By the end of this lesson you should be able to
- Define diffusion, osmosis and active transport and give examples
- Explain what happens to plant and animal cells in different solutions
- Calculate percentage change in mass from an osmosis experiment
- Compare the three ways substances move across membranes
Why substances must move
Cells need to take in oxygen, glucose and water, and to remove waste such as carbon dioxide. These substances cross the cell membrane in three main ways: diffusion, osmosis and active transport. The cell membrane is partially permeable, meaning it lets some substances through and blocks others.
Key ideas
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement.
Osmosis is the net movement of water molecules from a dilute solution (high water potential) to a concentrated solution (low water potential) through a partially permeable membrane.
Active transport is the movement of particles against a concentration gradient, from a region of lower concentration to a region of higher concentration, using energy from respiration.
Diffusion needs no energy, so it is a passive process. Examples: oxygen moving from the air in the alveoli into the blood, and carbon dioxide moving the other way.
The rate of diffusion is greater when:
- the concentration gradient is steeper,
- the temperature is higher,
- the surface area is larger,
- the distance the particles travel is shorter (a thinner exchange surface).
Osmosis is a special case of diffusion that involves only water. It also needs no energy.
Active transport needs energy, so cells that do a lot of it have many mitochondria. Example: root hair cells absorb mineral ions such as nitrate from the soil, even though the concentration of these ions is higher inside the cell than in the soil.
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| What moves | Any particles, such as gases and dissolved substances | Water only | Ions, glucose and other substances |
| Direction | High to low concentration | Dilute to concentrated solution | Low to high concentration |
| Energy from respiration needed | No | No | Yes |
| Membrane needed | No | Yes, partially permeable | Yes, with carrier proteins |
What osmosis does to cells
| Cell placed in | Animal cell | Plant cell |
|---|---|---|
| Pure water or a dilute solution | Takes in water and may burst | Takes in water and becomes firm, called turgid. The cell wall stops it bursting |
| Concentrated solution | Loses water and shrivels | Loses water and becomes soft, called flaccid. Eventually the membrane pulls away from the wall, called plasmolysis |
Turgid plant cells push against each other and help support the plant. When plants lack water, the cells become flaccid and the plant wilts.
The potato experiment
Cylinders of potato of equal size are placed in sugar solutions of different concentrations. Each cylinder is weighed before and after.
Example. Five potato cylinders, each 5.0 g at the start, were left in sugar solutions for one hour.
| Sugar concentration (mol per litre) | Final mass (g) | Change in mass (g) | Percentage change |
|---|---|---|---|
| 0.0 | 5.6 | +0.6 | +12% |
| 0.2 | 5.3 | +0.3 | +6% |
| 0.4 | 5.0 | 0.0 | 0% |
| 0.6 | 4.7 | −0.3 | −6% |
| 0.8 | 4.4 | −0.6 | −12% |
Check one row: for 0.8, .
Interpretation. Below 0.4 the potato gains mass, because water enters by osmosis and the solution is more dilute than the cell sap. Above 0.4 it loses mass, because the solution is more concentrated than the cell sap. At 0.4 there is no change, so the solution and the cell sap have the same concentration.
Common mistakes
Saying water moves “to where there is more water”. Water moves from the dilute solution (more water) to the concentrated solution (less water). Say this in terms of concentration or water potential.
Saying plant cells burst in pure water. They do not, because the cell wall resists. They become turgid. Animal cells have no wall, so they can burst.
Forgetting the sign in percentage change. A loss in mass is a negative percentage. Always divide by the initial mass.
Saying active transport goes “from high to low”. It is the opposite: against the gradient, from low to high, and it needs energy.
Practice questions
- Define diffusion.
- State two factors that increase the rate of diffusion.
- Explain why a plant cell placed in pure water does not burst.
- A cell is placed in a very concentrated sugar solution. Describe what happens to (a) an animal cell and (b) a plant cell.
- A potato cylinder of mass 4.0 g increases to 4.6 g. Calculate the percentage change in mass.
- Another cylinder decreases from 4.0 g to 3.2 g. Calculate the percentage change and say whether the solution was more or less concentrated than the cell sap.
- Explain why root hair cells use active transport to absorb nitrate ions.
- State two differences between diffusion and active transport.
Show answers
- Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.
- Any two of: a steeper concentration gradient, a higher temperature, a larger surface area, a shorter distance.
- It has a strong cell wall that stops it expanding too far, so it becomes turgid instead.
- (a) The animal cell loses water by osmosis and shrivels. (b) The plant cell loses water, becomes flaccid, and the membrane may pull away from the cell wall (plasmolysis).
- . The potato lost water, so the solution was more concentrated than the cell sap.
- The concentration of nitrate is higher inside the root hair cell than in the soil, so the ions must be moved against the concentration gradient, which needs energy.
- Any two of: diffusion is passive and active transport needs energy from respiration, diffusion goes from high to low concentration and active transport goes from low to high, diffusion needs no carrier proteins and active transport does.
Summary
- Diffusion and osmosis are passive, and they move substances down a gradient.
- Osmosis is the movement of water from a dilute to a concentrated solution through a partially permeable membrane.
- Plant cells become turgid in water, and flaccid or plasmolysed in strong solutions. Animal cells burst or shrivel.
- Active transport moves substances against a gradient and needs energy from respiration.
Now practise this topic
Real questions show you what still needs work. Try a past paper under timed conditions, or take a short quiz.