Edexcel Biology Unit 1

Water is the medium in which all the reactions take place in living cells. Without it substances could not move around the body. Water is one of the reactants in the process of photosynthesis, on which all life depends.

The importance of water to biological systems is due to the basic chemistry of its molecules. You already know that the chemical formula for water is H20 from GCSE. However at this stage you need to know that water molecules are slightly polarised which means they have a slightly negative part (the oxygen atom) and slightly positive parts (the hydrogen atoms). This is due to the different electronegativities (electron pulling power) of the atoms which you don't need to know unless you do Chemistry. So to conclude we have a molecule with both negative and positive bits. This seperation of charges is called a dipole.

Now what happens is the negative oxygen part of one molecule is attracted to the positive hydrogen parts of another water molecule as opposite poles attract. This weak electrostatic attraction is called a hydrogen bond. THEREFORE, water molecules stick together better then you may otherwise expect. Although each individual hydrogen bond is weak, there are a great many of them!

Water has a relatively high melting andf boiling point compared with substances that have molecules of a similar size, e.g. C02, H2, NH3and 02 are all gases at room temperature and I'm sure you can think of more! In water it takes more energy to overcome the attractive forces of all the hydrogen bonds and that is why it is a liquid at room temperature.

Water is an unusual and excellent solvent

The fact that the water molecule has a dipole means that many ionic substances such as sodium chloride, which are made up of positive and negative ion, will dissolve in it. The water molecules surround each ion making them hydrated and seperate the substance ion by ion. For example the positive sodium ion in the sodium chloride will attract the negative charges of the oxygen atoms of the surrounding water molecules. When the water has surrounded the ion is breaks apart from the ionic lattice and becomes hydrated. This same ideology applies for other polar substances.

However, a molecule doesn't need to be charged for it to dissolve in water. They may form colloids, with solute particles larger than the solvent particles. The solute particles are spread through the water but do not seperate. This is important as chemical reactions in the body take place in water and is useless if the required solvents can not be transported together and are loosed on the way.

Some substances that do not dissolve in water are important in the body. Insoluble particles form emulsions (tiny droplets of one liquid suspended in another liquid). If you try putting oil over water it floats on drop which you may know if you ever tried cooking before. This is as the fat in the oil is hydrophobic which I shall explain later. Also suspensions is a solid mixed with a liquid, in which the particles will seperate out if the mixture is not constantly moved or stirred. Blood is an example of a suspension in plasma.

Water has one of the highest know surface tensions

Surface tension is a property of liquids, when they behave as if the surface is covered by a thin elastic skin. Water has a high surface tension due to the many hydrogen bonds that hold them together which was discussed earlier. Surface tension is of great importance in plant transport systems which you learn in more detail in Unit 4.

The water molecule is amphoteric

Not as scary as it sounds honest! Amphoteric is just a fancy word meaning that a water molecule can act both as an acid and a base.

Acids form H+ ions and act a proton donor, takes electrons.

Bases form OH- ions and is a proton acceptor, gives electrons away.

This ability of molecules to both donate and receive protons/electrons makes it an ideal medium for the biochemical reactions occuring in cells. It acts as a buffer, helping to prevent reactions in progress from changing the pH inside the cell. Any excess H+ or OH- ions are mopped up.

SUMMARY

• Water molecules are dipole which results in hydrogen bonds being formed between other water molecules.
• Water has a relatively high melting and boiling point compared to other substances that have molecules of a similar size as it takes more energy to overcome the attractive forces of all the hydrogen bonds.
• Most ionic substances dissolve in water.
• Substances that are non polar can form colloids.
• Insoluble particles form emulsions or suspensions in water.
• Water has one of the highest known surface tensions.
• The water molecule acts as a buffer as it can act both as a base and an acid.

Questions

1) How are hydrogen bonds formed between water molecules and what affect to they have on the properties of water?

2) Plasma is a solution, cytoplasm is a colloid and blood is a suspension. Explain how the properties of solutions, colloids and suspensions adapt these biological materials to carry out their functions in the body.

Question one is pretty straight foward but question two requires a bit more thinking.

Carbohydrates

Carbohydrates are important in organisms as an energy source. Some are broken down to provide usable energy in plant and animal cells, while others are important for storing energy in the cells. The best known carbohydrates are sugars, such as sucrose, glucose and starches.

The basic structure of carbohydrates are the same as they are made out of carbon, hydrogen and oxygen molecules. There are three main groups of carbohydrates I will be going over which are: monosaccharides, disaccharides and polysaccharides.

Monosacchrides - the simple sugar

The monosaccharides are known as the simple sugars. Their molecules containt carbon, hydrogen and oxygen molecules. A general formula for a simple sugar is (CH2O)n

Here n can b any number but it is usually small. Triose (n=3) sugars have the general formula C3H6O3. These are important in mitochondria, where glucose is broken down into triose molecules in respiration. The pentose (n=5) sugars which you may no better as ribose and deoxyribose are important in the nucleic acids of DNA and RNA which is involved in your genetic makeup. Thewse sugars have the general molecular formula C5H10O5.

The important monosacharide which you need to know however is the hexose sugars(n=6), such as glucose, galactose and fructose, which have the general formula C6H12O6.

You also need to know how a displayed formula looks like.