9 Membranes and Membrane Transport

Question 1

Give a quick recap on what we know about membranes

Answer 1

• fluid mosaic model
• thin film of lipids and proteins held together by non covalent attractions
• lipid bilayer
• impermeable to polar/water soluble molecules

Question 2

give a quick recap on what we know about fatty acids

Answer 2

• long hydrocarbon chains
• carboxyl group at the end
• forms ester bond with glycerol in condensation reaction
• unsaturated when contain double bonds

Question 3

describe the overall structure of a phospholipid

Answer 3

• 2 fatty acids covalently bonded to a glycerol
• 1 fatty acid is saturated and the other is unsaturated
• the third hydroxyl group of the glycerol is bonded to a phosphate group
• the phosphate bonds to either a choline/serine/ethanol amine group

Question 4

how many types of phospholipids are there? and whats the difference between them

Answer 4

• 3 types

- whether the phosphate is bonded to a choline, serine or ethanol amine

Question 5

What other structures are there that are similar to phosphlipids

Answer 5

• sphingomyelin and sphingosine

Question 6

Describe the structure of sphingomyelin

Answer 6

• 2 fatty acid tails, one very long unsaturated and one regular and pretty much unsaturated
• sphingosine instead of glycerol
• phosphate head
• choline group bonded to the phosphate

Question 7

describe the structure of sphingosine

Answer 7

• one long fatty acid chain

- and a sphingosine body

Question 8

explain how membrane lipids are amphipathic

Answer 8

• The are partly hydrophobic and partly hydrophilic

- having a polar hydrophilic head and nonpolar hydrophobic tail

Question 9

what determines polarity

Answer 9

• how well covalently bonded atoms share electrons, the more equal the sharing the less polar the molecule

Question 10

how do polar molecules and non-polar molecules interact with water

Answer 10

• water is a very polar molecules so interacts favourably with other polar molecules
• so its energetically favourable for polar molecules to dissolve in water
• non-polar molecules do not interact like water and oil

Question 11

how do micelles form

Answer 11

• hydrophobic tails cluster together disorderly exposing only the hydrophilic heads to water
• the overall system is more disordered so this is energetically favourable and happens naturally

Question 12

What happens to cell membranes if they become dameged?

Answer 12

the smooth ER traffics phospholipids to the damaged site

Question 13

Describe why the membrane is so fluid and dynamic

Answer 13

• the phospholipids are free to move
• phospholipids can move along laterally (lateral diffusion)
• phospholipids can rotate a full 360
• the tails of the phospholipids can move freely (flexion)

Question 14

In which direction can phospholipids not travel

Answer 14

• Across to the other side of the membrane

Question 15

How does temperature effect membrane fluidity

Answer 15

• at low temperatures the lipid bilayer undergoes a phase transition and becomes closely packed and rigid

Question 16

what factors prevent the membrane becoming packed and rigid

Answer 16

• cholesterol in the membrane bilayer

- c=c double bonds in the fatty acid chain of the phospholipids space out the phospholipids

Question 17

How do membranes with lots of double bonds in the fatty acid tails compare to a membrane with phospholipids that are unsaturated

Answer 17

• the c=c double bonds put kinks in the fatty acid tails
• this means the membrane is thinner but the fatty acids are harder to pack together so resist a phase transition due to low temps

Question 18

Describe the structure of cholesterol

Answer 18

• polar region with interacts with the phosphate head

- rigid steroid region which interacts with the phospholipid tails stiffening them

Question 19

• What role does cholesterol play in the lipid bilayer

Answer 19

• cholesterol stiffens up the upper region of the fatty acid tail which prevents phospholipids packing together tightly
• stiffening the tail reduces fluidity and lateral movement but reduced packing increases fluidity by more
• overall increase in fluidity

Question 20

Describe a lipid raft and how it forms

Answer 20

• a lipid raft is a region of the membrane with a high concentration of sphingolipids
• Sphingolipids have extra long fatty acid tails, as a result the van der vaals forces are much stronger and hold adjacent molecules together as a lipid raft

Question 21

What is a glycocalyx

Answer 21

• This is a protective layer of glycoproteins and glycolipids situated on the outer surface of a cell membrane, this is a coat

Question 22

What is the purpose of a glycocalyx

Answer 22

• this layer is used for protection and more importantly recognition

Question 23

Where do glycoproteins and glycolipids get synthesised in the cell

Answer 23

Glycoproteins: proteins which are glycosylated in the RER and Golgi
Glycolipids: lipids from the SER and are glycosylated in the Golgi

Question 24

What catagories can membrane proteins be put under?

Answer 24

• Integral/transmembrane proteins (through the lipid bilayer)
• peripheral membrane proteins (on 1 layer of the lipid bilayer)
• single pass (passes through the membrane once)
• multipass (passes through the membrane more than once)

Question 25

How well do non-polar, small polar, large polar and ions travel across the cell membrane

Answer 25

- non-polar diffuse across the cell membrane easily - small polar molecules can travel through the membrane but very slowly - large polar molecules mostly cant and need a transporter - All ions cannot travel through the cell membrane without a specialised mechanism

Question 26

give some examples of molecules that can travel through the membrane easily

Answer 26

- non-polar molecules | - eg O2, CO2, hormones, steroids

Question 27

can glucose and sucrose travel through the cell membrane

Answer 27

- they are large polar molecules so no without a transporter

Question 28

what factors contribute to how well molecules diffuse through the cell membrane

Answer 28

- the more polar the molecule the less likely it is to diffuse across - the larger the molecule the less likely it is to diffuse across the membrane

Question 29

describe the structure of all transport proteins

Answer 29

- transmembrane - multi-pass - either alpha helix or b-sheet/barrel shaped

Question 30

What 2 types of transport proteins are there

Answer 30

- carrier proteins | - channel proteins

Question 31

how do carrier proteins work

Answer 31

- they bind to a solute and undergo a conformational change to transfer the solute

Question 32

How so channel proteins work

Answer 32

- they interact very weakly with the solutes forming an aqueous pore that solutes can pass through

Question 33

what factors determine facilitated diffusion

Answer 33

- the concentration gradient | - the electrochemical gradient

Question 34

What are the 2 main mechanisms behind active transport

Answer 34

ATP driven pumps - primary active transport | Coupled carriers - secondary active transport

Question 35

What proteins mediate coupled transport

Answer 35

- symporter proteins | - antiporter proteins

Question 36

What is the difference between primary active transport and secondary active transport

Answer 36

- Primary active transport uses energy such from things like ATP hydrolysis to pump molecules against the concentration gradient - secondary active transport uses coupled carriers and as one molecule passes down its concentration gradient, this provides energy to pump another ion against its electrochemical gradient

Question 37

Describe the mechanism behind how Sodium-Potassium-ATPase pumps work

Answer 37

- 3Na+bind to the protein pump, ATP is hydrolysed releasing energy causing a conformational change spitting the 3Na+ out the cell - 2K+ from outside bind to the open protein pump which is kept open by the phosphate released from ATP hydrolysis, once both the K+ have bound the phosphate dissociates spitting K+ into the cell

Question 38

Give another example of where ATPase pumps are used

Answer 38

- on lysosomes to pump H+ ions into the lysosome to reduce the pH for the enzymes inside

Question 39

channel proteins form pores on cell membranes, are these selective? name a few types

Answer 39

yes - voltage gated - ligand gated - mechanically gated