w7 txtbk (CH 11) Flashcards
plasma membrane
protein-containing lipid bilayer that surrounds a living cell (self-healing)
-serves as a barrier to prevent the contents of the cell from escaping and mixing with molecules in the surrounding environ.
to facilitate exchange, the membrane is penetrated by highly selective channels and transporters- proteins that allow specific, small molecules/ion to be imported/exported
how does the cell membrane function as a barrier?
Regardless of their location, all cell membranes are composed of lipids and proteins and share a common general structure
The lipids are arranged in 2 closely apposed sheets, forming a lipid bilayer
Serves as a permeability barrier to most water-soluble molecules, while the proteins embedded within it carry out the other functions of the membrane
lipid bilayer structure and function
The structure of cell membranes is determined by the way membrane lipids behave in a watery (aq) environment
Lipid molecules are not very soluble in water, although they do dissolve readily in organic solvents (e.g. benzene)
Lipid bilayer is a thin pair of closely juxtaposed sheets, composed mainly of phospholipid molecules, that forms the structural basis for all cell membranes
Membrane lipids form bilayers in water
The most abundant lipids in cell membranes are the phospholipids, which have a phosphate-containing, hydrophilic head linked to a pair of hydrophobic, hydrocarbon tails
hydrophilic
Hydrophilic molecules dissolve readily in water because they contain either charged groups or uncharged polar groups that can form electrostatic attractions or hydrogen bonds with water molecules
hydrophobic
Hydrophobic molecules are insoluble in water because almost all atoms are uncharged and nonpolar; they cannot form favourable interactions with water molecules
Instead, they force adjacent water molecules to reorganize into a cage like structure around them
Because this cage like structure is more highly ordered than the rest of the water, its formation requires free energy
The energy cost is minimized when the hydrophobic molecules cluster together, limiting their contacts with the surrounding water molecules
Purely hydrophobic molecules coalesce into large fat droplets when dispersed in water
amphipathic part of membrane
Amphipathic molecules are subject to 2 conflicting forces: the hydrophilic head is attracted to water, while the hydrophobic tails shun water and seek to aggregate with other hydrophobic molecules
–the same forces that drive the amphipathic molecules to form a bilayer help to make the bilayer self-sealing
–any tear in the sheet will create a free edge that’s exposed to water
Because this situation is energetically unfavourable, the molecules of the bilayer will spontaneously rearrange to eliminate the free edge
–if tear is SMALL, this spontaneous rearrangement will exclude the water molecules and lead to repair of the bilayer, restoring a single sheet
–if tear is LARGE, the sheet may begin to fold in on itself and break up into separate closed vesicle
consequence of removing free edges
The only way an amphipathic sheet can avoid having free edges is to bend and seal, forming a boundary around a closed space
Therefore, amphipathic molecules such as phospholipids necessarily shape into self-sealing containers that define closed compartments
By-product of the nature of membrane lipids: hydrophilic at one end and hydrophobic at other end
flip flops
In synthetic lipid bilayers, phospholipid molecules rarely tumble from one half of the bilayer, or monolayer, to the other
Without proteins to facilitate this energetically unfavourable movement, it’s estimated that such “flip-flops” occur less than once a month for any individual lipid molecule under conditions similar to a cell
As the result of random thermal motions, lipid molecules continuously exchange places with their neighbours within the same monolayer
This exchange leads to rapid lateral diffusion of lipid molecules within the place of each monolayer
unsaturated
The chain that harbors a double bond does not contain the maximum number of hydrogen atoms that could, in principle, be attached to its carbon backbone; it is thus said to be unsaturated with respect to hydrogen
saturated
The hydrocarbon tail with no double bonds has a full complement of hydrogen atoms and is said to be saturated
why do membranes contain more unsaturated hydrocarbon tails?
Each double bond in an unsaturated tail creates a small kink in the tail, which makes it more difficult for the tails to pack against one another
For this reason, lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid than those with lower proportions
cholesterol
In animal cells, membrane fluidity is regulated by the inclusion of the sterol cholesterol
With its short and rigid steroid ring structure, cholesterol can fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated hydrocarbon tails
In this way, cholesterol can stiffen the bilayer, making it less flexible, as well as less permeable
importance of membrane fluidity
-allows many membrane proteins to diffuse rapidly in the plane of the bilayer and interact (cell signalling)
-permits membrane lipids and proteins to diffuse from sites where they’re inserted into the bilayer after their synthesis to other regions of the cell
how do new phospholipids make it to the opposite monolayer?
flip-flops that move lipids from one monolayer to the other rarely occur spontaneously
phospholipids are transferred by a protein called SCRAMBLASE, type of transporter protein that randomly removes selected phospholipids from 1/2 of lipid bilayer and inserts them in the other
as a result of this scrambling, newly made phospholipids are redistributed equally b/w each monolayer of the ER membrane
function of flippases
Use the energy of ATP hydrolysis to transfer specific phospholipids from one side of the bilayer to the other
–including moving selected lipids from the monolayer facing the exterior space to that facing the cytosol
Initiates and maintains the asymmetric arrangement of phospholipids that is characteristic of the membranes of animal cells
–this asymmetry is preserved as membranes bud from one organelle and fuse with another/ or w/ PM
all cell membranes have distinct faces
cytosolic monolayer always faces cytosol
noncyto. monolayer exposed to either the cell exterior (PM) or the interior space (lumen) of organelle
once a glycolipid molecule has been created and confined in PM…
it remains trapped in this monolayer, as there are NO flippases that transfer glycolipids to the cytosolic side
thus, when a glycolipid molecule is finally delivered to the PM, it displays its sugars to the exterior of cell
which group in a membrane phospholipid carries negative charge
phosphate group in the middle of the top 3 groups
membrane proteins
a protein associated with the lipid bilayer of a cell membrane
function of transporters
actively pumps Na+ out of cells and K+ in
function of ion channels
allows K+ ions to leave cells
functions of anchors
link intracellular actin filaments to extracellular matrix proteins
function of receptors
binds extracellular platelet-derived growth factor (PDGF) and consequently, generates intracellular signals that direct the cell to grow and divide
function of enzymes
catalyzes the production of the small intracellular signaling molecule cyclic AMP in response to extracellular signals
integral membrane proteins
-proteins that are directly attached to the lipid bilayer (whether they’re transmembrane, associated w/ lipid monolayer, or lipid-linked) can be removed only by disrupting the bilayer with detergents
peripheral membrane proteins
can be released from the membrane by more gentle extraction procedures that interfere with protein-protein interactions but leave the lipid bilayer intact
when is hydrogen bonding maximized in membrane
if the polypeptide chain forms a regular alpha helix, and so the great majority of the membrane-spanning segments of PP chains travel the bilayer as alpha helices
detergents
used to solubilize lipids and membrane proteins
-amphipathic molecules that only have a single hydrophobic tail
membrane domains
functionally and structurally specialized region in the membrane of a cell/organelle; typically characterized by the presence of specific proteins
how can the lateral mobility of PM proteins be restricted
Proteins can be tethered
-to the cell cortex inside cell
-to extracellular matrix molecules outside cell
-to proteins on surface of another cell
-Diffusion barriers can restrict proteins to a particular membrane protein
FRAP fluorescence recovery after photobleaching
involves uniformly labeling the components of the cell membrane with some sort of fluorescent marker
-the rate of this discovery is a direct measure of the rate at which the protein molecules can diffuse within the membrane
membrane proteins in artificial lipid bilayers vs cell membranes
membrane proteins diffuse more freely and rapidly in artificial lipid bilayers than in cell membranes
-most proteins show reduced mobility in cell membrane
