Site author Richard Steane
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Although we often think of respiration in terms of humans and breathing, it is important to note that it is one of the 7 characteristic processes shared by all living organisms, so it must have a universal definition: "processes leading to, and including the chemical breakdown (oxidation) of food materials to provide energy for life".

These processes occur inside the living cells of every type of organism - plant and animal. For this reason, it is also called INTERNAL or CELLULAR RESPIRATION.
In aerobic respiration, oxygen is needed, so this must be taken from the surroundings (air or water) by a process of gaseous exchange because carbon dioxide is got rid of at the same time. This exchange process is sometimes loosely described as "respiration" itself, but the term EXTERNAL RESPIRATION is better, because it stresses that the entry of oxygen and exit of carbon dioxide happen at a different place than where energy is released.

The place where this exchange of gases occurs is called the respiratory surface (lungs in mammals, gills in fish, skin in earthworm, cell surface in Amoeba).

Fill in the table below to explain why respiratory surfaces usually share some basic design features.
You will probably find it worthwhile to refer to diffusion.

Feature Explanation
large surface area maximises efficiency of transfer
moist gases must enter/leave via solution
in contact with blood capillaries blood carries oxygen away and brings CO2 back
extremely thin movement is via diffusion so the shorter the path the better
(possibly) mechanism to
provide a steady flow of oxygen (from air or water)
in order to maximise uptake of oxygen and match it to the body's needs, e.g. in exertion

Breathing is a means of maximising this process of gaseous exchange, which involves pumping movements to ventilate the respiratory surface (e.g. chest movements to inflate and deflate lungs in mammals, and mouth movements to pass water over gills, in fish). This may not occur in simpler organisms, e.g. Amoeba, and the earthworm.

Why do you think that these simpler organisms do not need to carry out pumping movements?
>they need less energy - have a lower metabolic rate

Do trees (and other green plants) breathe? Explain your answer.
> No > oxygen enters, carbon dioxide leaves by diffusion

Aerobic respiration

This is the normal process by which "food substances" are broken down and oxidised to provide energy, in Man and most living organisms. Do not confuse this with digestion. However, most food substances are converted to glucose before being respired.

The word aerobic means "requiring air", but the only important component of air (as far as respiration is concerned) is the gas oxygen , which makes up about one fifth ( 21%) of the total. The overall reaction of aerobic respiration process may be expressed as a chemical equation as follows.

Complete the word equation by adding the names below this.

C6H12O6   +    6 O2           ---->         6 CO2          +   6 H2O    (+ about 2800 kJ per mole of glucose)
   glucose      +  oxygen     ----> carbon dioxide  +   water  + energy released                                 

[n.b. Energy is not a "substance"]

Although this reaction resembles the one for burning, the aerobic respiration process is different in several respects:
1) The energy is gradually given out, not all released in one go.
2) It is not a simple one-step process - there are dozens of separate reactions, each controlled by different enzymes inside the cells.
3) Not all the energy is released as heat. Much is stored in chemical form.

List 3 uses to which the energy may be put:
>walking > thinking!> digesting food, etc.

Anaerobic respiration

Different organisms have several alternative ways of getting energy from food, without using oxygen, but all are less efficient than aerobic respiration, and do not break food down fully into the simplest substances. For example, when working hard, some animal cells do not receive enough oxygen from the blood, so they convert glucose into lactic acid without producing carbon dioxide or water.
C6H12O6 ----> 2 C3H6O3 ( + about 210 kJ per mole of glucose)
glucose ----> lactic acid + much less energy released
After exertion, the body takes some time to convert this lactic acid into useful substances, and repay the so-called oxygen debt due to lack of oxygen in the blood in the first place. See later notes.
In what tissue of the body will this reaction occur most often?
> muscle
What is the advantage of the human body not producing carbon dioxide in anaerobic respiration?
> lungs and circulation cannot provide enough oxygen, so removing CO2 would be a problem!
The single-celled fungus yeast may produce alcohol (ethanol) as a result of anaerobic respiration, although it will respire aerobically (the same as above), if given air.

