Biology Unit 1 & 2 AQA

Basic biology unit 1 information for the AQA exam board.

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  • Created by: Sabina
  • Created on: 17-04-12 13:57


Proteins are made from long chains of Amino Acids.

All Amino acids have the same general structure consisting of a, Carboxyl group (-COOH) and an amino group (-NH2) attached to a carbon atom. Different Amino acids have different variable groups (R):


                    H2N    C    COOH


Amino acids are joined together by peptide bonds to form dipeptides or polypeptides.

It is a condensation which means water is released during the reaction. The reverse of the reaction adds a molecule of water to break the peptide bond. This is called a hydrolysis reaction.

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Structure of a Protein

Proteins have four structural levels which are Primary, Secondary, Tertiary & Quaternary.

Primary: the sequence of amino acids in the polypeptide chain.

Secondary: Hydrogen bonds form between the amino acids in the chain which cause the chain to automatically coil into a alpha helix or fold into a beta pleated sheet.

Tertiary: the polypeptide chain folds or coils further and different kinds of bonds form Ionic bonds, Disulfide bonds and hydrogen bonds creating a 3D structure.

Quaternary: More than one polypeptide chain in a protein.

A protein's 3D structure allows it to carry out its function e.g. insulin's function to make cells take up glucose from the blood. It's a compact protein which makes it easy to transport around the body in the blood so it can carry out it function.

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Types of Proteins

Proteins can be Fibrous or Globular:


  • Round, compact proteins made up of multiple polypeptide chains.
  • The chains are coiled so that the hydrophillic (water attracting) parts are on the outer side of the molecule and hydrophobic (water repelling) parts inwards.
  • Soluble for easy transportation in fluids.

E.g. Haemoglobin made of 4 polypeptide chains, carries oxygen around the body in the blood. Its soluble so can easily be transported in the blood & has iron containing haem groups that bind to oxygen.


  • made up of long, insoluble polypeptide chains tightly coiled round each other to form a rope shape held together by lots of bonds, disulfide and hydrogen bonds making the proteins strong.
  • Because they are strong fibrous proteins are found in supportive tissue.

E.g. Collagen is strong, fibrous protein that forms supportive tissue in animals made of 3 polypeptide chains tightly coiled into a triplet helix, the chains are interlinked by strong covalent bonds. Minerals can bind to the triple helix to increase its rigidity.

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Protein functions

Proteins have a variety of different functions. There are loads of different proteins found in living organisms with different structures and shapes which makes them specialised to carry out particular jobs. A common proteins are:

Enzymes: spherical in shape due to tight folding of the polypeptide chains, they're soluble  and often have roles in metabolism e.g. break down large food molecules and other enzymes help to synthesise (make) large molecules.

Antibodies: involved in the immune response made up of 2 light polypeptide chains and 2 heavy polypeptide chains bonded together. They have variable regions where the amino acid sequences vary greatly due to there being many different antibodies for specific antigens.

Transport Proteins: present in cell membranes consisting of hydrophobic and hydrophillic amino acids causing the protein to fold up and form a channel. They transport molecules and ions across cell membranes.

Structural Proteins: physically strong proteins consisting of long polypeptide chains lying parallel to each other with cross links between them e.g. keratin found in hair and nails and collagen found in connective tissue.

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Carbohydrates are made from monosaccharides.

Most carbohydrates are large, complex,molecules composed of long chains of monosaccharides. Glucose is a hexose sugar or a monosaccharide with 6 carbon atoms in each molecule.

There are 2 forms of glucose - alpha and beta glucose:


Glucose structure is related to its function as the main energy source. It's structure makes it soluble so it can easily be transported.

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Disaccharides are 2 monosaccharides joined together by glycosidic bonds in a condensation reaction.

A condensation reaction is where H on one monosaccharide bonds to OH on another monosaccharide releasing a molecule of water. The reverse of this is a hydrolysis reaction where a molecule of water reacts with the glycosidic bond breaking it apart. Glycosidic bonds form at points 1-4


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Polysaccharides are chains of more than 2 monosaccharides joined together by a condensation reaction whereby a molecule of water is released.

Lots of alpha glucose molecules are joined together by glycosidic bonds to form amylose.

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