The roles water in living organisms and water as a habitat for organisms
The roles water in living organisms and water as a habitat for organisms
Introduction
Water is the most abundant liquid on the planet. It covers 2/3rds of the earth's surface, and at least 60% of living organisms is water. This value may be as large as 99% - in a jellyfish. The reason for such a large amount of water making up organisms is the fact that it is a major cell constituent - more on that later.
Water is a simple molecule yet has many interesting properties which contribute to its importance.
Structure
The molecule is polar, because the electrons in the bonds between O and H are pulled slightly toward the O. this means that the O atom is slightly negative and the two H atoms are slightly positive (shown by ?+ and ?-). The electrons are shown as small dots; note they are closer to the O.
These properties are described more in the solvent section. db.
Water is made up of two hydrogen atoms bonded to one oxygen atom. They are covalently bonded and the intramolecular covalent bonds are very strong. The structure is non-linear, due to the electron pair repulsion of the two lone pairs of electrons on the oxygen atom.
Major cell constituent
Often 2/3rds or more of living cells are water. The water is found mainly in the cytoplasm, and it plays a vital role in many functions of the cell: in all organisms - metabolism, and in plants photosynthesis and support. The actual roles of water in these functions are described more fully later.
Polar solvent / universal solvent
The pulling of the electrons towards the O makes the O electropositive and the H's electronegative. As opposites attract, not only are the hydrogen atoms attracted to their O atom electrostatically, but also to the O atoms in neighbouring molecules. These bonds are called hydrogen bonds, and are responsible for the great strength and unexpectedly high melting and boiling points of water.
The diagram above shows the differing electronegativities, and it is for the polar nature of this molecule that water is such a good solvent. More substances than any other can dissolve in it, and it has been named the universal solvent.
In plants, mineral salts can only be obtained when in solution, and water makes this possible. Likewise in humans, digestion only occurs when the food is in solution. The reactions of metabolism take place in solution, and also waste products are removed in solution.
Also, gas exchange in the alveoli requires a moist surface, and gas exchange takes place in solution.
Transport
Both blood and sap are essential for the transportation of food, waste products and many more substances in animals and plants. These two mediums consist of mainly water, and it ensures that the substances to be transported are in a solution (as water is the universal solvent).
Lubricant
Outside of cells, water is also useful by reducing friction and providing protection and cushion. In joints, water is a major part of synovial fluid, which prevents the bones from grinding against each other.
Cerebro-spinal fluid, amniotic fluid, mucus and pleural fluid (the latter in the lungs). All of these are mainly water, and provide either lubrication or protection and cushion, in different ways.
Support
The turgidity of plants cells is what keeps them upright, and is down to the fact that the cell exerts a force equal to the force of the water entering the cell by osmosis. This balance of force makes the cell stiff. This turgidity helps to support the leaves and stems of many plants.
Reactant
Reactions are constantly occurring in an organism, catalyzed by enzymes. Water allows many of these reactions to occur, as often the reactants need to be ionized.
Water also plays a major part in gaining energy from the sun, as it is a key part in the reaction by which green plants obtain light energy from the sun and store it as chemical energy:
6H2O + 6CO2 C6H12O6 + 6O2
Monosaccharides (such as alpha glucose) join together to by the process of condensation to form larger molecules of starch. In this reaction, a glycogen bond is formed and a water molecule is released when the two hydroxyl groups join (leaving one O atom). The reverse of this is called hydrolysis, and in order to break down the larger starch unit into its glucose sub-units for digestion, a water molecule is needed. This is another important reaction, where water plays a key part.
Cohesion of molecules
Water molecules are attracted to other water molecules. Water can also ...
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Monosaccharides (such as alpha glucose) join together to by the process of condensation to form larger molecules of starch. In this reaction, a glycogen bond is formed and a water molecule is released when the two hydroxyl groups join (leaving one O atom). The reverse of this is called hydrolysis, and in order to break down the larger starch unit into its glucose sub-units for digestion, a water molecule is needed. This is another important reaction, where water plays a key part.
