transport mechanisms Flashcards
define homeostasis
refers to any process that living things use to actively maintain fairly stable conditions necessary for survival
coined by walter cannon
how to preserve constancy of milieu interieur and the homeostasis
exchange nutrients, salts, gases and waste in and out of the body
what is the cell membrane highly permeable to
H2O
lipid soluble substances
dissolved gasses (O2 and CO2)
small uncharged molecules
what is the cell membrane less permeable to
larger molecules
charged particles
what is the cell membrane impermeable to
very large molecules
describe the cell membrane (plasma membrane)
6-10nM thick (very thin)
bimolecular phospholipid layer (phospholipid bilayer)
amphipathic = polar hydrophilic heads on outside and non polar hydrophobic tails in center
how much of the plasma membrane is phospholipids (by weight)
40-50%
is cholesterol hydrophobic
slightly amphipathic
describe what happens when cholesterol is inserted into the phospholipid bilayer
reduces packing of fatty acid tails
tends to increase membrane fluid it keeping fatty acid tails apart
describe what cholesterol may also be involved in
formation of vesicles that pinch off the plasma membrane in lipid rafts
what does cholesterol do at high temps
acts to stabilize the cell membrane and add firmness
what does cholesterol do at low temps
inserts into phospholipids and prevents them from interfering with each other to avoid aggregation
describe proteins
most diverse macromolecules
25-75% membrane by weight
name the 2 types of proteins
integral and peripheral
describe integral proteins
closely associated with phospholipids
mostly cross cell membrane
trans membrane and amphipathic
embedded transverse the whole membrane
not easily removable
describe peripheral proteins
more loosely associated
mostly on cytoplasmic side
easily removable
located on cell surface
what is glycocalyx (describe it)
layer of carbs formed by chain of monosaccharides
extend from extracellular surface of the cell membrane bound to proteins
what is glycocalyx associated with
glycoproteins and glycolipids
what is the purpose of glycocalyx
provides protection from infection
enables cells to identify each other and interact
describe fluid mosaic model or cell membrane
proteins are fixed and can move around
not a stable situation
membrane is always being replenished and so there’s much more turnover
name the functions of plasma membrane proteins (6)
selective transport channel
enzyme
cell surface receptor
cell surface identity marker
cell adhesion
attachment to the cytoskeleton
describe selective transport channel (functions of plasma membrane)
channels (ion channels) and transporters (ions and other molecules)
transport and diffusion of specific molecules into and out of cell
describe enzyme (functions of plasma membrane)
amino acid transport Na-K pumps
enzymes like ATPase
act as enzymes that catalyze membrane associated reactions
describe cell surface receptor (functions of plasma membrane)
G protein coupled receptors
insulin receptors
ACh receptors
for signaling within cells
serve as receptors for receiving and transducing chemical signals from the cell environment
describe cell surface identity marker (functions of plasma membrane)
proteins or cells
give each a marker/target for therapeutics
ex =
CD4 T lymphocytes
CD45 leucocytes
CD68 monocytes
describe cell adhesion (functions of plasma membrane)
CAMs
cadherins
integrins
cell-cell adhesion
describe attachment to the cytoskeleton (functions of plasma membrane)
proteins involved in attachment and stability
actin
microtubules
septins
name the 2 transmembrane transport pathways
via phospholipid bilayer
via interaction with protein cluster (channel or carrier)
name the 2 transport mechanism across cell membrane
passive
active
describe passive transport mechanism across cell membrane and name types
energy independent
diffusion
carrier mediated facilitated diffusion
osmosis
describe active transport mechanism across cell membrane and name types
energy dependent
carrier mediated active transport (primary and secondary)
pino/phagocytosis
what is diffusion
simple diffusion is the movement of molecules from one location to another as a result of random thermal movement
what is flux
amount of particles crossing a surface per unit time
net flux is from high concentration to lower
concentration
from high to low
at equilibrium describe diffusion fluxes and net flux
diffusion fluxes are equal
net flux is zero (molecules don’t stop moving but it’s equal and opposite movement)
does diffusion occur even in the presence of a mechanical partition (membrane)
yes as long as it is permeable to
the diffusing particles
describe the movement from high to low
downhill
until both are equal
describe flux from high to low
high to low = one way flux from high to low (large amount)
one way flux from low to high (small amount)
net flux = from high to low (medium amount)
describe what happens to intracellular concentration with constant extracellular concentration
intracellular concentration eventually equals extracellular concentration
equilibrium reached when evenly distributed across cell membrane
how to calculate rate of diffusion
ficks law of diffusion
J=PA(Co-Ci)
J=mol/cm^2/sec
