Respiration

Respiration is the chemical reaction that releases energy from glucose. It happens all the time in every living cell, in plants and animals alike, and the energy runs everything a cell does: moving, growing, keeping warm and making new molecules. It is not the same as breathing.

Key facts

  • Aerobic respiration (with oxygen): glucose + oxygen → carbon dioxide + water. It releases a lot of energy and happens in the mitochondria.
  • Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Respiration is exothermic: it releases energy.
  • Anaerobic respiration (without oxygen) in muscles: glucose → lactic acid. It releases much less energy.
  • In plant cells and yeast, anaerobic respiration is glucose → ethanol + carbon dioxide. In yeast this is called fermentation, used to make bread and alcoholic drinks.
  • During exercise, heart rate and breathing rate rise to bring more oxygen and glucose to the muscles and remove carbon dioxide.
  • In hard exercise the muscles respire anaerobically and lactic acid builds up. The oxygen needed afterwards to remove it is the oxygen debt.
  • Metabolism is the sum of all the reactions in a cell or the body; respiration provides the energy for many of them.

Worked examples

Example 1

Write the word equation for aerobic respiration, and say where in the cell it happens.

  1. Glucose comes from food (or, in plants, from photosynthesis); oxygen comes from the air.
  2. They react, releasing energy, and make carbon dioxide and water. glucose + oxygen → carbon dioxide + water
  3. Aerobic respiration happens in the mitochondria. It is the reverse of photosynthesis.

Answer: glucose + oxygen → carbon dioxide + water; in the mitochondria

Example 2

Compare aerobic and anaerobic respiration in human muscles.

  1. Aerobic: glucose + oxygen → carbon dioxide + water.
  2. Anaerobic: glucose → lactic acid, with no oxygen needed.
  3. Aerobic respiration releases much more energy from each glucose molecule. Anaerobic is used for short bursts, when oxygen cannot arrive fast enough.

Answer: aerobic needs oxygen, makes carbon dioxide and water, releases much more energy; anaerobic needs no oxygen, makes lactic acid, releases less energy

Example 3

Why does bread dough rise when yeast is added?

  1. Yeast is a fungus. Kept warm in dough, it respires the sugars.
  2. Without enough oxygen it respires anaerobically. glucose → ethanol + carbon dioxide
  3. Bubbles of carbon dioxide are trapped in the dough, so it rises. The ethanol evaporates when the bread is baked.

Answer: yeast respires anaerobically (fermentation) and makes carbon dioxide gas, which gets trapped in the dough

Example 4

In a respirometer, some germinating peas take in 3.6 cm³ of oxygen in 12 minutes. What is their rate of respiration?

  1. Germinating seeds respire quickly, using oxygen.
  2. Rate = volume of oxygen used ÷ time. 3.6 ÷ 12 = 0.3
  3. The rate of respiration is 0.3 cm³ of oxygen per minute.

Answer: 0.3 cm³ of oxygen per minute