Diffusion, Osmosis and active transport

Diffusion definition

Movement of particles from a region of high concentration to low concentration (down a concentration gradient)

Does diffusion require energy?

No, diffusion is a passive process

What happens at equilibrium in diffusion?

Particles are evenly spread but still moving randomly

Where does diffusion occur in the body?

Alveoli in lungs and villi in small intestine

Name 3 factors affecting rate of diffusion

Temperature, concentration gradient, surface area

Effect of temperature on diffusion

Higher temperature = more kinetic energy = faster diffusion

Effect of concentration gradient

Steeper gradient = faster diffusion

Effect of surface area

Larger surface area = faster diffusion

Examples of substances that diffuse

Oxygen, carbon dioxide, urea

Why multicellular organisms need exchange systems

Surface area to volume ratio is too small for diffusion alone

Features of efficient exchange surfaces

Large surface area, thin membrane, good blood supply, ventilation

Osmosis definition

Diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane

Does osmosis require energy?

No, it is passive

What is a dilute solution?

High water concentration, low solute concentration

What is a concentrated solution?

Low water concentration, high solute concentration

Direction of water movement in osmosis

From high water potential to low water potential

What must be mentioned in exam answers about osmosis?

Concentration of water or water potential

What is a partially permeable membrane?

Allows small molecules like water through but not larger solutes

What happens to a cell in a hypotonic solution?

Gains water, becomes turgid (plant cells)

What happens to a cell in an isotonic solution

No net movement of water

What happens to a cell in a hypertonic solution

Loses water, becomes plasmolysed

Active transport definition

Movement of substances from low to high concentration (against concentration gradient)

Does active transport require energy?

Yes, from respiration

Where does active transport occur?

Across cell membranes using carrier proteins

Example of active transport in plants

Absorbing mineral ions from soil into root hair cells

Example of active transport in humans

Absorbing glucose in the small intestine

Why is active transport important

Allows uptake of substances even when concentration is low

Osmosis required practical – independent variable

Concentration of solution

Osmosis required practical – dependent variable

Change in mass

Osmosis required practical – control variables

Size of potato cylinders, volume of solution, time

Osmosis calculation

Percentage change = (change ÷ original) × 100

What happens in osmosis practical

Water moves into/out of potato depending on solution concentration

Why mass changes in osmosis practical

Water enters or leaves cells by osmosis

A student investigated osmosis in pieces of potato.

What method do you use?

1. Cut a piece of potato into a cube.

2. Record the mass of the cube.

3. Place the cube into a beaker containing sucrose / salt solution eg 0.2 mol/dm³

4. After some time, remove the cube tap/blot with tissue paper to remove excess liquid and record the mass.

5. Calculate the change in the mass of the cube.

6. Repeat steps 1 to 5 using two more different concentrations of sucrose / salt solution.

Evaluate method