The Cell Surface

Question 1

what is the structure of the cell membrane?

Answer 1

• Composed of lipids ( mainly phospholipids) and proteins
• There are two opposing sheets of lipids into which proteins are inserted (called a bilayer)
• Hydrophobic tails face out towards fluids
• Fatty acid chains determine fluidity of membrane

Question 2

what is the structure of a phospholipid?

Answer 2

has a hydrophobic tail and a hydrophilic head

Question 3

what does amphipathic mean?

Answer 3

they have a polar and a hydrophobic part

Question 4

what is the function of a cell membrane?

Answer 4

barrier- between the cells/cell organelles and their environment, controlling which substances enter and leave.
compartmentalisation – maintain different internal environments
Site of chemical reactions – inner membrane of mitochondria (cristae) contains enzymes needed for cellular respiration.
Membranes also protect vital cell components e.g. DNA.
cell recognition- e.g. cells of the immune system. (Glycolipids)
cell communication - (cell signalling (Glycoproteins)

Question 5

what are the 4 major phospholipids in the mammalian PM?

Answer 5

• phosphatidylcholine
• phosphatidylserine
• phosphatidylethanolamine
• sphingomyelin

Question 6

how are all the phospholipids able to link together?

Answer 6

They all have similar structure so are able to link together
Only head group is different (EXCEPT
Sphingomyelin)

Question 7

what are intracellular signal transduction lipids?

Answer 7

They are a minor proportion of the phospholipid content of intracellular membranes.
Derived from lipids residing in the PM
Bind specifically to conserved regions found within many different proteins and once bound, induce conformational and/ or localisation and activity changes within these proteins

Question 8

what are the features of intracellular signal transduction lipids?

Answer 8

Rapidly generated/ destroyed by enzymes in response to a specific signal (to prevent continuous signals)
They are spatially and temporally (at a specific space at a specific time) generated giving a highly specific signal.

Question 9

what are 4 examples of intracellular signal transduction lipids?

Answer 9

▪ Phosphatidylinositol
▪ Diacylglycerol
▪ Ceramide
▪ Sphingosine-1-phosphate

Question 10

what are glycolipids?

Answer 10

located on non-cytosolic half of membrane
glycosylated in Golgi apparatus
Important for recognition and cell attachment

Question 11

what is the structure of cholesterol?

Answer 11

• 4-ringed ridged planar structure
• Polar head group interacts with the hydrophilic head
• hydrophobic hydrocarbon tail

Question 12

what is the role of cholesterol in the PM?

Answer 12

Inserts between membrane phospholipids
This tightens packing in the bilayer/ membrane rigidity and decreases membrane permeability to small molecules
Regulates cell membrane fluidity
At 37C cholesterol makes the membrane less fluid – stabilises interactions between neighbouring phospholipids

Question 13

what does cholesterol do to proteins?

Answer 13

Cholesterol also stabilises protein groups which makes communication easier

Question 14

what does the fluidity of the PM allow it to do?

Answer 14

Allow signalling lipids and membrane proteins to rapidly diffuse in the lateral plane and interact with one another
Fluidity is important in cell division- ensures membranes are equally shared between daughter cells following cell division
Allows membranes to fuse with other membranes e.g. in exo/endocytosis

Question 15

what are transmembrane proteins and why are they integral?

Answer 15

proteins that span the entire lipid bilayer
integral bc they go through both layers

Question 16

what are transmembrane domains and what do they consist of?

Answer 16

transmembrane domains - regions in the membrane of integral proteins

mainly contain hydrophobic amino acid side which are in contact with the phospholipid tails

the proteins are usually organised in alpha helical structures

Question 17

what are other examples of transmembrane proteins?

Answer 17

□ Beta barrel protein
□ Lipid linked- proteins that are post translationally modified (lipid is added) allowing it to anchor to the PM
□ Peripheral membrane proteins- proteins that interact with other integral proteins

Question 18

what are transmembrane protein functions? (SIGRATE)

Answer 18

• Signal transduction molecules: pass on and amplify signals.
• Intracellular joining – joining of two cells
• Glycoproteins – allows for cell-cell recognition
• Receptors – ligands bind to a receptor to allow for the transduction of a signal into the cell
• Anchors – integrins, link the extra cellular matrix with actin of the cytoskeleton (allows cellular motility)
• Transporters – channel and carrier proteins
• Enzymes

Question 19

What are the 2 different ways of transport across the PM?

Answer 19

Active- needs ATP
Passive- doesn’t need ATP

Question 20

What are the 2 examples of passive transport?

Answer 20

Simple diffusion
facilitated diffusion

Question 21

what is simple diffusion?

Answer 21

It is just driven by the concentration gradient.
- No membrane proteins involved
- Driven by concentration gradients
- Membranes are highly impermeable to ions

Question 22

what are the molecules involved in simple diffusion?

Answer 22

○ Hydrophobic molecules can freely diffuse across the membrane down their concentration gradients e.g. O2, CO2, N2 and benzene
○ Small uncharged polar molecules can also diffuse across the membrane (although this is to a lesser extent than hydrophobic molecules) e.g. H2O, urea, glycerol
○ Large uncharged polar molecules (e.g. glucose and sucrose) are too large to diffuse across the membrane
○ All ions are too polar to diffuse across the membrane

Question 23

what is facilitated diffusion?