describe ficks law of diffusion equation - J
J = net flux (rate of diffusion) - moles of solute crossing
describe the relationship between diffusion time and distance
diffusion time increases in proportion to the square of the distance travelled by the molecules
describe ficks law of diffusion equation - P
permeability or diffusion coefficient
constant based on the ease that a molecule moves through a membrane with
(property of the solute that is moving through the medium)
describe ficks law of diffusion equation - A
surface area of the membrane
cm^2 or area of whole cell
describe ficks law of diffusion equation - Co-Ci
concentration gradient of the diffusing molecule across the membrane
driving force - concentrations change on both sides and this drives the diffusion
T or F: diffusion is very inefficient over short distances
FALSE
diffusion is an effective transport method only over short distances
einsteins approximation equation
describe diffusion of glucose (distance and time)
diffusion is good over short distances
1um = 1 m sec
10 um = 100 m sec
100 um = 10000 m sec
name factors that affect diffusion across the membrane (5)
1- mass of the molecule
2- concentration gradient across cell membrane
3- lipid solubility (if going across lipid portion of cell membrane
4- electrical charge
5- availability of selective ion channels or membrane carriers
how do diffusion particles penetrate cell membrane - 2 ways
1 - dissolving in lipid component - for non polar molecules like O2 CO2 and fatty acids
2 - diffusing through channels for ions
what is needed for movement (diffusion of particles across cell membrane)
depends on existence of a concentration gradient
describe ion channels
consist of a single protein or clusters of proteins
show selectivity based on their diameter and the distribution of charges lining the channel
has hole down middle, subunits around it
what else is the movement of ions affected by
electrical gradient
simultaneous existence of an electrical and a concentration gradient for a particular ion = electrochemical gradient
what charge are the inside of cells
always negative
what determines membrane potential
concentration gradient
describe electrical gradient (movement of K+)
intracellular - K is high -90mV
extracellular - K is low so K is moved out of cell and negatives are moved into cell
describe the states ion channels can exist in
open or closed state as they undergo conformational changes = gating
name the 3 ways channels may be gated
ligan gated - cooper
voltage gated - hanrahan, lukacs shrier
mechanically gated - sharif
name and explain the 2 ways ion channels can be open or closed
conformational change - activation = open and gate closes = inactivation, physically closed off
occlusion of channel pore = blocking or closing with a gate - actual thing that blocks end of channel pore
describe voltage gated ion channels
cell undergoes action potential and cell membrane depolarizes
describe ligand gated channels
ligand that can bind to receptor and inactivate channel
ex= acetylcholine neurotransmitter
describe mechanically gated channels
transform mechanically into another type - pain receptors
name the main voltage gated ion channels
Na+
K+
Ca+
Cl-
what does the total number of ions that flow through these channels (generating ionic current) depend on - 3
channel conductance (how open it is = small/big)
how often channel opens
how long channel stays open
what is mediated transport
movement of ions and other molecules like amino acids and glucose by integral membrane proteins (transporters/carriers)
is ion movement across membranes via transporters slower or faster than through ion channels
transporters are MUCH slower than through ion channels
name types of mediated transport
facilitated diffusion
active transport (primary active and secondary active)
name the characteristics of mediated transport systems (3)
specificity
saturation
competition
describe specificity (characteristics of mediated transport systems)
systems usually transport one particular type of molecule only
describe saturation (characteristics of mediated transport systems)
rate of transport reaches a mx when all binding sites on all transporters are occupied
Tm = exists for a given substance across a given membrane
ex: seats on a bus analogy
the greater the gradient…
the higher the flux
describe competition (characteristics of mediated transport systems)
happens when similar substance compete for the same binding site on a membrane carrier
name the 4 factors that determine the flux magnitude of mediated transport system
1 - solute concentration
2 - affinity of transporter for the solute
3 - number of transporters
4 - rate of transporter conformational change
describe facilitated diffusion
involves presence of a transporter or carrier molecule
enables a solute to penetrate more readily than simple diffusion
molecule must associate with carrier to cross
driven by concentration gradient
can flip back and do opposite if conformations switch
describe steps of facilitated diffusion
solute binds transporter
transporter changes configuration
solute is delivered to other side of membrane
transporter resumes original configuration
describe facilitated diffusion (method, energy and flux?)
