Respiration is the oxidation of energy-containing organic molecules. The energy
released from this process is used to combine ADP with inorganic phosphate to make
ATP.
All cells obtain useable energy through respiration. Most cells use carbohydrate,
usually glucose, as their fuel. Some cells, such as nerve cells, can only use glucose as
their respiratory substrate, but others can use fatty acids, glycerol and amino acids.
Respiration may be aerobic or anaerobic. In both cases, glucose or another respiratory
substrate is oxidised.
– In aerobic respiration, oxygen is involved, and the substrate is oxidised completely,
releasing much of the energy that it contains.
– In anaerobic respiration, oxygen is not involved, and the substrate is only partially
oxidised. Only a small proportion of the energy it contains is released.
• glycolysis in the cytoplasm (cytosol) of the cell
• the link reaction in the matrix of a mitochondrion
• the Krebs cycle in the matrix of a mitochondrion
• oxidative phosphorylation on the inner mitochondrial membrane.
AEROBIC RESPIRATION
Glycolysis
Glycolysis (the breakdown of glucose) is the first stage of respiration. It takes place in
the cytoplasm and does not require oxygen. It begins with the 6-carbon ring-shaped
structure of a single glucose molecule and ends with 2 molecules of a 3-carbon sugar
called pyruvate and a net gain of 2 ATP. Glycolysis is summarised below.
A glucose (or other hexose sugar) is phosphorylated, using phosphate from 2
molecules of ATP, to give hexose bisphosphate. This phosphorylation converts an
energy-rich but unreactive molecule into one that is much more reactive, the chemical
potential energy of which can be trapped more efficiently.
The hexose bisphosphate is split into 2 triose phosphate molecules.
• Hydrogen atoms and phosphate groups are removed from the triose phosphate (by the
coenzyme NAD). The removal of hydrogens is an oxidation reaction, so triose
phosphate is oxidised to 2 molecules of pyruvate (pyruvic acid). During this step, the
phosphate groups from the triose phosphates are added to ADP to produce a small yield
of ATP.
• Overall, 2 molecules of ATP are used and 4 are made during glycolysis of one glucose
molecule, making a net gain of 2 ATPs per glucose. The pyruvic acid is then converted
to either lactic acid or alcohol and carbon dioxide without the production of any more
ATP.
The pyruvate formed in glycolysis is still energy-rich. It passes next to the link reaction.
This reaction and all subsequent stages of respiration occur inside a mitochondrion, and
can only occur in the presence of free oxygen. Respiration requiring free oxygen is
aerobic respiration. Pyruvate is transported into the mitochondrial matrix by a membrane
transport protein, which exchanges it for OH– in the matrix.
If the cell cannot catabolize the pyruvate molecules further, it will harvest only 2 ATP
molecules from 1 molecule of glucose. For example, mature mammalian red blood cells
are only capable of glycolysis, which is their sole source of ATP. If glycolysis is
interrupted, these cells would eventually die.
