Structure of the Cell Surface Membrane

Question 1

What is the Cell-Surface membrane?

Answer 1

The membrane surrounding cells and forms the boundary between the cytoplasm and the environment.

Question 2

What is the main role of the membrane?

Answer 2

It controls the movement of substances in and out of the cell.

Question 3

What is the role of the phospholipids of the cell surface membrane?

Answer 3

They form a bilayer.

Question 4

Why are phospholipids important key components of the membrane?

Answer 4

They hydrophilic heads point to the outside of the membrane, attracted by water.
The hydrophobic tails point to the centre of the membrane, repelled by water.

Question 5

What materials move through the membrane via the phospholipid portion of the membrane?

Answer 5

Lipid-soluble material.

Question 6

What is the function of the phospholipids in the membrane?

Answer 6

• Allow lipid-soluble substances in and out of the cell.
• Prevent water soluble substances in and out of cells.
• Make the membrane flexible and self-sealing.

Question 7

How are the proteins embedded in the phospholipid bilayer?

Answer 7

• Some occur in the surface of the bilayer to give mechanical support or act as cell receptors for molecules such as hormones.
• Others span the bilayer, in the form of protein channels, or carrier proteins.

Question 8

What is the role of protein channels?

Answer 8

They form water-filled tubes to allow water soluble ions to diffuse across the membrane.

Question 9

What is the role of the carrier proteins?

Answer 9

They bind to ions or molecules, such as glucose and amino acids, then change shape in order to move these molecules across the membrane.

Question 10

What are the functions of the proteins in the membrane?

Answer 10

• Provide structural support.
• Act as channels transporting water-soluble substances
• Allow active transport through carrier proteins
• Form cell surface receptors for identifying cells
• Helps cells adhere together
• Act as receptors, for example, for hormones.

Question 11

What does cholesterol do in the cell membrane?

Answer 11

Add strength and play a big part in preventing loss of water and dissolved ions as they are very hydrophobic. They also limit the movement of the bilayer molecules by pulling together the fatty acid tails.

Question 12

What are the functions of cholesterol in the membrane?

Answer 12

• Reduce lateral movement of other molecules, including phospholipids.
• Make the membrane less fluid at high temperatures.
• Prevent leakage of water and dissolved ions from cell.

Question 13

What do glycolipids do in the cell surface membrane?

Answer 13

They are made up of a carbohydrate covalently bonded with a lipid. Carbohydrate extends into the watery environment outside the cell and acts as a cell-surface receptor.

Question 14

What are the functions of glycolipids?

Answer 14

• Act as recognition sites
• Help maintain stability of the membrane.
• Helps attach to one another, forming tissues.

Question 15

What are the function of glycoproteins in the membrane?

Answer 15

• Act as recognition sites
• Help cells to attach to each other and so form tissues
• Allows cells to recognise each other, e.g. lymphocytes can recognise an organism’s own cell.

Question 16

Why do most molecules not freely diffuse across the membrane?

Answer 16

• They may not be lipid soluble and therefore not pass through phospholipid bilayer.
• Too large to pass through the channels in the membrane
• Of the same charge as on the protein channel, and so repelled.
• Electrically charged (polar) therefore have difficulty passing through non-polar hydrophobic tails in bilayer.

Question 17

Why is the cell-surface membrane known as the fluid-mosaic model?

Answer 17

Fluid - individual phosphate molecules can move relative to each other, giving it flexibility.
Mosaic - Embedded proteins vary in shape, size and pattern, similar to mosaic tiles.