AQA A Level Biology

Topic Questions

2.4 Cell Membranes & Transport

1a2 marks

State two functions of membranes in living cells.

1b3 marks

Figure 1 shows the structure of a cell surface membrane.

Figure 1

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Identify structures A-C in Figure 1. 

1c2 marks

Explain why structure A in Figure 1 forms in the aqueous (watery) environment of the cell cytoplasm. 

1d1 mark

The appearance of cell membranes means that they are sometimes described as having a mosaic-like structure. This mosaic is often referred to as a ‘fluid mosaic’. Explain why the mosaic of molecules that makes up cell membranes is referred to as a ‘fluid mosaic’.

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2a4 marks

Figure 1 below shows a cell surface membrane and some molecules of a small, nonpolar substance known as substance X

Figure 1

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Predict and explain what will happen to substance X in Figure 1.

2b1 mark

Give a possible name for substance X.

2c2 marks

Substance Y is a large molecule and is not lipid-soluble. Describe how substance Y  would travel across a cell membrane from an area of high to low concentration. 

2d2 marks

State and explain how an increase in temperature would affect the rate of movement of substance Y across a cell membrane. 

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3a3 marks

Figure 1 below shows a group of onion cells viewed under an optical microscope. 

Figure 1

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Explain the appearance of the cytoplasm that can be seen in Figure 1.  

3b1 mark

Predict the appearance of a herbaceous (non-woody) plant that contained cells looking like those seen in Figure 1.

3c2 marks

A student took a cheek cell sample using a cotton swab and placed the cells into a test tube containing pure water. Predict what would happen to the cheek cells in the test tube.

3d2 marks

The onion cells from part a) were also placed into pure water, but the effect on these cells were slightly different. State how the effect would be different and explain why.

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4a2 marks

Complete Table 1 by placing a tick ( ✔ ) in the appropriate boxes.

Table 1

 

Simple diffusion

Osmosis

Facilitated diffusion

Active Transport

Movement of glucose from high to low concentration

       

Movement down a concentration gradient

       

Movement of water from high water potential to low water potential.

       

Movement of oxygen from the plasma into a red blood cell

       
4b3 marks

Figure 1 shows a molecule (X) that is about to be transported across a cell membrane.

Figure 1

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Describe the events that would take place in order for molecule X in Figure 1 to move from the exterior of the cell into the cytoplasm.

4c2 marks

Figure 2 below shows the absorption of certain nutrients from the lumen of the small intestine into the blood.

Figure 2

q4c-2-4-cell-membrane-and-transportation-easy

Name the membrane transport processes taking place at A and B in Figure 2.

4d1 mark

State the purpose of the process taking place at the position labelled C in Figure 2.

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5a2 marks

A student wanted to investigate the effect of temperature on cell membrane permeability. 

They decided to carry out the experiment described in Figure 1 below.

Figure 1

  1. Cut five cubes or cores of beetroot to the same size.
  2. Rinse the beetroot cubes/cores.
  3. Place the beetroot pieces into five test tubes, each containing the same volume of water.
  4. Put each test tube into a water bath at a different temperature (10°C, 20°C, 30°C, 40°C and 50°C).
  5. Leave the test tubes in the water bath for 30 minutes.
  6. Remove samples of liquid from each of the five test tubes and transfer to cuvettes.
  7. Use a colorimeter to measure the absorbance for each of the five liquid samples.


Explain why beetroot is ideal for this experiment.

5b2 marks

Explain two control variables that have been included in the method in Figure 1.

5c2 marks

Table 1 below shows some of the student’s results.

Table 1

Temperature (°C)

Absorbance

10

0.15

20

0.19

30

0.21

40

0.22

50

0.27

Calculate the percentage increase in absorbance between 10°C and 50°C.

5d2 marks

Explain the relationship between temperature and absorbance shown in Table 1.

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1a3 marks

Explain the terms ‘fluid’ and ‘mosaic’ in the Fluid Mosaic model of membrane structure. Give one advantage of the structure being fluid. 

1b4 marks

Give four functions of proteins that occupy the plasma membrane. 

1c3 marks

Pieces of phospholipid bilayer were analysed from two different mammalian cell surface membranes. Sample A contained phospholipid molecules at a density of 5.0 x 106 molecules μm-2, whereas sample B contained phospholipid molecules at a density of 4.1 x 106 molecules μm-2. One sample was from a hormone-secreting liver cell and the other was from a skin cell. 

