Diffusion, osmosis and active transport

Overview

Diffusion as net movement down a concentration gradient without energy, the factors that increase its rate, osmosis through a partially permeable membrane and water entering root hair cells, and active transport against a gradient using carrier proteins and energy from respiration to take up mineral ions.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Define diffusion.

   Model answer

   Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.

2. 2

   Explain why diffusion does not require the cell to use energy.

   Model answer

   Particles move by their own random motion down the concentration gradient, so no energy from respiration is needed.

3. 3

   Name two substances that move into or out of a respiring cell by diffusion.

   Model answer

   Oxygen diffuses into the cell and carbon dioxide diffuses out of it.

4. 4

   State three factors that increase the rate of diffusion into a cell.

   Model answer

   A steeper concentration gradient, a larger surface area and a shorter diffusion distance all increase the rate.

5. 5

   Define osmosis.

   Model answer

   Osmosis is the net movement of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane.

6. 6

   Explain why a partially permeable membrane is essential for osmosis.

   Model answer

   It lets water molecules through but holds back the larger dissolved solute molecules, so only water can move to even out the concentrations.

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