Aerobic respiration

Cells release energy from nutrient molecules through a series of chemical reactions called aerobic respiration. These reactions use oxygen to break down the nutrient molecules.

When the nutrient is glucose, the overall reactants and products are shown by this word equation:

\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}

Glucose and oxygen are the reactants: they are used in the reactions. Carbon dioxide and water are the products: they are formed by the reactions.

Energy is released during the reactions, but it is not a substance, so it is not shown among the products in the word equation.

Anaerobic respiration

Cells can release energy from nutrient molecules without using oxygen. They do this through a series of chemical reactions called anaerobic respiration. The released energy can be used by the cell.

Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, which uses oxygen. The products of anaerobic respiration depend on the cells involved.

Anaerobic respiration in yeast

In yeast cells, glucose is broken down to produce alcohol and carbon dioxide:

\text{glucose} \rightarrow \text{alcohol} + \text{carbon dioxide}

Anaerobic respiration in muscles

During vigorous exercise, muscle cells can release energy from glucose by anaerobic respiration. In these cells, glucose is broken down to produce lactic acid:

\text{glucose} \rightarrow \text{lactic acid}

Energy transfer in cellular respiration

Living cells continuously transfer energy from nutrient molecules through cellular respiration. Respiration is an exothermic process: it transfers energy to the surroundings, including as heat.

Cells use some of the energy transferred to support living processes. For example, it supplies energy for chemical reactions that build larger molecules and for movement. Energy transferred as heat can also help organisms keep warm.

Energy from glucose

Glucose, $\ce{C6H12O6}$ , is a nutrient used in respiration. In aerobic respiration, glucose is oxidised using oxygen, $\ce{O2}$ . This process transfers energy and produces carbon dioxide, $\ce{CO2}$ , and water, $\ce{H2O}$ .

In anaerobic respiration, glucose is broken down without oxygen and is only incompletely oxidised. The products still contain some of the chemical energy that was in the glucose. As a result, anaerobic respiration transfers much less energy per glucose molecule than aerobic respiration.

Yeast fermentation in food production

Yeast cells can break down glucose without oxygen to release energy. This is anaerobic respiration. In yeast, it produces ethanol and carbon dioxide and is called fermentation.

Plant cells can also break down glucose anaerobically, producing ethanol and carbon dioxide.

Fermentation in bread and alcoholic drinks

Fermentation is economically important in food production because its products have different uses:

In bread dough, carbon dioxide forms bubbles that make the dough rise.
In alcoholic drinks, the ethanol produced by yeast forms the alcohol in the drink.