Biological Molecules (Edexcel IGCSE Biology)

Some cosmetic companies claim that adding argan oil to their shampoo increases the strength of human hair.

Design an investigation to find out if argan oil shampoo does increase the strength of human hair.

Include experimental details in your answer and write in full sentences.

Describe the structure of a protein such as an enzyme.

The mechanism of enzyme action is often described as being like a 'lock and key'.

Describe the mechanism of enzyme action.

During the digestive process, digestive enzymes break down larger molecules into smaller molecules.

Explain the role that digestive enzymes play in sustaining respiration in cells of the body.

Yeast cells are often used when making bread.

A student uses this method to investigate the effect of temperature on the height that bread dough rises.

• Place dough containing flour, sugar, water and yeast into a measuring cylinder

• Measure the height of the dough

• Place the measuring cylinder in a 25 °C water bath

• Measure the height of the dough after two hours

• Repeat the experiment at temperatures of 35 °C and 65 °C

The diagram shows the student’s apparatus.

The table shows the student’s results.

Calculate the percentage increase in the height of the dough after two hours at a temperature of 35 °C.

Explain why yeast causes the bread dough to increase in height.

Explain why the dough rises to a different height at 25 °C compared with the height at 35 °C.

Explain why the dough rises to a different height at 35 °C compared with the height at 65 °C.

The graph shows the effect of temperature on the activity of an enzyme.

Describe and explain the trend of the graph.

Explain how the evidence from the graph in part (a) indicates that the enzyme being studied is not likely to be one found in the human body.

A group of students wanted to investigate the effect that temperature has on the activity of pepsin, a protease found in the human digestive system.

They used the following method:

1. Put 4 cm³ of 1% protein solution into a boiling tube.

2. Put 4 cm³ of pepsin solution into a second boiling tube. Put both boiling tubes into a water bath at 20 °C.

3. After 4 minutes, add the pepsin solution to the protein solution together in one boiling tube, mixing together with a stirring rod.

4. After 20 seconds, add a drop of the mixture to a drop of Biuret solution in one well of a spotting tile.

5. Repeat step 5 until the Biuret solution no longer changes colour.

6. Repeat steps 1 – 6 at 10 °C (using an ice water bath), 30 °C, 40 °C and 50 °C.

Explain why the student left the protein and pepsin solutions in the water bath for 4 minutes in step 3.

The image below shows the incomplete results of the experiment carried out in part (c).

Complete the image below to show the results you would expect at 30 °C and at 40 °C

You should write a tick or a cross in each well of the spotting tile.

State where in the digestive system protease is secreted.

The table shows four samples of food that were tested for glucose (reducing sugar) using the Benedict's test. 

The results were observed as follows:

The Benedict's test is referred to as being semi-quantitative.

Use the information provided to suggest what the term 'semi-quantitative' means. 

Explain the purpose of the test tube containing just water in the experiment shown in part (a).

Explain why the test for lipids is known as an emulsion test.

Predict the colour changes (if any) of the iodine test on the food samples shown below:

Catalase is an enzyme found in many cells.

It speeds up the breakdown of hydrogen peroxide into water and oxygen.

The equation for the reaction is:

2H₂O₂ → 2H₂O + O₂

A teacher demonstrates the effect of increasing catalase concentration on the initial rate of the reaction.

This is the teacher’s method.

• Cut five equal size discs from a potato, each 0.2 mm thick

• Place the discs in a boiling tube with 5 cm³ of buffer solution

• Add 5 cm³ of hydrogen peroxide solution to the boiling tube

• Place a bung and delivery tube firmly into the boiling tube

• Position the other end of the delivery tube under an inverted measuring cylinder

• Start a timer as soon as the first bubble of oxygen enters the measuring cylinder

• Measure the volume of oxygen produced in one minute

Repeat this method three times.

The teacher uses this method with different numbers of potato discs, making sure that other conditions are unchanged.

Give the expected relationship between the named independent variable and the named dependent variable in this demonstration.

Give two variables that the teacher controls in this demonstration.

The table shows the teacher’s results.

(i) Calculate the mean volume of oxygen produced in one minute using 5 potato discs.

Give your answer in cm³.

(2)

(ii) Calculate the percentage increase in the mean volume of oxygen produced in one minute as the concentration of enzyme changes from 15 to 20 discs.

(2)

(iii) Explain the relationship between the concentration of enzyme and the mean volume of oxygen produced in one minute.

(2)

Suggest why the teacher measures the volume of oxygen after the first minute of the reaction rather than after 10 minutes.

