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25 Cards in this Set

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current flowing in from extracellular to intra is

negative current

current flowing in from intra to extra is

positive current

simple diffusion

flux is proportional to the concentration difference across the membrane, is proportional to the lipid solubility, is inversely proportional to the molecular size of the solute, and does not require metabolic energy

Facilitated diffusion differs from simple diffusion in specificity , competition, and saturability

specific - the carrier recognizes structural features of the solute and will carry only those substances that resemble it.

Facilitated diffusion also exhibits competition from similar but not identical solutes.

saturation- This is due to the fact that there are only so many carrier molecules in the membrane. When they are all busy, there can be no further increase in transport.

Comparison of kinetics of simple diffusion to facilitated diffusion

Simple diffusion is linearly related to [S] on the feed side when [S] on the output side is kept low. Facilitated diffusion is saturable, showing a definite maximum transport rate and characteristics [S] at half-maximal transport, called the Km.

The rate of transport by facilitated diffusion is described by

Qtrans = (Qmax C) / (Km + C)

If C = Km, you can see that Qtrans = ½ Qmax.


Facilitated diffusion and simple diffusion occur spontaneously without input of metabolic energy


delta G= nRTln Cr/Cl

delta G= nRTln Cr/Cl

If CR > CL, deltaG will be positive. This means that the opposite process will occur. That is, solute will move from the right to the left, opposite to the direction shown in Eqn. (3). If deltaG = 0, then no net movement occurs and CR = CL.

If Cr< CL then delta G would be negative and solute will move from left to right

The participation of the carrier, which remains unchanged by the transport reaction, does NOT alter the reaction energetics at all, whereas it DOES alter the reaction kinetics. The carrier is a catalyst. It speeds up the reaction, which in this case is transport, without entering into the stoichiometry of the reaction.

Examples of facilitated diffusion include the glucose transporters, GLUT-1-5, for which there are a variety of isoforms that differ in specificity and location in the body.

Ions can be passively transported across membranes by ionophores or by channels.

Charged species are poorly soluble in the lipid phase, and so they cannot merely dissolve in the lipid on one side of the membrane, diffuse across and then enter the compartment on the opposite side of the membrane. They need either carriers or channels to get across

Ionophores carry ions across membranes or form channels

Fungi and bacteria make a class of poison called ionophores. These are molecules that allow ions to cross membranes. The fungi and bacteria make these compounds to kill off competition by disrupting the permeability barrier of their competitors’ membranes. These ionophores are of two types: carriers and channel formers

ionophores examples

A23187- carrier

valinomycin (a K+ ionophore)

and nigericin (a H+ ionophore) .


Other examples of pore-forming antibiotics include amphotericin and nystatin. Amphotericin makes a channel by interacting with cholesterol in cell membranes.

Voltage-gated Na+ and K+ channels allow ions to flow across the membrane only under specific circumstances

The flow of ions is an electric current, because charged ions are moving. The membrane itself is a tiny capacitor

Other voltage-gated channels include the T, L and N-type Ca2+ channels
The designation “T” signifies that this channel opens “transiently” ; the “L” stands for “long-lasting”; and the N indicates that this type of channel is “neuronal”.

Where are ligand gated channels present?

Ligand-gated channels may be present on the surface membrane and also on interior membranes.

The endoplasmic reticulum of many cells contains a large tetrameric protein called the IP3 receptor

This receptor forms a channel for Ca2+ across the ER membrane, and opens in response to IP3 (inositol trisphosphate) that is liberated from the surface membrane as part of signal transduction. Gating by IP3 causes Ca2+ release from the ER and the increased cytoplasmic [Ca2+] alters cellular activity.

acetylcholine binds to nicotinic acetylcholine receptors

Binding of acetylcholine opens a large conductance pathway mainly for Na+.

Water moves passively through aquaporins

There are a variety of aquaporins and they are present on virtually every cell membrane.

the free energy was higher for the initial condition than for the final condition, and the free energy change delta mu = mu final - mu initial is negative ( delta mu < 0)

Passive diffusion through pores or the lipid bilayer, carriers and channels are all passive.

Primary active transport

moves materials against an electrochemical gradient by the direct involvement of ATP hydrolysis

Examples of molecules that are involved in active transport include

ion6pumps: the Na,K-ATPase, the Ca-ATPase, the H-ATPase.

Secondary active transport

moves materials against an electrochemical gradient by the indirect involvement of ATP hydrolysis.

Examples of secondary active transport

Na-glucose co-transport in the intestinal epithelium


renal proximal tubule and the Na-Ca exchange in the heart surface membrane.

The mechanism responsible for maintaining the resting concentrations of Na+ and K+ is the Na,K pump.

This pump moves 3 Na+ ions out of the cell at the same time that it transports 2 K+ ions into the cell.

Na+, K+ ATPase is electrogenic

it separates charge and contributes to membrane potential.