2(d) Movement of substances in and out of cells

Diffusion

The net movement of particles from an area of high concentration to an area of low concentration(down the concentration gradient) until there is an equal spread of particles(then it stops).

• It is a passive process. It doesn't require energy.

• It happens In both liquids and gases - particles move free and randomly

• Does not require cell membrane

What can diffuse through partially permeable cell membrane?

Only small molecules can diffuse through partially permeable cell membrane, e.g. glucose, amino acids, water, oxygen. Big molecules like starch and proteins can’t.

Factors affecting diffusion

• Difference in concentration

• Temperature

• Distance

• Surface area to volume ratio

Difference in concentration

The bigger the difference, the faster the diffusion rate.

(Because more particles move down the concentration gradient than against it.)

Temperature

The higher the temperature, the more kinetic energy the particles have. They move around a lot faster.

Distance

The shorter the distance, the faster the diffusion. (Because the particles travel less.)

Surface Area to Volume Ratio

The larger the surface area to volume ratio, the faster the rate of diffusion.

(Because there is more space for the particles to move through.)

Osmosis

The net movement of water molecules from an area of high water concentration to am area of low water concentration across a partially permeable cell membrane.

Partially permeable cell membrane

Has tiny holes(pores) though which only very small molecules(like water) can pass but not larger molecules(starch and proteins)

Osmosis for plant support

• If the plant has high water concentration in its external surroundings, its cells will absorb water by osmosis.

• The cell vacuole will swell and push against the cell wall.

• Cell is turgid→plant is supported

• If the plant has low water concentration in its external surroundings, its cells will absorb water by osmosis.

• The cell vacuole will shrink and there will be less pressure on the cell wall.

• Cell is flaccid→plant is unsupported and wilts. (cell membrane and cytoplasm will detach)

Active Transport

The movement of particles against a concentration gradient across a partially permeable cell membrane using energy released from respiration.

E.g. in digestive system- low concentration of nutrients in the gut, high concentration of nutrients in the blood.

Practical：Osmosis in living system

1. Use a cork borer to cut potato cylinderd out of the same potato. This ensures that only one species was used.

2. Use a sharp knife and a ruler to trim the cylinders to the same length(±3cm)

3. Measure and record the mass of each cylinder using a weighing scale.

4. Put the potato cylinders into boiling tubes containing solutions of different sugar concentrations.

5. Leave them for 15 minutes before removing them from the boiling tubes and drying them with paper towels.

6. Measure and record the new mass of each cylinder and calculate the percentage change of each potato cylinder.

7. Keep the temperature constant.

Visking tubing

Partially permeable membrane made from (regenerated) cellulose.

• It has tiny pores that allow small molecules(like water, glucose, oxygen, amino acids) to pass through.

• Larger molecules(like starch or protein) cannot pass through.

Practical: Osmosis in non-living systems

1. Make a bag from Visking tubing by tying a knot in one end.

2. Use a syringe to fill the bag with sucrose solution(±20-40cm³)

3. Place the capillary tubing inside the bag of sucrose.

4. Fix the visking tubing bag in position with a retort stand.

5. Fill a glass beaker with water

6. Ensure that the visking tubing bag is in the beaker of water.

7. Start the stop watch.

8. Mark the level of solution in capillary.

9. Calculate water movement rate by measuring the distance moved in mm divided by time in minutes.