Biochemistry
Water and Mineral
Salts
1. What is the approximate percentage (in mass) of water
in the human body? Is this percentage expected to be
larger in the adult or in the old individual?
Approximately 65% of the human
individual mass is water. The brain, for
example, has around 90% of water in
mass, the muscles, 85%, and the bones
have between 25% and 40% of water.
Younger adult individuals have
proportionally more water in mass than
older individuals.
2. Water has key participation in organic reactions. What are examples of two types of organic reactions in which
water is respectively incorporated or liberated in
the products of these reactions?
Photosynthesis is a biochemical process
in which water is incorporated into
organic molecules. In the reaction, the
hydrogen atoms from water go to the
produced glucose and the oxygen atoms
from water form the molecular oxygen
liberated: carbon dioxide + water +
light = glucose + molecular oxygen.
Aerobic respiration is an example of
biochemical reaction in which water is
produced: glucose + molecular oxygen
= carbon dioxide + water.
3. Which kind of polarity do water-soluble and fat-soluble
substances respectively have?
Water-soluble substances are polar
molecules, i.e., they have electrically
charged areas. These molecules get the
description “water-soluble” because
they are soluble in water, a polar
molecule too.
Fat-soluble substances are non-polar
molecules, i.e., they are electrically
neutral. They get the description “fat-
soluble” because they dissolve other
non-polar substances.
4. Can the heat capacity of water be considered small or
large? What is the biological significance of that
characteristic?
From Thermology it is known that the
quantity of exchanged heat (Q) is equal
to the mass (m) multiplied by the
specific heat of the substance (c)
multiplied by the variation of
temperature (T), Q = m.c.ΔT., and that
heat capacity is Q/T, hence, m.c. Heat
capacity, however, relates to a specific
body, since it considers mass, whereas
specific heat relates to the general
substance. Therefore it is more correct
to refer to specific heat in this problem.
Water has a specific heat of 1 cal/g.oC
which means that 1 oC per gram is
changed in its temperature with the
addition or subtraction of 1 cal of
energy. This is a very elevated value
(for example, the specific heat of
ethanol is 0,58 cal/g.oC, and mercury, a
metal, has a specific heat of 0,033
cal/g. oC) making water an excellent
thermal protector against variations of
temperature. Even if sudden external
temperature changes occur, the internal
biological conditions are kept stable in
organisms which contain enough water.
High specific heat is one of the most
important water properties.
5. What are the main water properties that make water
special for life?
The water properties that make water
biologically important are molecular
polarity, thermal stability (elevated
specific heat), fusion and ebullition
points that allow water to be liquid in
most environments, acid-base
neutrality, small molecular size and low
chemical reactivity. (Compared to other
substances, like ethanol or hydrogen
sulfide.)
6. What are ions? What are the two types of molecules
into which ions are classified?
Ions are atoms or substances
electrically charged by means of loss or
gain of electrons.
The two types of ions are the cations
and the anions. Cations are ions with
positive total electric charge and anions
are ions with negative total electric
charge.
7. How do mineral salts participate in osmotic
regulation?
Osmotic pressure depends on the
number of particles dissolved in a
solution and not on the nature of such
particles. Mineral salts, glucose,
proteins and urea are the main
regulating particles for the osmolarity of
the organism. These molecules along
with other particles inside and outside
the cell generate the larger or smaller
osmotic gradient between the
intracellular and the extracellular space.
8. Why is pH regulation important for living beings?
How do mineral salts
participate in this regulation?
The potential of hydrogen (pH) is a
measure of the amount of hydrogen
ions (H-
) in a solution. The regulation of
the pH according to the necessities of
each organ or tissue is extremely
important for the organism since
enzymes act only under some pH
ranges and many proteins are only
active under some pH ranges. Therefore
biochemical reactions depend on correct
levels of pH to occur.
Neutral pH is one of the water
properties.
9. How do mineral salts participate in enzymatic
activity?
Many mineral salts are cofactors of
enzymes, i.e., they are substances
without which enzymes do not work.
10. What is hemoglobin? What is the inorganic element that
is fundamental in the composition of hemoglobin?
Hemoglobin is the protein present in the
blood responsible for the transport of
oxygen from the lungs to the tissues
and cells.
The hemoglobin molecule is composed
of four protein chains, each with a heme
group containing an iron atom. The iron
is responsible for the binding of oxygen
in the lungs and also for the red color of
hemoglobin and thus of the blood.
