Structure of the cell-surface membrane

Question 1

What are plasma membranes?

Answer 1

All membranes within and around the cells.

Question 2

What is the cell-surface membrane?

Answer 2

The membrane that surrounds the cell and forms a boundary between the cytoplasm and the environment.

Question 3

What are phospholipids?

Answer 3

3

Question 4

How are phospholipids important components of the cell membrane?

Answer 4

They form a bilayer, the hydrophobic tail and a hydrophilic head meaning that water is repelled or attracted. Lipid-soluble material moves through the membrane.

Question 5

What are the functions of a phospholipid in the membrane?

Answer 5

Allow lipid-soluble molecules to enter and leave the cell and prevent water-soluble substances from entering or leaving. It also makes the cell flexible and self-sealing.

Question 6

What are the two main ways in which proteins are embedded in the cell?

Answer 6

1) occur in the bilayer and never extend fully across, they give mechanical strength or act as receptors.
2) Completely span the bilayer, like protein channels or carrier proteins.

Question 7

What are protein channels?

Answer 7

Proteins that form channels (water-filled tubes) that allow water-soluble molecules to pass through.

Question 8

What are carrier proteins?

Answer 8

Carrier proteins are proteins that bind to ions/molecules and the change shape in order to move the molecules across the membrane.

Question 9

What are the functions of proteins in the membrane?

Answer 9

Structural support/ transports water-soluble molecules/ allows for active transport to take place/ receptors/ makes cells adhere.

Question 10

What do cholesterol molecules play an important part in?

Answer 10

They add strength to the membranes and as they are very hydrophobic they play an important role in preventing loss of water and dissolved ions from the cell.

Question 11

What are the functions of cholesterol molecules in the membrane?

Answer 11

Reduce lateral movement of other molecules including phospholipids/ make the membrane less fluid at high temperatures/ prevent leakage of water and dissolved ions from the cell.

Question 12

What are glycolipids?

Answer 12

Carbohydrate covalently bonded with a lipid.

Question 13

What does the carbohydrate portion of a glycolipid do?

Answer 13

It extends from the bilayer into the watery environment outside the cell where it acts as a cell-surface receptor for certain chemicals.

Question 14

What are the functions of glycolipids?

Answer 14

Act as recognition sites/ help maintain the stability of the membrane/ help cells attach to one another and so form tissues.

Question 15

What are the functions of glycoproteins in the membrane?

Answer 15

Act as recognition sites/ help sites attach to one another and so form tissues/ allow cells to recognise one another, for example lymphocytes can recognise an organism’s own cells.

Question 16

Why might a molecule not be allowed to pass through the membrane?

Answer 16

It may not be soluble in lipids and can therefore not diffuse through the phospholipid bilayer, it may be too large/ of the same charge as the charge on the carrier protein and be repelled/ polar and so have difficulty passing through the non-polar hydrophobic tails.

Question 17

Where does the fluid-mosaic model get its name from?

Answer 17

Fluid- because the individual phospholipid molecules can move relative to one another, this gives the membrane a flexible structure and it is constantly changing shape.

Question 18

What is added to extrinsic proteins on the outer surface of the membrane?

Answer 18

Carbohydrate chains.

Question 19

What are the functions of membranes within the cell?

Answer 19

control entry exit/ separates organelles from cytoplasm so reactions can take place/ provide internal transport system/ isolate enzymes that might damage the cell/ provides surface for reactions.