Plasma Membrane Flashcards
Describe the current cell membrane model.
Fluid Mosaic model
Developed my singer/nicholson in 1972- it describes a “fluid” membrane which allows movement of the embedded proteins
Describe the phospholipid bilayer.
Double layered membrane of phospholipid. Electrophilic “heads” are oriented intra and extracellularly; electrophobic “tails” are hidden within the membrane itself
How thick is the phospholipid bilayer?
8/9 nm
Describe the structure of a phospholipid.
Glycerol- contains 3 oxygen binding sites, 2 of which are bound to carbohydrates and 1 of which is bound to a phosphate.
Carbohydrates- 2 of which combine to make up the hydrophobic “tail”.
Can be saturated (no double bonds) or unsaturated (double bonds), which has an effect on the fluidity of the membrane.
Phosphate- bound to glycerol and one other organic group to comprise the hydrophilic head. organic group differentiates phospholipids.
How are phospholipids different from normal fats?
Normal fats have 3 acyl groups and no phosphate.
What is a liposome? Why is it important?
In an aqueous solution, phospholipids have the property of forming energetically favorable spheres, or “liposomes.” This property is also the reason that if punctured, a membrane will reseal itself.
Describe what types of molecules (in terms of size and polarity) the selectively permeable membrane will allow and will not allow through passively.
small uncharged polar- passive
non-polar uncharged - passive
charged (ions)- no
large uncharged polar- no
Describe two ways to affect membrane fluidity.
- Unsaturated carbons create kinks in the carbohydrate chains, which does not permit the phospholipids to pack as closely together, leaving more space for mobility
- Cholesterol esterase- stiff d/t its rigid planar ring, if inserted into the membrane, it will decrease membrane fluidity.
What types of movement are permitted for phospholipids in the membrane?
They can move laterally and rotate in place, but they cannot flip from one side to the other because of the hydrophobic centers
Describe the symmetry in proteins found on either side of the membrane and how it is maintained.
Proteins associated with the external and internal membrane faces are asymmetrical. For example, glycolipids are more common on the outside than the inside. This asymmetry is maintained by flipase, an enzyme responsible for transferring phospholipids from one side of the membrane to the other.
How often is the membrane turned over and how does this process occur?
The entire membrane is redone every few hours. ER and Golgi generate new membrane via budding vesicles. The extracellular surface of the membrane is the inside of the vesicle
What is the phospholipid to protein ratio comprising the membrane? What is the respective weight ratio?
50: 1
1: 1
What is a lipid raft? What protein is often associated with them?
a clumping of proteins on the membrane, often with related functions. Glycosphigoproteins are often associated.
What are the 3 main types of membrane protein?
Peripheral- attached to the outside (or inside) of the membrane
Integral- partially embedded within the membrane itself
Transmembrane- a protein that spans from one side of the membrane to the other.
What is the importance of structures like alpha helicies or beta pleated-sheets to membrane proteins
Specifically in transmembrane or integral proteins, these secondary structures provide ways to “hide” hydrophilic structures within the hydrophobic membrane center.
