Biological membranes

Question 1

What are the properties of biological membranes?

Answer 1

Act as a barrier
Continuous
Self-repairing (repair after damage)
Flexible
Selectively permeable (only allow passage of certain molecules in and out the cell)
Made of lipids, proteins and carbohydrates

Question 2

What is the composition of biological membranes?

Answer 2

Lipids (phospholipids and cholesterol/other sterol)
Proteins (channel proteins and carrier proteins)
Carbohydrates(bonds with proteins or lipids to give glycoproteins or glycolipids)

(composition is pretty much the same for all membranes)

Question 3

What are oligosaccharides and what do they do?

Answer 3

They are a carbohydrate and they covalently bond with proteins (to give glycoproteins) or lipids (to give glycolipids)

They are made up of sugar residues that is linked covalently in branched chains

Question 4

What is the role of membranes?

Answer 4

Provides a barrier between the environment and cells

Question 5

What is a plasma membrane?

Answer 5

Provides a cell boundary and is selective with what enters and leaves the cell.
Prevent material movement in and out the cell

Question 6

What is an organelle membrane?

Answer 6

Provides a similar boundary
Divide the cytoplasm into compartments
Provide a boundary within the boundary. Inside environment is different to outside environment

Question 7

What are 3 examples of biological membranes?

Answer 7

1. Schwaan cell which have specialised plasma membranes to insulate the axon
2. Plasma membrane of epithelial cells
3. Plasma membrane of rod and cone cells in the eye, these have a stack of membranes

Question 8

What is a glycosidic bond?

Answer 8

Linkage between 2 glucose molecules between the alpha-1 carbon of one glucose to the alpha-2 position of the other glucose molecule

This gives a alpha 1,4-glycosidic bond

In general the glycosidic bond is between one carbohydrate molecule to another molecule (which can be a carbohydrate or something else)

Question 9

What is glycosylation?

Answer 9

the reaction where a carbohydrate is attacked to a hydroxyl or another functional group of another molecule

Question 10

What are phospholipids?

Answer 10

1x10^9 lipid molecules that form a bilayer around a small cell
Held by NON-COVALENT bonds
They are AMPHIPATHIC by nature

they provide structure and permeability barrier of membranes
have an important role in cell signalling and membrane interaction

Question 11

What does amphipathic mean?

Answer 11

hydrophilic and hydrophobic components in the same molecule

Question 12

How are phospholipids amphipathic?

Answer 12

Have a polar headgroup and 2 hydrophobic tails

Question 13

What are the fatty acid tails in phospholipids?

Answer 13

They have a linear hydrocarbon chain of 12-22 carbon atoms long
They have an even number of C atoms
They can be saturated or unsaturated
(If unsaturated then the C=C double bond is always CIS)

Question 14

Shape of fatty acid tail

Answer 14

Saturated HC chain = linear fatty acid
Introducing just ONE cis double bond will intorduce a significant kink in the chain
Fatty acyl chains differ in length and degree of unsaturated therefore varied shaped of fatty acid tails

Question 15

Name the 4 major membrane phospholipids

Answer 15

1. Phosphatidylthanolamide (neutral charge)
2. Phosphatidylserine (-ve charge)
3. Phosphatidylcholine (neutral charge)
4. Sphingomylin (neutral charge)

Question 16

Membrane phospholipid structure

Answer 16

Have a polar head which is linked by a -ve phosphate to a glycerol backbone then a fatty acyl tail

The fatty acyl chains associate with each other in a bilayer structure and the polar headgroup interact with the aqueous environment on the outside

Question 17

Structure of cholesterol

Answer 17

polar head group
rigid steroid ring structure
non-polar hydrocarbon tail (short chain)

the structure is similar to that of phosphatidylserine and phosphatidylcholine

Question 18

Hydrophobicity

Answer 18

hydrophobic and hydrophilic molecules react differently in aqueous solution

Acetone = partially polar, hydrophilic, dissolves well due to partial polarity which allows it to interact with H2O molecules and form electrostatic interactions

2-methylpropane = hydrophobic, cannot interact with H2O, forces water into ice crystals which requires high energy so fully insoluble in water

Question 19

What structures does forcing lipids in aqueous solution give?

Answer 19

1. Micelles = spherical structure, hydrophobic tail on inside and hydrophilic tail on outside
2. Lipid bilayer = form 2 layers, each layer called a leaflet of the plasma membrane. Polar headgroup on outside and hydrophobic fatty acid chain facing in

Question 20

The lipid bilayer

Answer 20

Have tendency to form closed spheres when in aqueous solution

Lipid bilayer (and biological membranes) prefer to form sealed compartments as it is energetically unfavourable to have a planar phospholipid bilayer where the edges are exposed to water

Question 21

What happens when the lipid bilayer is damaged?

Answer 21

When damaged it forms sealed compartments (as lipids are self-repairing) so they self-repair to a more energetically favourable structure which is a spherical structure

This contributes to the repair and flexibility of biological membranes

Question 22

What are liposomes?

Answer 22

Spherical vesicle of at least 1 lipid bilayer

Aqueous hydrophilic on inside and hydrophobic on outside

Question 23

Uses of liposomes

Answer 23

Drug therapy
Genetic engineering
Delivery DNA or RNA into cells
Cosmetics