LEARNING OBJECTIVES
What you will be able to do
- 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
AT A GLANCE
INTRODUCTION · THE BIG IDEA
How concentration gradients, membranes and energy control particle movement.
Cells survive only if useful substances enter and wastes leave at suitable rates. Particle motion, concentration gradients and membrane proteins determine how that exchange occurs.
The central comparison is between passive movement down a gradient and active transport against one. Water movement requires the more precise idea of water potential.
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
- 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 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
- 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.
STEP-BY-STEP EXAM EXAMPLE
Interpreting an osmosis investigation
- A potato cylinder changes from 4.80 g to 5.28 g.
- Find the change: 5.28 − 4.80 = 0.48 g.
- Divide by the initial mass: 0.48 ÷ 4.80 = 0.10.
- Multiply by 100 and connect the gain to net water entry.
Answer: The mass increased by 10%; water entered the cells by osmosis.
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.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- 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