Biological Molecules

Atom:          The smallest component of an element

                   Made up of electrons, protons and neutrons

                   e.g. Hydrogen

Molecule:    Consists of two or more atoms

                    Joined by a covalent bond

                    e.g. Iodine

Element:      A substance composed of only one type of atom

                    e.g. Carbon

Compound:  Combination of two or more elements

                    Joined by an ionic bond

                    e.g Water

Organic:     Any of a large class of chemical compounds which contain carbon

                  e.g. Glucose

Inorganic:  Any substance of which two or more chemical elements are are combined

                  e.g. Salt

Ion:            An atom/group of atoms which aare changed due to the loss/gain of an electron

                  e.g. Iron

Magnesium

• Important part of chlorophyll 
• Present in bones and teeth

Iron

• Vital part of haemoglobin 

Phosphate

• Found in ATP, nucleic acids, coenzymes and some proteins
• Role in the phospholipid component of cell membranes
• Structural role in bones and teeth

Calcium

• Main component of bones and teeth
• Important in plants - calcium pectate is a major component of the middle lamella

Water is essential because all reactions of life rely on water and key elements are found in aqueous solutions. 

Importance

A water molecule has no overall charge, therefore it is electrically neutral. Water is a polar molecule because the oxygen atom tends to pull electrons away from the nucleus of each hydrogen atom. Polar molecules allow the formation of hydrogen bonds. The hydrogen bonds stick together which is called cohesion and it is because of this, that water has many special properties, such as:

• Solvent - because water has a slightly positive and negative parts, it will attract other charged particles and other polar molecules which can then dissolve in the water. 
• Metabolite - in many reactions, water is either used up or produced.
• Surface tension - the cohesion between water molecules produces surface tension, this acts almost like a skin which covers the water.
• Thermal properties - the movement of water molecules is restricted by the hydrogen bonds, therefore a lot of energy is needed to raise its temperature (high specific heat capacity). Equally water has to lose a lot of heat energy to cool down.

Monomer:  a molecule which can be bonded to other identical molecules to form a polymer.

Polymer:   long chains of repeating monomer units.

A condensation reaction is when two monosaccharides join together to form a dissacharide and water.

A hydrolysis reaction is when a dissacharide is broken down to form two monosaccharides, a molecule of water is used up. 

Carbohydrates consist of carbon, hydrogen and oxygen. There is usually the same number of carbon and oxygen atoms but twice as many hydrogen atoms. It has the general formula C(H₂O)n. There are three main types of carbohydrates; monosaccharides or simple sugars, dissacharides or double sugars and polysaccharides.

Monosaccharides

These are the simplest form of carbohydrates and provide the building blocks for larger carbohydrate molecules. They also act as a respiratory substrate, providing cells with an energy source. 

Disaccharides

They are water-soluble and taste sweet. Disaccharides are more suitable for transport and storage than monosaccharides.

Polysaccharides

They are polymers made up of hundreds of monosaccharide units. Long chains of monosaccharides are held together by glycosidic bonds. The long molecules can be branched or unbranched. They are insoluble and not sweet to taste. 

Monosaccharides are monomers named according to the number of carbon atoms they have:

• Trioses have three carbon atoms (n=3)
• Pentoses have five carbon atoms (n=5)
• Hexoses have six carbon atoms (n=6) with the formula C₆H₁₂O₆

Monosaccharides usually exist as ring structures when they dissolve in water.

Disaccharides are formed by:

• two similar monosaccharides, e.g. glucose + glucose
• two different monosaccharides, e.g. glucose + fructose

Glucose exists as two isomers; alpha and beta glucose. These form two different polymers; starch (amylose and amylopectin) and cellulose.

Hydrogen bonding is important in maintaing the shape of biological molecules. 

Starch and Glycogen are storage polysaccharides because glucose can be added or removed easily and they have no osmotic effect in cells because they are insoluble. 

Cellulose and Chitin are similar structural polysaccharides with the alternating isomers allowing cross linking between chains, forming microfibrils. In Chitin, some OH groups are replaced by amino acids. 

Lipids are a large and varied group of organic compounds which include fats and oils. Fats and oils are chemically similar, but at room temperature, the fats are solid and oils are liquid.The elements which make up lipid molecules are carbon, hydrogen, oxygen and phosphorus. Lipids are immiscible with water but soluble in some organic solvents. Their main functions include; insulation, energy storage and protection. 

Triglycerides are one of the most common types of lipids. Triglycerides consist of one glycerol molecule with three fatty acid 'tails' attached. The formula for glycerol is C₃H₅ and the general formula for fatty acids is COOH. There are saturated and unsaturated fatty acids, saturated fatty acids contain a single bond whereas unsaturated fatty acids contain a double bond. 

A high intake of fat, especially saturated fat, is known to increase the risk of heart disease. 

Lipids are used instead of carbohydrates as an energy store in seeds and animals because of a high yeild of energy per gram. When a lipid is hydrolysed, the products are fatty acids and glycerol. 

Phospholipids consist of glycerol, fatty acids and a phosphate group. 

The glycerol is hydrophillic which means it is water loving, whereas the fatty acids are hydrophobic which means it is water hating. 

Amino acids are proteins and consist of an amino group (-NH₂) and a carboxyl group (-COOH). 

Proteins are polymers of amino acids of which there are twenty different types which differ by the 'R' group. 

Polymerisation occurs by condensation, to form peptide bonds giving rise to dipeptides and polypeptides.Proteins show a primary, secondary, tertiary and quaternary structure. Primary is the type, number and sequence of amino acids linked by peptide bonds only. Secondary is the most common is the alpha helix, formed by hydrogen bonding between peptide bonds in the polypeptide chain. Tertiary is the folding of the alpha helix, as shown by globular proteins, to form very specific three dimensional shapes. Projecting from the helix are -R groups which may interact to form bonds which help to maintain the tertiary structure's three-dimensional shape.There are four different types of bonds between -R groups; disulphide, ionic, hydrogen and hydrophobic. Quaternary is where two or more polypeptide chains in tertiary form combine to form complexes joined by bonds similar to those in tertiary structure. Only some proteins such as haemoglobin exhibit quaternary structures. 

Proteins can be classified according to function, which is determined by structure; globular proteins function as enzymes, antibodies and hormones; fibrous proteins such as keratin and collagen have alpha helices linked into strands.