2.5 Membrane Transport, 2.6 Facilitated Diffusion

Concentration gradient

when the concentration of one side of the membrane is greater than the other side

the selective permeability of the cell membranes regulates what enters and exits due to the hydrophobic interior(tails) preventing polar/charged molecules from passing through

this creates and maintains concentration gradient of solutes across the membrane

Passive transport

transport of a molecule that does not require energy from the cell because a solute is moving with its concentration or electrochemical gradient

no energy required

high concentration → low concentration

involved in the import of materials and export of waste

examples are diffusion,osmosis, and facilitated diffusion

Diffusion

spontaneous process resulting from the constant motion of molecules

substances move from a high to low concentration

move DOWN the concentration gradient

molecules diffuse directly across the membrane

different rates of diffusion for different molecules

even with diffusion, membrane is still selectively permeable

affected by SA, being non polar, being small

Osmosis

the diffusion of water down its concentration gradient across a selectively permeable membrane 

can also be thought of as the diffusion of water from areas of low solute concentration to areas of high solute concentration (hypotonic→hypertonic)

can use aquaporins in facilitated diffusion, which are specialized channel proteins for water

Facilitated diffusion

diffusion of molecules through the membrane via channel or carrier proteins

does not require energy because molecules are still moving down their concentration gradient

Channel proteins

form an open pore/channel in the membrane

move charged ions (like Na+ and K+)

most are not permanently open

“gates” can open/close in response to stimuli (ex signaling molecules, changes in electric potential across membrane, too much sugar, etc.)

ion channels are highly selective

direction of flow is determined by the electrochemical gradient(concentration and charge)

movement of ions can polarize the membrane(affect the charge across the membrane) meaning one side is more positive/negative than the other side

aquaporins are specialized channel proteins for water

Carrier proteins

alternate between 2 conformations

carry large polar molecules(like glucose/sugars, amino acids, nucleosides)