Above: a single equation symbolising and summarising the respiration of glucose sugar.

Respiration is the process whereby an organism obtains useful energy from foodstuffs by oxidising the chemical
constituents (such as sugars, fats and amino acids) of the food. Many organisms use oxygen as the oxidant
and organic chemicals as the foodstuffs. Respiration is similar to the process whereby a car engine or a candle
burns fuel - organic compounds in the petrol or wax are burned/combusted in oxygen, oxidising them to water
and carbon dioxide and releasing energy. Similarly, organic materials in foodstuffs are oxidised by oxygen to
carbon dioxide and energy, releasing energy which the cells can convert into a usable form.

Cellular respiration (respiration proper) is the process as it occurs in cells and is a distinct, though related,
process to breathing. Breathing or ventilation is the process whereby a multicellular organism gets oxygen into
its body. This oxygen is then transported to the cells by various means (in most vertebrates it is carried from
the lungs or gills to the tissues in the bloodstream bound to the iron-rich protein haemoglobin in red blood
cells). The cells then use this oxygen to respire or oxidise organic food materials which are also carried in the
bloodstream (from the small intestine, in vertebrates like you, where the final stages of food digestion occur).

Cellular respiration differs from combustion of fuel in a car engine in several important ways. First of all, a car
burns fuel in a controlled explosion - the petrol is burnt more or less completely in a single reaction. In contrast,
in respiration the fuel is oxidised in a number of stages in a very tightly controlled manner, such that the
available energy is released in small packets rather than explosively. In a car engine, the pressure of the
explosion drives the pistons, whereas in a cell the energy is stored in a number of molecules of a chemical
called ATP (adenosine triphosphate or more specifically adenosine trisphosphate). The energy stored in ATP
can be released when required by the cell in any part of the cell and is used to drive many different

Both combustion in a car engine and respiration use catalysts. Petrol contains negative catalysts (lead used to
be used) which slow the reaction to stop the petrol knocking or the engine from exploding. Each of the stages in
respiration is controlled by a biological catalyst (an enzyme) which can increase or decrease the rate of the
reaction as required. If the cell needs more energy and hence more ATP and if fuel is available then it will
increase its rate of respiration and decrease it when it is resting.
Above: the equation for combustion of octane, a component of petrol. Compare the equation to that for
respiration. In reality car engines do not burn fuel cleanly (completely) and carbon as soot (C) and carbon
monoxide (CO) are also produced.
Respiration is an example of a metabolic pathway. The metabolism of a cell or organism is the sum of all the
biochemical reactions occurring within it. These reactions may be anabolic, making more complex chemicals
out of simpler chemicals, such as building proteins from amino acids; or they may be catabolic, breaking down
more complex chemicals into simpler ones. Anabolic reactions generally require energy from ATP, whereas
catabolic reactions often release energy. A metabolic pathway is a chain of biochemical reactions, a reaction
pathway, in which a series of chemical reactions are linked in series as one or more of the products of the
preceding reaction become some or all of the reactants of the subsequent reaction.

Respiration is a catabolic reaction pathway that can be divided up into several sub-pathways that link together
in the following order:

1. Glycolysis
2. The link reaction
3. The Kreb's (tricarboxylic acid or TCA) cycle
4. The electron transport chain (ETC) also known as the electron transport system (ETS) or chemiosmosis

Living cells package related reactions together in the same compartment and separate certain reactions into
different compartments. Respiration occurs in several different compartments of the cell: glycolysis occurs in
the cytosol, the link reaction occurs in the matrix of organelles called mitochondria. The TCA cycle also occurs
in the mitochondrial matrix and the ETC is located in the inner membrane of the mitochondrion, which is
thrown into folds called cristae.


Mitochondria are organelles inside most animal, plant, fungal and protoctistan cells. The structure of a typical
mitochondrion is shown below:
Mitochondria and Cellular respiration - an example of a metabolic pathway
It is difficult to describe mitochondria (singular mitochondrion) in terms of numbers and sizes because they are
very dynamic organelles in living cells. Mitochondria continually fuse together, divide into smaller fragments
and move around the cell as and when they are required. More active cells or more active parts of the cell
contain more mitochondria (or mitochondrial fragments). It is best to think of mitochondria as forming a large
branching network which can bud off small sausage-shaped or spherical fragments as required.

Mitochondria are the power-plants of the cell, they use oxygen to complete the oxidation of food duels and
package the energy released into ATP. ATP (adenosine trisphosphate) is the energy currency of the cell.
Almost all energy requiring processes within the cell use energy packaged as ATP.

When divided up the mitochondria are probably increasing their outer surface area to volume ratio to enhance
transport of required materials into the mitochondria (e.g. pyruvate, amino acids, oxygen and NADH) and
export of materials into the cytosol (e.g. ATP).
  • Mitochondria are another membrane-bound organelle found in eukaryotes.

  • Mitochondria are enclosed in a double-membrane, each membrane is a typical phospholipid-bilayer
    type membrane.

  • The inner membrane is folded, each fold is called a crista (plural cristae) to increase its surface area
    for proteins that make-up the electron-transport chain (ETC) and ATP synthase. Electrons (electricity!)
    flowing through the ETC powers the ATP synthase. Cristae may have the appearance of plate-like
    folds, or tubular/finger-like projections with a round or angular cross-section, depending on cell-type
    and organism species.

