By Sanjay Sabu 10G
Enzyme revision resource
Digestive enzymes work outside of cells; they digest large, insoluble food molecules into smaller,
soluble molecules which can be absorbed into the bloodstream.
Metabolism is the sum of all the reactions happening in a cell or organism, in which molecules are
synthesised (made) or broken down.
Enzymes are biological catalysts made from protein.
Enzymes speed up chemical reactions in cells, allowing reactions to occur at much faster speeds than
they would without enzymes at relatively low temperatures (such as human body temperature).
Substrates temporarily bind to the active site of an enzyme, which leads to a chemical reaction and
the formation of a product(s) which are released.
Enzymes remain unchanged at the end of a reaction, and they work very quickly. Some enzymes can
process 100s or 1000s of substrates per second.
Required practical:
Aim: To investigate the effect of pH on the rate of reaction of amylase
You will:
Use the enzyme amylase to breakdown starch at a range of pH values, using iodine solution as an
indicator for the reaction occurring.
Use a continuous sampling technique to monitor the progress of the reaction.
Amylase is an enzyme that digests starch (a polysaccharide of glucose) into maltose (a disaccharide
of glucose).
Starch can be tested for easily using iodine solution.
Place single drops of iodine solution in rows on the tile.
Label a test tube with the pH to be tested.
Use the syringe to place 2cm3 of amylase in the test tube.
Add 1cm3 of buffer solution to the test tube using a syringe.
Use another test tube to add 2cm3 of starch solution to the amylase and buffer solution, start the
stopwatch whilst mixing using a pipette.
After 10 seconds, use a pipette to place one drop of the mixture on the first drop of iodine, which
should turn blue-black.
Wait another 10 seconds and place another drop of the mixture on the second drop of iodine.
Repeat every 10 seconds until iodine solution remains orange-brown.
Repeat experiment at different pH values – the less time the iodine solution takes to remain orangebrown, the quicker all the starch has been digested and so the better the enzyme works at that pH.
Improvements to method:
The above method can be adapted to control temperature by using a water bath at 35C.
All solutions that need to be used (starch, amylase, pH buffers) should be placed in a water bath and
allowed to reach the temperature (using a thermometer to check) before being used.
A colorimeter can be used to measure the progress of the reaction more accurately; with a solution
containing starch being darker and glucose light (as a result of the colour-change of iodine) – this will
affect the absorbance or transmission of light in a colorimeter.
Chemical Digestion:
The purpose of digestion is to break down large, insoluble molecules into smaller, soluble molecules
that can be absorbed into the bloodstream,
Large insoluble molecules, such as starch and proteins, are made from chains of smaller molecules
which are held together by chemical bonds. These bonds need to be broken.
Enzymes are biological catalysts – they speed up chemical reactions without themselves being used
up or changed in the reaction.
There are three main types of digestive enzymes – amylase, proteases, and lipases.
Amylase:
Amylase break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down
starch into maltose, which is then broken down into glucose by the enzyme maltase.
Amylase is made in the salivary glands, the pancreas, and the small intestine.
Proteases:
Proteases are a group of enzymes that break down proteins into amino acids in the stomach and
small intestine.
Protein digestion takes place in the stomach and small intestine, with proteases made in the
stomach (pepsin), pancreas and small intestine.
Lipase
Lipases break down lipids (fats) to glycerol and fatty acids.
Lipase enzymes are produced in the pancreas and secreted into the duodenum.
The role of Bile:
Cells in the liver produce bile which is then stored in the gallbladder.
Bile has two main roles:
It is alkaline to neutralise hydrochloric acid from the stomach. The enzymes in the small intestine
have a higher (more alkaline) optimum pH than those in the stomach. It breaks down large drops of
fat into smaller ones, increasing surface area. This is known as emulsification.
The alkaline conditions and larger surface area allows lipase to chemically break down fat (lipids)
into glycerol and fatty acids faster (the rate of fat breakdown by lipase is increased).
Products of Digestion:
The products of digestion are used to build new carbohydrates, lipids and proteins required by all
cells to function properly and grow.
Some glucose released from carbohydrate breakdown is used in respiration to release energy to fuel
all the activities of the cell.
Amino acids are used to build proteins like enzymes and antibodies.
The products of lipid digestion can be used to build new cell membranes and hormones.