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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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