AQA AS Biology 3.1.2

All about proteins and carbohydrates in compliance with the specification.

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  • Created on: 23-10-12 10:51
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What are proteins?
Proteins are made when amino acids joining together to form polypeptides. These polypeptides are joined together
by peptide bonds. Almost like glycosidic bonds except with amino acids as opposed to glucose. There are around 20
different amino acids but since they can be joined in any order, there are a huge number of proteins out there.
How can we test for proteins?
We can test for the presence of proteins through the use of the biuret test. This involves adding biuret solution
(consisting of Sodium Hydroxide Solution and Copper II Sulphate solution) onto whatever you're testing. If the
test is positive, a mauve/purple layer should be formed once shaken; if protein is not present the biuret solution
will remain pale blue in colour.
How are proteins formed?
A single monomer of a protein is the amino acid. The structure of an amino acid is as follows:
When two monomers join together they form a dipeptide:
A polypeptide is formed when more than two amino acids join together.
Proteins are one or more polypeptides.
All amino acids contain the carboxyl group (COOH) and on the left is the amino group (NH2). The R
represents the variable which is different for each amino acid (remember there are 20 naturally occurring
amino acids).
The above reaction shows a polypeptide being formed, this happens through a condensation reaction. This
is when water comes out of the original reactant. During hydrolysis the reverse happens and the amino acids
can be split by adding water.
The N terminus (end) is the amino group which is free at the end and the C terminus is the carboxyl group
that is free at the other end.
The structured levels of proteins
Primary- This involves more than one amino acids joining together to form a polypeptide
chain to form a sequence.

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Secondary- Hydrogen bonds form between amino acids in a chain and this causes it to coil into
an helix and/or a pleated sheet. The bonds form in the amino group and carboxyl group in
the polypeptide backbone.
Tertiary- More hydrogen bonds form and the structure coils/ folds even further. If the protein is only
made from one polypeptide the tertiary structure is the final 3D structure. The tertiary structure is held
together by bonds in the R group.…read more

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How do disaccharides form?
Disaccharides form when two monosaccharides are joined together by a glycosidic bond. This is a condensation
reaction as a molecule of water is released. The opposite of a condensation reaction is when water is added to
reform the two separate molecules. This is called hydrolysis. 1-4 Glycosidic bonds are when the first carbon of one
glucose molecule joins to the fourth carbon of another glucose molecule.…read more

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What is Lactose Intolerance?
This is caused by the fact that a human body may not produce enough lactase enzymes to break down the
disaccharide: Lactose. As a result the undigested lactose is fermented by bacteria. This can cause intestinal
discomfort, such as: cramps, excessive flatulence and diarrhoea. It's very common to be lactose intolerant.
What about starch?
Well, starch is a common carbohydrate which we can find all around us.…read more


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