Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.


Aerobic respiration

Respiration is a process which releases energy inside each of the body's cells.

It is not, as many people think, simply breathing - but see below.

Equation for the reaction

(3 sections below)
Reactants i.e. needed  to take part in the process

From digestion
of food, especially
+       oxygen

From air breathed in
Products i.e.  made by the process
arrow carbon dioxide

Into air breathed out
+     water

Left in cell/blood/
breathed out as vapour
Not "made" but released from molecules of reactant
+     energy

More efficient than anaerobic respiration

Energy is trapped in the molecular structure of ATP
Delivered to cells in bloodstream
Removed from cells in blood stream
Used to power all the cell's processes-
movement, electrical activity, synthesis
The energy is actually contained in the bonds between the atoms of the glucose molecule C6H12O6 which is the basic "fuel" for most cells in the body.
This comes from carbohydrates in food which are processed by the digestive system, absorbed into the blood and passed around the body.

The energy release is most efficient when the glucose is oxidised using oxygen derived from air, producing CO2 and H2O which are much simpler compounds.

Aerobic respiration is an almost universal process - carried out by most animals and plants.

It consists of several stages, the first of which is shared with anaerobic respiration and takes place in the cytoplasm of cells. The purely aerobic reactions take place inside mitochondria, small specialised organelles within the cytoplasm of all body cells. More active cells have more mitochondria.
Click for more information about the glycolysis, link reaction, Krebs/citric acid cycle and electron transport processes.

Green plants carry out respiration 24 hours of the day, but in the light it is masked by photosynthesis which seems to put it in reverse.

Some organisms can also perform anaerobic repiration as a less efficient alternative.

In the bodies of most (higher) animals, aerobic respiration is assisted by muscular movement performed by the breathing system (also known as the respiratory system) and the circulatory system, but at the single cell level diffusion takes over.

Respiratory Movements

based on humans - minor differences in other animals
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More detail
Breathing -

** advanced organisms only
Forced movement of air
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Movement of chest & diaphragm

18 per min?
From air outside body into alveoli in lungs,then out again
(Air movement: in and out)
More detail
Some of the oxygen in air breathed in dissolves and passes into blood, and most of the carbon dioxide in the blood passes into the air to be breathed out


** advanced organisms only
Forced movement of blood
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Contractions of heart

72 per min?
In blood from lungs to heart to body to heart, and back to lungs etc (gases)

From gut to liver and all around body (glucose)

(Blood movement: round and round)
More detail
Dissolved oxygen enters red blood cells and is carried inside them attached to haemoglobin, and dissolved carbon dioxide is carried in plasma

Glucose is also carried in the blood plasma

*simple organisms take in oxygen from, & give off carbon dioxide to, surrounding liquid
Gradual migration of molecules in solution

Very short distances involved
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Passive process random 3-D molecular movement
Blood passes into capillaries

Molecules leave capillaries and diffuse from high to low concentration
i.e down concentration gradient
More detail
Dissolved oxygen passes from red blood cells into plasma, through tissue fluid into respiring cells Carbon dioxide passes from respiring cells into tissue fluid and plasma

"At the lung surface" in alveoli oxygen dissolves then diffuses into the blood, and carbon dioxide diffuses in the opposite direction

Movement of substances into and out of a cell

See diagram below
a respiring muscle cell next to a blood capillary

Some questions about the diagram

What (tubes) is a capillary attached to?
>arteries/arterioles at the supply end, venules/veins at the other end

What (liquid) are the red blood cells suspended in?

Movement of substances into and out of the blood

See diagram below
An alveolus (air sac) in the lung

Some questions about the diagram

What (tubes) is a bronchiole attached to?
>bronchi branching in lung, connected to trachea (windpipe)

What is the function of the film of moisture?
>oxygen must pass from air and dissolve in water before it diffuses in aqueous solution, carbon dioxide diffuses out then leaves solution in the opposite direction

Other details

Breathing is ventilation of the respiratory surface (lungs in humans).

Gas exchange is oxygen in/carbon dioxide out.

Air breathed in is not 100% oxygen. Air breathed out is not 100% carbon dioxide.

Oxygen in blood is carried as oxy-haemoglobin in red blood cells.

Carbon dioxide in blood is mostly carried in solution in blood plasma, not much in red blood cells.

Some questions

What percentage of the air breathed in is oxygen?
> 21% - about one fifth

What percentage of the air breathed in is carbon dioxide?
> 0.04% - about 4 parts in 10,000

What makes up the rest of the air?
> nitrogen 78%, argon 0.9%, and a variable amount of water vapour

Air breathed out contains about 17% oxygen. How much carbon dioxide will it contain?
> about 4% (oxygen down by 4% from 21 to 17, so CO2 will be up by 4% from 0.04%: OK so that is 4.04% !)

Why is respiration inside cells called internal respiration, whereas breathing and gas exchange is called external respiration?
> Energy is released inside cells, but oxygen intake/CO2 output takes place somewhere else (lungs).

When oxy-haemoglobin gives up its oxygen, what else is produced, and where does it go?
> haemoglobin - stays inside red blood cells and picks up more oxygen from lungs

What sort of substance is glucose?
> Sugar/ simple carbohydrate

What sort of carbohydrate might be found in any meal (in bread/pasta/potatoes/rice)?
> starch

How would it be processed in the digestive system?
> digested - broken down into simpler (soluble) molecules - glucose

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