15.1 The Structure of the Plasma Membrane Flashcards
what are the main functions of a cell’s plasma membrane?
- defining the cell boundary → separates it from the environment
- serves as a selective barrier → determines the composition of the cytoplasm; allows only certain molecules through to the cytoplasm
- mediating the interactions between the cell and its environment (mostly through embedded proteins)
the fundamental structure of the plasma membrane
phospholipid bilayer
describe the electron micrograph of the phospholipid bilayer in the plasma membrane
- darkly stained electron-dense polar head groups
- bands of lightly stained hydrophobic fatty acids alternate with polar head groups
proteins embedded in the bilayer carry out functions including (1) and (2)
- selective transport
- cell-cell recognition
what are the 5 major phospholipids that make up the animal plasma membrane
Outer leaflet
1. phosphatidylcholine
2. sphingomyelin
Inner leaflet
3. phosphatidylethanolamine
4. phosphatidylserine
5. phosphatidylinositol
which PM phospholipids have an overall negative charge? neutra/no charge?
Negative: phosphatidylserine, phosphatidylinositol
Neutral: phosphatidylcholine, sphingomyelin, phosphatidylethanolamine
the overall negative charge of the inner leaflet of the plasma membrane is largely due to the predominance of ()
phosphatidylserine
() is the only major PM phospholipid that is synthesized in the Golgi
sphingomyelin
localization of phosphatidylserine in the inner leaflet plays an important role in ()
programmed cell death
In addition to the phospholipids, animal cell plasma membranes also contain:
- glycolipids (minor component)
- cholesterol (major component)
glycolipids are found exclusively on the () of the PM
outer leaflet
cholesterol exists as a major component of the PM having a () molar amount ratio as phospholipids
1:1
cholesterol is more concentrated on the (1) leaflet of the PM due to high affinity for (2)
- outer
- sphingomyelin
The two general features of lipid bilayers are:
- water-insoluble hydrophobic fatty acid chains → bilayer serves as a barrier between 2 aqueous environments by being impermeable to water-soluble molecules (ions and most biological molecules)
- viscosity (as a fluid) → lateral diffusion of lipids and proteins is possible because the membrane is soft and flexible
states that membranes are 2D fluids with proteins inserted into bilayers; proteins and lipids are able to diffuse laterally through the membrane
fluid mosaic model
the fluid mosaic model was proposed by () in 1972
Sanger and Nicolson
how was lateral movement of embedded proteins in the PM first shown?
- fusion of human and mouse cells to produce human-mouse cell hybrids
- antibodies of each cell were labeled with different fluorescent dyes to easily distinguish them
- after incubation, the differently labeled antibodies were shown to have mixed and were no longer separated from each other
what are the 2 classes of membrane-associated proteins (MAPs)?
- peripheral membrane proteins
- integral membrane proteins
() are MAPs that are indirectly associated to the plasma membrane through protein-protein interactions
peripheral membrane proteins
the bonds associating peripheral membrane proteins are usually ()
ionic
bonds associating the peripheral MAPs are easily disrupted by (1), allowing the proteins to be soluble in (2)
- polar reagents
- aqueous environments
examples of peripheral MAPs
proteins of the cortical cytoskeleton (spectrin, actin, band 4.1)
MAPs with hydrophobic portions that are directly inserted into the lipid bilayer
integral membrane proteins
integral MAPs can be dissociated from the PM only by reagents that disrupt ()
hydrophobic interactions
what characteristic of detergents allow them to disrupt the association of integral MAPs from the PM?
detergents have hydrophobic and hydrophilic portions → amphiphatic molecules that can solubilize integral proteins
integral membrane proteins are either:
- transmembrane proteins
- anchored to the plasma membrane by covalently attached lipids
these integral MAPs span the lipid bilayer with portions exposed on both sides of the membrane
transmembrane proteins
most transmembrane proteins of PM are (1) with their (2) exposed on the surface of the cell
- glycoproteins
- oligosaccharides
give 2 major examples of transmembrane protein structure (found in red blood cells):
- glycophorin
- band 3
glycophorin is a ()-pass transmembrane protein
single
band 3 is a ()-pass transmembrane protein
multiple
what are the mechanisms of proteins anchored to the PM by covalently attached lipids or glycolipids
- glycosylphosphatidylinositol (GPI) anchors (outer leaflet)
- N-terminal myristoyl group (inner leaflet)
- C-terminal (cysteine residues) prenyl group or palmitoyl group (inner leaflet)
describe the mechanism of how proteins are associated to the outer leaflet of the PM via GPI anchors
- glycosylated proteins from the ER are initially associated with the plasma membrane through their C-terminus
- C-terminus is cleaved and exchanged for GPI anchor → protein is associated to PM only through glycolipid anchor
describe the mechanism of how proteins are associated to the inner leaflet of the PM via the addition of an N-terminal myristoyl group
- addition of myristic acid (14 C fatty acid) to N terminus of polypeptides synthesized by free ribosomes
describe the mechanism of how proteins are associated to the inner leaflet of the PM via the addition of a C terminal prenyl/palmitoyl group
- addition of prenyl groups (15 or 20 C fatty acid) or palmitic acid (16 Cs) to C terminus of polypeptides synthesized by free ribosomes
carbohydrate coat formed by oligosaccharides of glycolipids and glycoproteins → covers cell surface
glycocalyx
the glycocalyx is formed from:
- heavy glycosylation of the extracellular portions of transmembrane proteins in PM
- exposure of the carbohydrate portions of glycolipids in PM on the cell surface
functions of the glycocalyx
- protects cell surface from ionic and mechanical stress
- forms a barrier to invading microorganisms
- oligosaccharides participate in cell-cell interactions
regions in which the mobility of MAPs are restricted
plasma membrane domains
what are the 2 distinct plasma domains of epithelial cells?
- apical domain
- basolateral domain
epithelial cell PM domain that faces intestinal lumen; covered by microvilli to increase surface area → efficiently facilitates nutrient absorption
apical domain
epithelial cell PM domain that faces underlying connective tissue; specialized to mediate transfer of absorbed nutrients into the circulatory system
basolateral domain
restrict PM proteins to the appropriate domains of the epithelial cell surface
tight junctions
motility of PM proteins can also be restricted as a result of association with the (1) or with (2) (location of lipid-protein interactions
- cytoskeleton
- specialized lipid domains
() are specialized lipid domains formed by clusters of sphingomyelin, glycolipids, and cholesterol
lipid rafts
the rigidity of lipid rafts is mostly due to () and its interaction with (2) to form small semisolid patches
- cholesterol
- sphingomyelin
lipid rafts are enriched in (1) and (2) that are involved in various functions
- GPI-anchored proteins
- transmembrane proteins
() are a subset of lipid rafts that start as invaginations in the PM; formation required high content of membrane cholesterol
caveolae
