2B - Cell Membranes

Question 1

Describe Cell-Surface membranes.

Answer 1

They act as a barrier between the cell an its environment, controlling which substances enter and leave the cell. They’re partially permeable- they let some molecules through but not others.

Question 2

Name three processes that allow substances to move across cell-surface membranes.

Answer 2

Diffusion, osmosis and active transport.

Question 3

What do the membranes around organelles do? (2)

Answer 3

-They divide the cell into compartments- they act as a barrier between the organelle and the cytoplasm. -They also control what enters and leaves the organelle as they are partially permeable.

Question 4

Explain why a cell membrane can be described as having a ‘Fluid Mosaic’ structure.

Answer 4

• Continuous bilayer, constantly moving, Fluid
• Proteins scattered throughout the membrane, Mosaic

Question 5

Describe how phospholipids form a barrier to dissolved substances

Answer 5

• The head is hydrophillic (attracts water)
• The tail is hydrophobic (repels water)
• The molecules automatically arrange themselves into a bilayer- the heads face out towards the water on either side of the membrane.
• The centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances (like ions) through it-it acts as a barrier to these dissolved substances.

Question 6

Describe and explain the role of Cholesterol in a cell membrane.

Answer 6

• Cholesterol is a type of lipid
• It’s present in all cell membranes (except bacterial cell membranes)
• Cholesterol molecules fit between the phospolipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.
• Cholesterol helps to maintain the shape of animal cells (which don’t have cell walls). This is particularly for cells that aren’t supported by other cells, e.g. red blood cells, which float free in the blood.