Aerobic vs Anaerobic Respiration

Cells can respire aerobically or anaerobically. Both processes break down glucose and transfer energy, but they differ in whether oxygen is used, how completely glucose is broken down, the products formed and the amount of energy transferred.

Aerobic respiration

When sufficient oxygen is available, cells usually respire aerobically:

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

The balanced symbol equation is:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}$

In aerobic respiration, glucose is completely broken down. It therefore transfers much more energy from each molecule of glucose than anaerobic respiration does. Most of its stages occur in the mitochondria.

Plants as well as animals carry out aerobic respiration. Photosynthesis stores energy in glucose, while respiration transfers energy from glucose for cellular processes; the two processes are not alternatives.

Anaerobic respiration in animals

If working muscles cannot receive oxygen quickly enough during intense exercise, they can transfer additional energy by anaerobic respiration. This occurs alongside aerobic respiration and does not require oxygen:

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

Glucose is only partially broken down, so considerably less energy is transferred per molecule of glucose than in aerobic respiration. No carbon dioxide is made in this animal pathway. Anaerobic respiration can therefore support a high energy demand for a short time, but it cannot sustain activity as effectively as aerobic respiration.

Anaerobic respiration in plants and yeast

Anaerobic respiration has different products in plant and yeast cells:

$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$

This pathway is called fermentation. It also breaks glucose down incompletely and transfers much less energy per glucose molecule than aerobic respiration. In bread-making, carbon dioxide from yeast fermentation forms bubbles that make dough rise; in brewing, ethanol is the desired product.

Comparing the processes directly

The central connection is the extent of glucose breakdown. With oxygen, glucose can be broken down completely and much more energy is transferred. Without oxygen, glucose is only partially broken down, leaving energy stored in lactic acid or ethanol, so less energy is transferred.