topic 1.3- membrane structure Flashcards
why do phospholipids form bilayers in water?
due to the amphipathic properties of phospholipid molecules
what is an amphipathic molecule?
contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions
describe the structure of a phospholipid
- a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
- and two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
describe how phospholipids spontaneously arrange into bilayers.
- hydrophobic tails face inwards and so are shielded from the surrounding polar fluids
- the two hydrophilic heads associate with the cytosolic (intracellular) and extracellular fluids
what is are phospholipids held together by?
weak hydrophobic interactions between the tails
give two properties of a phospholipid bilayer
- hydrophilic / hydrophobic layers restrict the passage of many substances
- fluidity and flexibility: individual phospholipids can move within the bilayer
why is the fluidity of the phospholipid bilayer important?
allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)
state the 6 functions of membrane proteins
Junctions
Enzymes
Transport
Recognition
And neurotransmitters
Transduction
Define ‘junctions’ as a membrane protein function
cell adhesion to connect and join groups of cells together in tissues and organs.
Define ‘enzymes’ as a membrane protein function
for immobilised enzymes with the active site on the outside, fixing to membranes localises metabolic pathways (eg small intestine)
Define ‘transport’ as a membrane protein function
act as channels to allow hydrophilic particles across by facilitated diffusion, and as pumps for active transport which use ATP to move particles across the membrane.
Define ‘recognition’ as a membrane protein function
May function as markers for cellular identification
Define ‘and neurotransmitters’ as a membrane protein function
cell-to-cell communication, for example receptors for neurotransmitters at synapses
Define ‘transduction’ as a membrane protein function
hormone binding sites (peptide hormone receptors), for example the insulin receptor.
what two types of membrane proteins are there?
- integral proteins
- peripheral proteins
describe integral proteins
- permanently attached to the membrane
- typically transmembrane (span across the bilayer)
- hydrophobic on at least part of their surface
- embedded in hydrocarbon chains at centre
describe peripheral proteins
- temporarily attached by non-covalent interactions
- associate with one surface of the membrane, not embedded in the membrane
- hydrophilic on surface
- may be attached to surface of integral proteins/have a single hydrocarbon chain in the membrane
the more active a membrane, the —– its protein content
higher
what group of lipid substances does cholesterol belong to?
steroids
why is cholesterol considered amphipathic?
Cholesterol’s hydroxyl (-OH) group is hydrophilic but the remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic
what does cholesterol’s hydroxyl group align with?
towards the phosphate heads of phospholipids
what do cholesterol’s steroid ring/hydrocarbon tail align with?
with the phospholipid tails
state two functions of cholesterol in mammalian membranes
- controls membrane fluidity
- reduces permeability to some solutes
how does cholesterol affect membrane fluidity in high/low temperatures?
at high temperatures, cholesterol reduces membrane fluidity through its interactions with the fatty acid tails, which stabilise the membrane . at low temperatures, cholesterol increases membrane fluidity by preventing the phospholipid tails from packing too close together.
why does the fluidity of animal cell membranes need to be controlled?
too fluid-> unable to control what substances pass through
not fluid enough-> movement of cell and substances would be restricted
describe the 2ary roles of cholesterol in mammalian membranes
- reduces permeability to hydrophilic particles such as Na+ ions and H+ ions
- helps membranes curve into a concave shape, which helps formations of vesicles during endocytosis
draw the fluid mosaic model
refer elsewhere
describe the Davson-Danielli model
- model whereby two layers of protein flanked a central phospholipid bilayer
- ‘lipo-protein sandwich’: the lipid layer was sandwiched between two protein layers
what were the dark segments under the microscope identified as?
dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers
Give the three main pieces of evidence for the falsification of the Davson-Danielli model
- membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size
- Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place
- Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane
How does the insolubility and variation in size of membrane proteins disprove the DD model?
Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane
how did antibody tagging disprove the DD model?
showed membrane proteins could move and did not form a static layer
how did freeze fracturing disprove the DD model?
rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure
