Cell Membrane Flashcards
Describe plasma cell membrane structure (11 points)
-Outer bilayered boundary of cell
-Composed of lipids and proteins
-Regulates passage of substances into and out of cell; Cell-to-cell recognition
-~7- 8 nm thick
-Inside of the bilayer – hydrophobic
-semi permeable
-Most membrane lipids are amphipathic, having a non-polar (hydrophobic) tail end and a polar (hydrophilic) head group.
-simple barrier function is modulated by the presence of membrane proteins that mediate the transport of specific molecules across this otherwise impermeable bilayer.
-hydrophobic effect and van der Waals interactions cause the tail groups to self-associate into a bilayer with the polar head groups oriented towards water.
-asymetric
- dynamic structures
Describe lipid arrangement of bilayer
- Polar hydrophilic heads - Exposed to
water inside and outside the cell - Non polar hydrophobic tails – Face each
other - Cholesterol - interspaced between molecules
Effect of cholesterol in high temp on membrane ?
cholesterol interferes with the movement of the phospholipid fatty acid chains, making the outer part of the membrane less fluid and reducing its permeability to small molecule
Effect of cholesterol in low temp on membrane ?
By interfering with interactions between fatty acid chains, cholesterol prevents membranes from freezing and maintains membrane fluidity (i.e. it maintains fluidity by preventing tight packing)
What are 3 categories of membrane proteins
Integral / transmembrane
Peripheral
Channel/carrier (transporter)
Describe structure of Integral membrane (transmembrane) proteins. Where they are found. Solubility?
-Most transmembrane proteins are glycosylated
with a complex branched sugar group attached to one/several amino acid side chains
-Span the phospholipid bilayer.
-Solubalized only by detergents/organic solvents
What are peripheral membrane proteins and the significance of their interactions ?
-Do not interact with the hydrophobic core of the phospholipid bilayer.
-Usually bound to the membrane indirectly by interactions with integral membrane proteins or directly by interactions with lipid head groups.
-Bound to hydrophilic regions of intrinsic proteins and/or polar heads of phospholipids through electrostatic and van der Waals forces.
- thus disrupted by adding salts / changing pH.
Why is it fluid?
Phospholipid Bilayer form a fluid sea (somewhat similar in consistency to vegetable oil) in which specific proteins float like icebergs.
Lipids & membrane proteins diffuse rapidly in the plane of membrane.
As temperatures cool, membranes switch from a fluid state to a solid state.
The temperature at which a membrane solidifies depends on types of lipids.
Membrane proteins DO NOT rotate across bilayer, but membrane lipids do so very slowly
Why at affects fluidity?
Membranes rich in unsaturated fatty acids are
more fluid than those rich in saturated
fatty acids.
Fluidity controlled by fatty acid composition & cholesterol content
Explain movement of membrane proteins and lipids
Membrane proteins DO NOT rotate across bilayer, but membrane lipids do so very slowly
Lateral movement occurs ~10^7 times per second.
Flip-flopping across the membrane is rare (~ once per month).
Why mosaic
Because many different proteins embedded on or within the
phospholipid bilayer
5 functions of plasma membrane ?
1) Separate cytoplasm inside the cell from the extracellular fluid outside.
2) Separate cells from each other
3) Provide a surface in which chemical reactions
can occur
4) Regulate the passage of material into and
out of the cell
5) has membrane proteins
6 functions of membrane proteins ? And. Explain
a. Membrane transport:
forming hydrophilic membrane channels or by acting as carriers that are selective for a particular solute. (Channel protein)
b. Signal Transduction:
A membrane protein may have a specific
binding site (receptor) for a chemical
messenger (signal molecule).
The signal molecule may cause a conformational
change in the protein (receptor) which relays
the message across the membrane to the
inside of the cell. (Cell surface receptor)
c) Enzymatic activity:
A membrane protein may act as an enzyme with
its active site exposed to substances in the vicinity.
d) Cell –cell recognition:
Several glycoproteins aid in recognizing other
cells.
e) Intercellular joining:
Membrane proteins of a cell may combine
with another through adherens, gap or tight junctions.
f) Attachment to the cytoskeleton and extracellular matrix :
Some components of the cytoskeleton (e. g. microfilaments) may be bound to membrane
proteins, thus helping to maintain cell shape and stabilize the location of certain membrane proteins.
4 functions of carbohydrate in membrane ?
a. Participate in cell recognition and adhesion
(cell-cell signaling or cell-pathogen
interactions).
b. Structural role as a physical barrier.
c. Blood groups antigens- cell surface
carbohydrates of red blood cells.
d. Ability to trigger immune responses.
Describe permeability of membrane ? With examples
selectively permeable barrier between the cell and its external environment
- Impermeable to polar molecules.
-But hydrophobic molecules, N, O2 can move
across.
-Also, small molecules e.g. Water, Urea, Glycerol, CO2 passes through
Gases ( co2 nd o2) , hydrophobic molecules (benzene) , small polar molecules ( h20 and ethanol) pass thru.
Large polar molecules ( glucose ) and charged molecules ( amino acids, ions like cl-, ca2+, na+, H+) don’t pass
