1 - membrane structure

plasma membrane functions

cell communication, import/export, cell growth and motility

two membraned organelles

nucleus and mitochondria

membrane phospholipid

hydrophilic head and two hydrophobic tails

phosphatidylcholine

most common phospholipid made of:

• hydrophilic head: choline linked to phosphate

• two hydrocarbon chains: hydrophobic tails that are fatty acids (hydrocarbon chain w/ cooh grp and glycerol)

• glycerol links head to the tails

three types of membrane lipids

• phospholipid: phosphatidylcholine

• sterol: cholesterol

• glycolipid: galactocerebroside

phospholipids spontaneously close in on themselves to form sealed compartments

forms a stable structure bc it avoids the exposure of the hydrophobic hydrocarbon tails to water (energetically unfavorable)

liposomes

phospholipids that form closed spheres

membrane phospholipids move like

lateral diffusion, flexion, rotation, flip flop

• mostly move in their own monolayer

cholesterol

stiffens cell membranes by filling gaps between phospholipid tails

biosynthetic enzymes

bind to the cytosolic monolayer of the ER to make new phospholipids from free fatty acids & insert them into the cytosolic monolayer

scramblases

transporters that randomly transfer phospholipids from one monolayer to another, allowing the membrane to grow as a bilayer

flippases

transporters that selectively remove phosphatidylserine and phosphatidylethanolamine from the noncytosolic monolayer and flip them to the cytosolic side (after the membranes leave the ER and go to golgi)

• leaves phosphatidylcholine and sphingomyelin concentrated in the noncytosolic monolayer —> curvature of membrane for vesicle budding

phospholipids and glycolipids: noncytosolic monolayer

phosphatidylcholine

sphingomyelin

glycolipids

cholesterol (equal)

phospholipids and glycolipids: cytosolic monolayer

phosphatidylserine

phosphatidylethanolamine

phosphatidylinositols

cholesterol (equal)

integral membrane proteins

embedded within the lipid bilayer

peripheral membrane proteins

stay on the membranes surface but don’t penetrate it (are attached by proteins like a-helices)

transmembrane hydrophilic pore

made of 5 amphipathic transmembrane a helices to from a water-filled channel across the lipid bilayer

lateral mobility of plasma membrane proteins can be restricted by:

• tethering them to the cell cortex inside the cell

• tethering them to the EC matrix outside the cell

• tethering them to the proteins on the surface of another cell

• using diffusion barriers to restrict proteins to a specific domain

lectins

transmembrane proteins made by endothelial cells lining the blood vessel in response to chemical signals emanating from a site of infection

recognize sugar grps carried by glycolipids and glycoproteins on the surface of neutrophils