Ch. 5: Membrane Transport and Cell Signaling

clathrin

fibrous protein, lines and reinforces the coated pit on the cytoplasmic side and probably helps deepen the pit to form a vesicle

turgid

firmness or tension as found in walled cells that are in a hypo-osmotic environment where water enters the cell by osmosis

hypotonic environment (animal cell)

gain water by osmosis, swell and maybe lyse (cell destruction)

G protein bound to GDP

inactive

hypotonic solution

solution with a lower solute concentration compared to that inside a cell

paracrine signaling

- one cell secretes the signal into extracellular fluid and the signal acts on nearby target cells

signal transduction

- a membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone - the external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein

synaptic signaling

- a nerve cell releases a signal (neurotransmitter) into a synapse, the narrow space between the transmitting cell and the target cell, such as another nerve cell

enzymatic activity

- a protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution

G protein linked receptor structure

- a single polypeptide chain that is threaded back and forth in the PM in such a way as to have 7 transmembrane domains (epinephrine receptor)

fluid mosaic model

- accounted for amphipathic character of proteins - proteins are individually embedded in phospholipid bilayer, rather than forming a solid coat spread upon the surface (they are hydrophobic so they would be in water) - hydrophillic portions of both proteins and phospholipids are maximally exposed to water resulting in a stable membrane structure - hydrophobic portions of proteins and phospholipids are in the nonaqueous environment inside the bilayer - membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids

pinocytosis-

- cell drinking - endocytosis of fluid - droplets of EC fluid taken into small vesicles - process is nonspecific

phagocytosis

- cell eating - endocytosis of solid particles - cell engulf particles with pseudopodia and pinches off a food vacuole - vacuole fuses with a lysosome containing hydrolytic enzymes that will digest the particle

what happens when a ligand binds to G protein linked receptor

- changes its conformation and it interacts with a G protein - this interaction causes the GDP bound to the inactive G protein to be displaced by GTP, thereby activating the G protein

tyrosine kinase receptors

- characterized by extracellular ligand binding domain and a cytosolic domain possessing tyrosine kinase enzyme activity

forces that drive passive transport of ions across membranes

- conc grad of ions - effect of membrane potential on ion

direction of osmosis

- determined by differences in TOTAL SOLUTE CONC regardless of the type or diversity of solutes in solution

electrochemical gradient

- diffusion gradient resulting from combined effects of membrane potential and conc. gradient - ions may not always diffuse down conc gradients, but they always diffuse down their electrochemical gradients - at equilibrium, the distribution of ions on either side of the membrane may be different from the expected distribution when charge is not a factor - uncharged solutes diffuse down concentration gradients because they are unaffected by membrane potential

facilitated diffusion

- diffusion of solutes across a membrane with help of transport protein - passive transport

G protein linked receptor: propagate signal?

- does this by interacting with a variety of proteins on the cytoplasmic side of the PM called G proteins because they bind guanine nucleotides GTP and GDP

active transport

- energy - against conc. grad - not spontaneous - sodium potassium pump

Exocytosis

- exporting macromolecules from cell by fusion with PM - vesicles bud from ER or GA - used by secretory cells

cholesterol

- found in eukaryotic membrane - modulate membrane fluidity by making membranes less fluid at warmer temperature and more fluid at cooler temperatures - cells may alter membrane lipid concentration in response to changes in environment (cold plants increase unsat. phospholipid concentration in autumn which prevents solidification in winter)

integral proteins

- generally transmembrane proteins with hydrophobic regions that completely span the hydrophobic interior of the membrane

Amphipathic

- hydrophobic and hydrophilic regions

solutions separated by selectively permeable membrane

- if 2 solutions of different concentrations are separated by a selectively permeable membrane only permeable to water then water will diffuse from the hypoosmotic solution (lower osmotic concentration-total solute) to the hyperosmotic solution (one with higher osmotic concentration-total solute)

receptor mediated endocytosis

- importing specific macromolecules into cell by inward budding of vesicles in coated pits - in response to specific ligands to receptors on cell surface - substances that are not very concentrated

plant cell

- in HYPOTONIC environment water moves by osmosis into plant cell, causing it to swell until internal pressure against the CW equals the osmotic pressure of the cytoplasm; a dynamic equilibrium is established

transport proteins

- integral membrane proteins that transport specific molecules or ions across biological membranes - may provide hydrophilic tunnel through membrane or bind to a substance and physically move it across the membrane - specific

hydrocarbons enhancing membrane fluidity

- kinks in C-C double bonds hinder close packing - membranes solidify if temperature decreases to a critical point - critical point is lower if membranes have a greater concentration of unsaturation phospholipids

propagation of tyrosine kinase receptors

- ligand binding causes aggregation of 2 receptor units - aggregation activates endogenous tyrosine kinase activity on cytoplasmic domains - endogenous tyrosine kinase catalyzes the transfer of phosphate groups from ATP to AA tyrosine contained in a particular protein... in this case, tyrosines which are phosphorylated are in the cytoplasmic domain of the tyrosine-kinase receptor itself (autophosphorylation)

transport

- may provide hydrophilic channel across membrane that is selective for a solute - other transport proteins shuttle a substance from one side to the other by changing shape.