Identify which cell type corresponds to samples A and B. Give reasons for your choice. 

1d2 marks

Researchers have discovered that an individual phospholipid molecule can exchange places with its neighbouring phospholipid molecule in a monolayer as frequently as 107 times per second. By contrast, phospholipid molecules almost never exchange places with each other from one monolayer to the other within a bilayer, referred to as a ‘flip-flop’ exchange. The ‘flip-flop' takes place around once a month for a typical phospholipid molecule. 

Use your knowledge of membrane structure to explain this difference in molecular behaviour. 

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2a6 marks

Explain how three different factors can affect the fluidity of membranes

2b5 marks

Identify the structures labelled in Figure 1. In each case, state one function of the structure that has been identified.

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Structure

Name

Function

A1

   

A2

   

B

   

C

   

D

   

E

   

F

   

G

   

2c4 marks

Outline the main factors that govern the rate of diffusion across a phospholipid bilayer. 

2d4 marks

Distinguish between the features of channel proteins and carrier proteins and their roles in membrane transport. 

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3a6 marks

Design an experiment to estimate the glucose concentration of the cell contents of sweet potato tuber. The following equipment has been provided. 

  • Sweet potato
  • 1 mol dm3 glucose solution
  • Cork borers
  • Other common laboratory supplies and reagents.

Outline the steps you would take and the way that you would process the data that you would generate. 

3b3 marks

Sketch a graph of the expected results from the experiment in question 3a). 

3c2 marks

Define the term, ‘water potential’ and explain why values of water potential are expressed in kilopascals (kPa).

3d2 marks

Explain why an aqueous solution has a lower water potential than pure water. 

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4a3 marks

Figure 1 shows two configurations of a voltage-gated membrane protein that is found in the cell surface membranes of nerve cells. This protein allows the transport of ions when open. Its role is in the generation and transmission of nerve impulses. 

Figure 1

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Suggest the mode of transport of ions that this protein employs, giving reasons for your answer. 

4b2 marks

State and explain one benefit and one drawback of using beetroot tuber as a tissue in studies of factors that affect the permeability of plant cell surface membranes. 

4c2 marks

Explain one advantage to medicine of studying the permeability of the cell surface membranes of bacteria. 

4d2 marks

Active transport via a carrier protein is not the only active mechanism by which substances can cross cell boundaries against their concentration gradients. Describe and explain one other of such mechanisms. 

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5a4 marks

In a landmark scientific experiment in the 1920s, two Dutch scientists made a new claim about the structure of membranes. They calculated the area of the red blood cell membrane and then extracted the lipids that were present. These were dissolved in petroleum ether and allowed to spread into a tightly-packed layer one molecule thick on a surface of water and the area was measured.

Their data is shown in Table 1

Table 1

Mean surface area of one red blood cell 

/ μm2

Red blood cell count 

/ cells cm-3 blood

Water surface area covered by lipids extracted per cm3 of blood 

/ cm2

101

4.67 x 109

9,430

 

Explain what the data in Table 1 reveals about the alignment of lipids in the cell surface membranes of red blood cells. Reinforce your explanation with suitable calculations.

5b3 marks

Suggest three refinements of the Dutch scientists' claim that have arisen from subsequent research by other scientists, which have led us towards the currently-accepted model of the structure of membranes. 

5c4 marks

Predict and explain the consequences to digestion if mammalian intestinal epithelial cells were to stop performing active transport of sodium ions. 

5d3 marks

Malabsorption of glucose in the small intestine may lead to diarrhoea. Explain why. 

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1a2 marks

The diagram in Figure 1 shows part of a cell surface membrane. The arrows show the path taken by potassium ions and by substance A when they diffuse through the membrane out of a cell.

Figure 1

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Explain why an optical microscope would not be a suitable piece of scientific apparatus for viewing the cell surface membrane in Figure 1

1b1 mark

State how substance A in Figure 1 can diffuse through the cell surface membrane.
Refer specifically to the molecules of substance A in your answer.

1c2 marks

A scientist was investigating how the rate of diffusion of potassium ions across the cell surface membrane (as seen in Figure 1) is affected by the concentration of potassium ions within the cell. Their results can be seen in the graph in Figure 2.

Figure 2

q1c-fig-2-2-4-medium-aqa-a-level-biology

Describe the limiting factor on the rate of diffusion of potassium ions across the membrane between W and X on the graph in Figure 2, and state how the graph provides evidence for this?

