What are three actions that the plasma membrane is involved in?
- receiving information
- import / export of small molecules
- capacity for movement and expansion
What does amphipathic mean?
Both hydrophilic + hydrophobic parts
ex. phospholipids have hydrophilic heads + hydrophobic tails
Examples of membrane lipids?
phosphatidylcholine (chloine group in head)
phosphatidylserine (serine group in head)
cholesterol
glycolipids
How / why do hydrophobic molecules avoid water?
molecules like to be in a less energetic state –> (i.e more entropic + disordered)
When individual hydrophobic molecules are “caged” by water molecules –> it is highly ordered and requires free energy
–> therefore, they aggregate + cluster together to minimize energy cost
Properties of phospholipids
- lipid bilayer: energetically favorable
- self-sealing: reverts to bilayer when severed (b/c free edge exposes hydrophobic tails to water)
how do membrane phospholipids move?
- lateral diffusion
- flexion
- rotation
- flip-flop (from one mono-layer to the next mono-layer)
Purpose of cholesterol
stiffen cell membranes by inserting itself between phospholipids and locking / preventing lipid movment
Process of membrane assembly in ER
- portion of membrane on ER is modified (phospholipid synthesis adds to cystolic half of bilayer)
- protein SCRAMBLASE randomly transfers phospholipids to the other monolayer)
- results in symmetric growth of lipid bilayer
What happens when vessicles fuse with membrane of other organelles?
- membrane delivered from ER
- protein FLIPPASE transfers specific phospholipids to cytosol monolayer
- Golgi apparatus + other cell membranes have asymmetric distributions
How do membrane proteins insert themselves in/on the lipid bilayer?
Transmembrane (integral membrane):
1. singlepass (signal receptors) *alpha
2. multipass (channels) *alpha
3. beta barrel (channels)
- often requires 16 beta sheets
Monolayer-associated (integral membrane)
Lipid-Linked (integral membrane)
Protein-Attached (peripheral memrbane)
(transmembrane + mono-layer associated proteins insert themselves directly into membrane)
How to isolate membrane proteins?
use detergents: SDS + TritonX-100
- detergent monomers have 1 hydrophobic tail + hydrophilic head
- separates protein from lipids in homogenous solutions (still difficult to crystalize)
Function of cell cortex
meshwork of fibrous proteins that reinforces plasma membrane
- often made of spectrin –> forms a lattice and helps maintain biconcave shape of RBC
If phospholibds move laterally, how and why do membrane proteins remain stationary?
Proteins are tethered:
1. to cell cortex
2. to extracellular matrix
3. to proteins on surface of another cell
4. diffusion barriers (ex. tight junctions)
Why?
proteins serve different functions –> maxmimized function at certain locations
ex. Ca2+ voltge gated channels are localized near the end of a nerve cell axon at presynaptic membrane
Why are cells slimy?
coated with sugars
1. glycolipid
2. glycoprotein
3. proteoglycan
