Movement of substances

Question 1

Define ‘diffusion’.

Answer 1

• The net movement of a substance from an area of high to low concentration.
• Diffusion does not require energy (is passive).

Question 2

Explain what the term ‘concentration gradient’ means.

Answer 2

• The difference in concentration between two areas. Greater gradient = greater difference.

Question 3

Explain the difference between simple diffusion and facilitated diffusion across cell membranes.

Answer 3

• Simple diffusion: the movement of molecules directly across the plasma membrane (e.g. oxygen, carbon dioxide).
• Facilitated diffusion: the movement of molecules across the membrane through protein channels/carrier proteins.

Question 4

What is osmosis?

Answer 4

• The net movement of water from an area of low solute concentration (high water concentration) to an area of high solute concentration (low water concentration).

Question 5

Explain what would likely occur if cells were placed in:
a. A highly concentrated salt solution.
b. A solution of pure water. Animal cells may burst as they gain water

Answer 5

A* Water would move out of the cells by osmosis from a region of low salt concentration (inside the cells) to a region of high salt concentration (outside the cells).
* Hence, cells would decrease in size and mass.
B* Water would move into the cells by osmosis from a region of low salt concentration (outside the cells) to a region of high salt concentration (inside the cells).
* Hence, cells would increase in size and mass, and may burst (in the absence of a cell wall).

Question 6

Explain the key differences between passive transport and active transport.

Answer 6

• Passive transport: does not require energy, molecules move with/down the concentration gradient.
• Active transport: requires energy (supplied by ATP), molecules move against the concentration gradient.

Question 7

Describe the process of endocytosis.

Answer 7

• Particles approach extracellular membrane.
• Membrane invaginates / folds inwards around the substance
• And pinches off forming an intracellular vesicle (diagram may be helpful)

Question 8

Describe the process of exocytosis. What types of molecules would use exocytosis to leave the cell?

Answer 8

• A vesicle containing substance (e.g. proteins) is packaged at the Golgi body and sent towards the cell membrane.
• The vesicle fuses with the cell membrane and expels contents into extracellular environment.