1. Substances enter and leave cells through the cell membrane
WJEC Module 2

2. Questions you’ll be answer by the end!
How do substances enter and leave cells?
What is diffusion?
What is osmosis?
Why does some transport need energy?

3. Learning Outcomes
know that diffusion is the movement of substances down a concentration gradient. The process does not require energy and only certain substances pass through the cell membrane in this way

4. Diffusion Diffusion is
The movement of particles in a gas or any dissolved substance from a region of higher concentration to a region of lower concentration
In cells, diffusion occurs across the cell membrane. The cell membrane can be described as partially permeable. It will only allow some substances to pass through it.

5. Learning outcome
know that osmosis is the diffusion of water through a selectively permeable membrane, from a region of high-water concentration to region of low-water concentration.

6. Osmosis
Osmosis is the diffusion of water, through a partially permeable membrane, from a region of high water concentration to a region of low water concentration.

7. Learning outcome
carry out experimental work using living plant material and visking tubing as non-living material and be able to interpret experimental results in terms of membrane pore and particle size: the pore size is large enough to allow water molecules through but restricts the movement of solute molecules.

8. Osmosis Practical Activity 1 Changing the rate of osmosis
Two students carried out an investigation using cylinders of potato. They cut out 5 potato cylinders and measured the mass of each one. They made up five different glucose solutions with a different concentration and left a potato cylinder in each solution for 24 hours.
After 24 hours they dried each potato cylinder and measured its mass again.
Look at the table of results
Answer the questions that follow

9. Osmosis in potato Solution Mass before (g) Mass after (g)
Change in mass (g)
%age change in mass
Water
0.1
0.2
0.5
1.0

10. Osmosis practical A model Cell Visking tubing is partially permeable.
Cut 2 pieces of Visking tubing to 12cm long
Tie a knot in one end
Fill “cell A” with sugar solution
Fill “cell B” with water
Tie the other end of each piece of visking tubing
Weigh each “cell”
Put “cell A” into a boiling tube of water
Put “cell B” into a boiling tube of sugar solution
After 30 minutes reweigh each “cell”

11. Conclusions Describe and explain your results for each cell
Cell A increases in mass because water has moved into the cell by osmosis, from a high water concentration in the boiling tube to a low water concentration inside the visking tubing.
Cell B decreases in mass because water has moved by osmosis

12. Osmosis Practical Osmosis in potato cells
Cut three potato cylinders to the same length
Measure their mass and write it down
Set up the following test tubes
Test tube A – distilled water
Test tube B – weak sugar solution
Test tube C – strong sugar solution
Place one potato cylinder in each test-tube and leave it for 30 minutes
Re-weigh each chip

13. conclusions Calculate the %age increase in mass for each potato chip
Which chip has increased in size? Why?
Which chip has decreased in mass? Why?
Describe the “feel” of each of the potato cylinders
Explain why the potato cylinder in A feels different to the potato cylinder in C.

14. Learning outcome
understand active transport as an energy requiring process whereby substances can enter cells against a concentration gradient.

15. Active Transport
Active transport is the movement of particles across a membrane up a concentration gradient.
Particles move from where they are in low concentration to where they are in high concentration.
This requires energy from respiration.

16. Comparing diffusion and active transport
Substances move from a region of higher concentration to a region of lower concentration
This relies on the random movement of molecules
Diffusion is like freewheeling down a steep hill
Substances are moved from a region of lower concentration to a region of higher concentration
This process required energy
Active transport is like cycling up a steep hill, you use lots of energy