Transport in Animals (Edexcel GCSE Biology)

Unicellular organisms such as bacteria, can obtain oxygen from their surrounding environment through simple diffusion across the cell membrane.

In larger multicellular organisms, the distance from their surface to their centre is relatively long. Consequently, atmospheric oxygen cannot diffuse into the cells in the centre of the organism as they are too far away.

Calculate the surface area : volume ratio of a cube that has a side length of 3cm.

Describe the features that make a gas exchange surface efficient.

Figure 1 shows three cubes with sides of length 1 cm, 2 cm and 3 cm, the cubes represent imaginary cells.

Figure 1

During an exam, a student looked at Figure 1 and wrote the following statement:

Figure 2

Explain which factors affect the rate of diffusion of molecules into and out of cells. Make reference to Fick's law in your answer.

A cell membrane has a thickness of 10 nm.

Explain how the rate of diffusion will change if the cell membrane thickness changed to 5 nm

Many small molecules such as carbon dioxide and oxygen can pass across the cell membrane through diffusion.

Figure 1 shows two scenarios A and B in which molecule X can diffuse across the cell membrane.

Figure 1

Figure 2

Monoglycerides are a type of lipid molecule. They are absorbed into the epithelial cells of the small intestine by diffusion across the cell membrane.

Some people suffer from coeliac disease, a condition that affects the lining of the small intestine. Figure 1 compares the lining of the small intestine of a healthy person with that of a person suffering from coeliac disease.

Figure 1

Use the information provided to explain the effect that coeliac disease would have on the absorption of monoglycerides.

The graph shows the rate of uptake of monoglycerides in the small intestine.

Describe and explain the trend shown in the graph.

Human core body temperature is approximately 37 °C.

Suggest why a temperature of 37 °C helps to maximise the rate of transport of substances across cell membranes.

Figure 1 below shows a cross section taken through a capillary using an electron microscope.

Public domain, via Wikimedia Commons

Figure 1

Identify the structures labelled X and Y in Figure 1.

Describe the pathway taken by oxygen in Figure 1.

Table 1 below contains information about the oxygen concentration in different locations within the human body.

Table 1

Between which two locations in Table 1 would the fastest diffusion take place?

(1)

(ii)

Use your knowledge of Fick's law to explain your answer to part (i)

(2)

Oxygen diffuses from the arterial end of a capillary into pancreatic cells at a rate of 0.0002 g/s.

Calculate the rate of diffusion if the exchange surface changes in the following ways:

Give your answers to 2 significant figures.

The diffusion distance becomes 3x longer

(1)

(ii)

The available surface area increases by 40 %

(1)

(iii)

The difference in concentration decreases by 20 % and the diffusion distance becomes 1.5x longer

(2)