Diffusion, Acrive Transport, Osmosis

Question 1

Describe two factors than can increase the rate of diffusion

Answer 1

Increasing the temperature
Increasing the concentration gradient

Question 2

Define osmosis and explain how it is different from diffusion

Answer 2

Osmosis is the movement of water molecules down a water concentration gradient through a partially permeable membrane.
Unlike diffusion, osmosis specifically mentions involves water and requires a membrane.

Question 3

A student puts a potato cylinder into pure water and measures its mass before and after. Explain what happens to the mass and why. (4 marks)

Answer 3

The mass of the potato increases because water moves into the potato by osmosis. The water moves from a higher water concentration in the pure water to a lower water concentration in the potato cells through a partially permeable membrane.

Question 4

What is active transport and how is it different from diffusion?

Answer 4

Active transport is the movement of substances against a concentration gradient using energy from respiration.
Unlike diffusion, it requires energy and goes against the concentration gradient.

Question 5

Explain how root hair cells in plants use active transport.
(4 marks)

Answer 5

Root hair cells absorb mineral ions from the soil, where the concentration of ions is lower than inside the root. Active transport moves the ions into the root cells against the concentration gradient, using energy from respiration.

Question 6

Why is it important to dry the potato cylinders before reweighing them?

Answer 6

To remove excess surface water which could affect the mass and lead to inaccurate results

Question 7

A student places identical potato cylinders into sugar solutions of different concentrations.
They measure the mass before and after 24 hours.

Solution concentration (mol/dm³) Initial mass (g) Final mass (g)
0.0 5.2 5.8
0.2 5.1 5.4
0.4 5.3 5.2
0.6 5.2 5.0
0.8 5.3 4.8
Describe the trend shown in the table
(3 marks)

Answer 7

As the concentration of the sugar solution increases, the final mass of the potato cylinder decreases.

Question 8

How could the student ensure the experiment is a fair test? Give two variables to control. (2 marks)

Answer 8

Use potato cylinders of the same length and diameter
Keep the time in solution the same for each cylinder

Question 9

The student repeats the experiment and finds inconsistent results. Suggest 3 possible sources of error.
(3 marks)

Answer 9

Inaccurate measuring of the mass or length
Not drying the potato properly before weighing
Different surface area due to irregular cutting of cylinders

Question 10

How could the student determine the concentration of the potato’s cytoplasm using this practical?

Answer 10

Plot a graph of percentage change in mass against concentration. The point where the line crosses the x-axis (where there is no change in mass) is the concentration of the potato’s cytoplasm — because water is moving in and out at the same rate (isotonic point)

Question 11

Describe the graph of the osmosis experiment

Answer 11

The graph would show a decrease in percentage change as concentration increases. It would cross the x-axis at the isotonic point, where there is no net change in mass — this indicates the concentration of the potato’s cytoplasm.

Question 12

What are the independent and dependent variables in the osmosis experiment?

Answer 12

Independent variable: Sugar solution concentration
Dependent variable: Percentage change in mass of the potato cylinder

Question 13

A student investigates the effect of sugar solution concentration on osmosis in potato cylinders. They measure the mass of each cylinder before and after placing them in different concentrations of sugar solution for 24 hours.

Here are their results:

Sugar solution concentration (mol/dm³) Initial mass (g) Final mass (g)
0.0 4.2 5.0
(2 marks)

Answer 13

Percentagechange

Calculation of percentage change:

Finalmass
−
Initialmass
Initialmass
×
100
=
5.0
−
4.2
4.2
×
100
=
0.8
4.2
×
100
≈
19.05
%
Percentagechange=
Initialmass
Finalmass−Initialmass
×100=

5.0-4.2/4.2 ×100≈19.05%

Question 14

A student wants to investigate how different concentrations of sugar solution affect the mass of potato cylinders.

Describe how the student should carry out this experiment. Write the steps in the correct order.
(4 marks)
Write in bullet points

Answer 14

1. Cut equal sizes of potato cylinders using a cork borer and scalpel.
2. Measure and record initial masses of each cylinder
3. Place each cylinder in different concentration sugar solutions
4. Leave to sit for 30m to 1h
5. Measure and record final mass

Question 15

A student writes the following method to investigate how different concentrations of sugar solution affect the mass of potato cylinders:

Cut several potato cylinders of different sizes.
Pour different amounts of sugar solution into unlabelled beakers.
Weigh all the cylinders together and record their total mass.
Leave them in the beakers for random amounts of time.
Take the potatoes out, don’t dry them, and weigh them again.
Calculate how much mass they gained or lost.

Identify three mistakes in the student’s method and explain how to correct each one
(6 marks)

Answer 15

3. Weigh each potato separately before and after the experiment
4. Leave cylinder out for the same amount of time
5. Calculate percentage change in mass to fairly compare different results

Question 16

Describe what is meant by “water potential.”
(2 marks)

Answer 16

The measure of how freely water molecules move. Pure water has the highest water potential.

Question 17

Compare what happens to a plant cell and an animal cell when placed in pure water

Answer 17

In pure water, both cells take in water by osmosis.
The animal cell swells and may burst because it has no cell wall.
The plant cell becomes turgid, but doesn’t burst due to the rigid cell wall.

Question 18

A student places plant tissue into three sugar solutions of different concentrations. After 24 hours, they measure the length of the tissue and find the following:

Solution Change in length
A (pure water) +2 mm
B (medium sugar) 0 mm
C (concentrated sugar) –3 mm

Explain the results using ideas about osmosis and water potential.

(5 marks)

Answer 18

In solution A, water moved into the plant tissue by osmosis because the water potential was higher outside the tissue.
In solution B, there was no net movement of water. The water potential inside and outside the tissue was equal (isotonic).
In solution C, water moved out of the tissue into the sugar solution because the water potential was lower outside the tissue.

Question 19

Describe what happens to a red blood cell placed in a hypertonic solution.
(2 marks)

Answer 19

Water leaves the cell by osmosis.
The cell shrinks and becomes crenated

Question 20

Explain why plant cells do not burst in a hypotonic solution but animal cells can.
(3 marks)

Answer 20

Water enters both cells by osmosis.
Plant cells have a cell wall, which provides support and prevents bursting.
Animal cells do not have a cell wall, so they can burst (lysis) if too much water enters

Question 20

Compare what happens to a plant cell and an animal cell in:

(a) A hypotonic solution
(b) A hypertonic solution

(5 marks)

Answer 20

In a hypotonic solution, water enters both cells by osmosis.
Plant cell becomes turgid but does not burst due to the cell wall.
Animal cell swells and may burst (lysis) because it has no wall.
In a hypertonic solution, water leaves both cells by osmosis.
Plant cell becomes plasmolysed as the membrane pulls away from the wall.
Animal cell shrinks and becomes crenated.

Question 21

Define the following terms as they relate to osmosis:

(1) Isotonic
(2) Hypertonic
(3) Hypotonic
3 marks

Answer 21

1.A solution that has the same solute concentration as the cell
2. A solution that has a higher solute concentration than the cell.
3.A solution that has a lower solute concentration than the cell.