Author: Elizabeth O'Brien
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NAME: 							Membranes homework 2
Q1.Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer. Describe other ways by which substances cross this 
membrane.
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(Total 5 marks)
Q2.Strawberries may be dehydrated by removing most of the water they contain. Dehydrated strawberries have many different uses in the food industry. Food scientists investigated the effect of using osmosis to dehydrate strawberries. The results of the investigation are shown in the table.
1.      The scientists weighed a sample of strawberries and then cut them into 10 mm thick slices.
2.      They put the strawberry slices into a 1.2 mol dm^–3 solution of sucrose at a temperature of 25 °C.
3.      After 1 hour, they removed the slices from the sucrose solution and washed them in water. They dried the slices by blotting them and then weighed them.
4.      They also measured the texture of the strawberry slices.
5.      The scientists repeated steps 1 to 4, but they left the strawberry slices in the sucrose solution for different amounts of time.

Length of time in
sucrose solution / hours

Percentage loss in mass

Texture / arbitrary units

0 | Not applicable | 1.2
1 | 15.96 | 0.9
2 | 22.88 | 0.7
4 | 32.36 | 0.7
6 | 38.78 | 0.7

(a)     (i)      In this investigation, the scientists cut the strawberries into slices (step 1).
Explain the advantage of this.
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(ii)     The scientists blotted the strawberry slices dry before weighing them (step 3).
Explain why.
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(b)     In the second column of the table, the percentage loss in mass for one of the values has been recorded as not applicable. Explain why.
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(c)     Use the table to describe how the length of time in the sucrose solution affected the strawberries.
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(d)     You could use the data in the table to predict the time that strawberries should be left in sucrose solution to dehydrate them fully. Describe how you could use a graph to do this.
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Q3.If red blood cells are placed in pure water, water enters the cells by osmosis and they burst. This is called haemolysis. As red blood cells burst they release pigment.Scientists placed samples of red blood cells in different concentrations of sodium chloride solution for the same period of time. They used red blood cells from four different mammals: dog, guinea pig, rabbit and sheep.
If haemolysis had taken place, the solution turned red. The scientists measured the intensity of the red colour using a colorimeter. The more intense the red colour, the greater the amount of haemolysis.The scientists calculated the percentage of red blood cells that were haemolysed in each sodium chloride solution.The following figure shows the scientists’ results.

(a)     Use the figure to give two differences between the results for dog and sheep.
Difference 1 _________________________________________________________
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Difference 2 _________________________________________________________
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(b)     Calculate the difference in the percentage of haemolysed cells between sheep and rabbit at a sodium chloride concentration of 0.5%.

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(1)

(c)     Explain the relationship between the depth of the red colour of the solution and how much haemolysis has taken place.
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(2)
(d)     During treatment in a veterinary surgery, any of the mammals in the figure above may be given an infusion of sodium chloride solution directly into a vein. The concentration of sodium chloride solution used is 0.9%, rather than 0.5%, regardless of the species of mammal.
Explain the advantage to the vet of using this concentration.
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 (2)
(Total 7 marks)