Biological Membranes Flashcards
Due to the amphipathic nature of phospholipids, different structures are formed ?
In water, including a surface film, micelles, bilayers and liposomes/vesicles
Non-polar portions aggregate so that ?
Fewer water molecules are ordered and entropy increases
What part is exposed ?
Only polar “head groups” are exposed
This hydrophobic effect powers ?
Membrane formation, and van der Waals interactions between the hydrophobic tails stabilise membranes
All cells have a cell membrane, which separates ?
The cell from its surroundings: sheet-like fluid structure, 30–100 Å (3–10 nm) thick
Main structure is composed of ?
Two leaflets of lipids (bilayer)
- Except archaebacteria: monolayer of bifunctional lipids
Eukaryotic cells have various ?
Compartmentalising internal membranes
Protein molecules span the lipid bilayer, mitigating?
Permeability and the transfer of information
what does an asymmetric membrane mean ?
The outer surface is always different from the inner surface
Some lipids are found more commonly ?
Inside and some outside
Carbohydrate moieties are attached ?
On the carbohydrate
They are usually electrically polarised due to ?
Ion gradients across the membrane
What are functions of membranes ?
- Define the cell boundaries and provide compartmentalisation within it
- Allow import and export, e.g. of nutrients and waste
- Sense external signals and transmit information into the cell
- Separate energy-producing reactions from energy-consuming ones
- Keep proteolytic enzymes away from important cellular proteins
- Produce and transmit nerve signals
- Store energy as a proton gradient
- Support synthesis of ATP
Some bacteria are surrounded by ?
Two membranes, with a cell wall lying between the
Some bacteria are enclosed by ?
A single membrane surrounded by a thick cell wall
What is Periplasm ?
The space between the two membranes
Bacteria do not have ?
Internal membrane-bound organelles
What three major classes of membrane do eukaryotic cells contain ?
- The plasma membrane, separating cell from environment
- Endomembrane system, transporting components through cytoplasm
- Mitochondrial membrane (+chloroplast in plants), which are energy conversion factories
Explain the Fluid Mosaic Model ?
- Proposed in 1972 by Singer and Nicholson
- Lipids form a viscous, two-dimensional solvent into which proteins are inserted and integrated more or less deeply
- Proteins can either be embedded in or associated with the membrane
What are three main classes of membrane lipids?
All amphiphatic:
- Phosphoglycerides (most abundant)
- Glycolipids
- Sterols
Phospholipids are composed of ?
Four components: one or two fatty acid tails, a glycerol or sphingosine platform, a phosphate, and an alcohol
Glycolipids are ?
Carbohydrate-containing lipids derived from sphingosine
Cholesterol is a ?
Sterol that is modified on one end by the attachment of a fatty acid chain and at the other end by a hydroxyl group
What is Phosphatidylcholine ?
This is the major component of most eukaryotic cell membranes
What does it consist of ?
It consists of a hydrophilic head (choline + phosphate), two hydrophobic tails (fatty acid chains) and a connecting glycerol
Many prokaryotes, including E. coli, cannot ?
Synthesise this lipid; their membranes do not contain phosphatidylcholine
How are phospholipids and glycolipids distributed?
They are distributed asymmetrically in the lipid bilayer of an animal cell plasma membrane: note that they retain their orientations during transfer between cell compartments
E.g. phosphatidylcholine and sphingomyelin are concentrated in ? whereas phosphatidylserine is concentrated in ?
Phosphatidylcholine and sphingomyelin are concentrated in the noncytosolic monolayer, whereas phosphatidylserine is concentrated in the cytosolic monolayer
Glycolipids are found exclusively in ?
In the non-cytosolic monolayer
Cholesterol is evenly ?
Distributed
Cells synthesise new membranes by ?
The expansion of existing membrane
Phospholipids are made in?
The ER membrane
During trafficking, what happens ?
During trafficking, both the lipid composition of the bilayer and the disposition of specific lipids between inner and outer leaflets change considerably
Protein content and lipid content, and types of sterol can all vary between?
Species
Bacteria do not make ?
Sterols
The functional specialisation of each membrane type in a cell is reflected in ?
Its unique lipid composition
Lipid composition affects ?
Membrane fluidity
Archaeal membranes are built from ?
Ether lipids with branched chains
Better membrane stability by ether linkages and branching
structures prevent ?
Hydrolysis and oxidation of membranes
The lipid bilayer can be in what form ?
