membrane structure and function

Question 1

define boundaries between cells and within cells ?

Answer 1

flexible
self-sealing
selectively permeable
define boundaries and divide the internal spaces

Question 2

why do biological membranes form bilayers

Answer 2

• Biological membranes are composed largely of lipids and proteins
• The lipids are amphipathic meaning they have a hydrophilic and hydrophobic part
• The majority of lipids that make up the membrane are phospholipids.
• phospholipids in the presence of an aqueous environment, will oritenate themselves such that the hydrophoibic tails are kept away from the water and hydrophilic head interacts with the water. Many of these molecules together form first a sheet and then a lipid bilayer.
• Formation of the lipid bilayer is spontaneous
• Made out of many Non-covalent assemblies.

Question 3

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

Answer 3

phospholipids:Phospholipid has a glycerol backbone with hydrophobic fatty acids. Glycerol is then linked to a polar head group via a phosphate bride (hydrophilic portion of the molecule ) .
The polar head group can take many forms: choline, serine, ethanolamine, inositol.
The name of the molecules then become incorporated into the lipid, eg phosphatidyl choline.

cholesterol : - Cholesterol – component of the membrane. It’s a flat molecule with a small hydrophilic head group.

ceramide : Based on sphingosine which is attached to a fatty acid, forming ceramide. This molecule can either be combined with a carbohydrate, forming a glycolipid.or it can be linked via a phosphate group to a choline molecule, forming sphingomyelin.

Question 4

where is sphingomyelin present ?

Answer 4

in small amounts in most membranes but in significant amounts in neuronal tissue.

Question 5

how are the lipids distributed in the membrane ?

Answer 5

Cl =cholesterol : distributed evenly between inner and outer leaflet.

Whilst the other lipids are assymetrically distributed ie theyre expressed more on one side than the other.

• PC and SM , majority are expressed on the outer surface of the cell membrane.
• PS and PE and PI ( which fits with its role with intracellular signalling) majority are expressed on the inner surface.

Question 6

what is sphingomylein ?

Answer 6

is a phospholipid where the glycerol

backbone is replaced by sphingosine

Question 7

what does floppase do ?

Answer 7

moves phospholipids from the inner to the outer leaflet

Requires atp

Question 8

what does flippase do ?

Answer 8

moves phospholipids from the outer to the inner leaflet

Requires ATP

Question 9

what does scramblase do ?

Answer 9

bidirectional movement.

Question 10

what are key facts about membranes what are the major classes, what do they spontaneously form , where does synthesis take place and what does the distribution of the newly formed lipids require?

Answer 10

• Membrane lipids are amphipathic
• Major lipid classes are phospholipids, glycolipids and cholesterol
• Lipids spontaneously form bilayers in aqueous solutions
• Phospholipid synthesis takes place on the cytosolic surface of the endoplasmic reticulum
• Distribution of newly formed lipid requires the enzymes flippase and floppase

Question 11

describe the two movements of lipids

Answer 11

• Lateral movement of lipids in the membrane is rapid

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

Question 12

how does temperature alter fluidity

Answer 12

• As temperature increases the energy associated with the molecules within the membrane will increase and so will their movement, increasing fluidity
• Low temperature : molecules can pact more tightly together, so low fluidity due to the increasing intermolecular interactions
• At high temperatures,
  molecules will have more energy associated with them and are more motile.

Question 13

How does chain length affect membrane fluidity

Answer 13

the longer the chain length, the more interactions there are between the tails.

therefore the membrane is less fluid.

this is because intermolecular interactions between fatty acid tails add rigidity to the membrane.

Question 14

how does the degree and extent of saturation alter fluidity ?

Answer 14

• saturated fatty acids have no double bonds and therefore have straight unkinked tails. this allows the fatty acids to stack together. This means the distance between the phospholipids is small which allows the formation of non-covalent bonds and therefore fluidity is low.
• unsaturated fatty acids tend to be bent as they have kinked tails. this affects the degree to which the molecules can pact together, increasing the distance between the tails. therefore fewer non-covalent bonds form and degree of fluidity increases.