Answer 23

Membrane proteins are involved
They transport inorganic ions/ small molecules across the membrane passively (downhill) along their concentration/ electrochemical gradient
It is also driven down a concentration gradient.

Question 24

what are the 2 classes of channels involved in facilitated diffusion?

Answer 24

1) Channels - discriminates mainly on size and charge
2) Uniporter carrier proteins - involves a binding site for solutes

Question 25

what is the structure of the channels?

Answer 25

○ Most are non-directional ion channels
○ Hydrophilic pores are formed through the membrane (Hydrophilic surfaces)
○ Show some selectivity on molecules that pass through based on size and charge
○ Fast

Question 26

what are uniporter carrier proteins and give an example?

Answer 26

○ Carries one type of molecule
○ Highly selective (as molecule needs to bind to a specifically shaped site)
○ Binding of the molecule to a specific site causes a small conformational change to the carrier protein resulting in movement of molecule from inside to outside or vice versa
○ Carrier proteins are required for almost all small organic molecules
○ Relatively slow
E.g. glut 2 in gut epithelium

Question 27

define an electrochemical gradient

Answer 27

Defined as.. the net force driving a charged solute across a membrane.

Question 28

define an electrochemical gradient

Answer 28

Defined as.. the net force driving a charged solute across a membrane.

Question 29

what is the electrochemical gradient composed of?

Answer 29

It is composed of a concentration gradient and the voltage across the membrane.
An electrochemical gradient combines the concentration gradient and membrane potential
The force driving a charged solute (e.g Na+ ions) across a membrane is the concentration gradient and the membrane potential

Question 30

what are the reasons to maintain an electrochemical gradient?

Answer 30

○ It drives transport across the membrane
○ Maintains the osmotic balance - without active transport to maintain EC gradient, ions would flow down their gradients through leak channels so the osmotic balance is disturbed which results in cell death

Question 31

what is the difference between primary and secondary active transport?

Answer 31

primary active transport: the energy is derived directly from the breakdown of ATP.

secondary active transport: the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences between the two sides of a membrane

Question 32

what is active transport?

Answer 32

Moves solutes against their chemical gradient by expanding energy (primary active transport)
requires ATP

Question 33

Explain the Na+K+ ATPase example (Primary active transport)

Answer 33

Na+ and K+ electrochemical gradient:
· In the absence of Na+ / K+ ATPase ions would flow down their gradients, disturbing osmotic balance and preventing ‘secondary active transport’
· Found in the plasma membrane of all animal cells
· There is a higher concentration of K+ inside the cell than outside and a higher concentration of Na+ outside the cell than inside
· 3 Na+ pumped out of the cell for every 2 K+ moved in.
· 3 Na+ bind to the catalytic subunit, this activates ATPase
· ATP undergoes hydrolysis resulting in the phosphorylation of a subunit of ATPase. ATPase undergoes a conformation change which causes the release of 3 Na+ out of the cell
· 2 K+ then bind extracellularly
· The K+ binding causes the subunit to undergo dephosphorylation so the pump changes conformation to its original and 2 K+ moves into the cell
· If one step is blocked then pump is halted
· whole cycle takes 10 milliseconds

Question 34

what are coupled transporters?

Answer 34

Couple the transport of one solute with its conc gradient to another against its gradient
Do not depend directly on the hydrolysis of ATP (secondary active transport)

Question 35

what is the difference between symport and antiport transporters?

Answer 35

· symport – both solutes move in same direction
· antiport – solutes move in opposite direction

Question 36

explain the Na+/ glucose symporter in the gut epithelia (secondary AT)

Answer 36

• Higher Na+ concentration & lower glucose concentration in the Gut (extracellular)
• The binding of glucose is dependent on the binding of Na+ (co-operative binding)
• As there is a higher concentration of Na+ outside the cell glucose more likely to bind to the symporter when its in its initial state – allows for binding of extracellular molecules.
• As a result there is an overall net flow of glucose (against its concentration gradient) and Na+ (along its concentration gradient) into cell.
• One of the reasons you get a build-up of Na+ in cells is because it is being used in symporters to transfer other molecules

Binding of Na+ and glucose is cooperative i.e binding of glucose is dependant on Na+
Bc Na+ conc is much higher outside the cell, glucose is more likely to bind in state A than state B
Over all result is that Na+ and glucose enter the cell more than leave it therefore NET FLOW is into the cell

Question 37

explain the Na+/Ca2+ antiporter (secondary active transport)

Answer 37

• Important in cardiac muscle
• Cardiac muscle contraction is triggered by rise in intracellular Ca2+ concentration
• There is a higher concentration of Na+ and Ca2+ outside the cell than inside the cell
• Therefore, the antiporter moves 3 Na+ in to the cell (along its concentration gradient) for every 1 Ca2+ out (against its concentration gradient)
• The antiporter therefore rapidly reduces intracellular Ca2+ concentration which reduces the strength of cardiac muscle contraction i.e. causes cardiac muscle relaxation