Gel or fluid phase, dependent on composition and temperature
What does heating cause?
Phase transition from the gel to fluid
Under physiological conditions, membranes are more ?
Fluid-like than gel-like (necessary for function)
Explain Liquid-ordered state (i.e., “gel phase”):
Individual molecules do not move around
Explain Liquid-disordered state: (i.e., “fluid phase”):
Individual molecules can move around
Define melting temperature (Tm) ?
The temperature at which a membrane transitions from being highly ordered to very fluid
What is Tm dependent on?
Tm is dependent on the length of the fatty acids in the membrane lipid and the degree of cis unsaturation.
What does a higher and a lower temperature mean ?
- higher °C: need more long, saturated fatty acids
- lower °C : need more unsaturated fatty acids
What does cholesterol help to maintain ?
Cholesterol helps to maintain proper membrane fluidity in animals
How can Protein content and lipid content, and types of sterol vary ?
Between species
What does bacteria not make ?
Sterols
The functional specialisation of each membrane type in a cell is reflected in its ?
Unique lipid composition
What does lipid composition affect ?
Membrane fluidity
How are Archaeal membranes built ?
From ether lipids with branched lipids
What are the different membrane protein and what do they interact with?
- Integral membrane proteins interact with the lipid bilayer Transmembrane and Monolayer-associated
- Peripheral membrane proteins interact with the polar head groups of the lipids (amphitrophic/lipid-linked) or bind to the surfaces of integral proteins
Spontaneous flips from one layer to another are rare because ?
The charged head group must cross the hydrophobic tail region
“Flip-flop” diffusion of specific lipids is catalysed by ?
Enzymes such as flippases, floppases and scramblases, depending on direction of movement
What is the use of Fluorescence recovery after photobleaching (FRAP) ?
It allows us to monitor lateral lipid diffusion by monitoring the rate of fluorescence return
FRAP allows us to determine ?
The diffusion coefficient of a lipid/protein in the leaflet by plotting a graph of the rate of fluorescent recovery
Biological regulation results in ?
Attachment to, or cleavage from, lipids
Amphitropic proteins are conditionally attached to the membrane by ?
Covalent interaction with lipids or carbohydrates attached to lipids
Cytosolic proteins may be anchored via ?
Acylation or Prenylation
Extracellular proteins can be anchored by ?
A GPI lipid anchor
What are some examples of transmembrane proteins ?
- Bacteriorhodopsin: Crosses the membrane as seven alpha-helices (a common motif) and acts as a proton pump
- Porin: consists of a 16-stranded β sheet curved around on itself to form a transmembrane water-filled channel. Three porin proteins associate to form a trimer with three separate channels (not shown)
Biological regulation results in ?
Attachment to, or cleavage from, lipids
Transmembrane proteins are usually composed of ?
Alpha helices
Explain what the hydrophobic residues and hydrophilic parts of the polypeptide backbone do?
Hydrophobic residues interact with the lipid bilayer, while hydrophilic parts of the polypeptide backbone form hydrogen bonds in the helix interior
An α helix containing about 20 amino acids is required to ?
Completely travel through (traverse) a cell membrane: multiple helices commonly form a pore/channel
Hydropathy plots can predict ?
Hydrophobic Domains
Detergents are amphipathic molecules that disrupt ?
Membranes by intercalating into phospholipid bilayers
Og / TX-100 are gentle detergents used to ?
Solubilise and reconstitute functional membranes.
SDS is a strong detergent that will ?
Unfold proteins as well
Membrane proteins and phospholipids in the outer leaflet are ?
Glycosylated, having short chains of sugars called oligosaccharides covalently linked to them
Caveolin forces ?
Inward curvature of a membrane
Stable associations of sphingolipids and cholesterol in the outer leaflet produce ?
- A microdomain, slightly thicker than other membrane regions, that is enriched with specific types of membrane proteins
- May function in signal transduction
Inwardly curved rafts called? and are enriched in what?
Inwardly curved rafts called caveolae are especially enriched in the protein caveolin
Define Caveolae ?
Small invaginations of plasma membrane
What influences the passive transport of charged solutes ?
Both the concentration gradient and membrane potential
Explain with example what endocytosis and exocytosis do ?
Larger proteins and signaling molecules may be internalised in vesicles (endocytosis) and neurotransmitters are released via vesicles (exocytosis)
