Respiration
How cells release the energy in nutrient molecules, with or without oxygen.
Uses of Energy
Living organisms need energy to stay alive. The energy comes from respiration, which takes place in every living cell.
| Use of energy | What happens |
|---|---|
| Muscle contraction | Muscles shorten, causing movement |
| Protein synthesis | Cells build proteins |
| Cell division | Cells divide to make new cells |
| Active transport | Substances are moved across membranes |
| Growth | Organisms increase in size |
| Passage of nerve impulses | Messages travel along nerves |
| Maintenance of a constant body temperature | Heat is released to keep the body warm |
Worked example (naming the use): A student runs for a bus. Name the use of energy in the leg muscles. Step 1. The leg muscles shorten to move the legs. Step 2. This use of energy is muscle contraction. Answer: Muscle contraction.
Respiration goes on all the time
Respiration takes place in every living cell, all the time, not only during exercise. Cells that are very active, such as muscle cells, release energy at a faster rate than cells that are resting. The energy is used for the processes in the table above.
Worked example (naming a use): A mammal keeps its body at the same temperature on a cold day. Name the use of energy involved. Step 1. The body releases heat from respiration to stay warm. Step 2. This use of energy is the maintenance of a constant body temperature. Answer: Maintaining a constant body temperature.
Respiration in Yeast
Yeast is a single-celled organism. In a solution of glucose it respires and releases carbon dioxide, which can be seen as bubbles.
Method
Put equal volumes of yeast and glucose solution into tubes, and place each tube in a water bath at a different temperature, for example from 20 to 60 °C. Leave the tubes for a few minutes to reach the temperature. Count the bubbles of gas released in the same time, for example one minute, and repeat to find a mean. Keep everything except the temperature the same.
Result
As the temperature rises, the rate of respiration increases up to a highest rate. At higher temperatures the rate falls.
Aerobic Respiration
In aerobic respiration, nutrient molecules such as glucose are broken down using oxygen, releasing energy.
| Substance | Where it comes from or goes |
|---|---|
| Glucose | From digested food, carried in the blood |
| Oxygen | From the air, carried in the blood |
| Carbon dioxide | Released and removed by the lungs |
| Water | A product of the reactions |
ExtendedBalanced chemical equation
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
One molecule of glucose reacts with six molecules of oxygen to give six molecules of carbon dioxide and six molecules of water.
Worked example (checking the balance): Show that the balanced equation has the same number of oxygen atoms on each side. Step 1. Left side: 6 in glucose plus 6 × 2 = 12 in oxygen gives 18. Step 2. Right side: 6 × 2 = 12 in carbon dioxide plus 6 in water gives 18. Answer: 18 = 18.
Anaerobic Respiration
In anaerobic respiration, nutrient molecules are broken down without oxygen. It releases much less energy per glucose molecule than aerobic respiration.
| Organism | Word equation |
|---|---|
| Yeast | glucose → alcohol + carbon dioxide |
| Muscles (vigorous exercise) | glucose → lactic acid |
ExtendedBalanced equation in yeast
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
One glucose molecule gives two molecules of alcohol (ethanol) and two molecules of carbon dioxide.
Worked example (choosing the equation): Which products form when muscles respire anaerobically during vigorous exercise? Step 1. The word equation for muscles is glucose gives lactic acid. Step 2. No carbon dioxide is formed in this equation. Answer: Lactic acid.
Comparing Respiration
Both kinds of respiration break down nutrient molecules such as glucose to release energy, but they differ in several ways.
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen | Used | Not used |
| Energy released per glucose | Much more | Much less |
| Products in yeast | Carbon dioxide and water | Alcohol and carbon dioxide |
| Products in muscles | Carbon dioxide and water | Lactic acid |
Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration. Aerobic respiration is the main way that cells release energy.
Worked example (comparing the two): Give one similarity and one difference between aerobic respiration and anaerobic respiration in muscles. Step 1. Similarity: both break down glucose to release energy. Step 2. Difference: aerobic uses oxygen and releases much more energy; anaerobic does not use oxygen. Answer: Same fuel; oxygen and energy differ.
The Oxygen Debt
ExtendedHow the debt arises
During vigorous exercise the muscles also respire anaerobically. Lactic acid builds up in the muscles and blood, causing an oxygen debt.
ExtendedHow the debt is removed
(a) The heart rate stays fast, to transport lactic acid in the blood from the muscles to the liver.
(b) Breathing stays deeper and faster, to supply oxygen for aerobic respiration of the lactic acid.
(c) The lactic acid is broken down by aerobic respiration in the liver.
Worked example (explaining recovery): Explain why a runner keeps breathing heavily after a race has ended. Step 1. Lactic acid built up in the muscles during anaerobic respiration. Step 2. Oxygen is needed to break the lactic acid down by aerobic respiration in the liver. Step 3. Deeper, faster breathing supplies this oxygen. Answer: Repaying the oxygen debt.
Exam advice
Common mistakes
Model answer
Recall checklist
- Define respiration and list the uses of energy.
- Describe the yeast investigation and its result.
- Define aerobic respiration; give its word equation.
- Define anaerobic respiration; give both word equations.
- Compare the energy released by the two kinds.
- Write the balanced equations (Extended)
- State that lactic acid builds up and causes an oxygen debt (Extended)
- Outline how the oxygen debt is removed (Extended)
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