BOTANY Higher Secondary Second Year - Textbooks Online

5.3.1. Mechanism of respiration
Oxidation of glucose involves following four distinct stages –
glycolysis, oxidative decarboxylation of pyruvic acid, Krebs cycle and
Electron transport chain. In the first three stages, the hydrogen acceptor
Nicotinamide adenine dinucleotide – oxidized form (NAD + ) and Flavin
adenine dinucleotide – oxidized form (FAD + ) are reduced to NADH and 2
FADH respectively. Both the coenzymes, (NAD 2 + ) and (FAD + ) act as
hydrogen carriers from respiratory substrate to electron transport chain,
where H + and electrons are transferred to oxygen to form water. This
electron transport results in the release of energy, which is used to
phosphorylate ADP to ATP. Hence, the electron transport chain reactions
are referred to as oxidative phosphorylation.
5.3.2. Glycolysis
The process by which the glucose (6C compound) is split into two
molecules of pyruvic acid (3C compound) is called glycolysis. Three
German Microbiologists – Embden, Meyerhof and Parnas, first
demonstrated this process in yeast cell. Hence, it is otherwise known as
EMP pathway. It occurs in cytoplasm. It is common in all organisms. It is
divided into two phases – hexose phase and triose phase. Glyceraldehyde
3-phosphate and DHAP are the products of hexose phase and two molecules
of pyruvic acid are the products of triose phase. The overall reaction of
glycolysis is given in the following equation.
2ATP 2ADP
Reactions involved in glycolysis are as follows
1. The glucose is phosphorylated with ATP to form glucose-6phosphate.
The reaction is catalyzed by the enzyme hexokinase.
2. Glucose-6-phosphate is isomerized to form fructose-6-phosphate
by phosphoglucoisomerase.
3. Fructose-6-phosphate is then phosphorylated using ATP to form
fructose 1,6-bisphosphate. This reaction is catalyzed by
phosphofructokinase. The ATP is dephosphorylated to ADP.
4. Fructose 1,6-bisphosphate is cleaved by the enzyme aldolase to
two molecules of 3C compounds – dihydroxy acetone phosphate (DHAP)
and glyceraldehyde 3-phosphate. These two trioses are isomers.
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