osmotic pressure

- measure of the tendency for a solution to take up water when separated from pure water by a selectively permeable membrane - pure water: 0 - osmotic pressure of a solution is proportional to osmotic concentration (direct relationship)

membrane fluidity

- membrane held with hydrophobic, weak interactions - drift laterally within membrane - don't flip - membrane proteins drift more slowly than lipids - membrane must be fluid to work properly

intercellular joining

- membrane proteins of adjacent cells may hook together in various kinds of junctions - more long-lasting

attachment to cytoskeleton and ECM

- microfilaments or other elements of cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins

factors that contribute to cell membrane potential

- negatively charged proteins in cell interior - PM selective permeability to various ions - sodium potassium pump translocates 3 Na+ out to every 2K+ which is a net loss of 1 positive charge per cycle

passive transport

- no energy - spontaneous - rate of diffusion is regulated by permeability of membrane, so some molecules diffuse more freely than others - water diffuses freely across most cell membranes - osmosis = passive transport of water

plant cells in isotonic

- no net movement so the plant cell becomes flaccid or limp - loss of structural support from turgor pressure causes plants to wilt

cell markers

- oligosaccharides that are branched - some covalently bonded to lipids (glycolopids) - most covalently bonded to proteins (glycoproteins)

sodium potassium pump

- oscillates between 2 conformations - high affinity for Na+ with binding sites oriented towards cytoplasm - high affinity for K+ with binding sites towards a cell's exterior - ATP phosphorylates the transport protein and powers the conformational change - as the transport protein changes conformation, it translocates bound solutes across the membrane - Na+ K+ pump translocates 3 Na+ out of the cell for every 2K+ into the cell

net directional movement

- overall movement away from the center of concentration - results from random molecular movement in all directions

cotransport

- process where a single ATP powered pump actively transports one solute and indirectly drives the transport of other solutes against their conc. gradients - ATP powered pump actively transports one solute and creates PE in the gradient it creates - another transport protein couples th solute's downhill diffusion as it leaks back across the membrane iwth a second solute's uphill transport against its concentration gradient

peripheral proteins

- proteins not embedded but attached to membrane surface - may be attached to integral proteins or held by fibers of ECM

osmometer

- pure water is separated from a solution by a selectively permeable membrane that is permeable to water but not solute - the tendency for water to move into the solution by osmosis is counteracted by applying enough pressure with a piston so the solution's volume will stay the same - the amount of pressure required to prevent net movement of water into the solution is the osmotic pressure

3 stages of signaling

- reception - transduction - repsonse

diffusion

- result from intrinsic KE of molecules - random movement continue until dynamic equilibrium is reached - spontaneous process (decreases free energy) and increases entropy

polar (hydrophillic) molecules

- small polar uncharged molecules like water and ethanol that are small enough to pass between membrane lipids, easily pass thorugh synthetic membranes - large polar uncharged molecules (glucose) will not easily pass through synthetic membranes - all ions have difficulty passing through the hydrophobic layer

hypertonic solution

- solution with greater solute concentration than that inside a cell

cell-cell recognition

- some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells - this is usually short lived

hydration shell

- some solute molecules can reduce the proportion of water molecules that can freely diffuse - water molecules form a hydration shell around hydrophilic solute molecules and this bound water cannot freely diffuse across membrane - in dilute solutions including most biological fluids, it is the difference in the proportion of the unbound water than causes osmosis rather than the actual difference in water concentration

transport proteins vs. enzymes

- specific for solutes - can be saturated with solute so max transport rate occurs when all binding sites are occupied - can be inhibited by molecules that look like the solute - difference: protein has no catalytic functions

epinephrine

- stimulates glycogen breakdown by stimulating the cytosolic enzyme, cytosolic phosphorylase (cellular response) - epinephrine could only stimulate glycogen phosphorylase activity when presented to intact cells, suggesting that (1) the plasma membrane is critical for transmitting the signal (reception) and (2) activation of glycogen phosphorylase required the presence of of an intermediate step or steps inside the cell (signal transduction)

activation of G-protein

- temporary because the active G-protein possesses endogenous GTPase activity which hydrolyzes the bound GTP to GDP

transduction

- the binding of the signal changes the receptor in some way - usually a change in conformation or shape - the change in receptor initiates a process of converting the signal into a specific cellular response - the process is called signal transduction but it has many steps

signal reception

- the signal molecule acts as a ligand which binds to a specific cellular protein called a receptor

cellular response

- the transduction system triggers a specific cellular response - the response can be almost any cellular activity

functions of membrane proteins

- transport - enzymatic activity - attachment to cytoskeleton and ECM - cell cell recognition - intercellular joining - signal transduction

electrogenic pump

- transport protein that generates voltage across a membrane - Na+/K+ ATPase in animal cells - proton pump in plants, bacteria, fungi - voltages created by electrogenic pumps are sources of PE available to do cellular work

signal transduction pathway

- type a cells secrete a a factor chemical signal - type b cells secrete a type b factor - the binding of one a factor to type b cells and the binding of B factor to type A cells induces type B to move toward one another and fuse

membrane potential

- voltage across membrane - inside of cell is negatively charged with respect to outside - favors diffusion of cations into cell and anions out of cell - anions and cations are unequally distributed across PM

animal cell in isotonic environment

- volume of cell will remain stable with no net movement of water across PM

plasmolysis

- walled cell shrivels and plasma membrane pulls away from cell wall as the cell loses water to a hypertonic environment

plant cells in hypertonic

- walled cells lose water by osmosis and will plasmolyze (lethal)

Endocytosis

-importing macromolecules by forming vesicles from PM - pinch off - incorporate extracellular substances

how does substance move?

MORE CONC --> LESS CONC

G protein bound to GTP

active

nonpolar (hydrophobic) molecules

dissolve in the membrane and cross it with ease (hydrocarbons, oxygen, carbon dioxide) - if 2 molecules are equally lipid soluble, the smaller crosses fastest

hypertonic environment (animal cell)

lose water by osmosis and crenate (shrivel)

ligand

molecule that binds to specific receptor site of another molecule

osmotic concentration

total solute concentration of a solution