Biological Membranes (OCR A Level Biology)

A student investigated the uptake of two different substances in cultured cells.

The rate of uptake was measured at different concentrations of each substance in the medium surrounding the cells.

The results are shown in Fig. 16.2.

The student used a colorimeter to measure the concentration of each substance in the liquid surrounding the cells.

The colorimeter had an analogue display. The reading was indicated by a needle moving across a scale. The smallest divisions on the scale corresponded to 0.1 absorbance unit.

After the investigation the student suggested some improvements.

Draw a line between each of the improvements to the corresponding justification.

Water moves by osmosis in living organisms.

The apparatus shown in Fig. 16 can be used to demonstrate osmosis.

Fig. 16

When the capillary tube with visking tubing bag was placed in solution Y, the level of solution X inside the capillary tube rose from 10.5mm to 26.5mm.

uncertainty = ............................. % [2]

[2]

A group of students used the following method to investigate osmosis in plant cells.

• Cut pieces of plant material of equal surface area ensuring no skin is present.
• Rinse to remove cell debris.
• Gently pat the plant pieces dry with a paper towel.
• Weigh each piece and record mass.
• Put the plant piece in a 200cm³ beaker.
• Cover plant piece with 50cm³ of sucrose solution.
• Use sucrose solutions of 0, 0.1, 0.3, 0.5, 0.7moldm^–3.
• Leave for 24h.
• Remove the piece of plant material.
• Dry carefully using a paper towel.
• Weigh the plant piece and record the mass.
• Calculate the percentage change in mass for each piece.
• Repeat twice for each sucrose concentration.

The students investigated material from three different plants: carrot, courgette and potato.
Their results are shown in Table 16.

Table 16

Water has many properties that are essential for living organisms.

Explain how properties relating to the density of water contribute to the survival of organisms.

Soluble mineral ions are present in soil.

Explain why water molecules can form hydrogen bonds with nitrate (NO₃^-) ions.

Fig. 18

Explain how the events shown in Fig. 18 cause water to enter the endodermis.

[2]

Fig. 1 shows a diagram of part of a cell surface membrane.

Fig. 1

Identify molecules A and B.

Describe the role of molecule C in Fig.1.

State two functions of molecule D in Fig. 1.

Explain the importance of molecule E in Fig. 1 when the cell is exposed to high temperatures.

Fig.1 and the key below give information about some molecules found inside and outside a cell.

Using the information provided, identify the mechanism by which each of these molecules can enter the cell across the cell membrane. Give an explanation for your choice.

Fig. 2 shows the process which occurs when a phagocyte encounters a pathogen in the blood stream.

Fig. 2

Describe the mechanism by which the pathogen is taken into the cell.

[3]

Studies have indicated that high cholesterol levels may lead to a reduced non-specific immune response in patients.

Leukocyte Adhesion Deficiency (LAD) Syndromes are a group of disorders affecting white blood cells in the body.

One type of LAD that has been identified is caused by a reduction in cell membrane proteins of neutrophils and is characterised by a reduced ability of the body to respond to bacterial infection leading to increased susceptibility to recurrent infection.

Using your knowledge of cell membrane function and the information from this question, suggest how LAD may result in recurrent infections. 

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 1.1. In each case, state one function of the structure that has been identified.

Figure 1.1

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 part (a). 

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 1.1 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 1.1

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 the table below.

Explain what the data in the table 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.