3.1 Diffusion, Osmosis & Active Transport

They cut a potato into cubes and recorded the change in mass over a 30 minute period.

Fig. 1 shows the results the student obtained from one cube of potato.

Describe and explain the trend shown on the graph.

The potato cube from Fig. 1 had an initial mass of 2.5 g.

Calculate the percentage increase that the potato cube had shown after 15 minutes.

The student repeated the investigation but with the beaker placed in a water bath at 30°C.

Draw a second line on the graph to show the trend expected from these results.

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Plants transport sugars in the form of sucrose in phloem sieve tubes. Companion cells are located next to phloem sieve tubes and are responsible for moving sucrose into the tubes for transport.

Fig. 1 shows part of a companion cell and the mechanism by which it moves sucrose into the cytoplasm so that it can be transported into the sieve tubes.

Fig. 1

Sucrose enters the companion cells via a cotransporter protein which will only transport sucrose in the presence of hydrogen ions (H⁺). This is represented by process B in Fig. 1.

The companion cells will transport H⁺ out of the cytoplasm by process A so that it can re-enter the cell via the cotransporter protein, allowing sucrose to enter the cell at the same time.

Use the information in Fig. 1 to describe how H⁺ is transported out of the companion cell by process A.

Process B in Fig. 1 is a passive process.

Explain what is meant by the term 'passive process' with reference to process B.

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High temperatures will change the shape of the proton pump involved in process A.

Explain why sucrose won't be able to enter the companion cell in this scenario.

Based on your knowledge of diffusion, suggest a possible explanation for this quick transport out of the companion cells.

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Diabetes insipidus is a disease that results in a large amount of dilute urine being excreted by the kidneys. This leads to blood which is more concentrated than normal.

Explain the effect that diabetes insipidus may have on red blood cells.

One of the symptoms of diabetes insipidus is blue fingernails, which indicates a lack of oxygen.

Suggest an explanation for this.

Central diabetes insipidus is a form of the disease that is believed to have a negative effect on the bone density of a sufferer which may leave them with an increased risk of sustaining fractures.

A study was conducted on the effect of central diabetes insipidus on the risk of sustaining fractures.

• The study was done at a hospital and included men and women between the ages of 45 and 65 that were admitted with bone fractures.
• Men and women were divided up into two groups each; those with diabetes insipidus and those without.
• Each of the four groups consisted of 250 individuals.
• The data gathered was used to calculate the percentage prevalence of fractures in each group.

Fig. 1 shows the results of this study.

Fig. 1

Calculate the difference in the number of men suffering from fractures between those with diabetes insipidus and those without. Show your calculations.

[3]

The scientists concluded that diabetes insipidus does increase the risk of sustaining fractures.

Evaluate this conclusion.

[4]

Some people suffer from coeliac disease that affects the lining of their small intestine.

Fig. 1 compares the lining of the small intestine of a healthy person and a person suffering from coeliac disease.

Fig. 1

Based on the information provided, explain the effect that coeliac disease would have on the absorption of monoglycerides.

Fig. 2

Describe and explain what the graph shows.

Human body temperature is approximately 37 °C, which provides the optimum temperature for activity of protein molecules such as enzymes. 

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

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Fig. 3

Describe and explain the movement of water between the cells in Fig. 3.

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Table 1 shows some of their measurements.

Table 1

Ions move into guard cells by active transport.

Describe how the ions move into the guard cells.

[2]

Describe and explain how the change in ion concentration causes the guard cell volume to change.

Use the information in Table 1 in your answer.

(iii)

The botanists left the broad bean plants unattended for three days. During this time the broad bean plants wilted. 

Suggest two environmental factors that can cause plants to wilt.

[2]

Table 2 shows the concentrations of four mineral ions in the roots of a plant and in the surrounding soil.

These minerals are all essential to the health of the plant.

Table 2

Based on the information in Table 2, explain why the plant roots cannot absorb these mineral ions by diffusion.

Calcium ions play a structural role in cell walls and membranes of the plant cell.

Calculate the percentage difference between the concentration of calcium ions inside and outside the plant root, using the data from Table 1.

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They used the following method:

1. Five eggs were placed in hydrochloric acid for 24 hours to dissolve the eggshell.
2. The mass of each egg was measured and recorded.
3. Five beakers were set up; four with 200 cm³ of sugar solution of different concentrations (0.2, 0.4, 0.6 and 0.8 mol dm^-3) and one with distilled water (0 mol dm^-3).
4. An egg was place in each of the beakers for 1 hour.
5. The eggs were removed and dried with a paper towel whereafter the mass of each egg was measured and recorded.
6. The percentage change in the mass of each egg was calculated and a graph was drawn from these results.

Fig. 1 represents the graph that was drawn of the percentage change in mass.

Fig. 1

Use the information in Fig. 1 to describe the results obtained in each beaker.

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Text

Use the graph in Fig. 1 to identify the concentration of the sugar solution where the mass of the egg would not change.

[1]

Explain why the mass of the egg would not change at the concentration identified at part (i).

[2]

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Diffusion and osmosis are two ways in which molecules can move.

Compare diffusion and osmosis with each other.

A group of scientists investigated the rates of absorption of different sugars using two pieces of the intestine.

One of these pieces were treated with cyanide, which is a toxin that prevents cellular respiration from occurring.

The results of the investigation are shown in Table 1.

Table 1

Suggest the method by which sugar A, B and C are absorbed by the intestine.

The scientists concluded that sugar D is absorbed by active transport.

Explain how they came to this conclusion by using evidence from Table 1.

One of the scientists states that 'All four of the sugars investigated could be absorbed by diffusion'.

Determine whether this statement is correct by using the information in Table 1 and your knowledge of diffusion.

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The wall of the intestine contains smooth muscle cells which convert glucose into glycogen. Glycogen is insoluble in water which makes it very suitable as an energy storage molecule.

Using your knowledge of osmosis, suggest an explanation as to why it is important for glucose to be stored as glycogen in muscle cells.

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Fig. 1

Which of the plant cells above (A or B) was placed in a solution that was more dilute than the cell contents?

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Fig. 2 below shows a red blood cell.

Fig. 2

Describe what would happen if a red blood cell was placed in a solution that was more dilute than the cell contents.

[2]

Explain why the description at part (i) would be different from what would happen to a plant cell.

[2]

Hereditary spherocytosis is a rare inherited condition that produces red blood cells with an abnormal spherical shape, as shown in Fig. 3.

Fig. 3

Suggest the possible impact of this condition on a sufferer.