BI110 Lecture Notes - Lecture 14: Active Transport, Electrochemical Gradient, Cell Membrane
BI110 Lecture #14 (Wednesday October 18th)
Transport and Cells
Note: CG – concentration gradient
• The plasma membrane is selectively permeable (semipermeable)
• Diffusion:
o Net movement of solute from an area of high concentration to an area of low solute
concentration
o When diffusion is occurring and solute concentration is same on both sides, movement
does not stop since particles are always in motion, but net movement stops
o Ex. Walking by a store and you smell and a really strong cologne/perfume, the smell is
diffusing to areas of lower concentration
• Passive transport: always along concentration gradient
o Passive transport is the movement of molecules along their CG
o Simple diffusion: allows molecules to freely cross membrane, along CG (needs no help)
o Facilitated diffusion (channels, carriers): allow molecules to move through plasma
membrane, along CG. Just need a helper to get across
▪ Beneficial for molecules that have properties that do not allow it to cross
membrane on its own
▪ Carriers change shape to fit molecule
• Active Transport: against concentration gradient
o Is the uphill movement of molecules against the CG, requires energy (ENERGY)
o Primary active transport: uses energy (ATP) directly to move molecules against CG.
o Secondary active transport: movement of molecules against their CG and is not coupled
with ATP directly
▪ Energy is still used, but normally in a two step process. One molecule is moved
across membrane through use of ATP and the movement of this molecule drives
the movement of a second particle against its CG (the second molecule uses
energy indirectly to move across = secondary)
▪ Example: protons are pumped across CG by primary active transport, generating
an electrical gradient (since protons are charged) as well as a CG/proton
gradient and these two gradients together make an electrochemical gradient.
An antiporter moves protons outside of cell against their CG and at the same
time, moves a second molecule against its CG inside the cell
o Antiporters: move two molecules in opposite directions at same time
o Symporters/uniporters: move molecules in same direction
o Sodium-potassium pump: example of primary active transport
▪ Potassium goes from area of low to high (into cell)
▪ Sodium goes from low to high (out of cell)
▪ This pump uses ATP directly, so it’s primary
