Random stuff Flashcards
(30 cards)
What are the four different phospholipids
Phosphatidylcholine
Phosphatidylserine
Phosphatidylethanolamine
Sphingomyelin
Why is fluidity important?
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)
How do bacteria and yeast stop themselves freezing in colder environments?
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
How does cholesterol effect lipid bilayers?
Inserts in between membrane phospholipids and tightens the packing within the bilayer and so decreases membrane permeability to small molecules
How are lipid bilayers assembled in the ER?
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
What is the role of the enzyme scramblase?
catalyses flip-flop i.e transbilayer movement so that the phospholipids distribute equally between the outer and inner leaflets of the ER membrane
What is the role of the enzyme flippase?
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
Glycolipids
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
How are integral proteins inserted into the membrane?
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
How are single pass transmembrane proteins inserted into the membrane?
Start (signal peptide) and Stop transfer sequence
Once translocator interacts with STOP; discharges and the protein laterally into the bilayer
How are double pass transmembrane proteins integrated into the membrane?
Both signal peptide and STOP remain part of the protein
Glycoproteins
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
Glycocalyx
Carbohydrate rich layer surrounding cells
Composed of glycolipids and glycoproteins
Protects cells against chemical and mechanical damage
Why is membrane transport important?
Regulation of intracellular ion concentrations
Uptake of nutrients
Excretion of metabolic waste products
Em
The difference in charge between the inside and the outside of cells
The PM is negatively charged on the inside face
What are the four factors that determine whether a fluid is able to cross the membrane?
Conc grad, electrochemical grad, hydrophobicity, size
Facilitated diffusion
Channels (ion channels), transporters
Transport inorganic ions and small molecules across the membrane
PASSIVELY, along CONC grad
Transporters
exhibit selectivity, oscillate between two conformations: A and B regardless of whether a solute is bound:
Glucose transporters
Channels
Voltage gated, ligand gated, mechanically gated (stretching may open channel)
K+ channels
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
Glucose transporters (Gluts)
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
Transport of glucose into erythrocytes by Glut 1
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
Active transport
Two classes of membrane protein involved: ATP-driven pumps; Coupled transporters Hydrolyses ATP (Na+/K+ ATPase) Utilises 30% of available energy in most cells
Na+/K+ ATPase
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)
