Membranes Flashcards
What are the 5 roles of a cell membrane?
- Define boundaries of cell/organelles
- Separate cellular and extracellular environment
- Control movement of molecules & signal transduction
- Separate aqueous regions of different compositions within cells
Which components have a higher intracellular concentration?
- K+
- H+
Which components have a higher extracellular concentration?
- Na+
- Mg2+
- Ca2+
- Cl-
Briefly describe the fluid mosaic model
- Sea of phospholipids which is movable and flexible
- Protein molecules which are slightly more fixed but are also movable and can cross the bi-layer
Describe the phospholipid structure
Hydrophilic head composed of: choline (or others), phosphate and glycerol
2 Hydrophobic hydrocarbon tails: At least one tail always contains at least 1 C=C bond which determines fluidity
What 3 components can form the hydrophilic head? Which one produces a negative head?
Ethanolamine, Serine (-), Choline
What other molecule can choline form a hydrophilic head with?
Phosphate and sphingosine (in place of glycerol)
Produces sphingomyelin
What is the length range of hydrocarbon tails and how does this affect fluidity?
14-24 hydrocarbons long
Shorter tailed hydrocarbons are often more fluid
What is the primary role of sphingomyelin?
Forms the myelin sheath
What 3 characteristics define the properties of the plasma membrane?
- Head components
- Saturation
- Length
What is the effect of sterols on the phospholipid membrane?
- Attach themselves to the head of a phospholipid, with sterol rings interacting with hydrocarbon tail making it stiffer (ambiphilic molecules)
- Allow lipid-soluble molecules to cross the membrane
- Cholesterol increases the viscosity of membranes by limiting flexion, rotation and lateral diffusion
What is cell membrane viscosity believed to affect?
Cell signalling
What is cell thickness thought to influence?
Trafficking of proteins across cells
How does the composition of the interior and exterior cell membrane differ?
- Outside composed mostly of choline and sphingomyelin, interior mostly serine
- Causes difference in charge
Describe lipid microdomains
- Groups of lipids aggregate together for a short time period causing a microdomain to regulate a cell function, particularly cellular vent
- Regulates release of hormones through interactions with proteins
Name 6 types of transmembrane protein
- Single-pass transmembrane (alpha-helical)
- Multiple-pass transmembrane (alpha-helical)
- Beta-sheet barrel
- Associated with one leaflet of membrane (cytosolic) with amphiphilic alpha-helix
- Covalently bonded to a hydrocarbon
- Lipid-anchored
Describe Glycophorin A
Amphiphilic alpha helical transmembrane protein, passing once with hydrophobic membrane spanning section
Describe bacteriorhodopsin
- transmembrane protein with 7 helical transmembrane regions
- first membrane-bound protein to have its structure determined
- oritentates itself to stay within the hydrophobic region
Describe aquaporin
- alpha helical, composed of 2 smaller proteins that span only half of the membrane
- acts as gates for the water channel in the kidney
Where are beta-barrel transmembrane proteins commonly found and what is their function?
Found in: mitochondria, chloroplasts and bacteria
Allow controlled movement of hydrophilic molecules
How are lipid-anchored proteins attached to the phospholipid membrane?
- Protein is covalently attached to lipid on extracellular side of the membrane by GPI anchor composed of inositol
- Can be attached via amide linkage ( myristoyl anchor) or thioester linkage (palmitoyl anchor/farnesyl anchor)
How can the fluidity of the plasma membrane be demonstrated?
- Attach fluorescently tagged protein to plasma membrane (GFP)
- Bleach area and measure recover time
- Allows the measure of lateral mobility by diffusion constant
