The diagram, illustrates the polar nature of the water molecule.
The H and O represent hydrogen and oxygen atoms respectively,
and the black circles represent the electrons which are shared
(one from each hydrogen atom, and two from the oxygen atom, one for each bond),
and they are shown nearer to the oxygen atom, as it attracts them.
The δ+ represents the small electropositive charge on each of the hydrogen atoms, and the δ− represents the electronegative charge on the hydrogen atom.
The polar nature of the water molecule is the reason that water is such a good solvent,
as it can easily ionize substances. Water has been called the 'universal solvent',
as more substances can dissolve it in that in any other solvent.
Often in living organisms, substances must be solution, and water is the solvent
which makes this a possibility. For example, plants can only obtain mineral salts in
solution, and human digestion will only dissolve soluble foods, so large starch
molecules must be broken down in soluble sugars, such as glucose.
The crucial reactions of metabolism take place in the protoplasm with the materials
in solution. Waste products can also be removed in solution, for example in urine.
Gas exchange requires a moist surface, since gas exchange takes place in solution. In mammals, the alveoli in the lungs are moist with water, to allow gas exchange, and many plants have moist surfaces in their leaves (the Mesophyll cells) for gas exchange.
Many organisms living in water spend much or all of their time under the surface. They require oxygen gas to respire, and as water is such a good solvent, the required oxygen gas is dissolved in the water. Four parts of oxygen will dissolve in 100 parts of water, and this is sufficient oxygen for most marine life.
Substances produced in organisms often need to be transported to other parts of the organism. In humans, blood is used to transport food, hormones, oxygen, waste products and so on, and similarly in plants, sap is used to transport food and other substances. Both of these mediums for transports (blood and sap) are mainly water, as this is the solvent which can dissolve the products to be transported. Blood allows oxygen to be transported to tissues, and waste products to be quickly removed. It also facilitates the transport of hormones, to control various organs.
At any one time, many reactions are occurring in an organism, catalyzed by enzymes. Water, being an excellent solvent, allows many of these reactions to occur, as often the substances need to be ionized. However, water does not just allow reactions to take part, it can also take part as a reactant in reactions. The most basic example of this is photosynthesis. The energy from the sun must be harnessed and stored in organic molecules, such as glucose. Apart from some bacteria and algae, green plants are the only organisms which can do this, by the process of photosynthesis, and all other organisms rely on plants to trap the energy, which is then used by them when the plants are consumed. Without photosynthesis, there would be no way for organisms to obtain energy, so life would be impossible. The equation for photosynthesis shows that water is a vital reactant in the reaction.
Water + Carbon dioxide Glucose + Oxygen
6H2O + 6CO2 C6H12O6 + 6O2
The water is actually used in the light reaction. The oxygen contained in the water molecule
is vital to life, as it is required for all organisms which respire aerobically.
As mentioned earlier, substances can only be absorbed into the gut, when in solution.
However, large molecules such as starch have been made by combining smaller units such as glucose. In this process known as condensation, water molecules are released. The reverse of this reaction, hydrolysis, is carried out in digestion, and as water molecules are released in condensation, they must be supplied in hydrolysis. This is another example of an important reaction where water is a reactant. Water enables metabolism, by allowing the required substances to ionize, but it also provides the hydrogen necessary for the processes of metabolism. As mentioned earlier, substances can only be absorbed into the gut, when in solution. However, large molecules such as starch have been made by combining smaller units such as glucose. In this process known as condensation, water molecules are released. The reverse of this reaction, hydrolysis, is carried out in digestion, and as water molecules
are released in condensation, they must be supplied in hydrolysis. This is another example of an important reaction where water is a reactant. Water enables metabolism, by allowing the required substances to ionize, but it also provides the hydrogen necessary for the processes of metabolism.
Plant cells have a cell wall in addition to the usual cell membrane, so the cells are not likely to burst, as can happen in animal cells when too
much water enters them by osmosis. Plant cells can therefore become turgid - this occurs when water diffuses in the cell by osmosis up to the point where the cell wall prevents further water intake, by exerting a force equal to the osmotic force, by which the water was diffusing in. This makes the cell stiff, or turgid. The turgidity of plant cells in important in support, as it helps to support leaves, and the stems of some plants.
If there is a weaker solution outside, water As the cell becomes turgid the cell wall exists a
enters the cell by osmosis, moving down its 'wall-pressure'. This resists further
concentration gradient. The cell does not water entry by osmosis. The turgidity of the cell
resist the entry of water, as there is little or gives support to non-woody organs of a plant. no 'wall-pressure'
