The water molecules 'sticking together' or the cohesion has a number of effects on living systems.
This image shows how the cohesion of the water molecules at the interface of air and water has enough structural strength to support the mass of the Insect.
The insect has exploited this niche opportunity through the evolution of structures adapted to improve its ability to move on the surface of the water.
Cohesion of water molecules is critical in the way plants transport water through their tissues in the process known as transpiration.
Solvent Properties:
The polar water molecules are attracted to the strong ionic charges of the ions in solution.
Often the ion, in this instance the Na+ , exert a strong enough attraction on water molecules that they form shells around the ion. Here there appears to be concentric shells of water molecules around the sodium ion.
This has the effect of isolating one ion form another which is essentially what is known as solubility.
This image attempts to show the isolation of ions in solution caused by the 'cloud' of water molecules attracted through extended hydrogen bonding.
This hydrogen bonding reduces the over all movement of water molecules in a solution. Their total kinetic energy is reduced, which can be measured (Kpa) as a reduction in the pressure exerted by the water molecules.
Osmosis: A reduced tendency of the water molecules to move in a solution is another way to explain processes such as osmosis.
Transport and reactions: Water of course is the solvent of blood, tissue fluid and the cytoplasm. It therefore allows the transport of the soluble minerals, carbohydrates, amino acids etc that are required to be transported around the body.
Thermal properties of water.
You will be familiar with the concept of the phases of matter, solid, liquid and gas.
The transition form one phase to another (in the above sequence) requires energy to be added.
From the structure of any molecules we can predict the energy required to make each transition e.g. liquid to a gas.
The predictions made for water (based on the empirical formula) turn out to be too low. With experiments showing higher values than predicted.
To make the phase transition from a liquid to a gas requires that extra energy be added to overcome the hydrogen bonding.
Water is described as having a high specific heat capacity.
The biological consequences include using water as a coolant. Body heat can be absorbed by the water molecules before entering the gas phase evaporating and in so doing reducing body temperature.
A body of water such as a pond, lake or ocean tend to be thermostable with considerably less variation in daily temperatures than land. This has consequences for the ecology and evolution of aquatic organisms.
3.1.6 Explain the relationship between the
properties of water and its uses in
living organisms as a coolant, medium
for metabolic reactions and transport medium .
Explain means to give a detailed account of causes, reasons or mechanisms.
Thermal Properties
Heat capacity is a measure of the energy input required for a certain rise in temperature.
It is important to define what is meant by temperature at this point. Temperature is the average kinetic energy possessed by a particle in a substance. For substances of large molecules, a certain mass of that substance consists of fewer molecules than a substance with smaller molecules. This means that for a certain energy input, the substance of larger molecules with receive more energy per molecule, than a substance of smaller molecules.
Thus the temperature of the substance of larger molecules should increase more than the substance of smaller molecules.
Following this argument through would lead to the explanation that water has a high specific heat capacity because its molecules are small, which compared to most other (more complex) substances is true.
The hydrogen bonding in water also provides another means of storing both KE and PE.
Water absorbs a lot of heat before there is a change in temperature
Therefore water is a useful substance for living things both physiologically and ecologically.
Physiology:Cooling effects
The heat generated by the body needs to be removed to prevent a denaturation of enzyme systems.
Water absorbed a great deal of energy before entering the vapour phase. This makes it an effective agent for the removal of heat and the maintenance of body temperature.
Blood which is mainly water can absorb and carry heat away from hot parts of the body to the cooler parts.
Ecological effects:
Oceans, lakes and ponds have fairly stable temperature (since it can absorb alot of heat without a temperature change) which means that poikilothermic organisms need not waste energy on thermoregulation. Surrounding air temperatures may show marked changes but water water in the same vicinity will remain relatively stable.
The graph show the relative temperature change of air and water in a pond from a temperate climate.
Medium for metabolic reactions:
Most biochemical reactions take place in solution where water is the solvent. The basis of this ability of water molecules to act as a solvent in covered in section 3.1.5. Remember that the cytoplasm is largely water and all cells exist in a medium which is water based e.g. tissue fluid, blood, pond.
Transport Medium:
Transport systems require something to do the 'transporting', carrying of material from one place to another. Blood carries nutrient, gases and waste all of which are dissolved in the water of blood. Once more this is achieved by taking advantage of the solubility of water.
All living things are composed of matter that contains the same core elements of carbon, hydrogen, oxygen and nitrogen. These elements are combined along with a few less common elements to give the 'biological molecules' of carbohydrates, proteins and lipids and DNA. These are not the only biological molecules but they are the main ones in a biological system.
Water is not a biological molecule but it is essential to living things. Water is often called the universal solvent. It forms the main component of solutions such as cytoplasm, blood and tissue fluid. Solutions form in the water of cells and it is here that the chemical reactions or metabolic processes of the cell take place. Water also provides an important habitat for living things. These aquatic habitats take diverse forms including streams, rivers, lakes and oceans. There are many more aquatic habitats but this list covers the major ones. The chemistry of water is surprisingly complex and there are still aspects of properties that remain unclear to science.
This image attempts to show the isolation of ions in solution caused by the 'cloud' of water molecules attracted through extended hydrogen bonding.