OCR GCSE Chemistry A (J248) — Monitoring and controlling chemical reactions
You can speed up a chemical reaction in a laboratory by turning up the heat, but living cells would be destroyed if they tried the same trick.
To survive, organisms rely on an enzyme to safely speed up chemical reactions. Enzymes are complex, globular proteins that act as a biological catalyst. They control an organism's metabolism, which is the sum total of all the chemical reactions occurring within a cell or body to sustain life.
Like the inorganic catalysts you use in chemistry experiments, enzymes speed up reactions without being consumed or permanently changed in the process. Because they are not used up, a single enzyme molecule can be reused multiple times, meaning only a tiny amount is needed to catalyse the reaction of thousands of molecules.
Every time you digest a meal, enzymes are breaking down the food millions of times faster than it would happen naturally.
Enzymes achieve these incredibly fast rates by providing an alternative reaction pathway. This new pathway requires a much lower activation energy () than the uncatalysed reaction. Activation energy is the minimum amount of energy that reacting particles must possess when they collide to successfully react.
By lowering the activation energy, enzymes allow vital metabolic reactions to happen very quickly at relatively low temperatures, such as the standard human body temperature of 37°C.
Enzymes are highly specific to the reactions they control. A single enzyme typically only speeds up one specific reaction.
This specificity is due to the unique three-dimensional shape of the protein. On the surface of every enzyme is an active site. The reacting molecule, known as the substrate, has a shape that is perfectly complementary to the active site. They fit together exactly like a key fitting into a lock.
When the substrate binds to the active site, it forms a temporary enzyme-substrate complex. The enzyme weakens the bonds in the substrate, the reaction occurs, and the products are released, leaving the enzyme unchanged.
If the environment becomes too hot or highly acidic, the bonds holding the protein's 3D structure together can break. This causes denaturation, which is an irreversible change in the shape of the active site. Once denatured, the substrate no longer fits, and the enzyme cannot function.
Different enzymes operate in different locations and perform entirely different jobs, ranging from breaking molecules apart (catabolic) to building them up (anabolic).
In a laboratory experiment, a student adds catalase to hydrogen peroxide. The reaction produces 72 cm³ of oxygen gas over a period of 4.5 minutes. Calculate the rate of reaction.
Step 1: Identify the values.
Step 2: Substitute into the equation.
Step 3: Calculate.
Students often describe the enzyme and substrate as having 'the same shape'. You must use the word 'complementary' to describe how they fit together.
In exam questions asking how enzymes speed up reactions, examiners will look for the exact phrases 'provides an alternative pathway' and 'lowers the activation energy'.
When explaining denaturation, never say the enzyme 'dies' or 'melts'. Always state clearly that the 'shape of the active site changes' so the 'substrate no longer fits'.
While biology often focuses on human enzymes working at 37°C, remember in chemistry that enzymes from extremophile organisms can have entirely different optimum temperatures.
Enzyme
A complex globular protein that acts as a biological catalyst to speed up metabolic reactions without being changed or used up.
Globular protein
A protein formed from long chains of amino acids folded into a specific, complex three-dimensional shape.
Biological catalyst
A substance produced by a living organism that speeds up a chemical reaction by providing an alternative pathway with a lower activation energy.
Metabolism
The sum of all the chemical reactions that occur within a cell or a living organism.
Alternative reaction pathway
A different sequence of steps for a chemical reaction to take place, which requires less energy to start.
Activation energy
The minimum energy required for particles to react when they collide.
Active site
The specific region on an enzyme's surface where the substrate binds and the chemical reaction occurs.
Substrate
The specific reactant molecule that binds to the active site of an enzyme.
Complementary
A term describing how the unique shape of an active site perfectly matches the shape of its specific substrate, like a lock and a key.
Enzyme-substrate complex
The temporary structure formed when a substrate molecule successfully binds to the active site of an enzyme.
Denaturation
A permanent and irreversible change in the shape of an enzyme's active site, caused by extreme temperature or pH, which prevents the substrate from binding.
Catalase
An intracellular enzyme that breaks down toxic hydrogen peroxide into water and oxygen gas.
Amylase
An extracellular digestive enzyme that catalyses the breakdown of starch into sugars.
Protease
An enzyme that breaks down protein molecules into amino acids.
Put your knowledge into practice — try past paper questions for Chemistry A
Enzyme
A complex globular protein that acts as a biological catalyst to speed up metabolic reactions without being changed or used up.
Globular protein
A protein formed from long chains of amino acids folded into a specific, complex three-dimensional shape.
Biological catalyst
A substance produced by a living organism that speeds up a chemical reaction by providing an alternative pathway with a lower activation energy.
Metabolism
The sum of all the chemical reactions that occur within a cell or a living organism.
Alternative reaction pathway
A different sequence of steps for a chemical reaction to take place, which requires less energy to start.
Activation energy
The minimum energy required for particles to react when they collide.
Active site
The specific region on an enzyme's surface where the substrate binds and the chemical reaction occurs.
Substrate
The specific reactant molecule that binds to the active site of an enzyme.
Complementary
A term describing how the unique shape of an active site perfectly matches the shape of its specific substrate, like a lock and a key.
Enzyme-substrate complex
The temporary structure formed when a substrate molecule successfully binds to the active site of an enzyme.
Denaturation
A permanent and irreversible change in the shape of an enzyme's active site, caused by extreme temperature or pH, which prevents the substrate from binding.
Catalase
An intracellular enzyme that breaks down toxic hydrogen peroxide into water and oxygen gas.
Amylase
An extracellular digestive enzyme that catalyses the breakdown of starch into sugars.
Protease
An enzyme that breaks down protein molecules into amino acids.
