physio cell membrane transport

selectively permeable

allow some substances to cross the membrane but not others

passive transport

does not require energy, occurs when a substance moves down its concentration or electrical gradient

active transport

requires energy, occurs when a substance moves against its concentration or electrical gradient

passive transport: simple diffusion

spontaneous transport of molecules across a permeable plasma membrane without energy or transport proteins

occurs with molecules that easily diffuse through the plasma membrane (lipid soluble hormones, fatty acids, small molecules, lipid soluble vitamins)

oxygen and carbon dioxide transport

factors affecting simple diffusion rate

magnitude of the driving force: larger driving force = faster diffusion

membrane surface area: larger surface area = increased diffusion rate

membrane permeability: more permeable membranes = faster diffusion

passive transport: diffusion through channels (pores)

occurs when substances diffuse down their concentration gradient through plasma membrane pores

passive transport: facilitated diffusion

utilizes transmembrane carrier proteins embedded in the cell membrane

substrate specific and transport substances down their concentration gradient without energy (glucose)

protein undergoes conformational change

active transport: primary active transport

ATP directly transports substances across a membrane

pump transporters act as ATPase enzymes

removing a phosphate group from ATP releases energy for this

pumps

transmembrane proteins using ATP energy to move molecules against concentration gradients

have greater affinity for molecules where least concentrated

resemble enzymes - specific to particular molecules with defines affinities

sodium-potassium pump

ATP energy moves sodium out and potassium in, against their gradients

sodium (high outside, low inside), potassium (high inside, low outside) essential for cell function and resting membrane potential

each cycle moves 3 Na ions out and 2 K ions in, consuming 1 ATP

active transport: secondary active transport

movement is powered by concentration gradients from primary active transport

ion diffusion provides energy to pump molecules against their gradients

one substance moves passively down its gradient while driving another’s active movement up its gradient

ATP isn’t directly used but previously created the concentration gradient’s potential energy

secondary active transport: cotransport (symport)

moves two substances in the same direction (like sodium-linked glucose transport, SGLT)

sodium moving into the cell down its gradient releases energy powering glucose inward against its gradient, sodium ions increase carrier protein affinity for glucose when the glucose binding site faces ECF

type 2 diabetes medications target kidney sodium-glucose transporters, blocking SGLT reduces glucose reabsorption and promotes urinary glucose excretion

this resembles mitochondrial electron transport chains, sodium flows down gradients through sodium-glucose transporters to move glucose into cells

secondary active transport: countertransport (exchange)

moves two substances in opposite directions - like sodium-proton exchange

inward sodium flow powers outward H+ movement by increasing carrier protein H+ affinity when facing the cell interior

osmosis

water transported passively through channels

always moves passively down its concentration gradient, from high to low areas

water movement is unaffected by membrane potential (Vm)

osmolarity

describes the concentration of solutes in solution with water - the total solute particle concentration of a solution

high osmolarity = high solute concentration = low water concentration

low osmolarity = low solute concentration = high water concentration

isoosmotic

equal osmolarities - same solute and water concentration

hyperosmotic

high osmolarity - higher solute concentration and lower water concentraion

hypoosmotic

lower osmolarity - lower solute concentration and higher water concentration

osmotic pressure

pressure required to prevent osmosis

as osmolarity increases, osmotic pressure increases

osmotic pressure increases when total solute concentration increases and decreases when total solute concentration decreases

vesicular transport

active transport of substances across membranes in vesicles or punches

vesicles contain phospholipids, cholesterol, glycolipids, and proteins like plasma membranes

endocytosis

brings materials into cells via endosomes

the plasma membrane invaginates, forms a vesicle around material outside the cell, and brings it into the cytoplasm

exocytosis

material removal from cells via secretory vesicles

vesicles containing secretion materials form within cells, move to plasma membranes, fuse with them, and release contents into extracellular fluid

in neurons, neurotransmitter-filled vesicles fuse with plasma membranes and release into synapses