Question 15

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

Answer 15

• Cholesterol in membranes help to stabilise the membrane by maintaining the interactions that are necessary to keep the membrane intact.
• As you increase the amount of cholesterol in a membrane at low temperature, this reduces the interactions between the phospholipid fatty acid chains, this increases fluidity.
• Cholesterol at low temps has a significant effect on increasing membrane fluidt.y
• At high temperatures, the cholesterol pulls the phospholipids together, increasing intermolecular forces and decreasing fluidity.
• This allows organisms to exist in extreme temperatures.

Question 16

what is spur cell anaemia and how does it rise.

Answer 16

• Plasma membrane content of cholesterol increased by 25-65% leading to decreased membrane fluidity and flexibility
• In rbc, it can have a severe effect on functioning – reduced flexibility – rbc tend to get trapped as they pass through capillaries and can subsequently rupture, leading to anaemia.

Question 17

describe the properties of integral membrane proteins

Answer 17

• They exist in two forms depending on the number of times the protein traverses the membrane : Single or multi pass
• Strong non-covalent bonds due to the large number of non covalent interactions between the membrane lipids and proteins. These interactions are hydrophobic in nature but there are some hydrophilic interactions at the external surface of the membrane.
• Trans-membrane domain often an alpha helix
- The r groups in these alpha helixes are hydrophobic in nature.

• can be predicted from sequence

Question 18

the trans-membrane domain of integral membrane proteins are often in what form ?

Answer 18

alpha helix .

Question 19

describe the properties of peripheral membrane proteins

Answer 19

• Located on both the extracellular or cytosolic membrane
• Associated by non-covalent bonds to the surface, not embedded into it.
• More transient in nature
• Interactions : with both phospholipid component and the protein component of the membrane.
• Interactions with other proteins - only exist between the membrane protein and the peripheral protein
• Interaction directly with the phospholipid component.

Question 20

describe the properties of lipid anchored membrane proteins

Answer 20

Covalently linked to a lipid molecule such as glycosyl-phosphatidylinositol
Or through association with a fatty acid.

Question 21

what are the two major types of membrane proteins

Answer 21

integral and peripheral

Question 22

what do integral proteins interact with

Answer 22

exclusively with the lipid bilayer

Question 23

what do peripheral proteins interact with

Answer 23

integral proteins or lipid polar head groups

Question 24

describe the properties of membrane carbohydrates

Answer 24

arbohydrates are associated with both membrane lipids and proteins
• Form 2-10% of the membrane weight

• The majority of glycolipids and glycol proteins are externally facing ( ie theyre on the external surface).
• They are often involved in cell-cell interactions or cellular recognition

Question 25

what are selectins

Answer 25

an important group of membrane glycoproteins

Question 26

describe the movement of proteins through the hydrophobic core

Answer 26

restricted or prohibited by the energy barrier

Question 27

how are membranes synthesised ?

Answer 27

• new membrane is synthesised on the cytosolic surface of the ER
• the phospholipids bud off the ER and fuse with the golgi. as they move towards the golgi, the membrane undergoes some modifications.
• as the new lipids are added, the cytosolic side enlarges and the bilayer becomes bow shaped.
• the lipids are then transferred to the luminal side so that both sides get enlarged and the membrane is straight.
• this is achieved by the abcTRANSPORTER.

Question 28

what is the ABC transporter

Answer 28

ATP-binding cassette transporter enables the movement of the charged head group across the hydrophobic region in the core of the membrane.this requires energy and enzymes f,f,s

Question 29

Give an example of membrane carbohydrates involved in cellular recognition or cell-cell interactions.

Answer 29

the difference in blood groups. the difference in carbohydrates attached to the membrane is the reason for the difference in blood groups. the blood group antigens are glycolipids

Question 30

How do the proteins and phospholipids interact in the fluid mosaic model?

Answer 30

• Proteins associated with the membrane interact in a dynamic way with the lipids and are able to move across the membrane
• The movement of proteins through the hydrophobic core is restricted or prohibited by the energy barrier .