Cell Membrane Structure

Question 1

What are the two forms of a cell membrane? (Phospholipids in water)

Answer 1

Micelle or bilayer

Question 2

What is the role of membranes?

Answer 2

• cover the surface of every cell
• surround most organelles and isolate them from the rest of the cytoplasm allowing cellular processes to occur seperately
• allow selected molecules to move in/out of cell
• site for biochemical reactions

Question 3

What is the fluid mosaic model?

Answer 3

It suggests proteins are found within not outside of the phospholipid bilayer. Phospholipids can move within the membrane. The membrane is two layered and the proteins are not uniformly arranged.

Question 4

What is the role of channel proteins in the membrane?

Answer 4

They are a hydrophilic channel which form water filled tubes to allow water soluble ions and polar molecules to diffuse across the membrane- passive movement

Question 5

What is the role of carrier proteins in the plasma membrane?

Answer 5

They can be used for passive or active transport. They bind to ions or molecules such as glucose and amino acids then change shape to move these across the membrane. They are transmembrane

Question 6

What does transmembrane mean?

Answer 6

It spans the membrane

Question 7

What are the role of glycoproteins in the cell membrane?

Answer 7

These are proteins with a carbohydrate chain. They act as recognition sites and help cells to attach to each other to form tissues.
E.g. lymphocytes can recognise an organisms own cells

Question 8

What is the role of glycolipids in the cell membrane?

Answer 8

These are lipids with a carbohydrate chain. They act as recognition sites and also help maintain the stability of the membrane. They also help cells attach to each other to form tissues

Question 9

What is the role of cholesterol in the cell membrane?

Answer 9

Cholesterol has hydrophobic and hydrophilic tails. It reduces the lateral movement of other molecules including phospholipids and makes the membrane less fluid at high temperatures. It prevents the leakage of water and dissolved ions from the cell and also interacts with the fatty acid tails

Question 10

What is the role of extrinsic proteins? (Surface proteins)

Answer 10

They are only on one side of the membrane and act as enzymes and receptors. They can attach to the cytoskeleton.

Question 11

What is the thickness of all cell membranes?

Answer 11

7.5nm

Question 12

Phospholipid bilayer model answer

Answer 12

The phospholipid bilayer is made of phospholipids with hydrophilic/polar phosphate heads and non polar/hydrophobic fatty acid tails.
The bilayer limits the movement of substances through the membrane so that only small non polar molecules can easily diffuse through.
Transmembrane proteins are embedded in the membrane and they can act as carrier or channel proteins.
Channel proteins allow facilitated diffusion of ions.
While carrier proteins can perform facilitated diffusion they can also transport molecules across the membrane using energy from ATP in active transport.
Glycoproteins are embedded in the membrane and act as antigens for cell recognition, they also increase stability and adhesion of the cells.
Glycolipids are part of the membrane and increase stability and allow for adhesion of cells.
Cholesterol is also found in the bilayer and acts to increase rigidity of the membrane.

Question 13

Can water soluble molecules (wsm) pass through the membrane?

Answer 13

Phospholipids and water dont mix so the membrane prevents many WSM getting through

Question 14

Can water pass through the membrane?

Answer 14

It is a polar molecule so can move through

Question 15

Can charged molecules pass through the membrane?

Answer 15

No

Question 16

Can larger uncharged molecules pass through the membrane?

Answer 16

Larger uncharged molecules like glucose and polar chains of fatty acids cannot easily pass through the membrane and are transported by proteins

Question 17

Can lipid soluble molecules pass through the membrane?

Answer 17

Yes (e.g. steroid hormones)

Question 18

How can polar and non polar molecules pass through the membrane?

Answer 18

Polar molecules require proteins to allow them to pass through whereas non polar molecules can diffuse directly through