Respiration » Aerobic and Anaerobic Respiration

Respiration: energy for life

Every living cell needs energy, all of the time. Plants, animals and microorganisms all get that energy from respiration. Respiration is not the same as breathing. Breathing moves air in and out of the lungs. Respiration is a chemical reaction that happens inside cells.

In respiration, glucose is broken down and energy is transferred. This is an exothermic reaction, which means it transfers energy to the surroundings. Some of that energy is released as heat, which is why your body feels warm.

Respiration is also continuously occurring in living cells. It never stops while the organism is alive. The energy it transfers supplies all the energy needed for living processes, so a cell that stops respiring soon stops working.

Key terms:

• Respiration: the reaction in cells that transfers energy from glucose.
• Exothermic: a reaction that transfers energy to the surroundings.

What do organisms use the energy for?

Organisms need the energy from respiration for three main jobs:

Remember that photosynthesis is endothermic and takes energy in. Respiration is the opposite. It is exothermic and transfers energy out. Plants do both: they photosynthesise in the light and respire all the time, day and night.

Aerobic and anaerobic respiration

Respiration can happen in two ways:

• Aerobic respiration uses oxygen.
• Anaerobic respiration happens without oxygen.

Both transfer energy from glucose. They do not give the same products or the same amount of energy.

Key terms:

• Aerobic respiration: respiration that uses oxygen to transfer energy from glucose.
• Anaerobic respiration: respiration without oxygen, which transfers energy from glucose.

Aerobic respiration

Aerobic respiration is the main way most organisms transfer energy. Glucose reacts with oxygen. The products are carbon dioxide and water.

You must recognise the chemical symbols:

Put together, the symbol equation is:

This is the reverse of the photosynthesis equation. Photosynthesis makes glucose and oxygen. Aerobic respiration uses them up.

Anaerobic respiration

If there is no oxygen, cells can still transfer some energy from glucose. The glucose is only partly broken down, so it is not fully oxidised. The products depend on the type of cell.

In muscles

In muscle cells, glucose is changed to lactic acid. No carbon dioxide is made.

In plant and yeast cells

In plant cells and yeast cells, glucose is changed to ethanol and carbon dioxide.

Key terms:

• Lactic acid: the product of anaerobic respiration in muscle cells.
• Ethanol: the alcohol made by anaerobic respiration in plant and yeast cells.

Comparing aerobic and anaerobic respiration

The oxidation of glucose is incomplete in anaerobic respiration. Because the glucose is only partly broken down, much less energy is transferred than in aerobic respiration.

A good way to remember it: aerobic respiration finishes the job and gets all the energy out. Anaerobic respiration stops half way, so energy is still left in the products.

Fermentation and industry

Anaerobic respiration in yeast cells is called fermentation. It has economic importance because it is used in the manufacture of bread and alcoholic drinks.

• Bread: yeast in the dough ferments sugar. The carbon dioxide forms bubbles that make the dough rise.
• Alcoholic drinks: yeast ferments sugar in a liquid such as grape juice. The ethanol made is the alcohol in the drink.

Ethanol and carbon dioxide are made in both cases, as the equation shows. Bakers want the carbon dioxide. Brewers want the ethanol.

Key term:

• Fermentation: anaerobic respiration in yeast cells.

Exam-style question

(a) Write the word equation for aerobic respiration. [2 marks]

(b) Yeast is used to make bread. Name the process that produces carbon dioxide in the dough, and explain why this is useful in baking. [2 marks]

(c) Compare aerobic and anaerobic respiration in muscle cells. [3 marks]