To emphasise the sheer importance of water in living things and to put the content of this essay in to perspective, I am firstly going to inform you that the human body is approximately 50-75% water. Without water our body would be unable to successfully perform the numerous complex reactions and processes required for our functioning. Throughout my essay I aim to present to you several uses of water particularly focusing on the human body’s reliance on water to function. To appreciate the important uses of water in living things it seems vital that we first explore the behaviour of water.
As water is a polar molecule, substances are easily able to dissolve in it and this results in the formation of a solute/solvent mixture where the water molecules cluster around the solute, meaning there are less water molecules that are free to occupy the space surrounding the cluster and hence we say the concentration is lower than before. Water potential is a measurement of the free water molecules that are not clustered around the solute; it measures the tendency of such molecules to move down a concentration gradient and in to another area, represented by the unit (potential energy per volume).
Pure water that does not contain any solute is said to be 0 on the concentration gradient scale and so if you were to then add solute to this water you would be decreasing the water potential, essentially making it less than zero and thus ‘more negative’. Water will always move from a region of higher water potential (where there is less solute) to an area of lower water potential to try and counteract the added solute and balance the concentrations.
Water is also able to diffuse across a partially permeable membrane, which is fundamentally a membrane that will allow specific molecules/ions to diffuse through it. We can link water potential to osmosis – Osmosis is defined as ‘the movement of water molecules from a region of higher water potential to a region of lower water potential through a semi-permeable membrane’, it only involves water molecules which is beneficial particularly within our bodies, as if there is
a high concentration of a solute within a cell, i. e. sodium chloride, then the water within our bodies can cross specific cell membranes unaccompanied by the solute in order to balance the concentrations. Osmosis is incredibly important in animal cells, as animal cells will expand when placed in a solution of higher water potential, and as these cells do not have cell walls it is possible that excess water could cause the cell to burst (haemolysed), which could prove fatal in particular cells such as red blood cells.
In the same concept, if too much water leaves an animal cell by osmosis then it will lose its stability and shrink (crenated), which highlights the important uses of water in living things as without processes such as osmosis our cells would be unable to function. Water is crucially important for two reactions that occur in living organisms – condensation and hydrolysis reactions. A condensation reaction is when two molecules containing hydroxyl groups (an OH group) are joined together to form a larger molecule with the removal of a water molecule.
In this reaction the hydroxyl group from one molecule is removed by an enzyme and joined with a hydrogen atom that has been removed from the other, speeding the formation of a bond between the two molecules at the exposed sites. An example of an imperative condensation reaction is the formation of a peptide bond between two amino acids which accumulate to make proteins; in this reaction the hydroxyl group is removed from the carbon atom of the amino acid and the hydrogen atom is removed from the nitrogen atom of the other.
Without this condensation reaction the proteins that contribute to our genetic make-up would not be formed. Hydrolysis is effectively the reverse of condensation and plays an equally important role. Defined as ‘the cleavage of chemical bonds by the addition of water’ hydrolysis, in most reactions, causes both the substance to which the water has been added to and the water molecule itself to split into two parts- resulting in a specific site of the target molecule gaining a hydrogen molecule. The use of water to break large molecules in to smaller ones is extremely useful in living things particularly in the digestive system.
The diet of an animal largely consists of carbohydrates, proteins and fats that have to be digested and absorbed by the intestines to gain nourishment, however as the molecules are complex in their structure they cannot be absorbed directly by the intestinal cells, hence they require breaking down. Despite these molecules being distinctively different the same hydrolysis reaction is able to break down each of their complex structures in to smaller molecules during chemical digestion, and so we will focus on the starch molecule to gain an understanding of this process.
Starch is a carbohydrate consisting of a large number of ? – glucose units that are joined by glycosidic bonds, and as glucose is a monosaccharide the units or ‘chains’ are formed through a condensation reaction where a molecule of water is removed. Thus, in order to break down the units in to simple glucose molecule an enzyme is required to return the molecule of water and break the glycosidic bond; in this specific reaction the enzyme that breaks down starch is amylase, which hydrolyses the bonds and breaks the polysaccharide first in to a disaccharide named maltose and then finally in to glucose.
The glucose can then be directly absorbed in to the intestinal wall. The hydrolysis reaction in digestion exaggerates the importance of water in living things; as without it we could not digest the important molecules found within our food that aid our survival. The final use of water in living things that I am going to cover in this essay, and in my opinion the most important, is the role of water in blood and tissue fluid. The role of tissue fluid (or interstitial fluid) within the body is to bathe and surround the cells, providing protection and allowing for the absorption of molecules or ions.
It is formed through hydrostatic pressure that is generated through the force of the heart muscle pushing water out of the arteriole end of the capillary. The blood in the capillaries contains plasma proteins which means it has moderately low water potential – this draws water out of cells and in to the blood. However, the hydrostatic pressure created by the heart muscle has a greater effect than the low water potential at the arteriole and consequently the fluid leaves the capillary to form tissue fluid.
The tissue fluid contains useful materials such as amino acids and glucose which diffuse in to the cells surrounding the vessels, providing them with the necessary substances they require to function. The effect of this loss of fluid means that the hydrostatic pressure at the venule end of the capillary is lower and the water potential is greater, and subsequently fluid drains back in to the blood once the useful substances have been transferred to the cells.
Hence, we see the absolute importance of water in living things particularly involving the blood vessels and transportation of molecules from cells in to the blood stream. To conclude, water is undoubtedly useful in living things. Without water we as human beings could not survive any longer than 3 days, as our vital organs and the chemical reactions that aid our survival would ultimately fail.
Courtney from Study Moose
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