CORE D - CELL MEMBRANE TRANSPORT

MODES OF MEMBRANE TRANSPORT A.    Cell Membrane Structure
1.    Phospholipid bilayer
Note: Lipid core is impermeable to charged particles (ions) and polar substances
2.    Proteins, including membrane-spanning proteins (integral proteins or transmembrane proteins) which can aid in membrane transport
3.    Carbohydrates, associated with membrane proteins (glycoprotein) or membrane lipids (glycolipid)
4.    Cholesterol (membrane flexibility)

Dissolve in Membrane Lipid
1.    Only applicable to lipid-soluble substances
2.    Permeability depends on degree of lipid solubility (sometimes measured by oil-water partition coefficient)
3.    Substances utilizing this mechanism include
a.    small, lipid-soluble organic molecules b.    dissolved gases (O2, CO2, etc.) c.    water (!)
C.    Simple, Passive Movement Through Channels
1.    Channel: membrane-spanning macromolecule with an aqueous pore in its center
2.    Aqueous-soluble particles can move through pores (charged particles and polar particles)
3.    Some channels are selective for one or a small group of substances (e.g. Na- channel, K-channel); some channels are non-selective (e.g. “leak” channel)
4.    Permeability depends upon
a.    particle size (generally only small particles can pass through channels) b.    channel density (number of channels/area) c.    presence and state of channel gates -- element of a channel that can
open (increase permeability) or close (decrease permeability) d.    gated channels are classified by their control
1)    voltage-gated or electrically-gated: open or close depends upon membrane potential
2)    ligand-gated: open or close depends upon a chemical substance binding to a membrane receptor
3)    physically-gated: open or close depends upon a physical influence -- mechanical, thermal, etc.

Facilitated Diffusion
1.    Define: passive movement by binding to specific sites (receptor sites) on membrane-spanning or membrane-mobile proteins
2. Mechanism
a.    particle on one side of the membrane binds to receptor site b.    channel reconfigures, exposing the receptor site to the solution
on the other side of the membrane c.    particle dissociates from the receptor and diffuses into the
solution on the other side d.    may move several particles simultaneously in the same direction (symport)
or in opposite directions (antiport)
3.    Selectivity: facilitated diffusion channels are generally selective because of specificity of receptor binding sites
4.    Permeability depends upon
a.    same factors as simple, passive movement through channels b.    affinity of receptor for transported particle c.    number of receptors; note saturation (maximum transport rate) as
concentration of transported particles increases and all receptors are
occupied d.    possible presence of other particles that can bind to the same receptors
(agonist, antagonist)


Active Transport
1.    Define: Movement of particles coupled to a metabolic energy supply, frequently ATP (adenosine triphosphate), which is degraded to ADP (adenosine diphosphate) and inorganic phosphate (Pi), releasing energy in the process)
2.    Similar characteristics and mechanism as Facilitated Diffusion, except
3.    May transport particles against their passive (concentration and electrical) gradients

VESICULAR TRANSPORT
A.    Vesicle:    Subcellular membrane-bounded container used in intracellular storage and/or membrane transport
B.    Exocytosis: Movement from cytoplasm to the interstitial fluid 1.    Vesicle migrates to the cell membrane and attaches (“docks”) to the membrane 2.    Vesicle membrane fuses with the cell membrane 3.    Vesicle contents are released into the interstitial fluid
C. Endocytosis: Movement from interstitial fluid to intracellular vesicle
1.    Substance to be transported binds to cell membrane receptors
2.    Cell membrane invaginates, forming vesicle containing the transported substance
3.    Vesicle migrates intracellularly D. Utility
1.    Membrane transport of large molecules 2.    Intracellular storage in vesicles (endocytosis)