Exam 1: Ch 4 Book Flashcards
fluid mosaic membrane
membranes are dynamic and complex
globular proteins are integrated with the membrane
evidence: freeze etch of membrane b4 and after subjecting it to proteolytic enzymes
plasma membrane
thin lipid-based structure that encloses the cytoplasm and the cell nucleus
held together by non covalent interactions
integral proteins
proteins that span the plasma membrane that include channels, pumps, pores, enzymes, and receptors
peripheral proteins
associated with the surface of the plasma membrane by electrostatic interactions
3 primary types of lipids in cell membranes
phosphoglycerides: glycerol backbone
sphingolipids: sphingosine base backbones
sterols: cholesterol (nonpolar and slightly soluble in H2O)
which two types of lipid are amphipathic?
phosphoglycerides and sphingolipids
fluidity
the ability of membrane components to move relative to one another
cholesterol makes membranes less fluid but stronger/stiffer
diffusion
random thermal motion of suspended or dissolved molecules causes them to disperse from regions of high concentration to low concentration until equilibrium is reached
Flick diffusion equation
rate of diffusion of a solute s
dQs/dt = DsA(dCs/dx)
DQs/dt = rate of diffusion (quantity of s diffusing per unit time)
Ds = diffusion coefficient of s
A = cross sectional area s is diffusing through
dCs/dx = concentration gradient of s (determines rate)
why is a concentration gradient important to diffusion
determines the rate at which solute s diffuses down the gradient
higher the gradient fast the diffusion
membrane flux
a solute on both sides of a membrane has a unidirectional flux
the flux (J) is the amount of solute that passes through a unit area of membrane every second in one direction
J = dQs/dt
permeability & permeability constant
the rate at which the substance passively penetrates the membrane under specified conditions
dQs/dt = P(C1 - C2)
C1 & C2 are concentrations on either side of the membrane
P, the permeability constant has the dimension of velocity (cm per second)
diffusion coefficient
how fast a solute diffuses through a membrane
more viscous the membrane the lower the value
partition coefficient
how well a solute dissolves in lipids vs water
K = [solute in lipid] / [solute in water]
higher means better lipid solubility
osmosis
the movement of water down its concentration gradient from more pure to less pure
hydrostatic pressure
a pressure gradient across a semipermeable membrane caused by osmosis
when equal to osmotic pressure, water net flux is 0
osmotic pressure
pressure applied by a solution to prevent inward flow of water
proportional to solute concentration and absolute temperature
pi = RTC
higher [solute] = higher osmotic pressure
osmolarity
theoretical comparison
osmotic membrane allows water to pass but not solutes
all solutions with the same number of dissolved particles per unit volume are isosmotic
isosmotic, hypoosmotic, hyperosmotic
two aqueous solutions that exert the same osmotic pressure
the solution that exerts less osmotic pressure than another solution
the solution that exerts more osmotic pressure than another solution
tonicity
the response of cells or tissues immersed in a solution
functional cell based comparison
isotonic
a solution is isotonic to a cell or tissue if the cell or tissue neither shrinks or swells when placed in it
there is no osmotic pressure difference and thus no net water gain or loss
hypotonic
if the tissue swells because it absorbs water the solution is said to be hypotonic to the tissue
hypertonic
if the tissue shrinks because it loses water the solution is said to be hypertonic to the tissue
what does membrane permeability to charged particles depend on
membrane permeability constant
electric potential across the membrane
what two forces act on charged atoms or molecules to produce net passive diffusion across a membrane
chemical gradient from difference in solute [ ]s across the membrane
difference in electric potential across the membrane
electrochemical gradient
the sum of the concentration gradient and electrical gradient
equilibrium potential
The voltage when net ion flux is 0
what affects the equilibrium potential
ratio of the ion concentrations on opposite sides of the membrane
when can an ion passively diffuse against its concentration gradient?
