OCR GCSE Biology B (J257) — Practical Skills
Think of trying to unlock your front door with your car key—it simply will not work because the shapes do not match. Enzymes are globular proteins that act as biological catalysts, speeding up metabolic reactions by lowering the activation energy required. They are not used up in the reaction.
The 3D shape of an enzyme is determined by its folded amino acid chain, which is held together by hydrogen bonds and ionic bonds. The Lock and Key Hypothesis explains how they work:
We can observe the invisible world of enzyme activity in a lab using a simple colour-changing dye. The standard enzyme practical (PAG 4) investigates how the rate of starch digestion by amylase is affected by changes in pH.
You can calculate the relative rate of an enzyme-controlled reaction using the formula:
In an experiment, amylase breaks down a sample of starch completely in 40 seconds at pH 7. Calculate the relative rate of reaction.
Step 1: Identify the formula and values.
Step 2: Substitute the values into the equation.
Step 3: Calculate the final answer.
Food spoils much faster on a hot summer day than in the fridge because chemical reactions are driven by heat. As temperature increases, both enzyme and substrate molecules gain kinetic energy and move much faster. According to collision theory, this results in a higher frequency of successful collisions between the substrate and the active site, increasing the rate of reaction. The temperature at which the enzyme works fastest is called the optimum temperature (around for human enzymes).
If the temperature rises significantly above the optimum, the increased kinetic energy causes the atoms within the enzyme to vibrate vigorously. This excessive vibration breaks the hydrogen bonds and ionic bonds holding the tertiary structure together. Consequently, the active site permanently changes shape and is no longer complementary to the substrate. The substrate cannot bind, no enzyme-substrate complexes can form, and the reaction stops. This irreversible process is called denaturation.
At very low temperatures, enzymes have very little kinetic energy. They are inactive and the reaction is incredibly slow, but importantly, they are not denatured.
Stomach acid is strong enough to damage tissue, yet the digestive enzyme pepsin actively relies on an extremely acidic environment to function. Every enzyme has an optimum pH where its rate of reaction is highest (e.g., pH 7 for most cellular enzymes, but pH 2 for pepsin).
If the pH moves too far above or below this optimum, the environment becomes either too alkaline (excess ions) or too acidic (excess ions). These excess ions interfere with and disrupt the ionic bonds and hydrogen bonds holding the enzyme's 3D structure together. Just like with excessive heat, this causes the active site to change shape. The active site is no longer complementary to the substrate, meaning the enzyme has denatured and the rate of reaction drops to zero.
Adding more cashiers to a supermarket only speeds up the queue until all the tills are occupied. The same principle applies to enzymes. At low substrate concentrations, increasing the amount of substrate increases the rate of reaction linearly. This is because there are many "empty" active sites available, and adding more substrate molecules simply increases the probability of successful collisions per second.
However, the rate of reaction eventually levels off and reaches a plateau. At this saturation point, every single active site is occupied by a substrate molecule at any given moment. The enzyme is working at its maximum possible rate. Adding more substrate will not speed up the reaction any further; at this stage, the enzyme concentration has become the limiting factor.
Students often state that high temperatures 'kill' enzymes. Enzymes are molecules, not living organisms. Always use the term 'denatured'.
In 6-mark mechanism questions on denaturation, examiners look for a clear causal chain: state the condition (e.g., high heat), explain that it 'breaks bonds', which 'changes the shape of the active site', resulting in a 'loss of complementary shape'.
When explaining why rate increases with temperature or concentration, you must use the phrase 'higher frequency of successful collisions' to get the mark.
Remember that low temperatures do not denature enzymes; they just reduce kinetic energy, making the enzymes temporarily inactive.
For practical validity in PAG 4, always state that the temperature must be controlled using a water bath, otherwise changes in rate could be due to heat rather than the pH buffer.
Enzymes
Globular proteins that act as biological catalysts, speeding up metabolic reactions without being used up.
Activation energy
The minimum amount of energy required for a chemical reaction to occur; lowered by enzymes.
Hydrogen bonds
Weak chemical bonds that help stabilize the specific 3D shape (tertiary structure) of an enzyme.
Ionic bonds
Electrostatic attractions between oppositely charged groups that help maintain the enzyme's structure.
Lock and Key Hypothesis
A model describing enzyme action where the substrate's shape is exactly complementary to the enzyme's active site.
Active site
The specifically shaped region on an enzyme molecule where the substrate binds and the chemical reaction takes place.
Substrate
The specific molecule upon which an enzyme acts.
Complementary
Having a matching shape that fits perfectly into another, like a key into a specific lock.
Enzyme-substrate complex (ESC)
The temporary intermediate structure formed when a substrate molecule successfully binds to the active site of an enzyme.
Denaturation
An irreversible change in the shape of an enzyme's active site caused by the breaking of internal bonds, preventing the substrate from binding.
Optimum temperature
The exact temperature at which an enzyme works at its maximum rate.
Buffer solution
A solution used in practical experiments that resists changes in pH, keeping the pH level constant.
Collision theory
The principle that chemical reactions occur when particles collide with sufficient energy and in the correct orientation.
Successful collisions
A collision between an enzyme and substrate that results in the formation of an enzyme-substrate complex.
Optimum pH
The specific pH value at which an enzyme-controlled reaction reaches its maximum rate.
Saturation point
The point at which all available enzyme active sites are occupied by substrates, meaning the reaction cannot proceed any faster.
Put your knowledge into practice — try past paper questions for Biology B
Enzymes
Globular proteins that act as biological catalysts, speeding up metabolic reactions without being used up.
Activation energy
The minimum amount of energy required for a chemical reaction to occur; lowered by enzymes.
Hydrogen bonds
Weak chemical bonds that help stabilize the specific 3D shape (tertiary structure) of an enzyme.
Ionic bonds
Electrostatic attractions between oppositely charged groups that help maintain the enzyme's structure.
Lock and Key Hypothesis
A model describing enzyme action where the substrate's shape is exactly complementary to the enzyme's active site.
Active site
The specifically shaped region on an enzyme molecule where the substrate binds and the chemical reaction takes place.
Substrate
The specific molecule upon which an enzyme acts.
Complementary
Having a matching shape that fits perfectly into another, like a key into a specific lock.
Enzyme-substrate complex (ESC)
The temporary intermediate structure formed when a substrate molecule successfully binds to the active site of an enzyme.
Denaturation
An irreversible change in the shape of an enzyme's active site caused by the breaking of internal bonds, preventing the substrate from binding.
Optimum temperature
The exact temperature at which an enzyme works at its maximum rate.
Buffer solution
A solution used in practical experiments that resists changes in pH, keeping the pH level constant.
Collision theory
The principle that chemical reactions occur when particles collide with sufficient energy and in the correct orientation.
Successful collisions
A collision between an enzyme and substrate that results in the formation of an enzyme-substrate complex.
Optimum pH
The specific pH value at which an enzyme-controlled reaction reaches its maximum rate.
Saturation point
The point at which all available enzyme active sites are occupied by substrates, meaning the reaction cannot proceed any faster.
