respiration does not make food. It uses food that photosynthesis made earlier.

Do plants breathe?

An animal has lungs, gills or a windpipe. A plant has no organ for gas exchange. Does a plant breathe at all?

It does exchange gases, and each part of the plant looks after itself.

• Leaves exchange gases through the stomata.
• Woody stems exchange gases through the lenticels, small openings in the bark.
• Roots take oxygen from the air that lies between the soil particles.

Three facts make this enough for a plant.

• Each part of the plant takes care of its own gas exchange, and very little gas moves from one part to another.
• Plants do not need much gas exchange. The rate of respiration in a plant is much lower than in an animal of the same mass.
• The distance that a gas has to travel is short. Even in a tall stem, the living cells lie in thin layers near the surface, and in a leaf almost every cell is near a stoma.

During the day, the oxygen that photosynthesis releases inside a leaf is used by the cells of the leaf. The carbon dioxide of respiration is used in photosynthesis. Yet respiration never stops, day or night. A cell that stops respiring is dead.

Burning and respiring

Glucose can be burnt in a flame. It can also be respired in a cell. The overall change is the same.

One mole of glucose gives 2870 kJ of energy. The cell does not let it out in a rush. If all of it were released at once, the cell would be cooked. So the cell takes the glucose apart in many small steps, and each step is controlled by an enzyme. In some of the steps, a part of the energy is trapped in ATP. One mole of ATP holds about 30.5 kJ.

The steps are oxidations. An oxidation takes electrons, with hydrogen, away from a compound. Two carriers hold these electrons: NAD$^+$ becomes NADH + H$^+$, and FAD becomes FADH$_2$. They will carry the energy to the next stage of respiration.

The three stages

Respiration begins in the cytoplasm and goes one of two ways.

• Glycolysis splits one glucose molecule into two molecules of pyruvic acid. It happens in the cytoplasm of every living cell, and it does not need oxygen.
• Fermentation follows glycolysis when there is no oxygen. Pyruvic acid is changed into ethanol and carbon dioxide, or into lactic acid.
• Aerobic respiration follows glycolysis when oxygen is present. Pyruvic acid goes into the mitochondria, where it is oxidised completely to carbon dioxide and water.

This course covers glycolysis and fermentation. The next course follows the pyruvic acid into the mitochondria.

You should find that even the best case keeps only about two fifths of the energy of the glucose in ATP. The rest is let out as heat, which keeps a living body warm. With 2 ATP, as in fermentation, the cell keeps only a very small share.

Try it in Python

The program works out the share of the energy of glucose that is kept in ATP, in fermentation and in aerobic respiration.

Practise with new numbers

Common mistakes

• Saying that respiration is breathing. Breathing is the exchange of gases. Respiration is the breakdown of food in the cells.
• Saying that plants respire only at night. A plant respires all the time. In the day the oxygen of photosynthesis hides this.
• Thinking that all the energy of glucose goes into ATP. Only part is trapped. The rest is heat.
• Calling ATP a long-term store of energy. It is the currency that is spent soon after it is earned. Starch and fat are the stores.
• Saying that a plant has no way to exchange gases. It has stomata, lenticels and the root surface.

Remember these

• Cellular respiration: stepwise breakdown of food in the cell, with the energy trapped in ATP.
• Plants have no special organs. Gases enter through stomata, lenticels and roots.
• One mole of glucose holds 2870 kJ. One mole of ATP holds about 30.5 kJ.
• Glycolysis is in the cytoplasm. Fermentation is without oxygen. Aerobic respiration is in the mitochondria.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the three places through which the parts of a plant exchange gases.

2. Why can a plant do without a respiratory organ like a lung?

3. What is the use of ATP in a cell?

4. Why is the energy of glucose released in many small steps and not at once?

5. Where does glycolysis take place?