1d2 marks

Explain why the graph in Figure 2 plateaus between points Y and Z.

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2a2 marks

A biologist was investigating how surface area affects osmosis in potato cubes.

Step 1: She cut two cubes of potato tissue, each with sides of 4 cm in length.
Step 2: She put one cube into a concentrated sucrose solution.
Step 3:  She cut the other cube into eight equal-sized smaller cubes and put them into a sucrose solution of the same concentration as the solution used for the large cube.
Step 4:  She recorded the masses of the cubes at intervals.


 Her results are shown in the graph in Figure 1.

Figure 1

q2a-fig-1-2-4-medium-aqa-a-level-biology

Explain why the potato tissue changed in mass.

2b2 marks

The biologist recorded the masses of the cubes at intervals. Before weighing the cubes at each interval, the biologist blotted dry the outside of each cube. Explain why.

2c4 marks

The loss in mass shown in the graph in Figure 1 in the first 20 minutes is faster in the eight small cubes than in the single large cube. At 20 minutes, the total mass of the eight small cubes was 50 grams and the total mass of the single large cube was 55 grams.

Calculate the rate of loss in mass per cm2 per minute for the single large cube and the eight small cubes during the first 20 minutes. Give your answers in grams per cm2 per minute.

2d2 marks

The scientist who collected the results in Figure 1 started with a 1.0 mol dm-3 sucrose solution but for the investigation, she needed to produce 40 cm3 of a 0.20 mol dm-3 sucrose solution. Describe how she could produce this using the original 1.0 mol dm-3 sucrose solution.

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3a2 marks

Some scientists investigated the uptake of magnesium ions in rice plants. They divided the plants into two groups and placed their roots in solutions containing radioactive magnesium ions.

   Group Y: plants had a substance that inhibited respiration added to the solution
   Group Z: plants did not have the respiratory inhibitor added to the solution

The scientists calculated the total amount of magnesium ions absorbed by the plants every 5 minutes. Their results are shown in the graph in Figure 1 below:

Figure 1 

q3a-fig-1-2-4-medium-aqa-a-level-biology

Calculate the ratio of the mean rate of uptake of magnesium ions in the first 20 minutes to the mean rate of uptake of magnesium ions in the second 20 minutes for group Z plants. Show your working.

3b4 marks

Consider the graph in Figure 1. Explain the results of the students’ investigation.

3c2 marks

Using the graph in Figure 1, calculate the rate of uptake of magnesium ions for group Y plants during the investigation. Give suitable units.

3d2 marks

Give two differences between the processes of facilitated diffusion and active transport.

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4a2 marks

Figure 1 shows a phospholipid.

Figure 1 

q4a-fig-1-2-4-medium-aqa-a-level-biology

Identify which of the fatty acids (A or B) in Figure 1 is saturated and explain how you identified this as a saturated fatty acid.

4b2 marks

Consider Figure 1.

i)
Name the bond-type between X and B.

ii)
Name the molecule from which X is formed.
4c2 marks

A group of biologists investigated the percentages of two types of lipid in plasma membranes from two cell types. Table 1 shows their results.

Table 1

Type of lipid Percentage of lipid in plasma membrane by mass

White blood cell of mammal

The bacterium Salmonella

Cholesterol 19 0
Phospholipid 70 80

Explain how the biologists could calculate the ‘percentage of lipid in plasma membrane by mass’ in Table 1.

4d2 marks

Cholesterol makes membranes less flexible. This means the presence of cholesterol in plasma membranes increase the stability of these membranes.

Explain how Salmonella cells maintain a constant shape, despite having no cholesterol in their cell-surface membranes (see Table 1).

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5a2 marks

The diagram in Figure 1 shows part of a plasma membrane.

Figure 1

q5a-fig-1-2-4-medium-aqa-a-level-biology

Complete Table 1 below by writing the letter from the diagram which refers to each part of the membrane.

Table 1

Part of membrane

Letter
Contains carbon and hydrogen only  

Surface/extrinsic protein

 

 

5b1 mark

Give a function of the structure labelled W in Figure 1.

5c2 marks

Plasma membranes (such as the one seen in Figure 1) are often described as having a fluid-mosaic structure. Explain why they are described this way.

5d2 marks

Together, structures X and Y in Figure 1 form a phospholipid. Together, these phospholipids form the phospholipid bilayer of the cell surface membrane.

State two functions of this phospholipid bilayer.

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