2.2 Cell membranes

Question 1

What is the structure of cell membranes?

Answer 1

• phospholipid bilayer
• imbedded/attached proteins and carbohydrates

Question 2

What is a phospholipid?

Answer 2

• 1 hydrophilic head containing phosphate group
• 2 hydrophobic fatty acid tails

Question 3

Why do they arrange in a bilayer?

Answer 3

• head is hydrophilic
• tail is hydrophobic
• arrange in 2 layers where the water loving heads face out (towards the water) and the water hating tails are on the inside
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Question 4

Why is the centre of the bilayer hydrophobic?

Answer 4

to prevent water soluble substances like ions from passing through the membrane

Question 5

What is the fluid mosaic model?

Answer 5

describes arrangement of molecules
- fluid because the phospholipids are constantly moving
- mosaic because protein molecules are scattered through bilayer (like tiles in mosaic)

Question 6

Can proteins move around in the bilayer?

Answer 6

yes - bilayer is fluid

Question 7

What else is a cell membrane made of other than phospholipids?

Answer 7

• glycoproteins
• glycolipids
• proteins (e.g. channel and carrier proteins)
• cholesterol

Question 8

How do proteins fit into the membrane?

Answer 8

• peripheral proteins are loosely attached on the outside surface of the membrane
• integral proteins are fully embedded and can span one or both layers

Question 9

What are glycoproteins?

Answer 9

proteins with polysaccharides attached

Question 10

What are glycolipids?

Answer 10

lipids with polysaccharides attached

Question 11

What is cholesterol and how does it affect membrane fluidity?

Answer 11

• a type of lipid
• fits between phospholipids
• maintains fluidity of membrane by affecting movement of phospholipids (at high temps makes them more rigid, at low temps makes them more fluid)

Question 12

What else affects the fluidity of the membrane?

Answer 12

• the greater the ratio phospholipids with unsaturated fatty acids : those with saturated fatty acids, the more fluid the membrane will be
• kinks in unsaturated fatty acid tails prevent close packing of phospholipids, meaning more movement is possible

Question 13

How is the membrane partially permeable?

Answer 13

small molecules can move through gaps between phospholipids but large molecules and ions can only pass through channel proteins or carrier proteins

Question 14

What was the old cell membrane model and why was it wrong?

Answer 14

• the sandwich model - phospholipid bilayer between two layers of protein
• does not show hydrophilic heads of phospholipids in contact with water

Question 15

What evidence suggests the cell membrane is a “mosaic”?

Answer 15

• discovery of embedded integral proteins using freeze fracture
• the bilayer was fractured and scanned with an electron microscope
• bumps suggested proteins were embedded not just on the surface, like tiles in a mosaic

Question 16

What evidence suggests the cell membrane is “fluid”?

Answer 16

• mouse and human cells were fused
• proteins in the mouse and human membranes labelled with different labels
• found that the proteins intermixed due to fluidity of membrane