PT15 LEC: Membrane Transport Mechanisms & Membrane Protein Channels

membrane transport mechanism

means by which substances passes through the plasma membrane into or out of the cell

plasma membrane

gateway between ECF and cytoplasm; dynamic structure that regulates concentration of substances in and out of the cell

ECF composition

sodium+, calcium++, chloride-, bicarbonate- dominant; oxygen pp greater; basic pH

ICF composition

potassium+, magnesium++, phosphates dominant; CO2 pp greater; greater protein- amount

interstitial fluid

between membrane and ECF

sodium

chief cation in ECF

chloride

chief anion in ECF

potassium

chief cation in ICF

phosphate

chief anion in ICF

lipid-soluble molecules that can pass through plasma membrane

fat-soluble vitamins, respiratory gases, alcohol

factors that affect passage thru plasma membrane

size of molecules & lipid-solubility

channel proteins

- watery-spaces all the way through the transport protein molecule
- rapid but not selective
- can be either gated or open

carrier proteins

- bind with molecules or ions that are to be transported
- slow but very selective

2 characteristics of transport protein molecules

(1) controls permeability based on shape, diameter, and nature of charge/bonds across inner surfaces
(2) movement depends on presence of gradient

electrical signals

control voltage-gated protein channels

ligands

control ligand-gated protein channels

leakage channels

non-specific with random ion movement

mechanically-gated channels

- tactile/ pressure channels
- can be physically opened

sodium protein gated-channels

- selective for sodium ions; small diameter channels
- Na+ is attracted to water; pulled away from hydrating molecules toward protein channel
- movement of sodium into cell changes cell polarity (reversed)

potassium protein gated-channels

- selective for potassium ions
- has a unique Tetrameric structure - consists of 4 identical protein subunits surrounding a central pore
- allows K+ to go out of the cell

voltage-gated protein channels

stimulated when change in membrane voltage is detected

voltage-gated sodium channels

- limits entry of sodium
- activation gate behaves more rapidly than potassium activation gate

voltage-gated potassium channels

- allows more potassium to exit
- only has one gate with slow activation

nerve impulses

give the signal from somatic motor neurons

ligand-gated protein channels

acetylcholine binds to plasma membrane to open/close gate

acetylcholine binding flow

- acetylcholine binds to receptor to open gated channels
- allows Na+ in and K+ out
- last step for muscle contraction
- acetylcholinesterase degrades AC into acetic acid & choline
- acetic acid reabsorbed to nerve ending
- choline is waste product

passive transport

- not energy-dependent
- high to low concentration/pressure
- using kinetic energy of motion

diffusion

passive, random molecular movement of molecules down their concentration gradient

factors affecting rate of diffusion

- permeability
- concentration gradient
- electric potential difference
- pressure difference

simple diffusion

- for lipid-soluble substances
- rapid over short distances
- directly related to temperature and inversely related to molecular size

Fick's Law of Diffusion across Plasma Membranes

states that the rate of diffusion is directly proportional to the surface area & concentration gradient and inversely proportional to membrane resistance & membrane thickness

facilitated diffusion

- limited by carrier protein
- limited as diffusion approaches Vmax and the concentration of the substance increases (saturation point/transport maxiumum); no more carrier proteins to transport solute

filtration

- dependent on hydrostatic pressure
- passive process that is not very selective
- high to low pressure

saturation point

carrier saturation or transport maximum

transport maximum

the point wherein the transport level of solutes reaches a plateau since the availability of the carrier proteins reaches saturation point and no more available carrier proteins to transport solute, even if there are still solutes to be transported

osmosis

process of net movement of water across a selectively permeable membrane caused by concentration difference/gradient of water

aquaporins

special membrane channels

osmolarity

total concentration of solute particles in a solution

isotonic

same solute conc [with cytosol]

hypertonic

greater solute conc [than cytosol]

hypotonic

lesser solute conc [than cytosol]

example of simple diffusion

movement of O2 through membrane

example of facilitated diffusion

movement of glucose into cells

example of osmosis

movement of H2O in & out of cells

example of filtration

formation of kidney filtrate

active transport

- uses ATP
- low to high concentration (against gradient)

ratio of Na+ entry and K+ exit

3:2

pressure

the sum of all forces of the different molecules striking a unit surface area at a given instant

hydrostatic pressure

force of water molecules exerted against the molecules in a solution (NaCl)

osmotic pressure

the net pressure required to stop osmosis

net movement of water

from "more watery-side" with less concentration of dissolved matter towards "less watery-side" with more solute concentration across a semi-permeable membrane

tonicity

ability of how a solution affects cellular fluid volume & pressure

primary active transport

- against concentration gradient using ATP
- utilizes hydrolysis of ATP
- Na+ K+ pump/Na+ K+ ATPase pumps

secondary active transport

- depends on ionic difference/gradient produced from primary AT process
- indirectly driven by energyy stored in ionic gradient from Na+ K+ pump
- symport/antiport system ("coupled systems"; same n diff direction)

vesicular transport

- "bulk transport" not using membrane carriers
- endocytosis and exocytosis

flow of sodium-potassium pumps

- transports 3 Na+ out & 2 K+ in per cycle
- binding of cytoplasmic Na+ to pump protein stimulated phosphorylation (change conformation) by ATP
- pump protein changes shape (ATP dissociates)
- Na+ gets expelled and extracellular K+ binds to a different receptor of the same pump protein
- K+ binding triggers release of the phosphate group
- phosphate loss restores original conformation
- K+ released back into the cell