transporter/carrier mediated
passive - no energy
net flux from high to low concentration
name and explain systems that use facilitated diffusion
hormones may increase the number and/or affinity of transporters in some membranes
glut-4 transports glucose in muscle that is increased by insulin
describe active transport - 4 statements
transporter mediated
requires supply of chemical energy - usually derive from enzymatic hydrolysis of ATP/GTP
susceptible to metabolic inhibitors
can transport solute against its concentration gradient - uphill transport
describe primary active transport
active transport involves hydrolysis of ATP by a transporter/carrier
phosphorylation of transporter changes the conformation of the transporter and its solute binding affinity
name and describe a very important active transporter
Na+/K+-ATPase
changing in the binding site affinity for a transported solute are produced by phosphorylation and dephosphorylation of Na+/K+-ATPase
describe Na+/K+-ATPase steps/changes in conformation
intracellular fluid - 3 binding sites for Na+
hydrolysis ATP conformation change and loses affinity for K+
sodium diffuses off
2 K+ binding sites then back to first change in conformation againn
name some active transporters and their functions
Ca2+-ATPase = maintain low intracellular Ca2+ levels
H+-ATPase = maintain low lysosomal pH
H+/K+-ATPase = acidification of stomach
is an ion channel a carrier
NOOOOO
ion channels allows ions to permeate down the concentration gradient
a carrier can transport ions but does not pass through a channel - it is a conformational change of a protein
describe secondary active transport
movement of Na+ down concentration gradient is coupled to the transport of another solute molecule (ion, glucose, amino acid) up hill against its concentration gradient
what energy does secondary active transport use
energy stored of the electrochemical gradient to move both Na+ and transported solute
creation and maintenance of electrochemical gradient depends on primary active transport
describe 3 steps of secondary active transport
1 - sodium binds to a transporter outside the cell (high sodium concentration) allowing glucose or amino acids or ions to bind to the same carrier
2 - through a change in configuration transporter delivers both sodium and glucose or amino acid into cell
3 - transporter then reverts to its original configuration and the sodium is extruded from cell by Na+/K+-ATPase
what is cotransport
when solute is transported in the same direction as sodium
aka symport
Na+-aas
what is countertransport
when solute is transported in opposite direction to Na+
aka antiport
Na+-Ca2+
name secondary active transport mechanisms (5)
Na+/H+ exchanger - antiport
Na+/Ca2+ exchanger - antiport
Na+/HCO3- symporter
Na+-amino acid symporter
Na+-glucose symporter (sodium-glucose linked transporter SGLT like glut4)
briefly describe endocytosis and exocytosis
active transport mechanisms
energy dependent
involves participation of the cell membrane itself
describe endocytosis (brief - process)
cell membrane invaginates and pinches off to form a vesicle
describe exocytosis (brief - process)
intracellular vesicle fuses with the cell
membrane and it’s contents are released
into ECF
describe and name the 2 types of exocytosis
exocytosis is the process of moving material from the inside to outside of cell
1- constitutive exocytosis
2 - regulated exocytosis
name the 3 types of endocytosis
pinocytosis
phagocytosis
receptor mediated endocytosis
describe pinocytosis
fluid endocytosis
involves endocytotic vesicle that engulfs extracellular fluid including solutes present
vesicles travel info cytoplasm and fuse with other vesicles like endosomes or lysosomes
describe phagocytosis
cells bind and internalize particulate matter (>0.75um), like small dust, cell debris and microorganisms
specific and triggered
extensions of cell membrane (pseudopodia) fold around particle and fully engulf it
pseudopodia fuse to form large vesicles (phagosomes)
phagosomes migrate and fuse with lysosomes where contents of phagosome are degraded
defends against infection and scavenge senescent and dead cells
involves macrophages, neutrophils and dendritic cells
describe receptor mediated endocytosis and name 2 types
molecules in extracellular fluid (ligands) bund with high affinity to specific protein receptors on the plasma membrane
clathrin dependent receptor mediated endocytosis
potocytosis
describe clathrin dependent receptor mediated endocytosis
when ligand binds receptor undergoes conformational change
clathrin recruited to plasma membrane
adaptor proteins link ligand receptor to clathrin complex forms a cage like structure that leads to aggregation of ligand bound receptors
then clathrin stuff (ex = LDL receptor)
describe LDL receptor
clathrin coated pit formed then invaginates and forms clathrin coated vesicle
vesicle pinches off and sheds clathrin coat
