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Anaerobic respiration

Breaking down glucose (food) without oxygen to provide available energy for the cells.


The amount of a substance (solute) in a solution


Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse


A distinct part of the cell, such as the nucleus, ribosome or mitochondrion, which has structure and function.


Reusable protein molecules which act as biological catalysts, changing the rate of chemical reactions in the body without being affected themselves


A type of sugar: a mono saccharide with 6 carbon atoms (a hexose sugar).


A large organ in the upper abdomen which manufactures, stores and breaks down substances as required by the body

Adenosine triphosphate

Molecule which acts as the common energy currency in all cells, providing the energy needed to drive chemical reactions in cells.

Cellular respiration

All cells need glucose and oxygen for respiration. The glucose is broken down into carbon dioxide and water and this makes energy available for all of the other reactions which take place in your cells. The energy source for cells is the molecule ATP. The energy in the chemical bonds in this molecule is used in other chemical reactions in the cells, building up large molecules or breaking them down. The most efficient way of producing ATP is during aerobic respiration:

glucose + oxygen -> carbon dioxide + water + energy

The chemical equation for this process is:

C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy (ATP)

Sometimes there is not enough oxygen to supply the needs of the cells for aerobic respiration. As a result cells undergo anaerobic respiration. The glucose is not broken down completely and so much less energy is released. In yeast cells the waste product of anaerobic respiration is ethanol, but in animal cells it is lactic acid.

glucose -> lactic acid + energy

Once the oxygen concentration in the cells goes up again, the lactic acid is broken down in aerobic respiration to produce carbon dioxide, water and more ATP.

lactic acid + oxygen -> carbon dioxide + water + energy
Amoeba Butterfly Hod Hill Gang

Almost all types of living organisms depend on aerobic respiration for most of their energy.
(Middle and right photo credit: Anthony Short)

Cells and mitochondria

Aerobic respiration takes place in the mitochondria, organelles which are found in every cell. They contain the enzymes which catalyse the breakdown of glucose using oxygen to produce carbon dioxide and water. This happens in a series of reactions which drive the production of ATP. The inside of a mitochondrion is adapted so it has a very large surface area. This makes it possible for as much aerobic respiration as possible to take place.

The structure of a mitochondrion is closely related to its structure

The structure of a mitochondrion is closely related to its structure

The number of mitochondria in a cell gives a good indication of how active the cell is. Cells that make lots of chemicals such as hormones or enzymes usually have many mitochondria. Cells that are relatively inactive have many fewer.

Comparison between a liver cell (hepatocyte) and a simple skin (epithelial) cell

Comparison between a liver cell (hepatocyte) and a simple skin (epithelial) cell