Cell Membranes

plasma membrane

governs what can enter/exit the cell, defines boundaries of cell, communication between cells

lipids

98% of plasma membrane molecules are ______

amphipathic

phospholipids are ___________, meaning their heads are hydrophilic and their tails are hydrophobic

cholesterol

also amphipathic, alters membrane fluidity

glycoprotein

carbohydrate chain attached to a membrane protein

glycolipid

carbohydrate chain attached to a lipid of the plasma membrane

membrane proteins

2% of the molecules, but 50% of the mass of membrane; transmembrane and peripheral

transmembrane (integral) proteins

go all the way through the plasma membrane, from one side to the other

peripheral proteins

only on one side of the plasma membrane

receptor proteins

usually highly specific to certain messenger molecules (ligands, proteins, steroids, ions, etc.)

signal transducrtion

ligand bonds to receptor protein, second messengers relay the message into the cell, eventually generating a cellular response

G-protein coupled receptor (GPCR)

discovered in 1980, ligand bonds to receptor protein, sets of series of reactions within the cell through the action of the G-protein, ~700 FDA-approved drugs act by binding to this receptor (antihistamines, morphine, CBD, etc.)

transport proteins

allow substances to move from one side of membrane to other; channel and carrier proteins

channel proteins

type of transport protein; act as a pore in the membrane to allow molecules to pass through (# of molecules is not limited when open); molecule-specific

carrier proteins

type of transport protein; have binding sites for specific amounts of certain molecules to pass through (can only move one molecule at a time); molecule-specific

channel proteins

two types: leakage/open channels and gated channel

leakage/open channels

type of channel protein; always open; ions move based on concentration gradient

gated channels

type of channel protein; open under certain conditions, closed if condition is not present

ligand-gated

type of gated channel; receptor and channel in one; hormones, neurotransmitters open them; channel will be closed if ligand is not present

mechanically-gated

type of gated channel; open due to mechanical change (temperature or pressure); pressure put on gate opens it, no pressure = closed

voltage-gated

type of gated channel; open due to change in charge (inside vs outside the cell); change in voltage may open or close the gate; e.g., sodium channel = open when no difference in charge, closed when charge changes

selectively permeable

the plasma membrane is ___________ __________, meaning it allows some things through, but prevents others

passive mechanism

mechanism used to cross the plasma membrane that don’t require ATP (diffusion, osmosis)

active mechanism

mechanism used to cross the plasma membrane that requires ATP

carrier-mediated mechanism

mechanism that uses a membrane protein to transport substances across membrane (active and passive)

solute

something that is dissolved in a solvent (e.g., salt)

solvent

dissolves solutes, most common in humans is water

solution

mixture of solute and solvent

concentration gradient

difference between concentration of a substance on either side of a membrane

passive transport

molecules move down/with their concentration gradient

active transport

molecules move up/against their concentration gradient

simple diffusion

passive movement of solute particles from place of high concentration to place of lower concentration; no membrane protein needed; small, hydrophobic (nonpolar) molecules, gases, small uncharged polar molecules, and some water can move

facilitated diffusion

passive movement of particles down their concentration gradient, no ATP needed, membrane transport protein required (channel or carrier); large uncharged polar molecules (glucose), charged molecules, charged polar molecules (small or large), and some water can move

osmosis

flow of water through a selectively permeable membrane; water moves from high to low concentration/moves toward the higher solute concentration (wants to dissolve the solute), may or may not be facilitated

aquaporin

water-specific transport protein; moves water much faster than water moving directly through the phospholipid bilayer

tonicity

comparison of solute concentrations in two solutions (one solution in the cell, one outside)

hypotonic

lower solute concentration outside cell will result in swelling of the cell

isotonic

equivalent solute concentration

hypertonic

higher solute concentration outside cell with result in crenation

active transport

solute moves up/against its concentration gradient, low to high concentration, transport protein required, can’t happen through phospholipid bilayer, ATP required

primary active transport

protein pumps use ATP directly to pump ions out of the cell

secondary active transport

uses ATP indirectly, one substance naturally diffuses, other substance “hitches a ride” , proteins are symporters or antiporters

symporter

protein that carries out cotransport in secondary active transport; two molecules move in the same direction

antiporter

protein that carries our countertransport in secondary active transport; one molecule moves into the cell while another moves out

sodium potassium pump

example of primary active transport, 3 Na⁺ exported from cell and 2 K⁺ imported into cell each cycle, maintains gradient across cell membrane (more K⁺ inside the cell and more Na⁺ outside the cell), inside of cell is negatively charged when at rest

sodium glucose transporter

example of secondary active transport, cotransport (both Na⁺ and glucose move into the cell through a symporter), kinetic energy of one substance moving with gradient provides ”power” to move other substance (sodium moves down its gradient and glucose hitches a ride), ATP is indirectly used in the sodium potassium pump to move sodium out of cell