Biological Membranes

Question 1

What are the leaflets that make up the plasma membrane?

Answer 1

Outer extracellular leaflet

Inner cytoplasmic leaflet

Question 2

What can diffuse laterally through the membrane?

Answer 2

Phospholipids and proteins

Question 3

Why is the plasma membrane like a mosaic?

Answer 3

Different lipids and proteins in different membranes

Question 4

Can lipids move from the extracellular leaflet to the inner cytoplasmic leaflet and vice versa?

Answer 4

no to both

Question 5

Why are phospholipids amphipathic (two moieties)?

Answer 5

They have two parts, non-polar hydrophobic part and a polar hydrophilic part

Question 6

What are lipids?

Answer 6

Insoluable fatty acid derivatives

Question 7

What are phospholipids?

Answer 7

Lipids with a phosphate group

Question 8

How many fatty acids are attached to each phosphate group?

Answer 8

Two

Question 9

What are the four major phospholipids in the mammalian plasma membrane?

Answer 9

Phosphatidyl-ethanolamine (amino alcohol)
Phosphatidyl-serine (amino acid)
Phosphatidyl-choline
Sphingomyelin

Question 10

Do the major phospholipids ionise at physiological PH?

Answer 10

yes

Question 11

Which of the four phospholipids has an overall negative charge?

Answer 11

phosphatidyl-serine

Question 12

What causes a kink in a fatty acid chain?

Answer 12

A double bond

Question 13

What is the structure of phosphatidylcholine?

Answer 13

Choline bonded to a phosphate bonded to a glycerol bonded to fatty acid chains (one with a cis double bond)

Question 14

How can you demonstrate that membranes are fluid?

Answer 14

1. Fluorescently tag a proteins on the membrane
2. Bleach an area of the proteins
3. some of the bleached proteins will diffuse out of the area and non-bleached proteins will move in

Question 15

What is more energetically favourable, a bilayer in a planar shape or a spherical shape?

Answer 15

Spherical because all the hydrophobic fatty acid chains are protected by the hydrophilic heads so they are not exposed to water

Question 16

Why is fluidity important?

Answer 16

• Allows lipids and proteins to diffuse in the lateral plane and interact with one another
• Allows membranes to fuse with other membranes
• Ensures membranes are shared equally between daughter cells following cell division
• Allows cells to change shape (cell motility) so they can move

Question 17

How do bacteria and yeast keep their membranes fluid in colder environments?

Answer 17

They synthesise:
-shorter fatty acid chains
-fatty acid chains with a degree of unsaturation (more double bonds)
This decreases the interactions between FA chains and means that membranes remain fluid

Question 18

What is the role of cholesterol in the cell membrane?

Answer 18

Cholesterol inserts between membrane phospholipids. This tightens packing in the bilayer and decreases membrane permeability to small molecules (membrane more tightly packed)

Question 19

Where are the lipid bilayers assembled?

Answer 19

In the ER

Question 20

Describe the synthesis of phosphatidylcholine

Answer 20

1. Fatty acids synthesised in cytosol are transported to ER by FABP (fatty acid binding protein) (highly hydrophobic so need to be transported in an aqueous environment)
2. FAs embed in membrane and in a succession of steps glycerol, phosphate and choline added (this happens in the outer cytosolic)