Food items can often be spoiled if saprotrophic microorganisms such as mould fungi grow on them.

Describe how a saprotrophic fungus such as mould obtains its food.

A student uses this method to investigate ways of preventing peas from being spoiled.

• Place three peas in each of three test tubes as shown in the diagram

• Cover the peas in test tube A with water and keep at 37 °C

• Cover the peas in test tube B with water and keep at 4 °C

• Cover the peas in test tube C with vinegar, which is a weak acid, and keep at 37 °C

• Leave the peas for 24 hours

The student observes the level of cloudiness of the solution to determine how spoiled the peas have become. The level of cloudiness can be used as a measure of fungal growth.

The table shows the student’s results.

(i) Suggest a problem with using the level of cloudiness of the solution to determine how spoiled the peas have become.

(1)

(ii) Explain the appearance of the peas in water at 4 °C.

(2)

(iii) Explain the appearance of the peas in vinegar at 37 °C.

(2)

A group of students carried out the following practical to establish the effect of temperature on protein breakdown:

1. Place a drop of biuret solution into every well in a spotting tile.

2. Put 4 cm³ of 1 % protein solution into a boiling tube.

3. Put 4 cm³ of pepsin solution into a second boiling tube.

4. Put both boiling tubes into a water bath at 20 °C.

5. After 4 minutes add the pepsin solution to the protein solution together in one boiling tube, mixing together with a stirring rod.

6. After 20 seconds add a drop of the protein/pepsin mixture to a drop of biuret solution in one well of the spotting tile.

7. Repeat step 6 until the biuret solution no longer changes colour.

8. Repeat steps 1–7 at 10 °C (using an ice water bath), 30 °C, 40 °C and 50 °C.

The concentration of protein present in the solution can also be measured using a colorimeter.

In a colorimeter, the amount of light that cannot pass through a solution is measured; giving the ‘absorbance’ of a solution. The darker the colour, the higher the absorbance.

Suggest advantages of using the colorimeter method rather than the method used by the students.

The graph below shows a calibration curve of concentration of protein versus absorbance.

An experiment was carried out to investigate the effect of a protease enzyme on a 1 % protein solution.

The absorbance at 30 °C was 0.8 arbitrary units after 20 seconds.Using the calibration curve, estimate the final concentration of protein in this solution after 20 seconds.

The students found that the concentration of protein in the solution at 10 °C after 1 minute was different to the concentration at 30 °C after 1 minute.

Explain the observation above.

The students repeated the experiment while incubating the protease and 1 % protein solution at 80  °C.

The absorbance was measured at approximately 1.14 arbitrary units.

Suggest a reason for this result.

The protein Rubisco is an enzyme. 

Identify where, in a cell, Rubisco would be made.

Rubisco has a unique 3D shape that enables it to carry out its roles in living organisms. 

Describe how the 3D shape of rubisco is determined.

Extreme hyperthermia is a condition in which human body temperature rises above the normal body temperature of 37 °C. Body temperatures above 40 °C can be life threatening.

Use your knowledge of proteins to suggest why body temperatures above 40 °C can be life threatening.

Sketch a graph to show how the rate of reaction of an enzyme such as Rubisco would change over a range of temperatures from 0 to 60 °C.

Use the axes provided below.

Separate: Biology Only

Catalase is an enzyme found in the liver which catalyses the breakdown of the harmful substrate hydrogen peroxide into the products water and oxygen.

Some students investigated the effect of pH on catalase activity. Each experiment was repeated at a different pH value (pH 2, 4, 7, 9 and 11) and was set up as follows:

• Five potato cubes of similar dimensions were used as a source of catalase.

• Each cube was added to 50 cm³ of hydrogen peroxide.

• The volume of oxygen released from this reaction was collected in a measuring cylinder.

• This was used to calculate the initial rate of the reaction in dm³s^-1

Which of the rows in the following table correctly identifies the variables in this experiment?

Separate: Biology only

The results below show the effect of pH 7 and pH 9 on the activity of the enzyme catalase.

The rate of reaction can be calculated by using the following formula:

Calculate the difference in the rate of reaction between pH 7 and pH 9 during the first 20 seconds. Give your answers with the correct units.

(3)

Describe the differences between the curves at pH 7 and pH 9 in the graph in part (b).

Separate: Biology only

Explain the shape of the curve for pH 7 from around 20 s onwards in the graph in part (b).

Separate: Biology Only

Predict the outcome of the experiment described in part (a) if the pH was increased to pH 11.

Explain your answer.