Adenosine triphosphate is a nucleotide derivative and consists of ribose, adenine and three
phosphate groups.

● Energy is released when ATP is hydrolysed to form ADP and a phosphate molecule.
This process is catalysed by ATP hydrolase.
● The inorganic phosphate can be used to phosphorylate other compounds, as a
result making them more reactive.
● Condensation of ADP and inorganic phosphate catalysed by ATP synthase produces
ATP during photosynthesis and respiration.

Respiration

Respiration is the breakdown of a respiratory substrate such as glucose to produce energy
in the form of ATP. There are two types of respiration:
● Aerobic- occurs in the presence of oxygen. Produces large amounts of ATP.
● Anaerobic- occurs in the absence of oxygen. Produces less ATP and is less efficient.
Some organisms and tissues are able to respire in both aerobic and anaerobic conditions.
When yeast and plants respire under anaerobic conditions, they produce ethanol and
carbon dioxide as end-products; mammalian muscle tissue produced lactate when oxygen is
in low supply, which causes fatigue.
Respiration is a multi-step process with each step catalysed by a specific intracellular
enzyme.

Glycolysis:
Glycolysis is the first process of both aerobic and anaerobic respiration. In aerobic
respiration which occurs in cytoplasm of cells.

In this process glucose is phosphorylated to
produce 2 molecules of pyruvate, 2 molecules
of ATP and 2 molecules of NADH.
In anaerobic respiration the pyruvate is
further converted into lactate with the help of
NADH. Lactate is then converted back to

pyruvate in the liver.
The link reaction:
The next step of aerobic reaction is the link
reaction where pyruvate is converted to acetyl
coenzyme A with the help of NADH.
Krebs Cycle
Acetyl-CoA then enters the Krebs cycle where
glucose is oxidised and carbon dioxide, ATP,
reduced NAD and reduced FAD are produced. The
Krebs cycle also includes decarboxylation (removal of
carbon dioxide) and dehydrogenation (removal of hydrogen) to convert 6-carbon citrate to
4-carbon oxaloacetate.

Oxidative phosphorylation

Oxidative phosphorylation is the process in which ATP is synthesised in the electron
transport chain in mitochondria. This process generates the majority of ATP in aerobic
respiration and it occurs as following:
● Reduced coenzymes carry hydrogen ions and electrons to the electron transport
chain which occurs on the inner mitochondrial membrane
● Electrons are carried from one electron carrier to another in a series of redox
reactions: the electron carrier which passes the electron on is oxidised whereas
the electron carrier which receives it is reduced
● Hydrogen ions move across the membrane into the intermembrane space – as a
result of this, the concentration of the hydrogen ions in the intermembrane
space is high
● Hydrogen ions diffuse through ATP synthase into the mitochondrial matrix down
the electrochemical gradient
● ATP is produced using ATP synthase, a stalked granule. Approximately 4
hydrogen ions produce one ATP molecule.
● Hydrogen atoms are produced from hydrogen ions and electrons. The hydrogen
atoms are then combined with oxygen to produce water

Respiratory substrates include carbohydrates, lipids and proteins which release varying
amounts of energy, depending on the number of hydrogens in the structure which are

oxidised to water. For instance, the number of hydrogens is greater in fatty acids than
carbohydrates.
The respiratory quotient (RQ) can be measured to determine which respiratory substrate is
being used and to determine if the organism is undergoing anaerobic respiration.
RQ = carbon dioxide produced / oxygen consumed
Different respiratory substrates have different RQ values e.g. carbohydrates have a value of
1.0, lipids – 0.8 and proteins 0.9.

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