9 Membranes and Membrane Transport Flashcards
Give a quick recap on what we know about membranes
- fluid mosaic model
- thin film of lipids and proteins held together by non covalent attractions
- lipid bilayer
- impermeable to polar/water soluble molecules
give a quick recap on what we know about fatty acids
- long hydrocarbon chains
- carboxyl group at the end
- forms ester bond with glycerol in condensation reaction
- unsaturated when contain double bonds
describe the overall structure of a phospholipid
- 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
how many types of phospholipids are there? and whats the difference between them
- 3 types
- whether the phosphate is bonded to a choline, serine or ethanol amine
What other structures are there that are similar to phosphlipids
- sphingomyelin and sphingosine
Describe the structure of sphingomyelin
- 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
describe the structure of sphingosine
- one long fatty acid chain
- and a sphingosine body
explain how membrane lipids are amphipathic
- The are partly hydrophobic and partly hydrophilic
- having a polar hydrophilic head and nonpolar hydrophobic tail
what determines polarity
- how well covalently bonded atoms share electrons, the more equal the sharing the less polar the molecule
how do polar molecules and non-polar molecules interact with water
- 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
how do micelles form
- 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
What happens to cell membranes if they become dameged?
the smooth ER traffics phospholipids to the damaged site
Describe why the membrane is so fluid and dynamic
- 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)
In which direction can phospholipids not travel
- Across to the other side of the membrane
How does temperature effect membrane fluidity
- at low temperatures the lipid bilayer undergoes a phase transition and becomes closely packed and rigid
what factors prevent the membrane becoming packed and rigid
- cholesterol in the membrane bilayer
- c=c double bonds in the fatty acid chain of the phospholipids space out the phospholipids
How do membranes with lots of double bonds in the fatty acid tails compare to a membrane with phospholipids that are unsaturated
- 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
Describe the structure of cholesterol
- polar region with interacts with the phosphate head
- rigid steroid region which interacts with the phospholipid tails stiffening them
- What role does cholesterol play in the lipid bilayer
- 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
Describe a lipid raft and how it forms
- 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
What is a glycocalyx
- This is a protective layer of glycoproteins and glycolipids situated on the outer surface of a cell membrane, this is a coat
What is the purpose of a glycocalyx
- this layer is used for protection and more importantly recognition
Where do glycoproteins and glycolipids get synthesised in the cell
Glycoproteins: proteins which are glycosylated in the RER and Golgi
Glycolipids: lipids from the SER and are glycosylated in the Golgi
What catagories can membrane proteins be put under?
- 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)
How well do non-polar, small polar, large polar and ions travel across the cell membrane
- 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
give some examples of molecules that can travel through the membrane easily
- non-polar molecules
- eg O2, CO2, hormones, steroids
can glucose and sucrose travel through the cell membrane
- they are large polar molecules so no without a transporter
what factors contribute to how well molecules diffuse through the cell membrane
- 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
describe the structure of all transport proteins
- transmembrane
- multi-pass
- either alpha helix or b-sheet/barrel shaped
What 2 types of transport proteins are there
- carrier proteins
- channel proteins
how do carrier proteins work
- they bind to a solute and undergo a conformational change to transfer the solute
How so channel proteins work
- they interact very weakly with the solutes forming an aqueous pore that solutes can pass through
what factors determine facilitated diffusion
- the concentration gradient
- the electrochemical gradient
What are the 2 main mechanisms behind active transport
ATP driven pumps - primary active transport
Coupled carriers - secondary active transport
What proteins mediate coupled transport
- symporter proteins
- antiporter proteins
What is the difference between primary active transport and secondary active transport
- 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
Describe the mechanism behind how Sodium-Potassium-ATPase pumps work
- 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
Give another example of where ATPase pumps are used
- on lysosomes to pump H+ ions into the lysosome to reduce the pH for the enzymes inside
channel proteins form pores on cell membranes, are these selective? name a few types
yes
- voltage gated
- ligand gated
- mechanically gated
