Membranes And Transport Flashcards
What are the three major kinds of membrane lipids?
What are all these membrane lipids described as and why?
Phospholipids,
Glycolipids,
Cholesterol.
They are described as amphiphillic because they have a hydrophilic and a hydrophobic end.
What do biological membranes do?
They define a cell’s boundaries by separating the cytosol from the extracellular environment and aid compartmentalisation. They control influx and efflux of substances.
What is the most abundant of the main membrane lipids?
What are the hydrophobic/philic regions of this molecule?
The most abundant are phospholipids. They have a hydrophilic polar head group and two hydrophobic fatty acids chains, one tail generally having more cis binds which influence how they pack together.
Phospholipids can be derived from two alcohol molecules?
Glycerol and sphingosine.
What are Glycolipids? What are there functions?
These are sugar containing lipid molecules found only in the non cytosolic mono layer of the lipid bilayer. In animal cells they have a sphingosine backbone.
They have many functions including being spread over the membrane to help the cell resist harsh conditions. They also can alter the electrical field across the membrane if they are charged.
What is cholesterol?
Cholesterol is a sterol which is present in eukaryotes but not in many prokaryotes. When mixed with phospholipids it alters their permeability when in a bilayer. Cholesterol packs the lipids closer together and increases the permeability for water soluble molecules.
Thin layer chromatography is a method used to what? How does it accomplish this?
Determine the lipid components of membranes. TLC separates lipids depending on their relative affinities for the stationary phase and the mobile phase.
What are the three formations of phospholipids?
Micelles- less than 20nm in diameter, very difficult to form due to fatty acid acyl chains being to bulky to fit inside.
Liposomes- these are aqueous compartment surrounded by a lipid bilayer with a approx. diameter of 50nm. They can be formed by suspending a lipid in an aqueous medium then sonicating.
Bilayer- these are the most common arrangements. They serve as permeability barriers. Van der waals forces and hydrogen bonding attractions stabilise the bilayer.
Our knowledge of the bilayer structure comes from what?
X-ray diffraction analysis. Low angle X-ray diffraction analysis of multi membrane myelin sheath (Schwann cells).
What are the two types of membrane proteins?
Integral- these are imbedded in or spanning the bilayer. They interact extensively with the hydrocarbon chains of the lipids.
Peripheral- these are bound to the membrane by electrostatic and hydrogen bond interactions.
Describe what freeze fracture/electron microscopy analysis has unearthed about the membrane and describe the process.
This analytical method has provided evidence for the ‘mosaic’ structure of the lipid membrane.
The membrane is frozen quickly and then struck by a knife to split it down the bilayer. The surface of the two mono layers is then coated with a heavy metal and then viewed under an electron microscope.
What does EM and X-ray crystallography of proteins unearth?
It is a method which allows the structures of proteins to be linked with their function.
How may nuclear membranes have evolved and why is this theory raised?
Nuclear membranes may have evolved through invagination of the plasma membrane as the lumen of the intracellular vesicle structure is topologically equivalent to the outside of the cell.
What are signal sequences?
These direct proteins to the correct compartment. They are continuous stretches of 15-60 amino acids encoded by a gene but removed after the sorting decision is made.
Describe the pores with the nuclear envelope. Describe the movement of molecules across the nuclear envelope.
They have a diameter approx. 120nm large and comprise a nuclear pore complex. Protein import/export requires specific amino acid sequences. Nuclear pores allow for passive diffusion of molecules smaller than 30kDa. Anything larger requires nuclear localisation signals.
