Biological Molecules

The structure of proteins can cause them to have two different shapes: globular and fibrous. Fibrous proteins are long parallel strands which are insoluble, and have important structural functions, e.g. hair, skin nails. Globular proteins are highly folded into a compact, rounded shape and are soluble in water. This makes them important in metabolic functions, e.g. enzymes, haemoglobin, insulin.

        

Carbohydrates can be divided into three main structures; monosaccharides, disaccharides and polysaccharides. All monosaccharides are sweet, soluble and crystallite. Different monosaccharides have different numbers of carbon atoms in their structure, therefore giving them different properties and functions within living organisms. Hexoses have six carbon atoms. There are many different isomers of hexose, which have the same number of carbon atoms but are structured in different ways, which gives them different properties and functions within the human body. For example glucose is an isomer of fructose. Although they have the same number of hydrogen, oxygen and carbon atoms, they are structured differently within the sugar. This gives glucose and fructose completely different properties. Glucose is a means of transferring energy around the body as blood sugar (amongst other things) whereas fructose is found in many sweet tasting fruits. Stereoisomers can also have completely different properties and functions even though their structures are slightly different, e.g. α glucose and β glucose.

Disaccharides consist of two monosaccharides bonded together by glycosidic bonds in a condensation reaction. A variety of different disaccharides can be formed (each with different properties and functions) depending on the monosaccharides involved. An example of a disaccharide is Maltose, which is formed by two α glucose molecules. Other disaccharides can be formed such as sucrose and lactose, each of which has different properties and functions.

Polysaccharides are formed from the linkages of many monosaccharide units. Examples include starch, glycogen and cellulose. All three are formed by the linkages of glucose units in long chains or branched chains. They have 1-4 and/or 1-6 glycosidic links. Starch and glycogen is made up of repeating α glucose units which are both in long chains and branched chains. They are structured like this so that they take up less space in living organisms. The result of this structure is a storage function in plants and animals. Cellulose is made up of repeating β glucose units lined up in long chains. The result is a strong structure used for cell walls. Although starch and cellulose is made up of a slightly different form of a glucose sugar, it has a completely different structure with different properties and functions within living organisms.

        

        Lipids are a combination of glycerol and 3 fatty acids in a condensation reaction. They are known as triglycerides. This general structure is modified in some lipids with the addition of other chemical groups, giving the molecule different properties. All lipids are made up of the elements Carbon, Hydrogen and Oxygen.

Fatty acids can either be saturated or unsaturated, depending on its structure. The difference between the two is that saturated fatty acids have no double bonds between the carbons whereas unsaturated fatty acids do have a double bond between the carbon atoms. They can have one or more double bonds in the chain.

The structure of a saturated fatty acid is relatively straight, so they pack together easily and become solid more readily therefore forming fats, e.g. animal fats. Unsaturated or polyunsaturated fatty acids (unsaturated fat with more than 1 double bond between the carbons) have bends in the hydrocarbon chain where there is a double bonded carbon, which increases the distance between the molecules, increasing fluidity. They have a low melting point and are usually liquid at room temperature, e.g. plant oils. So an addition of a double bond between the carbons in fatty acids causes them to have different structures and properties, giving them different functions in living organisms.

Some lipids are a combination of glycerol 2 fatty acids and a hydrophilic phosphate group. This is known as a phospholipid. The head (polar head) of the phospholipid contains a negatively charged phosphate group which is hydrophilic (water liking). The non-polar tail of the phospholipid is hydrophobic (water hating). For this reason phospholipids are important in cell membranes as they separate water from outside the cell from the water inside the cell. Therefore water cannot enter the cell. The replacement of one fatty acid by phosphate in a lipid changes its properties and functions completely.