Enzymes · GCSE Biology

Levels of organisation

Cells are the basic building blocks of living things.
A tissue is a group of cells with a similar structure and function, such as muscle tissue.
An organ is a group of tissues working together to do a specific job, such as the stomach.
An organ system is a group of organs working together, such as the digestive system.

Enzymes are biological catalysts: they speed up reactions in living things without being used up.
Enzymes are large proteins made of chains of amino acids folded into a particular shape.
Each enzyme has an active site with a unique shape that fits only one substrate, so enzymes are specific.

The substrate fits into the active site like a key into a lock, forming an enzyme–substrate complex.
The reaction happens and the products are released, leaving the enzyme free to catalyse another reaction.
If the shape of the active site changes, the substrate no longer fits and the enzyme stops working.

Rate of reaction increases with temperature up to the optimum, because particles collide more often and with more energy.
Above the optimum, bonds holding the enzyme's shape break, the active site changes shape and the enzyme is denatured.
Each enzyme has an optimum pH. Pepsin in the stomach works best at about pH 2; amylase works best at about pH 7.
A pH far from the optimum also changes the shape of the active site and denatures the enzyme.

Put a drop of iodine solution in each well of a spotting tile; iodine turns blue-black when starch is present.
Mix amylase, starch solution and a pH buffer in a water bath at a set temperature, and start a stopwatch.
Every 10 seconds, add a drop of the mixture to a fresh well of iodine. When the iodine stays orange-brown, all the starch has been broken down.
Repeat with buffers of different pH. Keep the temperature, volumes and concentrations the same.
Rate of reaction can be worked out as 1000 ÷ time (in seconds), or as change ÷ time.

Enzyme: A protein that acts as a biological catalyst, speeding up reactions without being used up.
Catalyst: A substance that speeds up a reaction without being changed by it.
Active site: The part of an enzyme where the substrate binds; its shape matches the substrate.
Substrate: The molecule an enzyme acts on.
Lock and key model: The idea that the substrate fits the active site of an enzyme like a key fits a lock.
Denatured: When an enzyme's active site changes shape so it no longer works.
Optimum: The temperature or pH at which an enzyme works fastest.
Tissue: A group of cells with a similar structure and function.
Organ: A group of different tissues working together to do a job.
Organ system: A group of organs working together to do a job.
Buffer: A solution that keeps the pH constant.