A2 REVISION NOTES ALL

·         Amino acids have an amino group (NH2) and a carboxyl group (COOH). The R group can be anything and makes each amino acid different. They may contain sulfur and phosphorus. The simplest R group is H, in glycine.

·         Amino acids have acidic and basic properties, making them amphoteric.

·         2 amino acids can join in a condensation reaction between the amino group of one amino acid, and the carboxyl group of the other. This makes a dipeptide. Water is eliminated and the bond between the 2 amino acids is a peptide bond. Lots of amino acids can join in a condensation polymerisation reaction to form a polypeptide.

·         The order of amino acids is the primary structure. Hydrogen bonds cause the chain to make an alpha helix or a beta pleated sheet which is the secondary structure. The R groups can form cross linkages, either disulphide bridges, ionic or hydrogen bonds to form the tertiary structure.

·         Proteins can either be globular and rounder, like haemoglobin, or fibrous and straighter, like gluten or collagen.

·         There are 20 naturally occurring amino acids, 8 are essential and cannot be mad by the body so must be eaten (9 in children) and 12 are non-essential which can be synthesis by the body by converting 1 amino acid to another in cells (11 in children).

·         The essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Histidine in children.

·         Biological value is the percentage of absorbed protein that is converted to body protein. So, proteins with more of the essential amino acids, have a higher biological value

·         High biological value proteins are animal sauces and soya and quinoa as they contain all the essential amino acids. Low biological value proteins are plant sources and are missing 1 or more of the amino acids. The amino acids that makes them low biological value is called the limiting amino acid.

·         The complementation of proteins is mixing low biological value proteins to make a high biological value dish. E.g. beans on toast, the beans lack in methionine and wholemeal toast lacks in lysine, so combining the 2 means neither are limiting. Chickpea curry and rice (lysine is lacking in the rice), peanut butter on toast (lysine is lacking in the nuts), dhal and rice, peas and rice, hummus and pitta bread.

·         As well as protein being used for growth and repair, it is also a secondary source of energy. Protein sources also contain other nutrients. Oily fish contains omega 3 fatty acids and the b vitamins, red meat contains iron and saturated and unsaturated fat, lentils contain fibre and some vitamin C, soya contains vitamin A and calcium.

·         Globular proteins are affected by heat, but fibrous ones are not (elastin and reticulin).

·         Heat causes cross linkages to break which hold the tertiary and secondary structure. The protein unfolds and…