Cell membranes Flashcards
define cell membranes
boundaries of cells
act as barriers defining the inside and outside of the cell
cell membranes prevent
imp molecules leaking out
unwanted molecules diffusing in
what do cell membranes contain to allow specific molecules to be taken up or removed
transport systems
transport systems confer on membranes
selective permeability (imp membrane property)
describe cell membranes
- sheet like (2 molecules thick)
- mainly lipids (form permeability barrier) and proteins (acts as a transport system of pumped channels - specific proteins mediate specific membrane functions)
- fluid structures
- lipids + proteins can diffuse rapidly in plane of the membrane
what is the relevance of fatty acids
- hydrocarbon chains
- hydrophobic properties
- can be saturated or unsaturated
give an example of
a) saturated fatty acid
b) unsaturated fatty acid
a) palmitate
- 16 carbon
- ionised form of palmitic acid
b) oleate
- 18 carbon
- 1 cis double bond between carbons 9 + 10
- cis-Δ 9octadecanoate
what are phospholipids
- major class of lipids (common membrane lipid)
- abundant in all biological membranes
- constructed from:
fatty acids (provide hydrophobic barrier)
platform fatty acids = attached to
a phosphate with an alcohol attached - non fatty acid components have hydrophilic properties
what is the name of phospholipids based on / derived from glycerol
name the simplest one
phosphoglycerides
phosphatidate
major phosphoglycerides are derived from
phosphatidates
- ester bond forms between phosphate group of phosphatidate and hydroxyl group of an alcohol
list some common alcohol moieties of phosphoglycerides
- amino acid serine
- ethanolamine
- choline
- glycerol
- inositol
what is sphingomyelin
- phospholipid in membranes
- not derived from glycerol BUT…
- has sphingosine backbone (amino alcohol w long unsaturated hydrocarbon chain)
glycolipids are
- sugar containing lipids
- derived from sphingosine in animal cells
- orientated asymmetrically with sugar residues on extracellular side of membrane
cholesterol is
lipid steroid (membrane lipid based on a steroid nucleus)
what is the structure of cholesterol
- 4 linked hydrocarbon rings linked to a steroid at one end and hydroxyl group at the other
- oriented parallel to fatty acid chains of phospholipids
- hydroxyl group interacts with nearby phospholipid heads
where is cholesterol present / absent
- NOT in prokaryotes
- found in varying degrees in most animal membranes
- almost 25% of membrane lipids in some nerve cells
- almost absent from some intracellular membranes
what type of molecules are membrane lipids and what does this mean
AMPHIPATHIC
contain both a hydrophilic and hydrophobic moiety
what is the favoured structure of phospholipids and what does this enable
bimolecular sheet (formed in an aqueous environment)
- allows membranes to form due to amphipathic nature as polar heads favour water and hydrocarbon tails interact with each other
what are the 3 biological consequences of the many reinforcing, non-covalent, hydrophobic interactions holding lipid bilayers together (makes them cooperative structures)
1) bilayer = extensive
2) lipids tend to close in on themselves so there are no edges with exposed hydrocarbon chains + so form compartments
3) lipid bilayers are self-sealing because a hole in the bilayer would be energetically unfavourable
what 2 categories can membrane proteins be split into
1) integral membrane proteins (intrinsic)
2) peripheral (extrinsic)
describe integral membrane proteins
- 1+ segments embedded in the bilayer (transverse it - transmembrane proteins)
- interact with bilayers hydrocarbon region
describe peripheral proteins
- dont interact with hydrophobic core of bilayer
- bound to membrane indirectly through interactions with integral membrane proteins
- bound directly through interactions with polar head groups of integral lipids
what can proteins span the membrane with
alpha helices
by integral proteins containing membrane spanning alpha helical domains (these are the most common structure motifs in membrane proteins)
what is bacteriorhodopsin
- consists largely of membrane spanning alpha helices embedded in membrane
- amino acids in these = non-polar
what is bacterial porin
- porin = membrane protein built entirely of beta strands that have hydrophobic + hydrophilic amino acids in adjacent positions
- each beta strand = H bonded to its neighbour in anti-parallel arrangement forming a single beta sheet
- beta sheet = curls up to form hollow cylinder that functions as the active unit
- SO form pores / channels in the membrane
what does the amino acid sequence of a porin demonstrate
- amino acids are adjacent
- diagonal lines indicate direction of H bonding along beta sheet
- hydrophobic residues (yellow) lie on outside of structure in contact with membranes hydrophobic core
what is prostaglandin H2 synthase-1 and where is it held
- membrane bound enzyme
- integral membrane protein BUT NOT a membrane spanning protein
- in the membrane by a set of alpha helices coated with hydrophobic amino acid side chains that extend from the bottom of the protein into membrane
- not largely embedded in the membrane
- lies along membranes outer surface + is bound by the set of alpha helices
what is converted to prostaglandin H2 and how
arachidonic acid
by prostaglandin H2 synthase-1
how can prostaglandin H2 synthase-1 be released from the membrane (strong link)
ONLY by the action of detergents
how can we predict transmembrane helices
