fluid: phospholipid bilayer in which individual phospholipids can move; membrane has flexible shape.
mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded.
explain the role of cholesterol and glycolipids in membranes
cholesterol: steroid molecule in some plasma membranes; connects phospholipids and reduces fluidity to make bilayer more stable.
glycolipids: cell signalling and cell recognition.
explain the functions of extrinsic and transmembrane proteins in membranes
binding sites/ receptors e.g for hormones and drugs.
antigens (glycoproteins)
bind cells together.
involved in cell signalling.
explain the functions of intrinsic transmembrane proteins in membranes
electron carriers (respiration/photosynthesis).
channel proteins (facilitated diffusion).
carrier proteins (facilitated diffusion/ active transport).
explain the functions of membranes within cells
provide internal transport system.
selectively permeable to regulate passage of molecules into/ out of organelles or within organelles.
provide reaction surface.
isolate organelles from cytoplasm for specific metabolic reactions.
explain the functions of the cell-surface membrane
isolates cytoplasm from extracellular environment.
selectively permeable to regulate transport of substances.
involved in cell signalling/ cell recognition.
3 factors that affect membrane permeability
temperature: high temperature denatures membrane proteins/ phospholipid molecules have more kinetic energy and move further apart.
pH: changes tertiary structure of membrane proteins.
use of a solvent: may dissolve membrane.
outline how colorimetry could be used to investigate membrane permeability
use plant tissue with soluble pigment in vacuole. tonoplast and cell-surface membrane disrupted = more permeability; pigment diffuses into solution.
select colorimeter filter with complementary colour.
use distilled water to set colorimeter to 0. measure absorbance/ % transmission value of solution.
high absorbance/ low transmission = more pigment in solution.
osmosis
water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established.
what is water potential
pressure created by water molecules measured in kPa.
water potential of pure water at 25 celsius and 100kPa: 0.
more solute= water potential more negative.
how does osmosis affect plant and animal cells
osmosis INTO cell:
> plant: protoplast swells = cell turgid.
> animal: lysis.
osmosis OUT of cell:
> plant: protoplast shrinks= cell flaccid.
> animal: crenation.
simple diffusion
passive process requires no energy from ATP hydrolysis.
net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration (i.e down a concentration gradient).
facilitated diffusion
passive process.
specific channel or carrier proteins with complementary binding sites transport large and/ or polar molecules/ ions (not soluble in hydrophobic phospholipid tail) down concentration gradient.
explain how channel and carrier proteins work
channel: hydrophilic channels bind to specific ions= one side of the protein closes and the other opens.
carrier: binds to complementary molecule= conformational change releases molecule on other side of membrane; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis.
active transport
active process: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape.
specific carrier protein transports molecules/ ions from area of low concentration to area of higher concentration (i.e against concentration gradient).
exocytosis and endocytosis
active process
involved in bulk transport and transporting large particles
vesicles fuse with cell surface phospholipid membrane
5 factors that affect the rate of diffusion
temperature
diffusion distance
surface area
size of molecule
difference in concentration (how steep the concentration gradient is)