membranes Flashcards
membrane definition
semi permeable barriers that surround the cytoplasm of a cell and control the movement of substances in and out
components of a cell surface membrane according to fluid mosaic model
phospholipid
glycolipid
glycoprotein
extrinsic protein
intrinsic protein
cholesterol
protein channel
function of phospholipids in a membrane
has a hydrophobic fatty acid tail and a hydrophilic polar phosphate head, enabling these molecules to form bilayers in aq solution - majority of biological membranes
fatty acid tails are unsaturated so have some kinks, this prevents molecules form stacking too closely therefore improving flexibility of membrane
phospholipids aren’t stationary, they constantly move around each other which gives the membrane flexibility
function of glycolipids in a membrane
a phospholipid with a carbohydrate attached
acts as antigens/markers that allow the cell to be recognised
function of glycoprotein in a membrane
a protein with a carbohydrate attached to it
this carbohydrate acts as a receptor, e.g. for chemicals/hormones
what is an extrinsic protein in a membrane
a protein that goes all the way through the membrane
what is an intrinsic protein in a membrane
a protein that is only partially submerged in the membrane
function of cholesterol in a membrane
hydrophilic end can interact with phosphate heads and hydrophobic tail can interact with fatty acid tails
pulls phospholipid molecules closer together
increases stability of the membrane as have kinks like fatty acids which takes up more space and ensures the other molecules don’t get too close together so membrane cannot crystallise
function of transport proteins in a membrane
water is polar but also very small so can squeeze through bilayer (passive diffusion via osmosis)
larger molecules, e.g. glucose - which may be non-polar and able to passively diffuse across, are too big to fit through the bilayer so need specific transport proteins
6 functions of membranes
compartmentalisation - making organelles/cells with specific environments, isolation of metabolic pathways
creation of concentration gradients
selective permeability - allows for active + passive transport of molecules
cellular communication - glycoprotein receptors and glycolipids that act as cell markers allow for identification and communication via chemical messengers/hormones
cell adhesion
fluidity - if phospholipids crystallise cell no longer works
2 factors affecting membrane permeability
temperature and solvent concentration
how does temperature increasing affect membrane permeability
as temp increases, lipids gain kinetic energy = become more fluid = effectiveness as a barrier decreases = diffusion rate increases - REVERSIBLE
as temp increases, proteins begin to denature = no longer specific = substances can freely pass through - IRREVERSIBLE
how does temperature decreasing affect membrane permeability
as temp decreases towards freezing channel proteins become deformed and ice crystals form which pierce membrane = increased permeability when cell is thawed again
how does solvent concentration increasing affect membrane permeability
increasing concentration of organic solvents increases membrane permeability as they dissolve lipids = membrane loses structure
partially permeable definition
a membrane that allows some substances to cross but not others
why is it called the fluid mosaic model
phospholipids can move around each other freely = fluid
proteins embedded vary in size, shape and posotion = mosaic
how can a colorimeter be used to show the permeability of membranes
a more permeable membrane means more dye/pigment leaks out
this means less light is absorbed by the detector
outline an experiment to investigate membrane permeability
- beetroot cells contain betalain
- when membranes are disrupted the pigment leaks out
- cut pieces of equal size and place in solvent (water at diff temps or organic solvents at diff concs)
- leave all pieces for same amount of time
- solutions should then be placed in a colorimeter and absorbance should be measured (must be recalibrated each time to 100% absorbance)
suggest 3 improvements to colorimeter experiment
- cut pieces from same place on beetroot
- blot surface of beetroot before experiment
- use same volume of solvent
how does a channel protein work
these provide a hydrophilic channel that allows the passive movement of polar molecules down a concentration gradient
water filled channel is lined with hydrophilic amino acids on the inside
channel proteins are specific
how does a carrier protein work
molecule binds to protein on outside of membrane, protein changes shape and releases the molecule on the other side
this conformational change requires energy - active transport
carrier proteins are specific
what is facilitated diffusion
passive diffusion across a membrane down a concentration gradient through protein channels - necessary for most polar molecules
what is endocytosis
the bulk transport of material into cells - a form of active transport
what are the 2 types of endocytosis
phagocytosis = solids
pinocytosis = liquids
