Movement of Substances Into & Out of CellsEdexcel IGCSE Biology: Revision notes
Section 1
What is diffusion?
Diffusion is the net movement of particles (in a gas or liquid) from a region of higher concentration to a region of lower concentration, down a concentration gradient, until evenly distributed. It requires no energy from the cell (it is a passive process).
Examples: oxygen diffusing into cells for respiration, carbon dioxide diffusing out of cells, digested food molecules diffusing into the blood.
Diffusion does not require energy from the cell — don't confuse it with active transport.
Section 2
What is osmosis?
Osmosis is the net movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution), through a partially permeable membrane.
Osmosis is a special case of diffusion that applies only to water moving across a membrane that lets water through but not larger solute molecules.
Placing a potato chip in concentrated salt solution causes it to lose mass, because water moves out of the potato cells by osmosis.
Section 3
What is active transport?
Active transport is the movement of particles from a region of lower concentration to a region of higher concentration (against the concentration gradient), using energy from respiration (ATP).
Unlike diffusion and osmosis, active transport can move substances 'uphill' against the gradient, which is essential when a cell needs to absorb something already in short supply outside, e.g. mineral ions by root hair cells or glucose reabsorption in the kidney.
Always state that active transport moves substances 'against the concentration gradient' and 'requires energy from respiration' — both phrases are commonly required marking points.
Section 4
What factors affect the rate of movement of substances?
Several factors affect how quickly substances move by diffusion, osmosis or active transport:
| Factor | Effect on rate |
|---|---|
| Surface area to volume ratio | Larger ratio → faster exchange (shorter average diffusion distance relative to volume) |
| Distance | Shorter distance → faster diffusion |
| Temperature | Higher temperature → particles move faster → faster diffusion |
| Concentration gradient | Steeper gradient → faster net movement |
Small or flattened organisms/structures have a large surface area to volume ratio, which allows efficient exchange without needing a transport system.
Section 5
Investigating diffusion and osmosis
Diffusion can be investigated using agar blocks containing an indicator (e.g. phenolphthalein) placed in acid — the time taken for the colour to disappear shows how far the acid has diffused, and this can be compared between blocks of different sizes to explore surface area to volume ratio.
Osmosis can be investigated using potato or plant tissue placed in a range of sugar or salt solutions of different concentrations, measuring the change in mass or length over time. Tissue in a solution more concentrated than the cell contents loses mass (water moves out); tissue in a more dilute solution gains mass (water moves in).
When describing an osmosis investigation, always mention using a control (distilled water) and repeating measurements to improve reliability.
Must Know
- Diffusion: net movement of particles from high to low concentration, no energy needed
- Osmosis: net movement of water from a dilute to a concentrated solution through a partially permeable membrane
- Active transport: movement against the concentration gradient, using energy from respiration
- A larger surface area to volume ratio, shorter distance, higher temperature and a steeper concentration gradient all increase the rate of movement
- Potato/agar block experiments are used to investigate osmosis and diffusion respectively
That's the notes covered.
Carry on to the next subtopic.