Membrane Structure

Question 1

What is the function of the membranes barrier?

Answer 1

regulates transportation i.e. nutrients in, waste out

prevents the contents of the cell from escaping and mixing with the outside environment

the lipid bilayer regulates transportation of substances in and out of the cell with the help of the proteins that are embedded in it

Question 2

What purpose does a plasma membrane acting as an interface serve?

Answer 2

by acting as an interface between a cell and its environment, it ensures that the cells can communicate and signals can be transferred from one cell to another

Question 3

How does the cytoskeleton provide mechanical support for the membrane?

Answer 3

the membrane is an attachment point for the intracellular skeleton

cytoskeleton is essential for the cell movements and the specific cell shape. with the membrane being the attachment point for the cytoskeleton, it allows the cell to move to specific directions and obtain a specific shape if required. membranes cant be rigid structures - must be able to bend and flex in 3D dimensions

Question 4

What types of lipids are there?

Answer 4

• phospholipids
• sphingolipids
• glycolipids
• cholesterol

Question 5

What 2 structures can phospholipids form when dispersed in water?

Answer 5

1) micelles - in micelles, hydrophobic tails are placed inwards and hydrophilic heads outwards.
2) bilayers - hydrophobic tails are sandwhiched between the hydrophilic heads.

Question 6

How are micelles formed?

Answer 6

formed as a result of the interactions of CONE-shaped amphiphilic molecules eg phospholipids with a single hydrocarbon tail

Question 7

How are lipid bilayers formed?

Answer 7

formed as a result of interactions of CYLINDER-shaped amphiphilic molecules eg phospholipids with 2 hydrocarbon tails

Question 8

what factors does the structure of the phospholipids in the absence of proteins depend on?

Answer 8

• the lenghth of the fatty acid chains in the hydrophobic tails
• the degree of saturation so how many double bonds there are
• the temperature

Question 9

describe the structure of a phosphoglyceride

Answer 9

a glycerol molecule where a phosphate group is attached and 2 fatty acid tails.

the head group with phosphate and glycerol is hydrophilic

the tails are hydrophobic

Question 10

what is the role of phosphatidylcholine (PC)

Answer 10

• the most abundant phospholipid in eukaryotic membranes and the source of diacylglycerol
• plays a major role in signalling

Question 11

what is the role of phosphatidylserine (PS)

Answer 11

• major anionic phospholipid class particularly enriched in the inner leaflet of the plasma membrane in neural tissues.
• plays a key role in several signalling pathways

Question 12

how and why is phosphatidylserine moved to the outer leaflet from the cytoplasmic leaflet of the plasma membrane?

Answer 12

• moves from the cytoplasmic leaflet of the plasma membrane to the outer leaflet with the help of the scramblase enzyme
• this gives a signal that the cell is dying through apoptosis.
• PS will then be recognised by phagocytes and result in the cell being phagocytosed.

Question 13

what is the role of phosphatidylethanolamine?

Answer 13

plays a role in the interaction of positively charged membrane proteins

Question 14

what are sphingolipids

Answer 14

• second major class of lipids found on membranes
• mainly found in brain and neurons
• only present in the non-cytosolic side of the bilayer

Question 15

what are sterols and what is their role?

Answer 15

• class of lipids
• 3 fused 6-carbon atom rings
• 1 5-carbon atom ring
• amphipathic
• role is to provide structural support to membranes and prohibit too close packing of the hydrocarbon tails of phospholipids
• allows the membranes to maintain the right level of fluidity and rigid structure.

Question 16

what are the properties of the phospholipid bilayer

Answer 16

• forms sealed close compartments no free edges
• impermeable to water soluble solutes
• stable
• lipids are laterally mobile

Question 17

what is the supporting evidence for the fluid mosaic membrane model

Answer 17

• studies that predicted transmembrane proteins
• studies that showed that lipids alone cannot make up complete biological membranes
• electron microscopy including freeze-etching, studies that suggested the presence of proteins within the membranes
• studies that predicted a favourable conformations for the membrane proteins

Question 18

what is the freeze fracture technique?

Answer 18

• a specimen is frozen rapidly and cracked on a plane through the tissue
• fracturing causes one half of the membrane to separate from the other
• proteins sticking out of the fractured membrane must have been embedded in the lipid layer

Question 19

what are the features of the fluid mosaic model?

Answer 19

• sheet like bilayer of lipids containing proteins
• held together by non-covalent bonds
• asymmetric in composition
• fluid in structure: proteins and lipids can diffuse laterally

Question 20

what is the link between cholesterol and membrane fluidity?

Answer 20

• fits in the gaps between phospholipid molecules and regulates membrane fluidity
• restricts the random movement of other phospholipid head groups at the outer surfaces of the leaflets. the interaction of cholesterol with the fatty acid tails of phospholipids immobilises those lipids and thus decreases the fluidity of the membrane

Question 21

in terms of membrane fluidity, what happens if the concentration of the cholesterol is low?

Answer 21

• the steroid ring will separate and disperse the phospholipid tails and thus increase fluidity

Question 22

what is the role of cholesterol in membrane fluidity?

Answer 22

• at low temperatures, it intercalates between the phospholipids and prevents them from clustering together and stiffening
• at high temperatures, it stabilises the membrane and raises its melting point