  • The mitochondria release energy from food ‘fuels’ like glucose and fatty acids by aerobic respiration
    by oxidising the foodstuffs with oxygen.

  • This released energy is stored in molecules of ATP (adenosine trisphosphate).
  • ATP is the universal energy currency of the cell.

  • Almost all energy-requiring processes in the cell take their energy from ATP.
  • Mitochondria are the ‘power-houses’ of the cell.

  • ATP synthase (stalked particles) are proteins that release energy from glucose/fats using energy
    from flowing electric charges – glucose is oxidised to release electrons and H+. The flow of the
    electrons through the ETC pumps H+ into the inter-membrane space. The H+ then flow through the
    ATP synthase, back into the matrix, releasing energy that is used to make ATP. This is aerobic
A Summary / Overview of Respiration

Respiration: the process by which living things extract energy from foods.

Respiration is an oxidation reaction (like the combustion of petrol or of a candle) – sugar is ‘burnt’ or oxidised (reacted with
oxygen) to produce carbon dioxide and water.

The oxidation of foods, like glucose and amino acids, releases some of the energy present in these foods, just as burning petrol
releases energy to power the engine of a car, or as burning a candle releases energy as heat and light. This energy comes
from the chemicals that are oxidised (gasoline, wax or glucose) and so is called chemical energy.

The chemical equation for the (aerobic) respiration of glucose is:
Notice that the sugar glucose is a chemical made up of molecules containing 6 carbon (C) atoms. These carbon atoms form the
skeleton of the molecule. Molecules with carbon skeletons are called organic molecules. Petrol and wax are also organic
molecules – it is a property of organic molecules that they can be burnt in oxygen (oxidised) to yield carbon dioxide (CO2), water
(H2O) and energy!
Q. Where does the glucose come from?
A. The glucose comes from carbohydrates, such as sucrose and starch, which are broken down by enzymes in the gut and
absorbed into the bloodstream. The glucose diffuses from the blood into the tissues and into the cells.

Where does the oxygen come from?
A. The oxygen comes from the air you breath in (which is 21% oxygen) in the form of O2 molecules (two O atoms joined
together). This process of breathing is also called ventilation or external respiration.

Internal respiration or cellular respiration is the actual chemical process of oxidising glucose and occurs inside the cells.
The end products of respiration of glucose are carbon dioxide and water which are breathed out and
ENERGY! Some of the
energy is used to make a chemical called ATP which stores the energy! The rest is lost as heat.
ATP is the universal energy currency of the cell! It is the immediate source of energy for almost all processes requiring energy,
such as cell movement and building materials (anabolic metabolism) such as proteins and DNA. Without ATP, all life ceases.

The process that makes most of the ATP in the cell is oxidative phosphorylation. Some ATP is also made directly from glycolysis
and the TCA cycle, but most is made by the mitochondria by using the energy stored in NADH.

Anaerobic respiration (does not require oxygen)

Aerobic respiration is preferred as it yields more ATP and hence more energy. When an athlete sprints fast or lifts a heavy
weight, the muscles are working to hard and too quickly for the usual aerobic respiration involving the mitochondria to produce
ATP fast enough.

Creatine phosphate system:

Instead the muscles rely upon glycolysis WITHOUT the TCA cycle and oxidative phosphorylation, as glycolysis alone does NOT
require oxygen! This results in a low ATP yield, however muscles have another store of energy which replenishes ATP –
creatine phosphate. There is enough creatine phosphate to replenish ATP levels for about 5 to10 seconds of intense
anaerobic exercise.

Lactic acid system:

If a sprinter is to run further, say 400 metres in about one minute, then both the ATP and the creatine phosphate stores in the
muscles will be depleted – the muscle must make more ATP! However, aerobic respiration is too slow, as it relies on the
transport of oxygen from the air into the lungs and into the bloodstream to the cells! Most of the energy required is still obtained
by anaerobic respiration by the lactic acid pathway shown below:
More detailed (though still greatly simplified) diagrams of the three main chemical pathways involved in aerobic respiration are
given below:
Note: the net production of glycolysis from one molecule of glucose is:

  • 2 molecules of ATP (4 produced, 2 consumed)
  • 2 molecules of reduced NADH ( more accurately 2NADH + 2H+ )
A summary of aerobic respiration is given below:
Mitochondria are the power-houses of the cell, helping the cell to convert fuel (like sugars, fats and amino acids) into a
useful store of energy that can be rapidly used when required, in the process of
aerobic respiration. This energy is in the
form of a molecule called
adenosine trisphosphate (ATP) which is the universal energy currency of the cell. Mitochondria
are often described as sausage-shaped organelles, one of which is shown in longitudinal section in our computer model
(above and below). In reality, however, mitochondria are rapidly changeable, switching in a matter of seconds between
networks of elongated, branching filaments that form when mitochondria fuse together, to a set of sausage-shaped discrete
organelles when the networks fragment (undergo controlled fission) according to the needs of the cell. Mitochondria consist of
an outer membrane (a phosohpholipid bilayer unit
membrane) and an inner membrane (also a unit membrane), the latter
bearing invaginations called cristae.