LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- explain diffusion and its rate
- plan diffusion and osmosis investigations
- describe osmosis using water potential
- predict effects on plant cells
- explain active transport and carrier proteins
THE BIG IDEA
How concentration gradients, membranes and energy control particle movement.
This chapter builds each process from its structures, inputs and outputs. Follow the connections rather than memorising isolated facts, then use precise biological terms when explaining cause and effect.
SECTION 01
Diffusion
Particles move randomly. When more particles begin in one region, random movement produces a net movement down the concentration gradient until distribution is more even.
DETAILED EXPLANATION
- Diffusion supplies oxygen to cells and removes carbon dioxide; dissolved substances also cross membranes by diffusion.
- A steeper gradient, higher temperature, larger surface area and shorter distance increase the rate.
- Energy comes from particles' kinetic energy, not directly from respiration.
- A diffusion investigation can compare how far a coloured substance travels through agar in a fixed time. Change one factor, such as temperature, while keeping agar size, concentration and time constant.
SECTION 02
Osmosis and water potential
Osmosis is the net movement of water from higher water potential in a dilute solution to lower water potential in a concentrated solution through a partially permeable membrane.
DETAILED EXPLANATION
- Water acts as a solvent in digestion, transport and excretion.
- Water entering a plant cell makes it turgid; the wall resists expansion and creates turgor pressure.
- Water loss makes a cell flaccid. Severe loss pulls the membrane from the wall, called plasmolysis.
- Osmosis supports plants and controls water uptake and loss in organisms.
- To investigate osmosis, cut equal plant-tissue pieces, record their initial mass or length, place them in different concentrations for the same time, blot them dry and calculate the change. Repeats improve reliability.
SECTION 03
Active transport
Active transport moves molecules or ions against their concentration gradient through a membrane, using energy released by respiration.
DETAILED EXPLANATION
- Carrier proteins bind particular particles and move them across the membrane.
- Root hair cells can absorb mineral ions even when the soil contains a lower ion concentration than the cell.
- Cells performing much active transport need a strong supply of respiratory energy.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Diffusion moves particles down gradients.
- Osmosis moves water through a partially permeable membrane.
- Water gain makes plant cells turgid.
- Active transport uses respiratory energy and carrier proteins.