Transport Mechanisms Flashcards
Permeability Characteristics of Cell Membrane
Highly permeable to: H2O, Lipid-soluble substances, dissolved gases (O2,Co2), small uncharged particles
Less Permeable to: larger molecules, charged particles
Impermeable to: very large molecules
Integral Proteins
closely associated with phospholipids, mostly cross the membrane (transmembrane)
Peripheral Proteins
more loosely associated, mostly on the cytoplasmic side
Glycocalyx
Carbohydrates and glycoproteins on outer side of membrane
Functions of plasma Membrane
Selective transport channel, enzyme, cell surface receptor, cell surface identity marker, cell adhesion, attachment to the cytoskeleton
Membrane Proteins Functions
- Transport specific molecules into and out of the cell
- Act as enzymes that catalyze membrane associated reactions
- Serve as receptors for receiving and transducing chemical signals form the cell environment
- Cell surface identity markers
- Cell- Cell adhesion
- Attachment to cytoskeleton
Passive
- Diffusion
- Carrier-mediated
Facilitated Diffusion - Osmosis
Active Transport
- Carrier-mediated
Active Transport
a. Primary
b. Secondary
Fick’s Law of Diffusion
Rate of diffusion can be calculated:
J = PA (Co -Ci)
J = Net Flux: net movement of molecules across a membrane
P = Permeability (or diffusion) coefficient: a constant based on the ease with which a molecule moves through a membrane
A = Surface area of the membrane
Co - Ci = Concentration gradient of the diffusing molecule across the membrane
Diffusion of particles may penetrate either by
a. Dissolving in the lipid component )for non-polar molecules) e.g. O2, CO2, fatty acids
b. Diffusing through channels(for ions)
Ion channels may consist of
- a single protein or more often, of clusters of proteins.
- Ion channels show selectivity based on their diameter as well as on the distribution of charges lining the channel
3 ways that channels can be gated
a. Ligand-gated
b. Voltage-gated
c. Mechanically-gated
Characteristics of Carrier-mediated transport
a. Specificity
b. Saturation
c. Competition
Secondary Active: Counter trasport Mechanisms
Na+/H+ exchanger
Na+/Ca2+ exchanger
Cl-/HCO3- exchanger
Secondary Active: Cotrasport Mechanisms
Na+-glucose cotransporter
Na-amino acid transporter+
Endocytosis
cell membrane invaginates, forming a channel, the end of which pinches off to form a vesicle
Exocytosis
an intracellular vesicle fuses with the cell membrane and its contents are released into the ECF
Pinocytosis
ingestion of dissolved materials by endocytosis. The cell membrane invaginates and pinches off placing small droplets of fluid in a pinocytic vesicle. The liquid contents of the vesicle is then slowly transferred to the cytosol.
Phagocytosis
ingestion of solid particles by endocytosis. The cell membrane invaginates and pinches off placing the particle in a phagocytic vacoule (phagosome). The phagocytic vacoule then fuses with lysosomes and the material is degraded.
Clathrin-dependent receptor-mediated endocytosis
-involves pinching off of “clathrin-coated vesicles” that often fuse with endosomes where the vesicles contents are sorted
Potocytosis
- type of clathrin-independent receptor-mediated endocytosis
- involves the pinching off of tiny vesicles called “caveolae” that deliver their contents to the cell cytoplasm via channels or carriers.
Osmotic Pressure
- the pressure required to prevent the movement of water across a semi-permeable membrane
- pressure is equal to the difference in hydrostatic pressures of the two solutions
Van’t Hoff Equation for Osmotic Pressure
P = nRT/V
n= # of particles R= gas constant T = abs. Temp V = Voume
Osmolarity(Osm)
-total solute concentration of a solution
1 osmol = 1 mol of solut particles
Isosmotic
solutions which have the same number of osmotically active particles as normal extracellular solutions
Hypoosmotic
solutions which have a lower number of osmotically active particles
Hyperosmotic
solutions which have a higher number of osmotically active particles
Hypertonic solution
solution has a conc. of nonpenetrating solute particles greater than 300 mOsm, water will leave the cell and the cell will shrink
Hypotonic
solution has a conc. of nonpenetrating particles less than 300 mOsm, water will enter the cell and the cell will swell.
Isotonic
solution has a conc. of 300 mOsm of nonpenetrating solute particles, there will be no net shift of water.
Capillary wall
-a single layer of flattened endothelial cells and a supporting basement membrane.
Bulk flow
-flow of molecules subjected to a pressure difference causing redistribution of extracellular solution
Filtration
-bulk flow across a porous membrane (which acts as a “seive”, withhold some particles).
