1.3 Membrane Structure Flashcards
Some substances are attracted to water –
hydrophilic (or polar)
Some molecules are not attracted to water –
hydrophobic (non-polar)
Phospholipids are amphipathic molecules:
Polar head (hydrophilic) containing glycerol and a phosphate molecule – a.k.a. phosphate head
Two non-polar tails (hydrophobic) made of fatty acid chains (hydrocarbons) – a.k.a. hydrocarbon tail
When put into water, an emergent property of phospholipids is that
they will self-organize to keep their heads “wet” and tails “dry”
Phospholipids spontaneously arrange into a
bilayer when in water
Phospholipid molecules can flow past each other laterally but
cannot move vertically
This fluidity allows for spontaneous breaking and reforming of membranes
Cell division
Releasing vesicles (exocytosis)
Taking in vesicles (endocytosis)
Gorter and Grendel (1920’s)
The Model: plasma membrane are composed of a lipid bilayer
Evidence: analyzed red blood cells
Extracted phospholipids from the plasma membrane
Calculated that there were twice as many phospholipids than would be needed for monolayer —> must be a bilayer
Many errors to methods, but luckily cancelled out to still get “true” results.
Model did not account for proteins
Davson-Danielli (1930’s)
The Model: “sandwich” model in which the lipid bilayer is “sandwiched” by protein molecules
Lipid bilayer composed of phospholipids whose outer surface are coated by proteins
Proteins do not permeate the lipid bilayer
Evidence: electron micrographs showed “railroad track” appearance
Proteins appear dark and phospholipids appear light
This explains: despite being very thin membranes are an effective barrier to the movement of certain substances.
Assumptions of the Davson-Danielli model
All membranes were the same thickness and would have a constant lipid-protein ratio
All membranes would have symmetrical internal and external surfaces
Freeze-etched electron micrographs
Rapid freezing of cells and fracturing them
Evidence:
Fracture reveals an irregular rough surface inside the phospholipid bilayer
Able to view “globular structures” interpreted as transmembrane proteins
Conclusion
Refutes the point that they are only found on the outside of the membrane
Structure of membrane proteins
Improvement in biochemical techniques allowed proteins to be extracted from membranes
Evidence
Proteins extracted found to be different in size and shape
Conclusion
Refutes uniform/continuous layer hypothesis
Evidence
Proteins found to all have hydrophobic section –> would embed in “tails”
Conclusion
Refutes proteins found only on outer layer
Fluorescent antibody tagging
Red or green fluorescent markers attached to antibodies which would bind to membrane proteins
The membrane proteins of some cells were tagged with red markers and other cells with green markers.
Evidence
The cells were fused together.
Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell.
Conclusion
Membrane proteins are free to move within the membrane rather than being fixed in a peripheral layer.
Singer-Nicolson fluid mosaic model
Our current model of the cell membrane
This model was first proposed in by Singer-Nicolson in 1972
Integral Proteins
Exposed to aqueous environments on both sides, used to transport molecules across the membrane
Partially hydrophobic and embed in the centre of the membrane
Monotopic or polytopic (transmembrane)
Peripheral Proteins
Hydrophilic – located on surface
Some attached to integral protein
Some attached to hydrocarbon chain that anchors to membrane
Monotopic
Glycoproteins
proteins with an oligosaccharide (oligo = few, saccharide = sugar) chain attached
They are important for cell recognition by the immune system and as hormone receptors
Protein content of a membrane varies depending on the function of the cell
Myelin sheath around nerve cells insulate —> 18% protein content
Most plasma membranes —> 50%
Mitochondria (cell respiration) and chloroplasts (photosynthesis) —> 75%
Functions of membrane proteins
Hormone binding sites (receptors)
Enzymatic activities (e.g. ETC)
Cell adhesion
Cell-to-cell communication/ recognition
Channels for passive transport
Pumps for active transport
Animal cells also contain
cholesterol (type of lipid)
Hydroxyl group (-OH) makes
the head polar and hydrophilic - attracted to the phosphate heads on the outer layer of the membrane.
Carbon rings
– it’s not classed as a fat or an oil, cholesterol is a steroid
Non-polar (hydrophobic) tail
– attracted to the hydrophobic tails of phospholipids in the centre of the membrane
cholesterol amount varies in animal cells
Membranes of vesicles that hold neurotransmitters at synapses (space between neurons) 🡪 30% content
Overall the membrane is…
fluid —> components can move freely
It is important to regulate the degree of fluidity:
Fluid enough that the cell can move
Fluid enough that the required substances can move across the membrane
If too fluid however the membrane could not effectively restrict the movement of substances across itself
Cholesterol’s role in membrane fluidity
Reduces membrane fluidity by restricting motion of phospholipids molecules
Disrupts tight packing of hydrophobic tails in the bilayer —> increase flexibility by preventing the tails from crystallising and hence behaving like a solid.
Reduces permeability to some solutes (hydrophilic)
Helps membranes curve into a concave shape —> formation of vesicles during endocytosis
