Membrane structure and function I

Question 1

What are some common features that membranes have, despite their diversity?

Answer 1

→they’re two molecules thick, and form the closed boundaries of the cell
→they consist of lipids and proteins, and contain specific proteins to mediate distinctive cell functions
→ they have a non-covalent arrangement and are asymmetric (aka, the fluid mosaic model)
→ they’re electrically polarised, which plays a key role in transport

Question 2

what are properties of membrane lipids?

Answer 2

→Biological membranes are composed of a lipid bilayer

→The majority of the lipids are phospholipids

→Phospholipids have a hydrophilic head group and a hydrophobic tail

→Non-covalent assemblies

Question 3

What are the three main lipids, and what do they consist of?

Answer 3

→PHOSPHOLIPIDS, with the phosphate head
→ CHOLESTEROL, with the -OH head
→GLYCOLIPIDS, with the large carbohydrate head and lipid group

Question 4

What two of the lipids are amphipathic, and what does that mean?

Answer 4

→PHOSPHOLIPIDS and GLYCOLIPIDS are amphipathic (meaning they possess both hydrophilic/water-loving/polar and lipophilic/fat-loving properties).

Question 5

what are some factors that alter the fluidity of the membrane?

Answer 5

→temperature -
→ fatty acid composition - 
→chain length - 
→degree, and extent, of saturation -
→ cholesterol content

Question 6

How does the extent of saturation affect membrane rigidity and fluidity?

Answer 6

→Saturated fatty acids can stack together well
→distance between the phospholipids is smaller
→ less fluid and more rigid.

→Unsaturated acids cannot pack together as closely due to the kinks created by the double bonds
→ distance between phospholipids increases
→more fluid and less rigid.

Question 7

how does chain length affect membrane fluidity?

Answer 7

The longer the chain length the more rigid the membrane becomes because there are more interactions between the chains (more bonds are formed)

Question 8

what is spur cell anaemia and how does it arise?

Answer 8

→cholesterol content is increased by 25-65% so the membrane becomes more rigid

→ cells that should be able to squeeze through capillaries are not able to because their membrane is not flexible enough.

Question 9

Describe how cholesterol and temperature interact to affect the membrane fluidity.

Answer 9

→When the temperature is low, the energy of molecules is low, so the motility of the membrane decreases

→ At higher temperatures,the molecules have more energy so the spaces between molecules increases and fluidity of the membrane increases.

→ At low temperatures cholesterol inserts itself between some phospholipids and it increases the distance between the phospholipids
→ increases membrane fluidity.

→At high temperatures, cholesterol works to stabilize the membrane and pulls the phospholipids closer together and reduces membrane fluidity by bringing the phospholipids closer together.

Question 10

Describe how membrane synthesis happens

Answer 10

→New membrane is synthesised in the ER.

→As it moves to the Golgi, it undergoes modifications.

→The enzymes responsible for making the membrane are only present on the inside of the ER.

→As new lipids are inserted, the bilayer starts to bow as one side gets more enlarged.

→Flippase then flips lipids from one side to the other, so that both sides get enlarged and the membrane is no longer bowed but straight.

Question 11

How does the membrane signal apoptosis?

Answer 11

→Cells that want to undergo apoptosis display ‘eat me’ signals for macrophages on their plasma membrane cell surface.

→ Phosphatidylserine is this marker
→it is on the cytosolic side of the membrane
→undergoes transverse diffusion when needed to be exposed on the surface.

Question 12

why is apoptosis important?

Answer 12

→It marks the cell for phagocytosis and the removal of these dying cells occurs in an orderly way without triggering inflammation which is important.

Question 13

what is the difference between lateral movement and transverse movement?

Answer 13

→Lateral movement of lipids in the membrane is rapid

→Transverse movement is slow and requires the action of three enzymes

Question 14

what do floppases do?

Answer 14

Floppase - moves phospholipids from the inner to the outer leaflet which requires ATP

Question 15

what do flippases do?

Answer 15

Flippase (flipase) – moves phospholipids from the outer to the inner leaflet which requires ATP

Question 16

what do scramblases do?

Answer 16

Scramblase – bidirectional movement

Question 17

what are key facts about membranes?

major classes & what do they spontaneously form?

Answer 17

Major lipid classes are phospholipids, glycolipids and cholesterol

Membrane lipids are amphipathic

Lipids spontaneously form bilayers in aqueous solutions

Question 18

what are some properties of integral membrane proteins?

Answer 18

→Single or multi pass

→Strong non-covalent bonds and they interact extensively with the lipid bilayer

→Trans-membrane domain often an α-helix with the R groups facing outwards (hydrophobic)

→can be predicted from sequence

→span the entire membrane and it is very difficult to remove them from the membrane.

Question 19

what are properties of peripheral membrane proteins?

Answer 19

→located on the extracellular or cytosolic membrane surface and are loosely associated by non-covalent bonds to the surface not embedded into it.

→They are simply bound to a phospholipid polar head group or integral membrane protein.

→ association is not that strong they associate transiently with the membrane.

Question 20

what are properties of lipid anchored membrane proteins?

Answer 20

proteins that are covalently attached to lipids embedded within the cell membrane, such as glycerol-phosphatidylinositol.

Question 21

describe properties of membrane carbohydrates

Answer 21

→Carbohydrates are associated with both membrane lipids and proteins

→Form 2-10% of the membrane weight

→Carbohydrate on all membranes faces away from the cytosol

→They are often involved in cell-cell interactions or cellular recognition

→In RBCs 8% of the weight is carbohydrate