Respiration as an exothermic reaction occurring continuously in living cells, the equations for aerobic and anaerobic respiration, the differences between them, and the uses of the energy released.

What this dot point is asking

AQA wants you to describe respiration as an exothermic reaction happening continuously in all living cells, recall the equations for aerobic and anaerobic respiration in animals, plants and microorganisms, compare them, and state how organisms use the energy released.

Respiration as an exothermic reaction

A key idea AQA stresses is that respiration is not the same as breathing. Breathing (ventilation) moves air in and out of the lungs; respiration is the chemical reaction inside cells that releases energy from glucose. Cells with high energy demands, such as muscle and liver cells, contain large numbers of mitochondria, the organelles where most aerobic respiration takes place.

The energy released by respiration is used for: muscle contraction (movement), keeping the body warm in mammals and birds, building larger molecules from smaller ones (such as proteins from amino acids, or starch and cellulose from glucose in plants), and active transport of substances against a concentration gradient.

Aerobic respiration

Aerobic respiration uses oxygen and releases the most energy per glucose molecule, because the glucose is completely oxidised:

Symbol equation (higher tier): $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$.

The carbon dioxide produced in animals diffuses into the blood, is carried to the lungs, and is breathed out. In plants, aerobic respiration occurs in every living cell, day and night, even though photosynthesis only happens in the light.

Anaerobic respiration

Anaerobic respiration happens without oxygen and releases much less energy because the glucose is not fully broken down (incomplete oxidation), so most of the energy is left locked in the products.

• In muscle cells: $\text{glucose} \rightarrow \text{lactic acid}$. This lets muscles keep contracting briefly when oxygen cannot be delivered fast enough.
• In yeast and plant cells: $\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$. This is called fermentation.

Fermentation in industry. Yeast fermentation is used to make bread (the carbon dioxide makes dough rise) and alcoholic drinks such as beer and wine (the ethanol is the alcohol). This makes anaerobic respiration in microorganisms economically important.

Try this

Q1. Write the word equation for aerobic respiration. [2 marks]

• Cue. Glucose + oxygen gives carbon dioxide + water.

Q2. State two differences between aerobic and anaerobic respiration in muscle cells. [2 marks]

• Cue. Aerobic uses oxygen and releases more energy and produces carbon dioxide and water; anaerobic uses no oxygen, releases less energy and produces lactic acid.