vesicles can dude with membrane of cellular organelles like endosomes and lysosomes
or they can fuse with membrane on another side of cell (transcytosis) receptors and clathrin protein are recycled back to cell membrane
describe potocytosis
molecules are sequestered and transported by tiny vesicles (caveolae)
vesicles are clathrin independent
caveolae can deliver contents directly into cell cytoplasm and to ER and other organelle and to plasma membrane on opposite side of cell (transcytosis)
implicates in uptake of low molecular weight molecules like vitamins
describe diffusion of water
water diffuses freely across most cell
membranes
facilitated by groups of proteins called aquaporins that form permeable channels
describe osmosis
net diffusion of water across a semipermeable membrane (permeable to solvent but not all solute)
must be able to calculate molarity (moles of solvent) also note that if solute then must account for that in moles
describe osmotic pressure
pressure required to prevent movement of water across a semi permeable membrane
pressure is equal to the difference in hydrostatic pressure of the two solutions
describe relationship between osmotic pressure and gas
osmotic pressure is related to temp in the same way as pressure of a gas
pv=nrt
osmotic pressure is proportional to the number of particles in the solution/unit volume and not their size/configuration/charge
what is osmolarity
total solute concentration of a solution
osmotic pressure is proportional to osmolarity (Osm)
1 osmol = 1 mole of solute particles
osm = osmol/litre
must consider dissociation (NaCl = 2 osmol of solute)
describe how to determine osmotic pressure of physiological saline 3
determine molarity
determine osmolarity
calculate osmotic pressure (osm to atm to mmHg)
describe isosmotic
solutions have the same osmolarity (concentration of osmotically active particles) as normal intracellular or extracellular solution
300 mOsm
describe hypoosmotic
solutions with osmolarity lower than 300 mOsm
describe hyperosmotic
solutions with osmolarity greater than 300mOsm
describe particles needed to be effective in exerting a sustained osmotic pressure
particles must not be able to cross the membrane
referred to as non penetrating
extracellular sodium = non penetrating bc sodium that moves into cell is pumped out by Na-K ATPase
describe what happens when you put rbc in isotonic extracellular solution
when solution has concentration of 300mOsm of non penetrating solute particles
no net shift of water
if penetrating - solute goes in and water goes in and then cells with burst
describe what happens when you put rbc in hypotonic extracellular solution
concentration of no penetrating solute less than 300mOsm
water will enter cells and they will swell
describe what happens when you put rbc in hypertonic extracellular solution
if solution has a concentration of solute non penetrating particles greater than 300mOsm then water will leave cell and it will shrink
where does main exchange take place
at level of the capillaries
describe capillaries physically
adult had ~40km of capillaries
capillaries contain ~5% of total circulating blood
each capillary is ~1mm long and has inner diameter of ~8um
describe capillary wall
single layer of flattened endothelial cells and a supporting basement membrane
name and briefly describe capillary structure and permeability characteristics (4)
1 - small water voluble substances pass through the pores
2 - lipid soluble substances pass through endothelial cells
3 - exchangeable proteins are moved across by vesicular transport
4 - plasma proteins cannot cross capillary wall
describe transport across capillary wall (1,2-diffusion)
across cell membrane
very important
also occurs through water filled channels (intracellular clefts and fused vesicle channels)
describe transport across capillary wall (3-transcytosis)
endocytosis on the luminal side followed migration of the vesicle across the cell and then exocytosis on the interstitial side
describe transport across capillary wall (bulk flow)
distributes extracellular fluid volume between plasma and ISF
magnitude of bulk flow is proportional to hydrostatic pressure difference between plasma and ISF
capillary wall acts as filter that permits protein free plasma to move from capillaries to ISF (difference in hydrostatic pressures)
describe constitutive exocytosis
non regulated
functions to replace plasma membrane
deliver membrane proteins to cell membrane
get rid of substances from the cell
describe regulated exocytosis
tends to be triggered by extracellular signals and increase of cytosolic calcium
responsible for secretion of hormones, digestive enzymes and neurotransmitters
ex= synapses and vesicles with acetylcholine
how is cholesterol transported in blood
lipid protein particles known as LDL
lipoprotein is recognized by PM LDL receptors and endocytosis follows
