Lecture 9 Membrane Transport Flashcards
Permeability
ability of a substance to pass through a membrane
What is permeability determined by?
the phospholipid bilayer
membrane transport proteins
Molecular size
smaller molecules are more permeable
Lipid solubility
non-polar molecules are lipid soluble meaning they are more permeable (ex: fatty acids)
polar molecules and ions are less permeable or impermeable (ex: H2O)
Membrane transport proteins
help ions and molar molecules to pass through
Highly permeable
02 & C02
Fatty acids
steroids
H20 (variable: pores)
less permeable
Na+, K+, Cl- (via channels)
glucose, a.a’s (via carriers)
Impermeable
proteins (except via vesicles)
ATP
DNA, RNA
Passive Transport
does not require energy
substances move down gradient
Simple diffusion, osmosis, diffusion through channels, facilitated diffusion
Active Transport
requires energy
transport against gradient
primary active transport, secondary active transport, transport via vesicles (endocytosis, exocytosis)
Protein mediated transport
diffusion through channels, facilitated diffusion, primary active transport, secondary active transport
Simple Diffusion
results from random molecular motion
net movement from high concentration to low concentration
Fick’s Law of diffusion
Fick’s Law of diffusion
gives the rate of diffusion
Rate=P A (Cout-Cin) / X
rate is proportional to permeability (P), surface area (A), concentration gradient (Cout-Cin)
inversely proportional to diffusion distance or membrane thickness (x)
Osmosis
passive movement of water across a membrane due to solute concentration difference
permeable to H20 but impermeable to solutes
primary mechanism for H20 transport across membranes
H20 ,moves from dilute to concentrated solution (solutes suck water)
Osmolarity
total concentration of all solutes in a solution
1 Osm = 1 mole of solutes per liter
Non-ionic solutes and salts
Non-ionic solutes
osmolarity = concentration
e.g. 1 M glucose = 1 Osm = 1,000 mOsm
Salts
ionize in H20
1M NaCl -> 1M Na+ + 1M Cl- = 2 Osm
Osmotic pressure
driving force for osmosis
depends on difference in total solute concentration
negative pressure pills water from dilute to concentrated solution
Tonicity
effect of an extracellular solution on cell volume, due to H20 movement by osmosis
Hypertonic
Hypotonic
Isotonic
hypo - H20 moves in cell expands
hyper - H20 moves out cell shrinks
Iso- no net movement of H20 cell volume stays constant
Diffusion through Channels
Ion channels are protein passageways for ions through the membrane
most channels are selective for certain ions
ions diffuse down electrochemical gradients
channels may be ungated or gated
Aquaporins
water channels found in most cell membranes
Electrochemical gradient
combination of concentration and electrical gradients
can act in same direction ( Na+)
or in opposite direction (K+)
Facilitated Diffusion
carrier proteins mediate diffusion of certain polar molecules across the membrane
down concentration gradient, no energy required
each carrier is specific to particular molecules
saturation- rate limited by number of carrier proteins in the membrane
GLUT proteins
Facilitated Diffusion
family of glucose transporters, present in many call membranes
most body cells take up glucose by FD using GLUT proteins
GLUT4
activated by insulin
is the insulin dependent glucose carrier of skeletal muscle, adipose tissue, liver, and connective tissue
Insulin promotes insertion of GLUT4 into the membrane -> glucose uptake via FD
Primary Active Transport
pumps are transport proteins that use energy from ATP directly
transport ions “uphill” against electrochemical gradients
Na+/K+ Pump
Transports Na+ OUT and K+ IN
maintains ionic composition of ICF and ECF
K+ and Na+ gradients are the basis of electrical properties of cells
Na+ gradient provides potential energy for transport of other molecules
Na+/K+ pump activity is stimulated by THYROID hormones
Other active transport pumps
Ca2+ -ATPase in muscles
H+ - ATPase in stomach
Secondary Active transport
uses potential energy stored in IONIC gradients to move other molecules
transport protein couples “downhill” flow of an ion to uphill transport of another molecule
Cotransport
SAT
movement of both molecules in the same direction
countertransport
SAT
movement in opposite directions
SGLT
SAT
is a Na+-glucose cotransporter in the small intestine and kidney epithelium
moves glucose against its gradient from the lumen into the epithelial cell
uses energy contained in the Na+ gradient
Endocytosis
phagocytosis
pinocytosis
Exocytosis
secretion of products out of the cell (mucus, neurotransmitters, hormones)
also functions for insertion of molecules into the plasma membrane (lipids, proteins)
Epithelial Transport
From lumen to ICF to ECF
Apical membrane
faces lumen
microvilli increase surface area (ficks law)
Basolateral membrane
faces ECF, attached to basement membrane
contains Na+/K+ pumps
Tight Junctions
join epithelial cells, near apical surface, prevent fluid leakage between cells
Transepithelial Transport
NaCl, glucose and H20 in the small intestines and kidneys
NaCl
transepithelial transport
apical membrane: Na+ enters via diffusion through channels
basolateral membrane: primary active transport (Na+/K+ pump) moves Na+ out to ECF
Cl- follows Na+ passively by diffusion through channels
Glucose
transepithelial transport
apical membrane: secondary active transport - cotransport with Na+ (SGLT)
basolateral membrane: faciliated diffusion of glucose out to ECF (GLUT)
Water
Transepithelial transport
moves by osmosis across apical and basolateral membranes; follows solute movement
pumping of Na+ to ECF between cells promotes H20 movement by osmosis