Cohesion of molecules
Water molecules are attracted to other water molecules. Water can also be attracted to other materials, this is known as adhesion. As explained above, the O atom of one molecule is attracted to the H atoms in the next. This attractive force is what gives water its cohesive and adhesive properties.
Surface tension
As the water molecules are cohesive to each other, this results in surface tension of the water as the molecules along the surface are 'holding' each other together.
This property allows organisms, such as the pond skater, to settle on or skate over water.
Sexual reproduction
In the process of fertilization, the sperm is transported to the egg in a fluid medium known as semen - this contains mostly water.
Density
One of the unusual things about water is the fact that the solid form (ice) is not as dense as the liquid form, and below 4?C the density of water decreases.
This property means that ice floats on water and insulates it, reducing the probability of the whole mass of water to freeze. This increases the chance of life surviving under such harsh conditions.
Temperature control
Water has a very high specific heat capacity (4200J/Kg?C). This means to raise the temp of water by 1?C, 4200J are needed, so the temperature of water is not easily changed.
This means that a large body of water like an ocean will heat up in the day and cool down in the night, but only slightly. This provides an ideal habitat for marine organisms.
When mammals get hot, they sweat. Sweat contains mostly water, and therefore a fairly large amount of energy required to evaporate it - this has a large cooling effect on the body.
Plants lose water via transpiration, which helps the uptake of nutrient rich water from the soil, and also prevents the temperature of the plant from rising too high.
Habitat
Water is an ideal medium for life because:
* It provides a protective shield
* It prevents the cell from drying out
* It provides support and buoyancy to organisms
* Its good solvency favours the supply of nutrients to the plants, and the removal of waste products from all organisms.
* It allows the oxygen required for respiration to be dissolved in it, and its good mobility means it can get virtually anywhere in an organism.
* Fertilization - a surrounding body of water disperses the offspring reducing the risk of competition.
* Aquatic organisms have very little need for temperature control mechanisms.
* Water filters out harmful UV rays from the sun.
Water is arguably the most important biochemical of all. Water is essential to life itself, without water life on earth would not exist. Water is a major component of cells, typically forming between 70 and 95% of the mass of the cell. This means that we are made from approximately 60% water by mass. Three-quarters of the planet is covered in water.
Water itself is a simple molecule made up of 2 hydrogen atoms and one oxygen atom, H20. The hydrogen and oxygen atoms are bonded covalently as shown in the diagram of waters molecular modal. Water is not a linear molecule; the two hydrogen atoms form a bond with the oxygen at the angle of 104.50.
Diagram of molecular modal of water:
Covalent bonds are formed by sharing electrons in the outer orbits of the quantum shells. In the case of water however the large number of protons in the oxygen nucleus have a stronger attraction for these shared electrons than the comparatively tiny hydrogen nuclei. This pulls the electrons slightly closer to the oxygen nucleus and away from the hydrogen so that the oxygen develops a slight negative charge and the hydrogen's a slight positive charge. This makes the molecules slightly polar. Molecules, which have charged regions, are called polar molecules. Water has both partial and negative charges and is therefore described as dipolar. Another thing to mention is that there is an unequal distribution of charge this is why it is called a dipole.
This slight charge means that when water molecules are close together the positively charged hydrogen atoms are attracted to the negatively charged oxygen atoms of another water molecule to form a weak hydrogen bond (H - bonds). This mutual attraction explains why water is a liquid at standard temperature and pressure (STP) whereas other, similar sized, non- polar molecules like methane (CH4), ammonia (NH3) and hydrogen sulphide (H2S) is gases.
The hydrogen bonds are weak individually but the sheer number of them means that the total force keeping the molecules together is considerable. The hydrogen bonds in water are broken when energy is supplied, e.g. when water is heated. When they break the liquid becomes a gas (vapour). At 00c and below the H - bonds are at their shortest and strongest and lined up to form an open regular crystalline structure, which is ice.
