Ch 6 - transport of ions across membranes Flashcards
diffusion
random molecular motion of solutes down a chemical or electrochemical gradient
osmosis
diffusion of water across a semipermeable membrane
protein-mediated transport
- more rapid than simple diffusion
- saturation kinetics (the rate of transport increases until it reaches a point at which it stabilizes
- chemical specificity and stereospecificity (only molecules of a particular shape and chemical structure are transported
- competitive inhibition (structurally related molecules can block transport)
- transport can be inhibited by compounds that bind to or structurally alter the transport protein
types of protein-mediated transport
- facilitated transport
- active transport
facilitated transport
- cannot move ions against an electrical potential difference
- cant move uncharged particles against a concentration gradient
- the transported molecule may alter transport by producing a conformational change in the transport protein
active transport
- ATP required
- metabolic inhibitors can block
- substances can move against concentration
types of transport proteins
carrier proteins and channel proteins
carrier proteins
molecules bind to these proteins and are moved across the membrane by a conformational change in the carrier
uniport
one type of molecule is transported across a membrane without help from a carrier
symport
two different molecules transported in the same direction
antiport
two different molecules transported in different directions
channel proteins
form pores that open and close to transmit certain molecules across the membrane
Na, K, ATPase
- maintains low Na and high K inside the cell by active transport
- 3 Na: 2 K: 1 ATP
- chemical potential energy created is used to drive other systems like amino acid transport
Ca ATPases
- maintain very low Ca in cytoplasm ( hi Ca will activate cells)
- phosphorylation and dephosphorylation fuels pumping
- activity of these ATPases may be regulated by the protein calmodulin
Na-Ca antiporter
- the energy of the Na gradient is used to transport Ca from the cell
- cause rapid and transient decreases in intracellular Ca
- activity stimulated by calmodulin
Na-H antiport
prevents acidification of cytoplasm
chloride-bicarbonate antiporter
- prevents cytosolic pH from becoming too basic
- this protein is important in red blood cells for CO2 transport and renal tubular cells for bicarbonate reabsorption.
Na-K-Cl symporter
- transports 1 Na: 1K: 2Cl into the cell
- can regulate cell volume, cell shrinkage acivates this symporter and the resultant ionic influx pulls in water osmotically
- in the kidney, this transporter is heavily involve dn the reabsorption of sodium. inhibition of the action of this symporter by furosemide (Lasix) increased sodium and water excretion (diuresis).
structure and function of ion transport channels
- one or more sets of transmembrane alpha-helices
- ion selectivity arises from the arrangement of charged amino acids spanning helices that line the pore
- the beta subunit of hte channel molecule forms a physical gate on the outside of the cell
two types of ion transport channels
- voltage gated
2. ligand gated
voltage gated
activated by changes in membrane potential
- voltage sensor in one of transmembrane helices and contains a row of 7 positively charged amino acids
- they are for Na, K, and Ca
ligand gated
activated by the binding of a ligand to the receptor portion of the channel molecule, which then opens the gate
what type of channel does ketamine work on? and what is its name?
- acts to block a ligand-gated ion channel for Na and Ca.
- this channel is called the NMDA channel and is normally activated by glutamate
- ketamine blocks depolarization of hte cell and thus the generation of action potentials which normally activate the NMDA channel.
protein-mediated transport of small molecules
- is faster than simple diffusion
- can be subject to competitive inhibition
- can be saturated
- displays chemical specificity
