membrane structure Flashcards
Phospholipids
phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules
structure of phospholipids
- polar head - hydrophilic polar head that is composed of a glycerol and a phosphate molecule
- non polar tails - hydrophobic two non polar tails composed of fatty acid (hydrocarbon) chains
- amphipathic - contains both hydrophilic and lipophilic regions
arrangement in membranes
phospholipids spontaneously arrange into a bilayer. the hydrophobic tail regions face inwards and are shielded from the surrounding polar fluids, while the two hydrophilic head regions associate with the cytosolic and extracellular fluids respectively.
properties of the phospholipid bilayer
- bilayer held together by weak hydrophobic interactions
- Hydrophobic/hydrophilic layers restrict the passage of many substances
- individual phospholipids can move within the bilayer
- fluidity allows spontaneous breaking and reforming of membranes
membrane proteins
phospholipid bilayers are embedded with proteins which may either be permanently or temporarily attached to the membrane
two proteins at membrane
- integral proteins - permanently attached to the membrane and are typically transmembrane
- Peripheral proteins - temporarily attached by non covalent interactions and associate with one surface of the membrane
phospholipid bilayer
Phospholipid bilayers are embedded with proteins, which may be either permanently or temporarily attached to the membrane
Structure of Membrane proteins
amino acids of a membrane protein are localised according to polarity; Non polar (hydrophobic) and polar (hydrophilic)
location of amino acids (polar and non polar)
1) Non-Polar amino acids associate directly with the lipid bilayer
2) Polar amino acids are located internally and face aqueous solutions
tertiary structure if Transmembrane
1) Single helices (helical bundles)
2) Beta barrels (common in channel proteins)
Integral Proteins
1) a-helix (Recognition, receptors)
2) Helical bundle (Enzymes, transporters, receptors)
3) b-barrel (Transporters, channel proteins)
Peripheral proteins
1) Enzymes
2) Anchorage
3) Transporters (carriers)
Functions of Membrane Proteins
1) Junctions - connect and join 2 cells together
2) Enzymes - Fixing membranes to localise metabolic pathways
3) Transport - facilitated diffusion and active transport
4) Recognition - markers for cellular identification
5) Anchorage - attachment points for cytoskeleton and extracellular matrix
6) Transduction - receptors for peptide hormones
Cholesterol
Cholesterol is a component of animal cell membranes where it functions to maintain integrity and mechanical stability (absent in plant cells since they have a rigid cell wall made of cellulose)
cholesterol molecule type
amphipathic molecule (both hydrophobic and hydrophilic regions)
- Cholesterol hydroxyl is hydrophilic and aligns towards the phosphate heads of phospholipids
- the remainder of the molecules is hydrophobic and associates with the phospholipid tails
Cholesterol function
1) interacts with the fatty acid tails of phospholipids to moderate its properties
2) immobilises the outer surface of the membrane, reducing fluidity
3) makes membrane less permeable to very small water soluble molecules
4) separate phospholipid tails and prevent membrane to crystallise
5) helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein
cell membranes respresented through fluid mosaic model du to:
1) being Fluid - phospholipid bilayer is viscous and individual phospholipids can move position
2) being Mosaic - phospholipid bilayer is embedded with proteins, resulting in a mosaic of components
components of the plasma membrane
1) Phospholipids - Form a bilayer with phosphate heads facing outwards and fatty acid tails facing inwards
2) Cholesterol - found in animal cell membranes and functions to improve stability and reduce fluidity
3) Proteins - May be either integral (transmembrane) or peripheral and serve a variety of roles
Danielli and Davson model
proposed in 1935, model whereby two layers of protein flanked a central phospholipid bilayer. Lipid layer was sandwiched between two protein layers.
problems of the Danielli and Davson model
1) constant lipid-protein ratio
2) all membranes had symmetrical surfaces
3) did not account permeability of certain substances
4) temperature was inaccurate
Singer-Nicolson Model
promoted in 1972, model preferred by scientist (accurate). Known as the fluid mosaic model
