Cell Membranes & Transport (AQA A Level Biology)

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

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

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.

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

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?

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

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

 Her results are shown in the graph in Figure 1.

Figure 1

Explain why the potato tissue changed in mass.

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.

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.

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.

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 

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.

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

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

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

Figure 1 shows a phospholipid.

Figure 1 

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.

Consider Figure 1.

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

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

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).

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

Figure 1

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

Table 1

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

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.

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.

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

Give four functions of proteins that occupy the plasma membrane. 

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. 

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. 

Explain how three different factors can affect the fluidity of membranes

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

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

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

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. 

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

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

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

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

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

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. 

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

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. 

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

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.

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. 

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

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