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Biology for Advanced Level Secondary Schools
relatively large amount of heat energy to
Exercise 1.15 freeze. Thus, this property is important in
maintaining natural states of cell contents.
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1. Briefly explain the process of Water has a density of 1000 kgm or 1 gcm at
ATP formation. standard temperature and pressure. However,
the density decreases with a decrease in
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2. Explain the importance of ATP in temperature. Thus, frozen water (ice) floats
our daily life. on liquid water. Water has a high surface
tension and cohesive force. Cohesive force
is the force by which individual molecules
1.5 Water as a constituent of the cell stick together.
Water is the most abundant molecule in
a cell, constituting about 80% of total Water being liquid at room temperature,
volume of cell. The liquid part of the provides a liquid environment inside
cell is called cytosol. Generally, without the cells. For example glycolysis and
water, life would not exist on this planet, synthesis of proteins take place in the
as water supports metabolic reactions. cytoplasm of the cell. Additionally, water
has a high latent heat of vaporisation. This
Properties of water means that water needs a lot of energy
Water is an excellent solvent for polar to evaporate hence, helps in cooling the
compounds such as ionic substances, like body through sweating.
salts, and non-ionic substances like sugars.
It has a high heat of vaporisation due to Roles of water
The roles of water in living organisms can
hydrogen bonds which hold molecules be categorised as follows:
together. Latent heat of vaporisation means
a measure of the heat energy required to The metabolic role of water
overcome the attractive forces between (a) It is used for hydrolysis of many
molecules and make them to escape in substances, such as proteins into
form of a gas (vaporise). This property is amino acids as well as fats into fatty
significant in minimising water loss from acids and glycerol. In the hydrolytic
the body. processes, water is brought in by
enzymes or hydrolytic reagents such
Water has high heat capacity which is the as mineral acids.
heat required to raise the temperature of (b) It is the medium for all cellular
1 kilogram of water by 1 C. By having metabolic processes, for instance,
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high heat capacity means that, increase water is an important raw material for
in heat energy can lead to relatively small photosynthesis.
rise in temperature. Also water has a high (c) It facilitates diffusion of materials
heat of fusion. It requires a relatively large across surfaces; for example, for
amount of energy to melt its solid state (the passage of food solutions into blood
ice). Conversely, liquid water must lose a stream across the walls of the ileum.
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