Random stuff

Question 1

What are the four different phospholipids

Answer 1

Phosphatidylcholine
Phosphatidylserine
Phosphatidylethanolamine
Sphingomyelin

Question 2

Why is fluidity important?

Answer 2

Allows lipids and proteins to diffuse in the lateral plane and interact with one another
Allows membranes to fuse with other membranes
Ensures membranes are shared equally between daughter cells following cell division
Allows cells to change shape (cell motility)

Question 3

How do bacteria and yeast stop themselves freezing in colder environments?

Answer 3

They synthesise SCFA’s and fatty acids with a greater degree of unsaturation (i.e increased number of double bonds)
DECREASES interactions between FA chains and the membrane will remain fluid at lower temperatures

Question 4

How does cholesterol effect lipid bilayers?

Answer 4

Inserts in between membrane phospholipids and tightens the packing within the bilayer and so decreases membrane permeability to small molecules

Question 5

How are lipid bilayers assembled in the ER?

Answer 5

Takes place on the outer cytosolic leaflet
FABP transports the FA to the ER
FAs embed into the membrane and in a succession of steps glycerol, phosphate and choline are added

Question 6

What is the role of the enzyme scramblase?

Answer 6

catalyses flip-flop i.e transbilayer movement so that the phospholipids distribute equally between the outer and inner leaflets of the ER membrane

Question 7

What is the role of the enzyme flippase?

Answer 7

Requires ATP to flip PE and PS from the EC leaflet to the cytosolic leaflet to create the characteristic asymmetric bilayer of the PM
Inner, cytosolic leaflet is negatively charged

Question 8

Glycolipids

Answer 8

Localised on the EC leaflet of the PM
Based on sphingosine (FA,FA, sphingosine, Sugar)
Glycosylation occurs in the lumen of the ER/Golgi as enzymes that add carbohydrate are only found in the lumen of the ER

Question 9

How are integral proteins inserted into the membrane?

Answer 9

Soluble proteins for export have a signal peptide at their N-terminal end (15-20 amino acids); hydrophobic
Signal peptide directs growing polypeptide chain as it emerges from the ribosome to a translocator in the ER membrane
Growing PP chain is threaded through the membrane
Signal peptide cleaved and protein is released into ER lumen

Question 10

How are single pass transmembrane proteins inserted into the membrane?

Answer 10

Start (signal peptide) and Stop transfer sequence

Once translocator interacts with STOP; discharges and the protein laterally into the bilayer

Question 11

How are double pass transmembrane proteins integrated into the membrane?

Answer 11

Both signal peptide and STOP remain part of the protein

Question 12

Glycoproteins

Answer 12

Some p’s are glycosylated
Glycosylation occurs in the lumen of the ER/Golgi
Located on EC side of PM
Important for cell recognition processes, inflammatory response etc

Question 13

Glycocalyx

Answer 13

Carbohydrate rich layer surrounding cells
Composed of glycolipids and glycoproteins
Protects cells against chemical and mechanical damage

Question 14

Why is membrane transport important?

Answer 14

Regulation of intracellular ion concentrations
Uptake of nutrients
Excretion of metabolic waste products

Question 15

Em

Answer 15

The difference in charge between the inside and the outside of cells
The PM is negatively charged on the inside face

Question 16

What are the four factors that determine whether a fluid is able to cross the membrane?

Answer 16

Conc grad, electrochemical grad, hydrophobicity, size

Question 17

Facilitated diffusion

Answer 17

Channels (ion channels), transporters
Transport inorganic ions and small molecules across the membrane
PASSIVELY, along CONC grad

Question 18

Transporters

Answer 18

exhibit selectivity, oscillate between two conformations: A and B regardless of whether a solute is bound:

Glucose transporters

Question 19

Channels

Answer 19

Voltage gated, ligand gated, mechanically gated (stretching may open channel)

Question 20

K+ channels

Answer 20

Most common, leak continuously
K+ originally hydrated
Negatively charged aa at pore entrance
Vestibule
K+ needs to be rehydrated (selectivity filter) carbonyl oxygens of aa
Energy lost through dehydration = energy gained by K+ interaction with C=O
Energetic cost for Na+ to go through channel

Question 21

Glucose transporters (Gluts)

Answer 21

Uniporters (only transport glucose)
12 pass membrane spanning proteins; alternate between two conformations
Glut 1: important for transporting glucose to brain; deficiency characterised by seizures, retarded development etc

Question 22

Transport of glucose into erythrocytes by Glut 1

Answer 22

Blood concentration of glucose higher than in erythrocyte, therefore transportation is ALONG the conc grad
Glut 1 can work in BOTH directions so it is essential that a concentration gradient in maintained: Glucose 6-phosphate
Not recognised by Glut 1

Question 23

Active transport

Answer 23

Two classes of membrane protein involved: ATP-driven pumps; Coupled transporters
Hydrolyses ATP (Na+/K+ ATPase)
Utilises 30% of available energy in most cells

Question 24

Na+/K+ ATPase

Answer 24

In the absence of Na+/K+ ATPase, ions would flow down their concentration gradient disturbing osmotic balance and preventing secondary AT
3Na+ bind (alpha sub-unit), phosphorylation, conformational change
2K+ bind, dephosphorylation, conformational change (back to original)

Question 25

Coupled transporters

Answer 25

Do not depend directly on the hydrolysis of ATP: secondary AT
Symport, Antiport

Question 26

Na+/Glu symporter and the absorption of glucose

Answer 26

E by Epithelial cells lining the GIT
Glu concentration is higher in epithelial cells than in lumen
Na+ electrochemical gradient: Na+/Glu BOTH go into the cell
Binding is co-operative: binding of Na+ increases the affinity of the transporter for glucose
Glucose more likely to bind on EC side where Na+ concentration is very high
Glut 2 transports glucose out of cells along its concentration gradient into the blood
Na+/K+ ATPase transports Na+ out of the cell into BS (goes round in a circle)

Question 27

Why are glucose simperers required for the absorption of glucose from the GI tract?

Answer 27

Glucose needed in BS
Can’t transport paracellularly due to the presence of TJ
Barrier and fence function: Rivot adjacent cells together preventing macromolecules moving through the paracellular pathway
Claudin/Occludin

Question 28

Na+/Ca2+ antiporter

Answer 28

Expressed by cardiomyocytes in the heart muscle
Ca2+ concentration greater in the interstitial fluid than the cardiomyocytes
3 Na+ in, Ca2+ OUT
decreases strength of contraction

Question 29

Treatment of cardiac failure

Answer 29

Digoxin: inhibit Na+/K+ ATPase
Increase in intracellular Na+; reduction in Na+ gradient
Fewer Ca2+ ions are exported, contraction maintained

Question 30

Aquaporins

Answer 30

Tetrameric complex made up of four monomers: each of which acts as a water channel in its own right
Each monomer is a six pass membrane spanning protein
Allows passage of water whilst preventing the passage of ions
Increases the efficiency of water movement