D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
MEMBRANE PHYSIOLOGY
Lecture 4: Membrane Transport (Chapters 2 & 4)
1.
Fluid Mosaic Model: (fig. 2 – 3 & ppt. 1)
• lipid bilayer –phosphoglycerides, organized as bimolecular layer (ppt. 2)
(hydrophilic heads, hydrophobic tails) form permeability barrier to water, ions and
polar substances; cholesterol is dissolved in this bilayer & exerts influence on
permeability properties and integrity of bilayer; fluid matrix permeable to non polar
substances
- proteins (integral & peripheral) – many combined with carbohydrate
(glycoproteins); serve as transport molecules for polar substances & ions (channels,
shuttles, pumps); serve as receptors for specific chemical messengers (e.g. hormones)
or receptors for specific substances to be taken in by endocytosis; serve as enzymes to
facilitate second messenger mechanisms
2.
Transport Processes: (fig. 4 – 2 & ppt. 3)
• diffusion: follows chemical (from high conc. to low) and electrical
gradients (like charges repel each other, unlike charges attract each other) (fig. 4 – 9
& ppt. 4); non polar substances diffuse through matrix; polar & ionic substances
(including water) diffuse through protein channels; rate of diffusion governed by
energy of gradient (conc. or voltage or pressure), velocity of diffusing substance &
surface area of available passageways
- gated channels (fig. 4 – 5 & ppt. 5) – voltage-gated (affected by voltage
change) and chemical-gated (activated by chemical, e. g. neurotransmitter); open or
close to increase or decrease permeability
Bio 659
- p. 2 -
- facilitated diffusion (carrier-mediated) (figs. 4 – 7, 4 – 8 & ppts. 6 & 7) –
diffusion assisted by shuttle mechanism; shows saturation kinetics
• osmosis (fig. 4 – 10, 4 – 11 & ppts. 8 & 9): water diffusion (follows water
conc. gradient = inverse of solute conc. gradient), direction of osmosis is from
solution of lower osmotic strength to one of higher osmotic strength (expressed as
osmotic pressure); osmotic strength of body fluids determines distribution of water
in body compartments
• active transport: primary pumps (require ATP), e.g. sodium/potassium
pump (fig. 4 – 12 & ppt. 10), calcium pump or hydrogen ion pump; secondary (figs. 4
– 13, 4 – 14 & ppts. 11 & 12) – cotransport or countertransport – systems deriving
energy from electrochemical gradient established by primary pump (e.g. glucose
transport following sodium diffusion, calcium or hydrogen ion transport moving
opposite to sodium diffusion); transcellular transport may entail diffusion at one
border & active transport at the other (fig. 4 – 15 & ppt. 13)
3.
Endocytosis: (figs. 2 – 11, 2 – 12 & ppts. 14 & 15)
• pinocytosis & phagocytosis (‘cellular drinking’ & ‘cellular eating’)
- receptors and coated pits (clathrin); invagination (contractile proteins)
- endosome + lysosome = digestion vesicle