1.1.2 - Cell Membranes (Crossing the Membrane) Part 10 & 11

Question 1

Explain what is meant by passive transport across the membrane and give an example of passive transport.

Answer 1

Passive transport - Molecules which possess kinetic energy that keeps them moving without the extra help of ATP are described as a passive process. Examples include:

• Simple diffusion
• Facilitated diffusion (using channel and carrier proteins)

Question 2

Explain the difference between simple diffusion and facilitated diffusion.

Answer 2

• Simple diffusion - When lipid-soluable / very small particles move from a high concentration to a lower concentration through the lipid bilayer.
• Facilitated diffusion - When larger or charged particles need to be carried accross the bilayer with the help of channel proteins (pores in the membrane made for one type of ion) or carrier proteins (shaped for specific molecules).

Question 3

1. Describe the term ‘active transport’.
2. How does this differ from facilitated diffusion that uses carrier proteins?

Answer 3

1. Active transport - This happens when going against the concentration gradient. When a specific molecule can fit into a carrier protein, the protein uses energy in the form of ATP to change shape so that the molecule is allowed through to the other side of the membrane.
2. This differs from facilitated diffusion because, even though they both use carrier proteins, facilitated diffusion does not require the energy to move large molcules against the concentration gradient, whilst active transport does.

Question 4

1. What is endocytosis?
2. How does this differ from exocytosis?
3. Why do both of these processes need ATP?

Answer 4

1. Endocytosis - Bulk transport of materials via vesicles that can fuse with the cell surface membrane.
2. This differs from exocytosis because endocytosis involves bringing the material into the cell, whilst exocytosis is the transportation of bulk material moving out of the cell.
3. They both need ATP because energy is used to move the membranes around to form vesicles and to be able to move these vesicles around the cell.

Question 5

Explain the term osmosis in terms of water potential.

Answer 5

Osmosis - The movement of water from a high water potential to a low water potential.

Question 6

A plant cell and an animal cell is placed in pure water.

Describe what might happen to the two cells and explain why this might happen.

Answer 6

• Plant cells in pure water will swell up with water due to osmosis where water is moving from a high water potential (the pure water) to a low water potential (inside the cell). The plant cell may not burst because the cell wall prevents it from bursting. Instead, the membrane pushes against the cell wall making the cell turgid.
• Animal cells will also swell up in pure water as water moves into the cell by osmosis. Because there is no cell wall, the animal cell may burst.

Question 7

A plant cell and an animal cell is placed in a concentrated sugar solution.

Describe what might happen to the two cells and explain why this might happen.

Answer 7

• Water will move out from both cells by osmosis, as water is moving from a high water potential (inside the cell) to a low water potential (the solution).
• The membrane of the plant cell will pull away from the cell wall as water leaves - the cell is plasmolysed.
• The animal cell will shrink and may become crenated when too much water leaves the cell.