Lecture 7: Transport of Solutes Across Membranes

integral/transmembrane proteins

span the membrane, embedded in the whole bilayer, transports solutes from one side to the other

peripheral membrane proteins

located on the inner or outer surface of the bilayer, sometimes associated with protein anchors

lipid-linked membrane proteins

have a fatty acid covalently linked to the polypeptide chain; stably associates proteins with the membrane

protein channel

move specific ions the fastest; does not undergo a conformational change for each ion, unlike other membrane proteins; result of random movement from thermal energy (passive transport, can only facilitate diffusion)

channel regulation

channels are typically gated

ligand in ligand gated channels

ligand is what bonds to the receptor, in this case the ion channel

ligand gated channel

when the ligand binds, the conformation is changed and the channel is opened, when the ligand leaves the channel conformation is changed and closed

voltage-gated channels

there is a measurable voltage across the plasma membrane; the voltage changes with the flow of ions and as that voltage changes, the structure of the proteins change as well

uniporter

moving one kind of solute, does not use a source of energy; undergoes a conformational change with each solute they use

symporter

carrier protein that transports 2 molecules across the plasma membrane in the same direction; form of active transport and cotransport

antiporter

carrier protein that transports 2 molecules across the plasma membrane in opposite directions, form of active transport and cotransport

symporter and antiporter working together

they exist to move one particular solute against its gradient using energy from moving the other solute along its gradient; one solute driving another solute, known as coupled transport or cotransport

pump

ATP-driven transmembrane protein, moving one or multiple solutes against its concentration or electrochemical gradient (active transport)

sodium-potassium pump

produces a sodium gradient; moves 2 different ions, both against their gradient, so it needs to use ATP; for every 3 sodium out, 2 potassium in; moving more positive charge out, so accumulating positive charge outside and leaving behind a negative charge inside

hypertonic

higher solute concentration

hypotonic

lower solute concentration

isotonic

a solution in which the solute and solvent are equally distributed

electrochemical gradient

the diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion's tendency to move relative to the membrane potential (an electrical force)

the effect of the electrochemical gradient on the diffusion of solutes

the larger the molecule and the more charged it is, the harder it is to move through the cell membrane

can ions easily move through the membrane?

yes, they are charged, but they are small so they will be able to move in easier than larger charged molecules

phagocytosis

uptake of the largest materials into cells; this is how single-celled organisms eat

phagocytosis in animals

in animals, very few cells perform phagocytosis, limited to certain white blood cells

pinocytosis

bringing in a membrane-bound compartment and whatever is in fluid phase in the environment; not specific in terms of uptake

receptor-mediated endocytosis

specific uptake of certain target molecules, molecules that bind to receptors on the surface of our cells; target is often a particular protein, large molecules that could not fit through a channel

exocytosis

cell releases substances by fusing a vesicle to the cell membrane, increases membrane surface area; how cells deliver proteins to the membrane and secrete proteins