CBS: Lipids + Membranes

Question 1

Give an example of how the composition of different membranes can vary

Answer 1

• The plasma membrane is the only membrane with carbohydrates (8%)
• The inner mitochondrial and nuclear membranes have lots of protein

Question 2

Describe lipids

Answer 2

• Insoluble in water, soluble in fat and organic solvents
• Functions: energy stores (triacylglycerol), precursors for vitamins and hormones (steroids), bile acids (cholic acid) and membrane structure (phospholipids)

Question 3

Describe phospholipids

Answer 3

• Essential and major components of all membranes
• Composed of a polar head group that is attached to a backbone (glycerol) through a phosphate group
• Two fatty acyl side chains are linked to the glycerol backbone via ester bonds
• They are amphipathic (both hydrophobic and hydrophilic)
• The two fatty acids in a glycerophospholipid can be fully saturated or mono/polyunsaturated (length ≈ 16/18)

Question 4

Give some examples of some phospholipid head groups/phospholipids

Answer 4

• Head groups: choline, serine, ethanolamine, inositol (sugar) - all have an OH group
• Phospholipids: phosphatidylcholine (head group is choline), sphingomyelin (has ceramide instead of glycerol backbone)

Question 5

What is an example of another amphipathic lipid?

Answer 5

Cholesterol - it has a hydrophilic (OH) group + hydrophobic region

Question 6

Describe the lipid bilayer

Answer 6

• They are asymmetrical - the two halves of the bilayer have a different lipid composition
• Cytosolic side is the side facing into the cell

Question 7

What is fluidity and why is it important?

Answer 7

• The ease with which lipid molecules move about in plane of the bilayer - movement normally on one side of membrane
• This is important in the regulation of membrane function e.g. movement of proteins, signalling, exocytosis
• The lipid composition of a membrane defines its fluidity

Question 8

What two features of fatty acids determine fluidity of a membrane?

Answer 8

1) chain length - short chain fatty acids reduce the tendency of hydrocarbon chains to interact and ∴ increase fluidity
2) degree of saturation - the kinks in unsaturated fatty acids result in less stable Van der Waals interactions with other lipids and ∴ increases fluidity

Question 9

Describe the effect of cholesterol on the membrane

Answer 9

• The % of cholesterol in a membrane changes the way it operates - high cholesterol content restricts the random movement of polar heads and orders the lipid bilayer ∴ decreases fluidity
• ∴ cholesterol is rich in highly ordered regions of the membrane (lipid rafts)

Question 10

What is the possible effect of impaired fluidity?

Answer 10

• Impaired fluidity can damage cells e.g. in cirrhosis (severe liver disease)
• In cirrhosis, increased levels of cholesterol by 20-60% in RBC membranes decreases the fluidity of the membrane
• This alters the cell shape, impairing O2 transport, leading to the destruction of RBCs and anaemia

Question 11

What are the 5 classes of membrane proteins and their functions?

Answer 11

1) structural - cell to cell contact, cytoskeletal organisations e.g. actin
2) receptors - signal recognition and transduction e.g. insulin receptor
3) ion channels - maintenance of ionic gradients, transmission e.g. nicotine ACh receptor
4) transporters - import and export of substances e.g. glucose transporter
5) enzymes - catalysis e.g. adenylyl cyclase

Question 12

What are the three ways to describe the structure of membrane proteins?

Answer 12

1) integral (intrinsic)
2) anchored
3) peripheral (extrinsic)

Question 13

Describe integral proteins

Answer 13

• Embedded in the lipid bilayer, most spanning the entire bilayer
• They have polar side chains facing into the channel to allow polar/ionic groups to pass
• They have hydrophobic side chains on the outside which interact with fatty acyl groups of the phospholipids ∴ anchoring the protein to the membrane
• The transmembrane spanning domains are either alpha helices or beta sheets

Question 14

Describe anchored proteins

Answer 14

They are not deeply embedded in the bilayer but are anchored to the membrane by covalently binding to fatty acid chains e.g. palmitic acid or on the cell surface to a glycolipid

Question 15

Describe two examples of anchored proteins

Answer 15

1) alkaline phosphate enzyme (glycolipid anchored protein)

2) RAS - signalling G-protein which relays info from the cell membrane (fatty acyl anchored protein)

Question 16

Describe peripheral proteins

Answer 16

• They attach to the membrane surface by ionic interactions with other membrane proteins (integral proteins) or with the polar head group of phospholipids

Question 17

Describe two examples of peripheral membrane proteins

Answer 17

1) spectrin - an important structural protein on the cytoplasmic surface of RBCs that interacts with other proteins e.g. ankyrin
2) phospholipase A2 (PLA2) - binds to the bilayer to cleave fatty acids from phospholipids
- phospholipids bind to and act on different sites in the membrane

Question 18

How can proteins be removed from the bilayer?

Answer 18

By detergents, high salt solution or treatment with phospholipases

Question 19

Which membrane proteins are removed from the bilayer by each technique?

Answer 19

1) detergent removes all types by surrounding the protein and separately the lipids with amphipathic detergent monomers, making them water soluble
2) high salt and urea removes only the peripheral proteins
3) phospholipases removed anchored proteins and some peripheral proteins