12.13.11+-+Membrane+Transport

Membrane Transport


 * 1. Know how transport is defined.**

Transport is defined as the movement of particles (ions, molecules) across the membrane. It is subdivided into active and passive transport.


 * 2. Know what drives transport across membranes.**

Like all processes, energy drives transport. Passive transport is the movement of ions or molecules down their concentration gradient, the transition from a higher energy state to a lower energy state. If the passive transport takes place through the lipid bilayer, without the assistance of a protein pore/channel, then it is called simple diffusion. If the passive transport takes place through a protein pore/channel, then it is called facilitated diffusion. Active transport is the movement of ions or molecules up their concentration gradient. This requires energy input. That energy can be applied directly by breaking a phosphate group off of ATP (what's called primary active transport) or it can be applied indirectly by pairing it with another molecule moving down its concentration gradient (what's called secondary active transport). It's important to remember that secondary active transport still uses ATP as its fundamental energy source. The concentration gradient that it is based upon was built by the work of primary active transporters.


 * 3. Be able to distinguish between passive transport and active transport.**

Passive transport is the movement of ions/molecules down their concentration gradient.

Active transport is the movement of ions/molecules up their concentration gradient (requires energy input).


 * 4. Be able to distinguish between simple diffusion and facilitated diffusion.**

Simple diffusion does not use a protein to transfer molecules down their concentration gradient. It is most common for small and uncharged/non-polar ions or molecules. Her lecture notes specifically mention gases and ethanol. Other lipids also can diffuse simply (sterols).

Facilitated diffusion is still mostly small ions or molecules, but since they are charged/polar, they require protein channels to create hydrophilic environments to travel through. Water, ions, sugars all pass through membranes via facilitated diffusion.


 * 5. Know the factors that affect membrane permeability.**

This is the flip side to what has been previously mentioned with regard to which solute factors determine their ability to diffuse through the membrane (polarity, charge, size). Rather than memorize the equation, I think it makes more sense to try to figure out which factors are important:

Area: the size of the membrane matters for how permeable it is (the area increases, so does permeability) Width: the thickness of the membrane matters (as thickness increases, permeability goes down) Concentration: the concentration of the solute matters (as concentration increases, so does permeability)

So then, if I remember that there are no exponential relationships, I can construct the equation using English.

The rate of passage of molecules through a membrane is equal to the area multiplied by the change in concentration divided by the width; also, there's a constant in there, but there is always a constant in modeling equations. Furthermore, since I know that the area and width of most cell membranes is constant I know that the only factors that affects permeability are concentration of the solute and whatever is the respective molecule's constant.


 * 6. Be able to describe a simple carrier model for membrane transport.**

The simple carrier model can be seen as a hole in a wall. There are holes that are always open and there are holes that have doors that must be opened.

Since there are only a certain number of facilitated transporters per membrane (rather than the diffuse simple diffusion that can occur any place on the membrane), increasing the concentration of the solute does not increase the rate of diffusion in a linear fashion. At low concentrations, increasing concentration dramatically increases rate. At high concentrations, increasing the concentration has a much smaller impact. This creates a hyperbolic graph.

There are a few different types of facilitated transporters: ionophores, ligand-gated channels, voltage gated channels, mechanically gated, and aquaporins:
 * Ionophores: mobile, non-protein, fast (e.g. used as antibiotics by microbes, highly specific K+ gradient killer)
 * Ligand-gated channels: must be opened by molecule binding to it at separate site which can be internal or external to cell membrane (e.g. neurotransmitter activated channels)
 * Voltage-gated channels: must be opened by voltage change to membrane (e.g. action potential on muscle cell)
 * Mechanically gated:
 * Aquaporins: can be regulated (AQP 2-4) or non-regulated (AQP 1); facilitate passage of H2O across membrane

Be able to define and describe a primary active transport system or pump.

Understand what is meant by secondary activetransportandtheroleof secondary active transport systems in cellular systems.

Know the function andstructuralorganizationofchannelsandbeableto define the different “gating” mechanisms. 10.Know the differences in transport characteristics of channels, and primary and secondary active transporters. 11.Understand the mechanism of ionophores.