Diagram of water molecules forming hydrogen bonds:
Water is an unusual substance, mostly due to its hydrogen bonds; its properties allow it to have different abilities.
Water can dissolve polar or ionic substances and can keep them in solution because of water's own polar properties. Substances that dissolve in water are known as hydrophilic substances. Ionic substances such as sodium chloride, NaCl, are made up of positive and negative ions. Sodium chloride is held in its structure by the strong attraction between its positive sodium ions and negative chloride ions. Normally these ionic attractions require a large amount of energy to break but when put into water the negative oxygen side of the water molecules cluster around the positive sodium ions Na+ and the positive hydrogen atoms cluster around the negative chloride ions Cl-. The attraction between the Na+ and Cl- ions is weakened as the ions are separated.
Diagram of sodium chloride dissolving in water:
Water can also separate covalently bonded molecules such as glucose and sucrose because the polar hydroxyl groups (- OH) in its structure forms hydrogen bonds with the water separating the molecules from each other.
As you can see the chemical structure of water determines its properties. The following table lists these properties with examples of their biological importance.
Water as a solvent: -
It is an excellent solvent for ions and polar molecules because of its polarity and its hydrogen bonding. This is also because the water molecules are attracted to them, collect around them and separate them. This is what happens when a chemical dissolves in water. When a chemical is in solution it is free to move around and react with other molecules. Any molecule with a polar region will dissolve, including sugars and alcohol, and water molecules gather around large protein molecules to form a special kind of solution called a colloid. Most processes in living cells take place in this way. Water's properties as a solvent are vital to life as most biochemical reactions such as respiration occur in solution. This is why cell cytoplasm contains about 90% water. Water cannot dissolve hydrophobic substances such as fats and oils; these are used by organisms as cell membranes to separate cells and also as waterproofing as they prevent water from entering the organism if it is covered in a hydrophobic (water-hating) substance. Hydrophobic interactions in protein structure and in membrane structure are where it increases the stability of these structures.
Thermal properties: -
Due to the hydrogen bonding it restricts the movement of water molecules, a relatively large amount of energy is needed to raise the temperature of water. Water also has many thermal properties as a temperature stabiliser. Water has a boiling point of 100oc and a melting point of 0oc, this is unusual for a molecule of it's size (water's RMM is 18) because other molecules of a similar size such as CO2 - RMM 44 and ammonia NH4 - RMM 18, are all gaseous at room temperature whereas water is a liquid. This is because of the hydrogen bonds, which hold the water molecules in a liquid state.
Water also has a high specific heat capacity; the result of this is that it takes 4.2 Joules of energy to raise one gram of water by 1oc. This means that it takes a lot of heat energy to raise the temperature of water significantly, but once warm it cools slowly. This is essential to life where internal body temperature has to be maintained at a constant temperature and fluctuations can result in a breakdown of essential processes. Large bodies of water will remain at an almost constant temperature with only very gradual changes which makes temperature regulations for organisms far more straightforward.
Because of the large number of bonds holding water molecules together, it takes 2 kJ per gram of water, which is a considerable amount of energy to separate the bonds and turn the liquid to vapour.
Water is therefore described as having a high latent heat of evaporation. Animals use this property of water by using excess body heat to evaporate water from their surfaces, resulting in them transferring a lot of energy into the environment but only losing a little water. Sweating and panting are based on this principle. Also because a large amount of energy must also be transferred from water to convert from liquid to solid (ice), it is quite difficult for water to freeze, which is an advantage both to organisms living in water, and to the bodies of living organisms.
Latent heat of evaporation: -
Water also has a high latent heat of fusion from solid to liquid. It requires 300 J per gram of ice to melt it to water. This means that water stays liquid. This is vital in the case of cytoplasm in cells, which is made of a high percentage of water because once frozen the cell would be irreparably damaged. The freezing point of water is also lowered by solutes because the soluble molecules disrupt the hydrogen bonds making the water freeze at a lower temperature and it easier to melt ice. As there are many solutes in cytoplasm the water will not freeze until well below 0 oc and the cells are protected until the temperature gets extremely low.