when there is a greater electrical potential than chemical concentration gradient
ex. if the interior of a cell has a negative charge greater than the equilibrium potential, K+ ions will diffuse into the cell even though its intracellular [ ] is higher than extracellular
what is the most concentrated inorganic ion in the cell
potassium K+
10-30 times higher inside than out
is Na+ higher inside or outside cell
outside
is Ca2+ higher inside or outside cell
outside
why can plant and bacterial cells withstand higher osmotic pressure and turgor pressure
they have rigid cell walls
3 types of mechanisms to move things across the membrane
passive diffusion (simple diffusion)
passive transport (facilitated diffusion)
active transport
which two mechanisms of transport across the membrane do not use ATP
passive diffusion and passive transport
passive (simple) diffusion
solute molecule comes in contact with membrane and passes through if thermal energy is high enough
breaks H-bonds with extracellular water to diffuse
rate of influx is determined by concentration gradient (high to low)
passive transport (facilitated diffusion)
protein pores, channels, or carriers move solutes down their concentration gradients into the cell
no ATP used
carrier protein
an integral protein that mediates the movement of solute across a membrane down its concentration gradient
this mechanism is called carrier-mediated passive transport
unitary current
sudden opening of channels that allow thousands of ions/sec to cross membrane
nystatin
a rod-shaped antibiotic that forms channels that allow water, urea, and chloride to diffuse
produces a 100,000 fold increase in membrane permeability to chloride
doesn’t allow large molecules or cations to diffuse
what do we learn from nystatin channels?
very little membrane area is needed to produce significant ion permeability changes through channels
aquaporin
channels that specifically permit passive diffusion of water but exclude ions and other substances
not permanent hourglass shaped channels controlled by hormone regulation
ionophores
small organic compounds that transport ions across the membrane
uniporter
carrier protein that transport a single solute
coupled transporter
carrier proteins that transport one solute and simultaneously transport a second solute
2 types (symporter and antiporter)
symporter
a coupled transporter carrier protein that transfers two solutes in the same direction
antiporter
a coupled transporter carrier protein that transports two solutes in opposite directions
carrier protein kinetics
a rate plateau is reached b/c michaelis menten
rate tapers off when all protein units are saturated
how was specificity of transporters discovered
in cystic fibrosis a defect in chloride transport channel protein (CFTR) is responsible for fluid imbalance in lungs
why is the distribution of ions across membranes of living cells never at true equilibrium
b/c all living cells continuously expend ATP to maintain a stable differential of transmembrane ion concentrations
active transport 2 types
eiher primary active transport or secondary active transport
primary active transport
ATP dependent pumps transport substances against their gradients
if the energy source is cut off, active transport stops
secondary active transport
movement of substance against its electrochemical gradient b/c it is moving down its own concentration gradient
doesn’t use ATP
Na/K pump models primary active transport
enzyme called an ATPase with binding sites for Na and ATP on cytoplasmic side, and K+ on extracellular side
pumps 3 Na out and cleaves ATP –> ADP to induce a conformational change, and pumps 2 K in to maintain higher extracellular Na
symporter example
transport of alanine is coupled to the transport of Na+
when Na+ is present, alanine is taken up by the cell until the cytosolic [ ] is 7-10 times higher than extracellular
block Na/K pump with inhibitor ouabain and effect on alanine transport
inhibitor diminishes extracellular Na [ ] and therefore the gradient
this stops transport of alanine into cell
antiporter example
Na/Ca antiport system maintains low cytosolic Ca levels
Ca expelled from cell in exchange for Na leaking in
Na/H cotransport
antiporter in proximal tubule of the kidneys
extrusion of H+ from inside cells into urine couples with Na uptake into cell in 1:1 ratio
avoids expending ATP to perform electrical work of exchanging two ions of the same charge
allows kidney to reclaim Na from urine and excrete excess H+
metabolic poisons bring what type of transport to a halt
primary active transport
rheogeneic
ionic pumps that produce net charge movement
produce a transmembrane electric current
electrogenic
describes a pump that produces an electric current with measurable effect on the voltage across the membrane
how much energy does the Na/K pump use
25% of total energy
50% in kidneys
the energy of ion gradients can be used to drive …
passive transport or secondary active transport
also needed to conduct info along the surface of the membrane
when does energy release occur in ion gradients
when the ions are allowed to travel down their concentration gradients
