AQA AS Biology 3.1.2 Study Guide

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3.1.2 Digestive System
The Digestive System
Salivary Glands
The salivary glands in the mouth contain amylase, mucus and mineral salts. The mucus lubricates the food
whilst the amylase breaks down starch into maltose.
Muscle contractions called peristalsis moves the food down and mucus lubricates the food.
The sphincter muscles cause expansion to allow the entrance and exit of foods. Gastric juice contains:
mucus, HCl and pepsin. This hydrolyses proteins and breaks food down. Through peristalsis the food is
turned into chyme.
Small Intestine
Chyme continues to move along by peristalsis. The small intestine is split into the Duodenum and the Ileum.
In the Duodenum pancreatic juices are secreted. This involves enzymes: Pepsin. Chymotrypsin, Trypsin,
Amylase and Lipase. It also contains Sodium Hydrogen Carbonate to neutralise the acidity of foods coming
out of the stomach.
In the Ileum, the small soluble products and absorbed through the villi lining the gut walls.
Large Intestine
The colon absorbs water, salts and minerals and the bacteria that ferment undigested foods are found
The sphincter muscles in the rectum carry the waste products to the anus in order for defecation to take
Digestion involves the hydrolysis of food products. This basically means turning larger molecules into smaller ones
by adding water. Enzymes are biological catalysts that speed up hydrolysis. These smaller molecules can be
absorbed and assimilated.
Proteins are made when amino acids joining together to form polypeptides. These polypeptides are joined
together by peptide bonds.
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.
A condensation reaction is what happens when two amino acids come together. 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.
Primary- This involves more than one amino acids joining together to form a polypeptide chain to form a

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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
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.…read more

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When temperature increases the rate of enzyme reaction increases (number of product molecules
This happens up to a certain point which is the optimum temperature.
This is because the enzymes gain kinetic energy and move faster. This means there are more successful
collisions and in turn more substrate and product complexes are formed.
After the optimum temp the rate of enzyme action goes down because the h bonds between R groups are
breaking.…read more

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This means that they don't block the active site but they do change the shape of it.
As a result the substrate can't fit on and form the enzyme-substrate complex. This in turn means that the
rate of reaction decreases.
An increase in inhibitors would decrease reaction heavily as all the active sites would eventually be filled. However
after a certain point it would no longer be a limiting factor.
Carbohydrate Digestion
Polysaccharides consisting of monosaccharides joined together by glycosidic bonding.…read more

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Amylose: Involves 1-4 glycosidic bonds and the h bonds cause the chain to spiral. It is a structural
Amylopectin: Involves 1-6 glycosidic bonding as well. This means branches are formed with each
glucose. It can be digested quicker because the enzyme amylase has more ends that it can bind to.…read more



thanks. really useful

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