Respiration by green plants

It is important to realise that green plants respire at all times, like all living organisms. However, they are unique in that they can also make their own food by photosynthesis . As a by-product of this process, they produce a waste substance, oxygen, which they release from the cells of their leaves into the air.
Of course, they may re-use some of this oxygen in aerobic respiration, so some people WRONGLY assume that plants do not need oxygen, or do not respire in the same way as other living organisms. What happens in these plants is merely a question of balance between the processes of photosynthesis and respiration. Click to see photograph

Experiment to show the release of heat energy by germinating seeds

Seeds are dormant stages of living organisms, and contain embryo plants, ready to grow when conditions are right. Most of the time, they appear to be doing nothing much, but they are respiring only slowly because they do not need much energy, and need to conserve the food reserves they contain.

However, before they start to grow into plants (and continue the life cycle by flowering and producing seeds again), seeds must germinate. In order to do this, seeds absorb water which they need in order to mobilise their food reserves using enzymes (basically the same process as digestion in animals), then they speed up their rate of respiration quite dramatically. They then use the energy released in order to sprout roots to absorb more water and minerals, and grow a shoot which takes the leaves above ground, so as to make food by photosynthesis. Of course this usually happens under the ground, but it is not necessarily so.

Dried peas (as packaged for food) are alive, and can be encouraged to germinate by soaking them in water and keeping them in airy conditions in fairly warm temperatures (room temperature is warm compared with soil!). Other seeds, such as wheat grains will also do.

The effect of respiring seeds in releasing heat can be shown by placing them into a vacuum flask - as used for keeping foods and drinks hot, or cold.
Peas in vacuum flask
The sprouting seeds can be divided into 3 portions:
1 left as they are, i.e. alive
2 boiled to kill them, then cooled
3 placed in concentrated disinfectant solution

Each portion is then placed into dilute disinfectant solution, to kill any bacteria or fungi on their surface, and poured into a vacuum flask. A thermometer is placed into each mass of seeds, held with a stand and clamp, and the mouth is "sealed" with a plug of cotton wool.
The temperature is then taken in each flask, and repeated (say twice a day) over a period of a few days. It is a good idea to record room temperature at the same time.
Use the table below to record the results, and then plot them as a graph.

Temperature / °C
Day and
Elapsed Time from
start of experiment
/ hours
Room i.e.
Vacuum Flask
containing live
Vacuum Flask
boiled peas
Vacuum Flask
containing peas in
strong disinfectant
day 0          
day 1          
day 2          
day 3          
day 4          
day 5          
day 6          
specimen data?

Using a respirometer to show uptake of oxygen by living tissue

A respirometer is an apparatus for measuring the amounts of gases used in respiration. There are many forms, but most consist of an air-filled chamber in which living tissue is placed, along with a substance to absorb gases, to which is attached a tube along which a liquid flows to show changes in the volume of the apparatus.

The respiring tissue could be anything small enough to fit inside the apparatus; germinating peas or other seeds or invertebrates such as locusts or maggots may be used.

The substance usually used to absorb carbon dioxide is soda lime; other alkalies such as sodium or potassium hydroxide (solid pellets or solution) could also be used. CAUTION: Alkalies are corrosive.

The apparatus is usually enclosed in a waterbath to keep the temperature stable. Temperature is also a factor which is likely to have a major effect on the rate of respiration (and all metabolic activity).

The apparatus is set up as in the diagram, and the liquid level noted. The time taken for the liquid to travel a certain distance is a measure of the respiration rate.

What is the purpose of the soda lime?
>to absorb CO2
In which direction does the liquid level move?
>upwards toward apparatus
Why is the liquid coloured?
> just to make it easier to see!
What would be the advantage if the tube was made of capillary (narrow bore) tubing?
> any movement would be maximised
Why is it necessary to keep the temperature stable?
>so the enclosed gas does not expand or contract
(as in a thermometer).

Does this experiment actually show what the title says? Explain why.
>no >oxygen is not tested for directly
What would be a better title?
> output of CO2 from living tissue

This topic has connections with other BioTopics units on:-
Aerobic respiration
Effects of exercise on the body
Much more complicated:
Krebs cycle, citric acid cycle or tricarboxylic acid cycle

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