UNIT 1.3 Flashcards
What is the structure of a phospholipid?
Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic
What is the arrangement of phospholipids 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.
What are the properties of the phospholipid bilayer?
The bilayer is held together by weak hydrophobic interactions between the tails
Hydrophilic / hydrophobic layers restrict the passage of many substances
Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility
This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)
What are the typical differences between Integral and Peripheral proteins ?
- Integral proteins are permanently attached to the membrane and are typically transmembrane (they span across the bilayer).
- Peripheral proteins are temporarily attached by non-covalent interactions and associate with one surface of the membrane
Outline the structure of membrane proteins
The amino acids of a membrane protein are localised according to polarity:
Non-polar (hydrophobic) amino acids associate directly with the lipid bilayer
Polar (hydrophilic) amino acids are located internally and face aqueous solutions
TRANSMEMBRANE proteins typically adopt one of two tertiary structures:
-Single helices (Recognition,Receptors) / helical bundles (Enzymes,transporters,Receptors)
Beta barrels (common in channel proteins/ Transporters)
INTEGRAL PROTEIN FUNCTIONS;
Enzymes,Anchorage, Transporters (Carriers).
Outline the functions of Membrane Proteins
JET RAT
Junctions – Serve to connect and join two cells together
Enzymes – Fixing to membranes localises metabolic pathways
Transport – Responsible for facilitated diffusion and active transport
Recognition – May function as markers for cellular identification
Anchorage – Attachment points for cytoskeleton and extracellular matrix
Transduction – Function as receptors for peptide hormones
Cholesterol is only present in animal cells, why do plant cells not need it ?
plant cell plasma membranes are surrounded and supported by a rigid cell wall made of cellulose.
What is the structure of Cholesterol ?
Amphipathic
Cholesterol’s hydroxyl (-OH) group is hydrophilic and aligns towards the phosphate heads of phospholipids
The remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic and associates with the phospholipid tails.
What section of the phospholipid bilayer does Cholesterol interact with ?
What does it moderate?
The cholesterol interacts with the fatty acid tails of the bilayer.
FUNCTIONS;
-Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity.
-It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross.
-It functions to separate phospholipid tails and so prevent crystallisation of the membrane.
-It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein.
Why is the plasma membrane so called the ‘Fluid Mosaic Model’ ?
Fluid – the phospholipid bilayer is viscous and individual phospholipids can move position
Mosaic – the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components
What are the components of the Plasma Membrane?
Phospholipids – Form a bilayer with phosphate heads facing outwards and fatty acid tails facing inwards
Cholesterol – Found in animal cell membranes and functions to improve stability and reduce fluidity
Proteins – May be either integral (transmembrane) or peripheral and serve a variety of roles
What did the Davson Danielli model propose the plasma membrane consisted of ?
A lipo protein sandwich, whereby two protein layers flanked a central phospholipid bilayer.
What were the problems with the Davson Danielli model ?
It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio
It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)
It did not account for the permeability of certain substances (did not recognise the need for hydrophilic pores)
The temperatures at which membranes solidified did not correlate with those expected under the proposed model
What was the falsification evidence for the Davson Danielli model ?
-Membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size; such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane.
-Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place;
-Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively.
-When the two cells were fused, the markers became mixed throughout the membrane of the fused cell
This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli).
-Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane; these rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure
