Biological Molecules (Edexcel IGCSE Biology)

A polysaccharide is formed when lots of monosaccharides join together.

True.

Starch, glycogen, and cellulose are all polysaccharides.

The basic unit of most fats (lipids) in the body is a triglyceride, which is one glycerol molecule bonded to three fatty acid chains.

False.

There are many different types of fatty acids with different structures.

Fats are solids at room temperature, while oils are liquids at room temperature.

An amino acid is a building block of proteins.

True.

The shape of a protein determines its function.

There are many types of proteins, including enzymes, haemoglobin, ligaments, keratin, and many more.

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

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 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.

False.

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

The starting colour of Benedict's solution is blue.

The starting colour of Biuret solution is blue.

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

An enzyme is a protein that acts as a biological catalyst to speed up the rate of a chemical reaction.

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

True.

Enzymes are necessary for all living organisms to maintain reaction speeds at a rate that can sustain life.

The enzyme-substrate complex is formed when a substrate attaches to the active site of an enzyme.

The active site is the region on an enzyme where a specific substrate attaches for the reaction to be catalysed.

False.

Enzymes are specific to one particular substrate(s) as the active site of the enzyme is a complementary shape to the substrate.

After the reaction has occurred, the products leave the enzyme's active site as they no longer fit it, and the enzyme is free to take up another substrate.

A substrate is the reactant of an enzyme-catalysed reaction.

False.

Enzymes are not changed or used up in the reactions they catalyse; they can be recycled and used in further reactions.

The optimum temperature for most enzymes in the human body is 37°C because this is our core body temperature.

When an enzyme is denatured its active site changes shape and can no longer bind to its specific substrate. This results in a loss of enzyme function.

False.

Denaturation is irreversible, meaning once enzymes are denatured, they cannot regain their proper shape and activity.

At low temperatures, enzymes work slowly due to a lack of kinetic energy and collisions between the substrates and active sites.

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

The optimum temperature is the temperature at which an enzyme works fastest.

Heating enzymes beyond the optimum temperature will break the bonds that hold the enzyme together, causing it to lose its shape and become denatured.

False.

Substrates cannot fit into denatured enzymes as the shape of their active site has been lost.

If an enzyme or substrate gains kinetic energy it moves around faster.

As kinetic energy increases, the rate of reaction increases due to more frequent collisions between molecules.

When investigating the effect of temperature on rate of reaction in amylase, iodine is added to detect the presence of starch. When all the starch has been digested into sugar the iodine will no longer turn blue-black.

True.

In a practical that investigates the effect of temperature on rate of reaction in amylase, the iodine solution will stop turning blue-black fastest at the optimum temperature; this is because the enzyme is working at its fastest rate breaking down the starch to sugar.

In a practical that investigates the effect of temperature on rate of reaction in amylase, the rate of reaction will be slow at low temperatures. This is due to low kinetic energy.

In a practical that investigates the effect of temperature on rate of reaction in amylase, the rate of reaction will decrease to zero at high temperatures. This is because the amylase enzyme has become denatured and cannot break down the starch.

The variable being changed (independent variable) is the temperature of the amylase and starch reaction mixture.

This is part of the CORMMS criteria for planning an investigation.

In the practical investigating how temperature affects the rate of reaction of amylase, the measurement needs to be repeated several times at each temperature.

For example the investigation might be carried out three times each at 20°C, 30°C, 40°C and 50°C and then an average taken of the results across the three repeats.

This is part of the CORMMS criteria for planning an investigation.

In a practical that investigates the effect of temperature on rate of reaction in amylase the variables that should be kept the same include:

• the concentration and volume of starch solution

• the concentration and volume of enzyme solution

• the pH

This is part of the CORMMS criteria for planning an investigation.

False.

Some enzymes that are produced in acidic conditions have a lower optimum pH, while those produced in alkaline conditions have a higher optimum pH.

At high or low pH, the bonds that hold the amino acid chain together can be disrupted, denaturing (changing the shape) of the active site and reducing the rate of activity.

Denaturation is the process of an enzyme losing its shape and becoming inactive due to high or low pH conditions away from their optimum.

Moving too far away from the optimum pH will cause the enzyme to denature, and activity will stop.

False.

Enzymes from the stomach have a low optimum pH (around pH 2) because the stomach is an acidic environment.

False.

Enzymes from the duodenum have a higher optimum pH (around pH 8 or 9) because the duodenum is an alkaline environment.

The iodine solution stops turning blue-black at certain pH levels because the starch is fully broken down to maltose (sugar) by the amylase so there is no longer starch in the solution to turn the iodine blue-black.

At the optimum pH, the iodine solution will stop turning blue-black and remain orange-brown within the shortest amount of time because the amylase is working at its fastest rate to break down the starch.

At very low pH values, such as pH 2, the iodine solution will continue turning blue-black throughout the investigation 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 starch and amylase solution must be brought up to the correct temperature before the investigation because this is a control variable for the investigation. The temperature must be kept the same throughout, including at the start.

The variable being changed (independent variable) is the pH of the amylase and starch reaction mixture.

This is part of the CORMMS criteria for planning an investigation.

The variable being measured (dependent variable) is the time taken for the iodine to stop turning black.

This is part of the CORMMS criteria for planning an investigation.

The variables being kept the same (control variables) are the concentration and volume of starch solution and iodine used in the investigation.

Also the temperature of the solution.

Avoid using the word amount.

This is part of the CORMMS criteria for planning an investigation.