2 D Movement of Molecules

Time: 43 minutes

Total marks available: 43

Total marks achieved:

Q1.(a) The diagram shows a plant cell drawn by a student.

(i) Label the diagram to show the selectively permeable membrane.

(ii) Name three parts of a typical plant cell that are not found in an animal cell.

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(b) The diagram shows a glass container divided in half by a selectively permeable membrane. One half contains water and the other half contains sucrose solution.

(i) Complete the diagram below to show the change to the level of the water and the level of the sucrose solution after one hour. (1)

(ii) Name the process that causes the change in the level of the water and the level of the sucrose solution.

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(1)(Total for question = 6 marks)

Q2.

John wanted to investigate the effect of the size of potato tissue on the rate of osmosis.

He cut three different sized cubes of potato, one 0.5 × 0.5 × 0.5 cm, one 1 × 1 × 1 cm and one 2 × 2 × 2 cm.

He weighed the potato cubes and recorded their masses.

He then placed each cube into a beaker of distilled water and left them for 1 hour.

He weighed them again and recorded their new masses.

In each case the mass of the potato cubes increased.

The table shows his results.

His teacher told him to calculate the percentage change in mass of each cube.

(a) (i) Calculate the percentage change in mass for the 1 × 1 × 1 cm cube. Show your working.

(2)

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(ii) Explain why John converted change in mass to percentage change in mass.

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(b) Use your knowledge of osmosis to explain why the mass of each cube increased.

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(c) John's teacher suggested that the different sized cubes have a different surface area and volume.Use the formulae below to calculate the surface area (SA) of each cube, the volume (Vol) of each cube and their SA:Vol ratios.SA = 6 × (cube side in cm)2 Vol = (cube side in cm)3

Write your answers in the table below. One cube has been done for you.

(d) Explain the effect of the different SA:Vol ratios on the rate of osmosis into the potato.

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(e) Potato tissue is made from plant cells.Draw and label a plant cell in the space below.

(3)

Q3. 

A student carries out an experiment to investigate the effect of different concentrations of a food dye on diffusion.

(a)  Describe what is meant by the term diffusion.

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(b)  The student adds a different concentration of coloured food dye to each of five wells in an agar plate.

The diagram shows the agar plate at the beginning of the experiment and after 24 hours.

The student does the experiment using three plates.For each plate he measures the diameter of each circle to see how far the food dye has diffused.The table shows his results.

(i)  Calculate the average diameter for 0.2% concentration of food dye.(2)

average diameter = ........................................................... cm

(ii)  Describe the effect of food dye concentration on diffusion.(2)

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(iii)  Explain the relationship between food dye concentration and diffusion.(2)

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(c)  State two variables that the student should control in this experiment to ensure that the results are valid.

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(Total for question = 9 marks) 

Q4. 

A chicken egg is a single cell protected by a shell on the outside.

A student puts three chicken eggs into dilute acid and leaves them for three days.

The acid dissolves the egg shells, leaving the contents of the eggs surrounded by the cell membrane.

The diagram shows the student's method.

The student removes the eggs from the dilute acid and uses water to wash the surface acid away.

The student then uses the eggs for this osmosis experiment.

  he measures the mass of each egg   he then puts one egg into a beaker containing distilled water   he puts another egg into a beaker containing 5% salt solution   he puts a third egg into a beaker containing 15% salt solution   after 15 minutes he removes each egg from its beaker and measures its mass again

(a)  The bar graph shows the results obtained by the student from the osmosis experiment.

(i)  Name the dependent variable in this experiment.(1)

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(ii)  Explain the result for the egg placed in distilled water.(2)

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(b)  The student calculates the percentage change in mass for the eggs placed in

distilled water and in 5% salt solution.(i)  Use the data from graph 1 to calculate the percentage change in mass for the egg placed in 15% salt solution. Show your working.

(2)

Percentage change in mass = ............................. %

(ii)  The student plots the percentage change in mass on graph 2.Complete the bar graph to show the percentage change in mass for the egg placed in 15% salt solution.

(1)

(c)  Give one way in which osmosis differs from diffusion.

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(Total for question = 7 marks) 

Q5. 

A student carries out an experiment to investigate the factors affecting the rate of diffusion.

(a)  In her first experiment

  she makes three agar jelly cubes, one with sides of 4.0 cm, one with sides of 2.0 cm and one with sides of 1.0 cm   she covers each cube in a solution of a coloured dye for three minutes   she then cuts the cubes to see how far the dye has diffused

The diagram shows a cross-section through the largest cube after three minutes.

