(d) Movement of substances into and out of cells
- Syllabus
- 2024
- Topic
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- Level
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Learning objectives
2.15Cell transport processesUnderstand the processes of diffusion, osmosis and active transport by which substances move into and out of cells.2.16Factors affecting cell transportUnderstand how factors affect the rate of movement of substances into and out of cells, including the effects of surface area to volume ratio, distance, temperature and concentration gradient.2.17Diffusion and osmosis practicalPractical: investigate diffusion and osmosis using living and non-living systems.
Compare diffusion, osmosis and active transport
Cells exchange substances with their surroundings by diffusion, osmosis and active transport. The three processes differ in what moves, its direction and whether cellular energy is required.
| Process | What moves | Direction | Membrane and energy |
|---|---|---|---|
| diffusion | particles of a substance | net movement from higher to lower concentration | no energy from respiration; a membrane is not essential |
| osmosis | water molecules only | from higher water potential (more dilute) to lower water potential (more concentrated) | through a partially permeable membrane; no energy from respiration |
| active transport | dissolved ions or other solutes | from lower to higher concentration, against the concentration gradient | across a living cell membrane using carrier proteins and energy from respiration |
Mineral ions can enter root hair cells by active transport when their concentration is lower in the soil than in the cell. Water can then move across partially permeable membranes by osmosis, while gases such as oxygen and carbon dioxide commonly move down concentration gradients by diffusion.
Osmosis is the movement of water, not solute. Active transport can move a substance against its gradient because it uses energy; diffusion and osmosis are passive. At equilibrium particles still move randomly, but there is no net movement in either direction.
Explain factors that change transport rate
The rate of movement into or out of cells depends on how much exchange surface is available and how quickly particles can cross it.
| Change | Effect on rate | Explanation |
|---|---|---|
| larger surface-area-to-volume ratio | faster | more membrane area is available for exchange for each unit of cell volume |
| shorter diffusion distance or thinner exchange surface | faster | particles have a shorter path to travel |
| higher temperature | faster, within a suitable range | particles have more kinetic energy and move more rapidly |
| steeper concentration gradient | faster net movement | the difference in concentration across the surface is greater |
Small cells have a larger surface-area-to-volume ratio than equally shaped large cells. Exchange surfaces can therefore be adapted by being folded to increase area and thin to reduce distance, while circulation or ventilation can maintain a steep concentration gradient.
These factors change rate, not the direction set by the gradient. Surface area alone is not the same as surface-area-to-volume ratio, and active transport additionally depends on carrier proteins and a supply of energy from respiration.
Investigate diffusion and osmosis
Diffusion and osmosis can be investigated in living tissue and in a non-living model. A valid investigation changes one factor, measures movement quantitatively and controls the other rate factors.
| System | Example method | Quantitative evidence |
|---|---|---|
| living | equal potato cylinders placed in a range of sucrose or salt concentrations | change in mass or percentage change in mass |
| non-living | partially permeable Visking or dialysis tubing containing solution, immersed in another solution | change in mass, volume or liquid-column height |
| Step | Action |
|---|---|
| 1 | cut equal potato cylinders, blot them in the same way and record each initial mass |
| 2 | place them in equal volumes of a suitable range of solution concentrations |
| 3 | keep time, temperature, cylinder dimensions and potato source constant |
| 4 | remove after the same time, blot consistently and record final mass |
| 5 | calculate percentage mass change; repeat each concentration and calculate a mean |
% ext{ change in mass}= rac{ ext{final mass}- ext{initial mass}}{ ext{initial mass}} imes100
A positive value means net water entry; a negative value means net water loss. The concentration giving approximately zero change has a similar water potential to the tissue. In the tubing model, a mass or height increase shows net water movement into the partially permeable bag without requiring living cells.
Blotting removes surface liquid that would otherwise distort mass, while percentage change allows samples with different starting masses to be compared. Use repeats to reveal variation, handle cutting tools safely, and use a water bath rather than direct heating when temperature is investigated.