Membranes Flashcards
What is membrane fluidity controlled by?
Fatty acid composition and cholesterol content
What are the two states lipid bilayer can exist in?
An ordered, rigid state
Relatively disordered, fluid state
When does the phase transition occur?
At Tm - melting temperature
Where do you find cholesterol?
Inserted between phospholipids - OH group of cholesterol aligns with polar head groups of lipids
What does cholesterol do to hydrocarbon chain? At high concentrations?
Rigid steroid ring partially immobilises hydrocarbon chain at point closest to head
At high concentrations, prevents hydrocarbon chains coming together blocks phase transition
How do phospholipids form sealed compartments?
Spontaneously
What do many integral membrane proteins have?
One or more domains embedded in lipid bilayer
Can have single or multiple transmembrane (TM) domains
What are transmembrane domains composed of?
20-30 hydrophobic residues that form an alpha helix that spans the lipid bilayer
How can transmembrane domains be predicted?
From primary sequences
-hydropathy plots can identify stretches of hydrophobic residues
Positive = energy required = hydrophobic
What are the hydrophobic amino acids?
F - phenylalanine
A - alanine
M - methionine
I - isoleucine
L - leucine
Y - tyrosine
V - valine
W - tryptophan
What can a transmembrane hydrophilic pore be formed by?
Alpha helixes
What do detergents do?
Solubilise membrane proteins
What is SDS?
Strong ionic detergent
What is Triton X-100?
Mild ionic detergent
What are peripheral membrane proteins?
Proteins indirectly associated with the membrane
More easily dissociated from membrane (eg salt wash)
Components of some signalling pathways are associated with cytoplasmic face of TM receptors
What are features of a typical single pass transmembrane protein?
Most transmembrane proteins glycosylated (sugar residues added in ER lumen and Golgi)
Disulphide bonds exist between folded parts, to help stabilise folded structure
Both features present on non-cytosolic side
What does freeze fracture electron microscopy do?
Visualise certain membranes
What is an experiment that demonstrates the diffusion of proteins in the plasma membrane?
Rhodamine labelled membrane protein (mouse) cells + fluorescein labelled membrane (human)
Fuse the two together
Incubate at 37 degrees
Different proteins are different colours
What does FRAP flurescence recovery after photo bleaching do?
A method to measure lateral mobility of proteins within membranes
What does single particle tracking allow?
Tracking of individual protein molecules
What are detergents used for?
Disrupt cells membranes to isolate proteins for study
What are small hydrophobic molecules in a lipid bilayer?
O2 CO2 N2 Steroid hormones
What are small uncharged polar molecules in a lipid bilayer?
H2O
Glycerol
Ethanol
What are the large uncharged polar molecules in a lipid bilayer?
Amino acids
Glucose
Nucleotides
What are the ions in a lipid bilayer?
H+
Na+
HCO3-
K+
Ca2+
Cl-
Mg2+
What are carrier proteins?
Bind molecule to be transported. Move through membrane by series of conformational changes
Only allow passage of molecules that fit binding site
What a re channel proteins?
Form hydrophilic pores through membrane
Discriminate on basis of size and charge
What is passive transport?
All channels and most carriers
Molecules move down concentration gradient
What is active transport?
Movement of molecules against electrochemical concentration gradient
Requires energy
Transport by carrier proteins can be either or passive - channels or passive
What is the hypothetical mechanism of carrier proteins?
1- specific molecule binds to carrier protein on one side of the membrane - binding site
2- binding triggers conformational change in carrier protein. Protein flips or shifts shape to the opposite side of the membrane
3- protein is release into that space. Binding site now has a low affinity so the molecule detaches
4- carrier protein returns to original shape
What is an electrochemical gradient?
Net driving force, composed of concentration and voltage
Affects ability of charged molecules to cross
Explain the Na+ K+ pump?
1- three Na+ Ions bind inside the cell
2- pump is phosphorylated - phosphate from ATP is used, giving it energy
3- energy causes shape change, and 3 Na+ to be released to outside
4- 2 K+ ions bind to pump (from outside)
5- phosphate released - shape change
6- 2K+ released
What does the Na+ / K+ pump do?
Maintains the electrochemical gradient essential for things such as nerve impulses and medulla contraction
How can active transport be driven by ion gradients?
Free energy released as ions move down electrochemical gradient essential
This free energy can be used to drive active transport of another molecule - coupled transporters
(Usually Na+)
What is uniport? How does it work?
Simple transport of soluble molecule from one side of the membrane to the other
Binds to protein, conformational change, molecule released
What is symport? How does it work?
Coupled transport of two types of solute (eg glucose and Na+ in kidney)
Bind to carrier protein, one goes with gradient, conformational change, both molecules released on other side
What is antiport? How does it work?
Two molecules bind on opposite sides membrane
Conformational change
Each molecule is released on other side that it started
One molecule against conc gradient and one with
What are the two types of glucose transporter in intestinal epithelial cells?
Apical
Basal
What is apical glucose transport?
Glucose / Na+ symport, allows uptake of glucose when conc in cell is already high
What is basal glucose transport?
Passive glucose transporters, glucose released as required
