Biochemistry Carbohydrates Monosaccharides join together to form disaccharides in a condensation reaction. During this reaction, a molecule of

water is released and a glycosidic bond forms between the two monosaccharides.

There are three main disaccharides that are needed in the exam:

Monosaccharide 1 Monosaccharide 2 Disaccharide

Glucose Glucose Maltose

Glucose Fructose Sucrose

Glucose Galactose Lactose

Many monosaccharides join together to form polysaccharides, again through condensation reactions.

There are three main polysaccharides needed for the exam:

Polysaccharide Structure Function Extra Info

Starch Amylose

A long, unbranched chain of -glucose. The bond angles give it a coiled structure.

The coiled structure makes it very compact, so it is a good for storage of glucose.

In flowering plants, starch granules are kept in organelles called plastids.

Amylopectin

1,6-glycosidic links between -glucose create a highly branched molecule.

The branched structure allows enzymes to get at more bonds, since the surface area increases.

Glycogen

Glycogen has more 1,6-glycosidic links than amylopectin so is much more branched.

Also a storage molecule, especially in liver and muscle cells.

Glycogen can be broken down more rapidly than starch, which is why animals have a higher metabolic rate.

Cellulose

A straight chain of -glucose. The straight chains enable them to lie parallel to each other and to be strengthened by hydrogen bonds between adjacent chains, forming microfibrils.

Microfibrils give cellulose its high tensile strength, and so it is a structural polysaccharide. The cell wall prevents the cell from bursting when they swell due to osmosis.

Cellulose is completely permeable. In wood, it is strengthened further by lignin.

Disaccharides and polysaccharides can be broken down into monosaccharides through a hydrolysis reaction, where a

water molecule is added to the molecule to break it apart.

Glucose is the most common monosaccharide, and has two main isomers: -glucose and -glucose.

To test a food for the presence of a carbohydrate, Benedicts test can be used. There are two forms of this test: one

for reducing sugars, and one for non-reducing sugars.

There is also a specific test for the presence of starch, called the iodine test. Simply add iodine dissolved in

potassium iodide solution to the test sample. If there is starch present, the sample changes from a brown-orange

colour to a dark, blue-black colour.

Amino Acids and Proteins All proteins are made up amino acids. All amino acids have the same basic structure:

Two amino acids can combine to form a dipeptide via a condensation reaction, and similarly they break apart

through a hydrolysis reaction. A peptide bond forms between the two amino acids.

When many amino acids join together, a polypeptide is produced. Eventually, a protein is produced, which have four

main structures.

Primary This is just a long, linear sequence of amino acids.

Secondary Hydrogen bonds begin to form between the amino acids, which leads to -helices or -pleated sheets.

Tertiary The coiled or folded chain is coiled and folded further. More hydrogen bonds form between different parts. Di-sulphide bridges form between cysteine molecules (due to their sulphur atom). This gives them a globular shape.

Quaternary Different polypeptides bond together to form quaternary proteins, such as haemoglobin, insulin, and collagen.

The shape of a protein influences its function haemoglobin is compact and soluble, so its good to transport.

Enzymes Usually spherical and soluble. They are an essential part in metabolism, and are involved in both anabolic and catabolic reactions. Theyre globular shape gives them the active site that can attach to the substrates.

Antibodies Made up of two short polypeptide chains and two long polypeptide chains bonded together. They have variable regions where the amino acid sequences differ greatly.

Transport Proteins

Present in the plasma membranes. They contain hydrophobic and hydrophilic amino acids, which causes the protein to fold up and form a channel, allowing molecules and ions to travel across the membrane.

Structural Proteins

Usually consist of long polypeptide chains lying parallel to each other with cross links between them. Examples are keratin and collagen.

The presence of a protein in a food molecule can be tested for using the biuret test.

1. Add a few drops of NaOH to the sample to make it alkaline.

2. Add some CuSO4 to the solution.

If a protein is present, the solution will turn from blue to purple.

Lipids and Phospholipids Triglycerides are lipids made from glycerol and fatty acids. Glycerol is an alcohol containing three carbon atoms, each

linked to a hydroxyl group, propan-1,2,3-triol. Fatty acids are simply carboxylic acids.

A triglyceride is formed when each hydroxyl group combines with a fatty acid in a condensation reaction to form

three ester bonds.

A saturated fatty acid has no double bonds, which makes it easier to pack together, so they are usually solid.

Unsaturated fatty acids have double bonds, and can be cis- or trans-. Triglycerides in cis- form tend to be oils due to

the kink, which prevents them from packing closely together.

Phospholipid molecules consist of glycerol attached to two fatty acids, and a phosphate group attached to the third

carbon atom. The phosphate group is polar and hydrophilic, whereas the fatty acids are oily and hydrophobic.

An emulsion test can be used to find out if lipids are present in a food.

1. Shake the test substance with ethanol for roughly a minute.

2. Pour the solution into water.

Any lipid present will show up as a milky emulsion.