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4.1 Why organisms undergo respiration
What is respiration and why do we need it?
Respiration is the process whereby energy is released from food molecules (most usually glucose). It takes place inside
living cells, both eukaryotic and prokaryotic. The process takes place inside the cytoplasm and the mitochondria. Energy is
used to synthesise adenosine triphosphate (ATP), which acts as a short-term energy store in cells. All living organisms
need energy to drive their metabolic reactions.
A molecule of ATP consists of one adenosine molecule and three phosphate groups. One molecule
can be hydrolysed to produce one molecule of adenosine diphosphate (ADP) plus one inorganic
phosphate (Pi). This hydrolysis reaction releases 30.6kJ of energy
so energy is immediately available to all cells in small amounts
that will not damage the cell or be wasted. P P P
ATP is described as the universal energy currency of cells. This is
because it is a high-energy molecule which can be used at any time to Ribose
release energy for all metabolic reactions (biological reactions which take place in living
organisms). It can be used for both anabolic reactions (building large molecules) and
catabolic reactions (breaking large molecules into smaller ones).
Aerobic and anaerobic respiration
There are two very different versions of respiration which occur under different conditions. Eukaryotic organisms undergo
aerobic respiration under conditions where oxygen is present. Where oxygen is not present, anaerobic respiration
occurs. Both types of respiration begin with one stage, glycolysis, which is also called the common pathway (as both
aerobic and anaerobic use it). Under aerobic conditions, there are a further three key stages which you will meet in this
chapter. Under anaerobic conditions, there is only an extension on the glycolysis pathway which you will again meet later.
Enzymes and coenzymes
During respiration (particularly under aerobic conditions), there are a number of enzymes and coenzymes involved. The
key enzymes used are:
decarboxylase ­ an enzyme which removes carbon dioxide from a molecule
dehydrogenase ­ an enzyme which removes hydrogen from a molecule
Although these enzymes are useful in catalysing metabolic reactions, they are not very useful for oxidation and reduction
reactions (adding and removing oxygen, hydrogen or electrons). For these reactions, a number of coenzymes are used.
The key coenzymes are NAD (nicotinamide adenine dinucleotide) and CoA (coenzyme-A). Also important during one stage
of aerobic respiration is the cofactor FAD (flavine adenine dinucleotide).
The common pathway series of reactions, or glycolysis, takes place not in the mitochondrion, but in the cytoplasm. During
this stage, one molecule of glucose is taken and broken down into two molecules of a compound called pyruvate (see
4.2 Glycolysis). The pyruvate produced is then transported into the mitochondrion, into the mitochondrial matrix, where
the following two stages occur. The final stage then occurs across the mitochondrial matrix, the inner mitochondrial
membrane and the intermembrane space of the mitochondrion.
It should be noted that as anaerobic respiration does not involve any of the steps beyond glycolysis, anaerobic respiration
does not concern the mitochondrion, as nothing takes place there.


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