3 important cellular processes that utilize the free energy of ion gradients
production of electrical signals
chemiosmotic energy transduction
uphill transport of other molecules
electrochemical energy is stored primarily as…
Na and Ca gradients
release of this energy controlled by gated ion channels that are normally closed, but open in response to chemical or electrical signals
this is the basis of the nervous system
chemiosmotic energy transduction
electron transport chain in mitochondria utilizes H+ gradient to synthesize ATP
two important factors producing membrane channel selectivity for ions other than size
ease of dehydration
charges within the channel pore
ease of dehydration
for an ion to enter a channel pore it must dissociate from water molecules
large ions dehydrate more easily than smaller ones so a pore with weak polar sites will admit large ions preferentially over smaller ones
charges within the channel pore
charged amino acids repel ions with the same charges
smaller ions can approach the polar sites more easily and interact with them more strongly than large ions
channel pore charge example
K channels in bacterium streptomyces have 4 identical protein subunits
K+ passes through in single file aided by negative aas (K is larger than Na)
Na cannot pass through b/c it is too small to interact with protein subunits
selectivity for nonelectrolytes
determined by molecular properties responsible for the partition coefficient
mechanisms for restricting nonelectrolyte flow through membranes haveq not evolved
drugs like a nicotine patch take advantage of this
endocytosis
ingestion of macromolecules by formation and fusion of membrane bound vesicles
called pinocytosis if fluid is ingested
called phagocytosis if solids are ingested
exocytosis
secretion from a cell of macromolecules
receptor mediated endocytosis
receptor molecules embedded in external membrane bind ligands that cannot pass through channels
receptors can diffuse laterally through membrane
coated pit
upon binding ligand, the receptor-ligand complex accumulates within depressions in the membrane
this pit invaginates and pinches off, forming a coated vesicle
clathrin
coats coated vesicles and covers the cytoplasmic surface of the vesicle membrane
directs budding off of vesicle from membrane
recycled to the plasma membrane after contents delivered
mechanism of exocytosis
fuse with plasma membrane and release substances into extracelular space
membrane recycling
endocytosis recovers extra vesicle material that fuses during exocytosis
tissue
cooperative assemblies
gap junctions
provide a means of communication between cells by allowing inorganic ions and small water-soluble molecules to pass directly from the cytosol of one cell to another
couple cells electrically and metabolically
clusters of hexagonal channels with 6 subunits
gap junction experiment
used fluorescent dyes to track diffusion into neighboring cells
tight junctions
seal cells together into an epithelial sheet but do not provide a channel
stops leakage of substances across the membrane from gaps inbetween cells (paracellular)
substances must cross the membrane transcellularly
zonula occludens
tight junctions in epithelial cells
thin band of protein molecules that encircles a cell like a gasket
paracellular pathway
path through a membrane between cells
transcellular pathway
path through a membrane going through the body of the cell
zonula adherens and desmosome
cell junctions that stabilize the structural bonding of neighboring cells
epithelia have several features in common
occur at surfaces that separate the internal space of an organism from the environment
cells of outermost layer are sealed by tight junctions
have a serosal (internal) and mucosal (external) side
salt transport across epithelium experiment
uses frog skin to demonstrate that active transport is needed to move salt across an epithelium
Na/K pump in serosal side creates normal net neg resting potential inside cell
mucosal side is impermeable to K and has channels or carriers that let Na in
major features common to transport epithelium
tight junctions disallow paracellular pathways so transport through transcellular pathways is used
mucosal and serosal portions exhibit functional differences; they are asymmetric in pumping and permeability
active transport of cations is accompanied by transport (active or passive) of anions in same direction to minimize buildup of electrical potentials
transport not limited to pumping of Na, other ions too
water balance in animals is achieved by the regulation of water transport via _______
epithelial sheets
can absorb or secrete aqueous fluids
2 hypothesis of uphill transport of water by epithelia
transported by a specific carrier mechanism driven by ATP (unlikely b/c no pump or carrier has been found)
transported by osmosis as a consequence of solute transport (validated as the Standing-Gradient Hypothesis)
osmosis through epithelia
water flows osmotically following Na active transport through portions of membranes facing the intercellular clefts
permeability constant equation
P = DmK / x
Dm = diffusion coefficient
K is partition coefficient
x = thickness of the membrane
