Page 313 - Biology_F5

Biology for Advanced Level Secondary Schools

namely glycolysis, Kreb’s cycle, and Carbohydrates
oxidative phosphorylation. Respiration is These are the main respiratory substrates
an important process as it yields chemical used by most respiring cells. Carbohydrates
energy in the form of ATP, which enables include polysaccharides, disaccharides, and
organisms to perform life activities, monosaccharides. Polysaccharides, such
such as movement, growth, excretion as glycogen, cellulose, and starch must be
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and reproduction. Respiration process hydrolysed into simple monosaccharides,
involves oxidation of organic compounds such as glucose; in contrast, disaccharides
known as respiratory substrates. These are have to undergo hydrolysis into three
carbohydrates, fats, or proteins. monosaccharides, namely glucose, galactose,
and fructose, depending on the type of the
6.3.1 Respiratory substrates and their disaccharide that has been hydrolysed.
energy value Monosaccharides are usually in the form
Different respiratory substrates release of hexose sugars. After their production, they
different amounts of energy (Table are then utilised in the respiratory pathways
6.2). The difference in energy values to release energy.
of respiratory substrates is due to the Lipids (fats and oils)
amount of hydrogen atoms present in each Lipids can be used as a respiratory substrate
substrate. The more hydrogen atom in the when the carbohydrates are exhausted. When
molecule of a respiratory substrate, the hydrolysed, each molecule releases three
more energy (ATP) is generated during fatty acids and one glycerol molecule. Fatty
respiration. Mitochondria synthesise acid are energy-rich compounds that enter
water using the hydrogen atoms from the respiratory pathways in order to release
organic molecules, such as glucose, and energy.
the oxygen atoms they take in as they
respire. So, the more the hydrogen bonds Protein
are broken, the larger the amount of Generally, protein is not used as an energy
energy released. Lipids’ energy density source unless the body has no other option.
is more than twice that of carbohydrates When all carbohydrate and lipid reserves
because of their long fatty acid tails with are fully utilised, proteins come into use.
large number of hydrogen atoms. Proteins are not used in the first place
because of the variety of vital roles they
Table 6.2: Respiratory substrates and their energy play in the body. Proteins are hydrolysed

values into amino acids and then deaminated.
S/N Respiratory Energy value Deamination involves the removal of the
substrate (KJ/g) amino group. The remaining acid may
enter the Kreb’s cycle directly or may
1 Carbohydrates 15.8 be converted into fatty acids before they
2 Lipids 39.4 enter the Kreb’s cycle. The amount of

3 Protein 17.0 ATP produced from protein metabolism is
slightly less than glucose metabolism for
equivalent weights.

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