Enzymes and digestion (CCEA)

• Enzymes are proteins that act as biological catalysts.

• They speed up the rate of reactions without being used up.

Each enzyme has an active site, which is where the reaction takes place.

The lock and key model explains how enzymes work:

• The enzyme is like a "lock".

• The substrate (the molecule it reacts with) is like a "key" that fits into the lock.

Key facts about enzymes:

• an enzyme’s active site and its substrate are complementary in shape

• enzymes and substrates collide to form enzyme-substrate complexes

• the substrates are broken down or built up quickly

• the products are released

• the enzyme can be used over again

Each enzyme is specific and acts on only one substrate to catalyse one reaction.

The active site has a specific shape that is complementary to the shape of its substrate, allowing them to fit together like a lock and key, meaning each enzyme works with only one specific substrate.

Inhibitors are molecules that fit into an enzyme’s active site but are not broken down.

They stop the substrate from fitting into the action and prevent enzyme-substrate complexes forming so reduce the rate of reaction.

As temperature increases the rate of enzyme activity increases, up to the optimum temperature, as more kinetic energy causes more collisions between the enzyme and substrate forming more enzyme-substrate complexes.

Above the optimum temperature the enzyme’s active site becomes denatured.

This means the active site changes shape and can no longer form enzyme-substrate complexes, leading to a decrease in enzyme activity.

Denaturation is a permanent change.

pH

Deviating from the optimum pH (too high or too low) causes the enzyme’s active site to become denatured and the active site changes shape.

It can no longer form enzyme-substrate complexes, leading to a decrease in enzyme activity.

As the enzyme concentration increases, the rate of enzyme activity increases because there are more enzymes to form enzyme-substrate complexes.

This happens up to a certain point. Enzyme activity then levels off (plateaus) as there are not enough substrate molecules to react with the extra enzymes.

The optimum value is the level at which the reaction rate is at its maximum, ie, fastest.

Procedure:

1. Add 20ml of hydrogen peroxide (H₂O₂) to each test tube

2. Place each test tube in a water bath set at different temperatures (eg 10°C, 20°C, 30°C, 40°C, 50°C)

3. Add 2g of raw potato (which contains catalase enzyme) to each test tube

4. Measure the volume of oxygen gas produced using a gas syringe for 1 minute

5. Record results

As the temperature increases, the rate of enzyme activity increases, up to 40°C, which is the optimum temperature for the enzyme.

Beyond 40°C, the enzyme becomes denatured.

Independent variable: temperature / °C

Dependent variable: volume of oxygen gas produced / ml

Control variables:

• same volume and concentration of hydrogen peroxide
• same mass of potato (enzyme)
• collect gas for same time

Enzymes are used in the digestive system to break large, complex, insoluble food molecules into small, simple, soluble molecules so they can be absorbed into the bloodstream.

Most food is digested in the ileum by enzymes. Its main function is to absorb digested food into the bloodstream for transport to body cells.

• large surface area – due to being long and folded
• thin – digested food doesn’t have to travel far to reach the blood
• permeable – digested food can pass through easily
• good blood supply – to maintain the concentration gradient for diffusion between the ileum and bloodstream
• villi – finger like projections that further increase surface area

• good blood supply – a villus has a large network of capillaries. Once blood becomes high in digested food products it is transported away and replaced with blood that is low in digested food products. This maintains the concentration gradient necessary for diffusion between the ileum and bloodstream.
• lacteal – a tube that absorbs the products of fat digestion before returning them to the blood
• single layer of surface epithelium cells – this reduces the diffusion distance that digested food products have to travel in order to enter the bloodstream
• permeable – digested food can pass through easily

Enzymes are used in biological washing powders to break down stains.

• Protease breaks down proteins (eg blood, egg stains).
• Lipase breaks down fats and oils (eg grease stains).
• Amylase breaks down starches (eg food stains).

Advantages:

• work quickly at lower temperatures, saving energy and money

Disadvantages:

• enzymes are denatured at temperatures above 40°C
• some people may be allergic to enzymes, leading to skin irritation

Play this game to explore how food is processed by the human body.