OCR GCSE Biology A (J247) — Cell level systems
Without special molecules to speed things up, the chemical reactions keeping you alive would happen far too slowly to sustain life. These crucial molecules are called enzymes, which function as biological catalysts.
For any chemical reaction to start, a certain amount of energy is needed, known as the activation energy. Enzymes speed up reactions by lowering this activation energy, providing an alternative reaction pathway that requires less energy.
According to collision theory, reactant molecules must collide with sufficient energy and in the correct orientation for a reaction to occur. Enzymes facilitate this by providing a specific environment for the reaction:
This can be summarised as:
Every enzyme has a highly specific active site. This is a small pocket or groove on the surface of the enzyme with a unique 3D shape.
The lock and key hypothesis is a model used to explain enzyme specificity:
Metabolism is the sum of all the chemical reactions happening in a cell or the body. Enzymes control these reactions, which can be divided into two types:
Enzymes are also classified by where they perform their work:
Students often state that extreme heat 'kills' enzymes. Enzymes are molecules, not living cells, so they cannot die. You must state that the enzyme undergoes denaturation and the 'shape of the active site has changed'.
In any exam question about enzyme specificity, you MUST use the exact phrase 'complementary shape'. OCR examiners will explicitly penalise answers that describe the active site and substrate as being the 'same shape' or simply saying they 'fit together'.
When asked to describe or explain the mechanism of enzyme action for 3-4 marks, always explicitly state that an 'enzyme-substrate complex' forms. This is almost universally a standalone marking point on the mark scheme.
If an exam question asks you to link liver function to metabolism, be sure to specifically mention its role in the catabolic breakdown of excess amino acids to form urea.
Enzyme
A biological catalyst made of protein that speeds up chemical reactions in living organisms without being changed or used up.
Biological catalyst
A substance found in living organisms that increases the rate of a chemical reaction without being consumed.
Activation energy
The minimum amount of energy required for a chemical reaction to successfully occur.
Substrate
The specific molecule or molecules upon which an enzyme acts and converts into products.
Active site
The specific 3D region on an enzyme's surface where the substrate binds and the reaction takes place.
Complementary
Describes two shapes that fit together perfectly, such as the relationship between a specific substrate and an enzyme's active site.
Enzyme-substrate complex
A temporary intermediate molecule formed when a substrate successfully binds to the active site of its specific enzyme.
Lock and key hypothesis
A model of enzyme action explaining that enzymes are specific because only one precisely shaped substrate (the key) fits into the active site (the lock).
Collision theory
The principle that chemical reactions only occur when reactant particles physically collide with sufficient kinetic energy and in the correct orientation.
Denaturation
An irreversible change in the 3D shape of an enzyme's active site, caused by extreme temperatures or pH, which prevents the substrate from binding.
Metabolism
The sum of all the biochemical reactions occurring within a cell or a living organism.
Anabolism
Metabolic reactions that require energy to synthesize complex molecules from simpler, smaller ones.
Catabolism
Metabolic reactions that release energy by breaking down large, complex molecules into simpler ones.
Product
A molecule produced at the end of an enzyme-catalysed reaction.
Intracellular enzymes
Enzymes that function inside the cells that produce them, such as those involved in DNA replication.
Extracellular enzymes
Enzymes that are secreted from the cell where they are made to work outside the cell, such as digestive enzymes.
Put your knowledge into practice — try past paper questions for Biology A
Enzyme
A biological catalyst made of protein that speeds up chemical reactions in living organisms without being changed or used up.
Biological catalyst
A substance found in living organisms that increases the rate of a chemical reaction without being consumed.
Activation energy
The minimum amount of energy required for a chemical reaction to successfully occur.
Substrate
The specific molecule or molecules upon which an enzyme acts and converts into products.
Active site
The specific 3D region on an enzyme's surface where the substrate binds and the reaction takes place.
Complementary
Describes two shapes that fit together perfectly, such as the relationship between a specific substrate and an enzyme's active site.
Enzyme-substrate complex
A temporary intermediate molecule formed when a substrate successfully binds to the active site of its specific enzyme.
Lock and key hypothesis
A model of enzyme action explaining that enzymes are specific because only one precisely shaped substrate (the key) fits into the active site (the lock).
Collision theory
The principle that chemical reactions only occur when reactant particles physically collide with sufficient kinetic energy and in the correct orientation.
Denaturation
An irreversible change in the 3D shape of an enzyme's active site, caused by extreme temperatures or pH, which prevents the substrate from binding.
Metabolism
The sum of all the biochemical reactions occurring within a cell or a living organism.
Anabolism
Metabolic reactions that require energy to synthesize complex molecules from simpler, smaller ones.
Catabolism
Metabolic reactions that release energy by breaking down large, complex molecules into simpler ones.
Product
A molecule produced at the end of an enzyme-catalysed reaction.
Intracellular enzymes
Enzymes that function inside the cells that produce them, such as those involved in DNA replication.
Extracellular enzymes
Enzymes that are secreted from the cell where they are made to work outside the cell, such as digestive enzymes.