Density and Freezing properties: -
Water is the only chemical where the solid is less dense than the liquid form. As water cools it's density increases and the hydrogen bonds between the water molecules take on a more latticed formation as ice. Yet ice floats on the surface of water which means its density must be lower than that of water. Water is at it's most dense at 4oc which is when its bonds are closest together. When water freezes the lattice arrangement of its structure move apart slightly and it floats on the surface. This means that the layer of ice insulates the water below, which stays at 4oc, and aquatic life can continue. This reduces the likelihood of large expanses of water such as lakes and oceans freezing over, and therefore increases the chances of organisms living in the water to survive. Changing densities with temperature, cause currents to form in masses of water, which help to circulate nutrients within oceans. The fact that water is most dense at 40c means a layer of water at this temperature will always sink to the bottom, ensuring a life-supporting band is maintained at the bottom of even the most heavily frozen lake.
Water as a transport medium: -
Water is the medium for many reactions, especially those, which occur in cells. Water is key in condensation reactions where water is removed from molecules to bond them together, this occurs with many sugars and carbohydrates. Water is a transport medium in blood, lymphatic, excretory and digestive systems of animals, and in the vascular systems of plants. In this role its solvent properties are essential.
Water used for hydrolysis: -
Water is also used to split up molecules by adding water. This is called hydrolysis and is essential to animals and plants because it allows them to utilise stored foods, which are in long chains by breaking off smaller molecules.
Water used for plants: -
Respiration produces water as a by-product. A large amount of energy is produced by oxidising hydrogen, which contains so much energy it is an explosive gas, into water. Photosynthesis uses water as a source of hydrogen atoms, which are needed to produce glucose, which is then stored in the plant cells as starch or used for respiration. Without water these two essential reactions would not occur and life would not be able to continue on earth.
Transparent: -
An important property of water is also it's transparency, it allows sunlight to pass through it so aquatic plants can photosynthesis and on a larger scale. This allowed life on earth to begin, since life started in the oceans with small organisms, which relied on sunlight for the reaction photosynthesis to take place.
Cohesion and surface tension: -
Water molecules are highly cohesive because of the hydrogen bonds between the molecules. Water forms spherical droplets, which have the maximum inner area and least surface area when in contact with a hydrophobic material. The cohesive properties of water allows plants to pull up water through xylem vessels from the roots to the leaves, this is called the transpiration stream. It also means that the water molecules where the water meets the air will be tightly held together and the water molecules below them to form an elastic film known as surface tension. Small creatures can get stuck in the surface water because they cannot break the water surface tension, creatures like pond skaters can move across the surface of the water without sinking as they have hydrophobic feet which stops them from breaking the surface tension.
Diagram of surface tension:
A pond skater standing on the surface of pond water. This was photographed through an interferometer, which shows interference patterns made by a pond skater as it walks on the water's surface. The surface tension of the water means the pond skater never breaks through the surface.
Incompressibility: -
Because of water's strong hydrostatic forces water is incompressible. This provides support for soft-bodied creatures such as worms slugs and jellyfish, which therefore do not require a supporting skeletal system. Water allows cells filled with water to become turgid and due to its incompressibility plants can support themselves.
Roles of water in the environment: -
Water is very important as an environment for living organisms to live in e.g. fish, insects. Some organisms spend the whole of their life cycles in water and are totally dependent on it (e.g. fish), whilst others are only dependent for part of their life cycles, usually the reproductive phase e.g. amphibians, mosses. Soft bodied creatures such as jellyfish are made of up to 96% water. Water also provides an environment for organisms to live in, 75% of the earth is covered in water.
Conclusion: -
In conclusion water's unique properties make it perhaps the most biologically important substance on the planet. No other substance shares similar properties to water and in the way that one single molecule can possess such varied and essential characteristics.