from amino acid sequences and their free energy changes for transfer of individual amino acid residues from hydrophobic to an aqueous environment
ie amino acids showing -ve values = more likely located in an aqueous environment
when is free energy change estimated
when a helical segment is transferred from the interior of a membrane to water
which membrane part is mostly non-polar residues
residues in alpha helices which span hydrophobic part of the membrane
what is glycophorin
membrane spanning helix
how can glycophorin be located
create hydropathy plot based on free energy values of transferring amino acid residues from hydrophobic environment to water (predicts single transmembrane domain)
- hydrocarbon core of a membrane is typically a length that can be transversed by an alpha helix
- free energy change measured when hypothetically alpha helix formed of residues transferred from membrane interior to water
what is the essence of the fluid mosaic model
- allows lateral movement but not rotation through the membrane
- membranes are 2D solutions of oriented lipids and globular proteins
what type of role does the lipid bilayer have, explain this
DUAL
its both
a) solvent for integral membrane proteins
b) permeability barrier
which membrane processes
a) can be rapid
b) are very slow
a) lateral diffusion of membrane components
b) transverse diffusion / flipflop - spontaneous rotation of lipids from one face of a membrane to another
what is membrane fluidity controlled by
- fatty acid composition
- cholesterol content (key regulator in animals)
how does cholesterol control membrane fluidity
- different shape to phospholipid so disrupts regular interactions between fatty acyl chains
- forms complexes which concentrate in specific regions in membranes with some phospholipids
- result = moderation of membrane fluidity (less fluid + less subject to phase transitions)
which fatty acid properties influence fluidity
- number of double bonds
- length
what do cis double bonds do and give an example
- disrupt ordered packing of fatty acid chains
- saturated fatty acids pack more tightly as do not have a kink in their structure
ie
- 3 molecules of stearate (C18, saturated) lie flat
- 1 oleate (C18, unsat) molecule between 2 stearate disrupts the layers
how are prokaryote cell membranes different
many bacteria have 2 membranes (ie e.coli)
others have 1
how can we classify bacteria based on cell membrane number
gram stain
1) 2 membranes = -ve result
2) 1 membrane = +ve result
how do animal cells take up cholesterol
receptor mediated endocytosis for cholesterol carrying complex LDL (low density lipoprotein - way that cholesterol is transported in the blood as cholesteryl ester)
what are the stages of receptor mediated endocytosis
1) ldl binding = ldl binds to specific ldl receptor
2) internalisation = the complex invaginates to form internal vesicle
3) after seperation from receptor, ldl containing vesicle fuses with lysosome
4) lysosomal hydrolysis = leading to degradation of ldl and cholesterol release
where is the reverse process of receptor mediated endocytosis key
key step in release of neurotransmitters from a neuron (fusion of vesicle to a membrane)
what is cholesterol taken up by receptor mediated endocytosis used for
to make new membrane (this method provides most of the cholesterol needed for this process)
what are some active and passive transport methods
- passive diffusion - small molecules dissolve in lipid bilayer
- many proteins require protein transporters
- active transport - ATP hydrolysis to pump ions across, creates a concentration gradient
- energy for transport can be generated by ion gradients
give an example of an enzyme which generates a gradient (active transport)
Na+/K+ ATPase pump
hydrolyses ATP providing energy needed for active transport
- 3 Na+ OUT
- 2 K+ IN
- generates a gradient
what are the 6 stages of P-type ATPase action
1) binding
2) phosphorylation of ATPase
3) leads to eversion of binding sites
4) release of 2Ca2+ to luminal side of membrane
5) hydrolysis of phosphoasparate
6) eversion - resets enzyme to initial state
what do P-type ATPases have the ability to do
transport lipid molecules across membrane
by flippase enzymes - maintain membrane asymmetry by ‘flipping’ phospholipids from outer to inner layer of membrane
what is MDR and what is it due to
multidrug resistance protein
expression and activity of a membrane protein which acts as an ATP dependent pump extruding small molecules from cells expressing it
what do amino acid sequences of MDR and related proteins (ie CFTR) reveal
- each comprises 2 membrane binding domains AND 2 ATP binding domains (ATP-binding cassettes)
- MDR, CFTR + ABC transporters are single polypeptide chains
what is the action of histidine permease
hydrolyses ATP which drives transport of histidine into the cell
what do secondary transporters do, give 2 examples
use one concentration gradient to power the formation of another
1) antiporters
2) symporters
what happens in antiporters and give an example of one
2 species flow in opposite directions
ie Na/Ca exchanger uses electrochemical gradient of Na to pump Ca out
what happens in symporters and give an example of one
2 species move in same direction
ie glucose symporter uses Na entry to power glucose entry to cells
how does the Na+/K+ pump use action of a secondary transporter
- converts free energy of phosphoryl transfer into free energy of Na+ gradient (energy transduction)
- Ion gradient used to pump materials into cell through secondary transporter (eg sodium glucose symporter)