(i)  Describe what is meant by the term diffusion.(1)

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(ii)  Measure the distance that the dye has diffused into the large cube.(1)

distance = ........................................................... mm

(iii)  Complete the diagram below to show how far the dye will have diffused in the smallest cube after three minutes.

(1)

(b)  The student then carries out a second experiment but this time she leaves the cubes in the dye for six

minutes.

Draw the results you would expect to see when the largest cube is cut open after six minutes.(1)

(c)  Other factors may affect the rate of diffusion of the dye.

Explain two factors that the student should keep constant in her investigation.(4)

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(d)  Explain how the results of the student's investigation support the idea that large organisms need a circulation system.

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(Total for question = 11 marks) 

Examiner's Report

Q1.

Part (a) examined basic knowledge of plant and animal cells. Most students were able to label the cell membrane and to name the cell wall, vacuole and chloroplasts as the parts of a typical plant cell not found in an animal cell. In part (b), it was pleasing to note that many students were aware that osmosis was responsible for the change in the level of water and duly completed the diagram to show a lower water level on the left hand side of the container compared to the right hand side of the container.

Q2.

The question gave data from a variation on a familiar experiment on osmosis in potato. In (a)(i), most were able to calculate the percentage change in mass but in only the best candidates were able to explain that calculating the percentagechange enables us to compare different starting masses in (ii). This is a common practice in many experiments and the examiners were disappointed that many candidates did not understand its purpose. However, in (c), almost all were able to explain why the potato cubes gained mass. Most could also calculate the surface area, volume and surface area to volume ratio of the cubes when given the formula. In (d), many could describe the relationship between cube surface area to volume ratio and rate of osmosis. In (e), candidates had to draw and label a plant cell, with most gaining full marks although a few confused cell wall and cell membrane.

Q3. This question described an experiment to investigate the effect of different concentrations of a food dye on diffusion. In part (a) most responses could correctly describe what is meant by diffusion. In part (b) (i) almost all candidates could correctly calculate the mean rate of diffusion. However, in part (ii) although most responses earned one mark, for describing an increase in rate of diffusion with increasing concentration of dye, only the very best could gain the second mark for noting that the rate of increase slows as concentration increases. In part (iii) only the best candidates seemed to appreciate the difference between an explanation of an effect and its description. Thus, many candidates merely repeated their answer to (ii) only the best explained the increase in diffusion in terms of more molecules and a steeper diffusion gradient. It is worth centres spending time with students looking at command words and what they mean. In part (c) most candidates could state at least one variable that the student should control in this experiment to ensure that the results are valid. 

Q4. 

This question described an investigation using eggs to study osmosis.

In part (a) (i) students had to identify the dependent variable in the experiment. The majority were able to correctly identify mass.

In part (ii) most were able to explain the result for the egg placed in distilled water in terms of water entering down a water potential gradient.

In part (b) (i) students needed to use the data from the graph to calculate the percentage change in mass for the egg placed in 15% salt solution. Almost all gained 1 or 2 marks with the majority correctly calculating -

10%. Some lost credit by not indicating a loss in mass.

In part (ii) students then had to plot their calculated value on the axis provided. Most were able to do this but some having calculated -10% then plotted it as 10%.

In part (c) almost all candidates could correctly give one way in which osmosis differs from diffusion.

 

Q5. 

Defining diffusion in (a)(i) posed little difficulty for most. Incorrect responses tended to state that 'substances' are moved from a high concentration to a low concentration. Candidates who made reference to diffusion gradient had to state 'down' rather than 'along' or 'across' to gain credit. Most were able to measure the 3mm distance moved by the dye in (ii) and also to complete the diagram correctly in (iii). A surprising number of candidates shaded outside the border provided or fully shaded the cube.

In (b) most were able to show that the dye would travel further but again some shaded outside the border provided or fully shaded the cube.

Part (c) was a challenging question, but most candidates were able to recall that temperature could affect the rate of diffusion by its effect on the kinetic energy and movement of molecules. The second most popular correct response was that of the concentration of the dye as this would affect the concentration gradient. The examiners also credited the concentration of the agar.

Part (d) challenged the most able students but the examiners were pleased to note a good number of knowledgeable responses. These candidates effectively linked the observations of the diffusion experiment to the problems facing a large organism. As such, they made correct reference to inadequacy of diffusion to supply oxygen to all body cells in large organisms with their small surface area to volume ratios. Weaker students were unsure as to whether large animals would have a large or small surface area: volume ratio.

 

Mark Scheme

Q1.

Q2.

Q3. 

 

Q4. 

 

Total 7 marks

 

Q5. 

 (Total for question = 11 marks)