By the end of this lesson you should be able to
- Define respiration and write equations for aerobic and anaerobic respiration
- Compare aerobic and anaerobic respiration in animals and yeast
- Explain oxygen debt and the effects of exercise on breathing and heart rate
- Describe simple experiments to show that respiration releases carbon dioxide and heat
Why cells respire
Every living cell needs energy for movement, growth, making new substances, keeping warm and transporting materials. This energy comes from food, mainly glucose, and it is released in a process called respiration. Respiration is not the same as breathing. Breathing brings oxygen to the body and removes carbon dioxide. Respiration is the chemical process inside cells that releases energy.
Key ideas
Respiration is the chemical reaction in all living cells that releases energy from glucose. It takes place in the mitochondria (aerobic stages) and in the cytoplasm.
Aerobic respiration (uses oxygen):
glucose + oxygen → carbon dioxide + water + energy
The energy released is used to build a chemical called ATP, which cells use as a store of usable energy. Some of the energy is also released as heat, which helps warm-blooded animals to keep their body temperature constant.
Anaerobic respiration takes place without oxygen and releases much less energy from each molecule of glucose, because the glucose is only partly broken down.
- In animal cells (for example, muscles): glucose → lactic acid + a small amount of energy.
- In yeast (fermentation): glucose → ethanol (alcohol) + carbon dioxide + a small amount of energy.
Yeast fermentation is used to make bread (the carbon dioxide makes the dough rise) and to make alcoholic drinks.
| Feature | Aerobic respiration | Anaerobic respiration |
|---|---|---|
| Oxygen needed | Yes | No |
| Glucose breakdown | Complete | Partial |
| Products in animals | Carbon dioxide and water | Lactic acid |
| Products in yeast | Carbon dioxide and water | Ethanol and carbon dioxide |
| Energy released | Large amount | Small amount |
| Where | Mainly mitochondria | Cytoplasm |
Exercise and oxygen debt
During hard exercise, the muscles use oxygen faster than the blood can supply it, so the muscle cells also respire anaerobically and make lactic acid. Lactic acid builds up and causes muscle fatigue and sometimes cramp. After exercise you keep breathing deeply and quickly for a while. You are taking in the extra oxygen needed to break down the lactic acid. This extra oxygen is called the oxygen debt.
Exercise also makes the breathing rate and heart rate increase, because muscles need more oxygen and glucose, and the carbon dioxide they make must be removed quickly.
Respiration in plants
Plants respire all the time, in the day and at night, in every living cell. In daylight, photosynthesis is usually faster than respiration, so plants take in carbon dioxide and give out oxygen overall. At night, when there is no photosynthesis, they only respire and give out carbon dioxide.
| Photosynthesis | Respiration | |
|---|---|---|
| Energy | Absorbed (light) | Released |
| Gases | Uses carbon dioxide, releases oxygen | Uses oxygen, releases carbon dioxide |
| When | In light, in cells with chloroplasts | All the time, in all living cells |
| Glucose | Made | Broken down |
Investigating respiration
Testing for carbon dioxide. Carbon dioxide turns limewater (calcium hydroxide solution) from clear to cloudy (milky). To show that we breathe out carbon dioxide, we blow through a tube into limewater and it turns cloudy. Air we breathe in has much less carbon dioxide, so limewater stays clear for longer.
Showing that yeast releases carbon dioxide. Mix yeast with a warm glucose solution in a flask, and connect the flask by a delivery tube to limewater. After some time, bubbles pass through and the limewater turns cloudy. A control flask with boiled (dead) yeast produces no bubbles, which shows that living yeast is responsible.
Showing that respiration releases heat. Soak seeds in water so they begin to germinate. Put one group in a vacuum flask with a thermometer, and boiled (dead) seeds, treated with disinfectant, in a second flask as a control. After a day or two, the temperature in the flask with the living seeds is higher. Heat is released by respiration.
Interpreting exercise data. A learner’s pulse was measured before, during and after exercise.
| Time | Resting | End of exercise | 3 minutes after | 6 minutes after |
|---|---|---|---|---|
| Pulse (beats per minute) | 72 | 148 | 110 | 80 |
- The pulse rises from 72 to 148 during exercise, because the muscles need more oxygen and glucose.
- It falls gradually afterwards as the oxygen debt is repaid.
- It is not yet back to normal at 6 minutes (80 compared with 72), so recovery takes time.
Increase in pulse: beats per minute. Percentage increase: .
Common mistakes
Confusing respiration with breathing. Breathing is a physical process that moves air in and out of the lungs. Respiration is a chemical reaction in cells.
Saying that respiration “makes” energy. Energy cannot be created. Respiration releases energy stored in glucose.
Saying that plants only respire at night. They respire all the time. It is only overall gas exchange that differs by day and night.
Writing that anaerobic respiration produces alcohol in humans. In humans and other animals it produces lactic acid. Ethanol is made in yeast and some plant cells.
Practice questions
- Define respiration.
- Write the word equation for aerobic respiration.
- Name the two products of anaerobic respiration in yeast.
- Explain why anaerobic respiration releases less energy than aerobic respiration.
- Explain why you keep breathing hard for some time after running fast.
- State how limewater is used to test for carbon dioxide, and the result.
- Give two differences between respiration and photosynthesis.
- A pulse rate rises from 70 to 140 beats per minute during exercise. Calculate the percentage increase.
Show answers
- Respiration is the chemical reaction in all living cells that releases energy from glucose.
- Glucose + oxygen → carbon dioxide + water + energy.
- Ethanol and carbon dioxide.
- In anaerobic respiration the glucose is only partly broken down, so much of the energy remains stored in the products (lactic acid or ethanol).
- During exercise the muscles respired anaerobically and produced lactic acid. Extra oxygen is needed to break down the lactic acid, so you must keep breathing quickly to repay the oxygen debt.
- Carbon dioxide is bubbled through limewater, and the limewater turns cloudy (milky).
- Any two of: photosynthesis absorbs energy and respiration releases it; photosynthesis uses carbon dioxide and releases oxygen while respiration does the opposite; photosynthesis happens only in light in cells with chloroplasts while respiration happens all the time in all living cells; photosynthesis makes glucose while respiration breaks it down.
- Increase . Percentage increase .
Summary
- Respiration releases energy from glucose in all living cells, all the time.
- Aerobic respiration uses oxygen and releases much energy. Anaerobic respiration releases little energy and produces lactic acid (animals) or ethanol and carbon dioxide (yeast).
- Hard exercise causes an oxygen debt, and faster breathing and heart rate.
- Limewater turns cloudy with carbon dioxide.
Now practise this topic
Real questions show you what still needs work. Try a past paper under timed conditions, or take a short quiz.