4.1 Structure of the cell-surface membrane

Question 1

What is the general role of the plasma membrane?

Answer 1

• It forms a boundary between the inside and the outside of the cell
• allows different conditions to be established inside and outside the cell.
• controls the movement of substances into and out of the cell

Question 2

What molecules make up the plasma membrane?

Answer 2

• phospholipids
• proteins
• cholesterol
• glycolipids
• glycoproteins

Question 3

Describe the structure of the phospholipid bilayer.

Answer 3

• the hydrophylic heads of both layers point to the outside of the bilayer and are attracted to water on both sides.
• the hydrophobic tails point into the centre of the bilayer repelled by water on both sides.

Question 4

What is the function of phospholipids in the membrane?

Answer 4

• allows lipid-soluble substances to enter and leave the cell.
• prevents water-soluble substances from entering and leaving the cell
• makes the membrane flexible and self-sealing

Question 5

How are proteins embedded in the phospholipid bilayer?

Answer 5

• some occur on the surface of the bilayer, and never extend completely across it. they offer mechanical support or (with glycolipids) act as cell receptors for molecules like hormones.
• others span the complete bilayer:
  PROTEIN CHANNELS: form water-filled tubes and allow water-soluble ions into the cell.
  CARRIER PROTEINS: bind to ions or molecules and change their shape to move them across the membrane.

Question 6

What are the functions of proteins in the membrane?

Answer 6

• provide structural support
• act as channels to transport water-soluble substances across the membrane
• allow active transport across the membrane through carrier proteins
• form cell-surface receptors for identifying cells
• help cells adhere together
• act as receptors for hormones

Question 7

What are the functions of cholesterol in the phospholipid bilayer of the cell-surface membrane?

Answer 7

• reduce lateral movement of other molecules, including phospholipids, by pulling their fatty acid tails together
• make the membrane less fluid at high temperatures
• prevent leakage of water and dissolved ions, as they are very hydrophobic

Question 8

What are the functions of glycolipids in the cell-surface membrane?

Answer 8

• act as recognition sites (as the carbohydrate extends outside of the cell)
• Help maintain membrane stability
• help cells attach to each other to form tissues

Question 9

What are the functions of glycoproteins on the cell-surface membrane?

Answer 9

• act as recogntion sites
• help cells attach together to form tissues
• allow cells to recognise each other

Question 10

Why can most molecules not diffuse across the cell-surface membrane?

Answer 10

• not soluble in lipids so can’t pass through the bilayer
• too large to pass through channels
• same charge as the protein channels so are repelled
• electrically charged (polar) so can’t pass through non-polar hydrophobic tails in the bilayer

Question 11

Why is the cell-surface membrane referred to as ‘fluid-mosaic’?

Answer 11

FLUID: Individual phospholipid molecules can move relative to each other, which gives the membrane a flexible structure
MOSAIC: the proteins embedded in the bilayer vary in shape, size and pattern.