Enzymes (Edexcel GCSE Biology)

Enzyme activity is low at low temperatures due to a lack of kinetic energy. This means that few successful collisions occur between enzyme and substrate molecules and so few enzyme-substrate complexes form.

Increasing the temperature towards the optimum increases enzyme activity as the molecules have more kinetic energy, leading to more successful collisions with substrate molecules and a faster rate of reaction.

Increasing temperature too far beyond the optimum reduces reaction rate to zero because heat breaks the bonds that hold the enzyme together, causing it to lose its shape and become denatured.

An increase in substrate concentration increases the likelihood that successful collisions will occur, resulting in an increased rate of reaction.

False.

Enzyme activity increases with substrate concentration until all of the active sites are occupied, at which point the rate of enzyme activity levels off.

The rate of an enzyme-controlled reaction decreases as the pH level moves further from the enzyme's optimum because extremes of pH break the bonds that hold the amino acid chain together, denaturing the active site and reducing the rate of reaction.

False.

Different enzymes function well at different pH levels, e.g. enzymes from the stomach have a low optimum pH (around pH 2) because the stomach is an acidic environment.

In a practical investigating the effect of pH on rate of reaction in amylase, the iodine solution stops turning blue-black at certain pH levels because the starch is fully broken down by the amylase. This means that there is no longer starch in the solution to turn the iodine blue-black.

In a practical investigating the effect of pH on rate of reaction in amylase, at the optimum pH the iodine solution will stop turning blue-black and remain orange-brown within the shortest time period. This is because the amylase is working at its fastest rate to break down the starch.

In a practical investigating the effect of pH on rate of reaction in amylase, at very low pH values the iodine solution will continue turning blue-black throughout the investigation. This is because the enzymes are denatured so the rate of amylase activity has slowed down/stopped.

A buffer solution is a solution that resists changes in pH upon the addition of small amounts of an acid or base.

Buffer solutions are used to maintain a specific pH environment for the amylase enzyme. This keeps the pH the same throughout the investigation for each of the pH conditions.

False.

The starch and amylase solutions should ideally be placed in a water bath at the optimum temperature before being used in the investigation, however it does not need to be the exact optimum as long as the temperature is kept constant throughout.

The formula for calculating reaction rate when we know how much product has been produced in a given time period is:

reaction rate = product producedtime taken

False.

When 5 g of substrate is used up in 10 seconds the rate of reaction is 0.5 g s^-1.

This is calculated as follows:

change (substrate used up)time

= 510

= 0.5

Rate of reaction can be calculated when the change in value for substrate used or product formed is not known using the following formula:

rate = 1 time taken

The formula rate = 1000time taken may also be used to generate larger numbers for graph plotting.

A catalyst speeds up the rate of a chemical reaction without being used up or changed in the reaction.

True.

Enzymes are needed to synthesise, or produce, large molecules from smaller molecules.

Carbohydrase enzymes break down carbohydrates into simple sugars, e.g. breaking starch down into glucose.

The type of enzyme that digests protein is protease.

False.

Protease enzymes break down proteins into amino acids.

Fatty acids are a product of lipid digestion.

Lipase enzymes carry out lipid digestion.

Lipids (fats) are broken down into glycerol and fatty acids by lipase enzymes.

The test for starch involves adding iodine solution to a food sample. A positive result is a colour change from orange-brown to blue-black.

False.

A positive test for starch using iodine will turn the solution from brown/orange to blue-black.

The test for glucose involves adding Benedict's solution to a sample and heating it in a boiling water bath. A positive result is a colour change from blue to brick red.

The starting colour of Benedict's solution is blue.

The protein food test involves adding Biuret solution to a food sample. A positive result is a colour change from blue to violet/purple.

The starting colour of Biuret solution is blue.

The test for lipids involves mixing a sample with ethanol and then adding the ethanol solution to cold water. A positive result is a cloudy white emulsion forming.

Note that a sample containing solid pieces of food may need to be strained before a positive result can be clearly seen.

The Benedict's test for glucose (a reducing sugar) must be heated to observe the results.

False.

Calorimetry involves measuring the energy content of food.

The equipment shown can be used to measure the energy content of a piece of food as follows:

• record the starting temperature of the water and the mass of the food

• burn the food underneath the test tube until the flame burns out

• record the final temperature of the water

• calculate the energy released per gram of food

When using calorimetry to measure the energy content of food, the independent variable could be the type of food, e.g. bread vs crisps vs nuts.

When using calorimetry to measure the energy content of food it is important to repeat each measure several times so that anomalous results can be identified and left out of the mean.

When using calorimetry to measure the energy content of food the variables that should be kept constant include:

• the distance between the food and the test tube

• the type of test tube

• the volume of water used

• the piece of food is always burned until it will no longer burn