Passive, Facilitated, and Active Transport Flashcards
the hydrophobic interior of a lipid bilayer prevents the passage of most
polar molecules
cells have specialized — — to transfer specific water soluble molecules and ions across their membranes
transmembrane proteins
the major classes of membrane transport proteins are known as (2)
transporters (carriers or permeases) and channels
passive/faciliated diffusion
channels and transporters which allow solute to cross the cell membrane down a concentration gradient
in the case of a single uncharged molecule, the — on each side of the membrane drives passive transport and determines its direction
concentration
high to low
in the case of a solute that carries a net charge, both its — — and — — — influence transport
concentration gradient
electrical potential difference
the concentration gradient and the electrical gradient combine to form the
net driving force/electrochemical gradient
smaller, hydrophobic molecules diffuse — across a bilayer
faster
types of molecules which can pass through channels depends on (2)
diameter
amino acids that make up the lining of the channel and how they interact with the molecule
energy used by carrier proteins (3)
ATP hydrolysis
mechanical energy from movement of H+ through the channel
light
electrochemical gradient
when one side of the membrane is more positively charged than the other
carrier proteins mediate passive transport via
conformational changes
for simple diffusion, the rate of transport is proportional to the
concentration of molecule being transported
for transporter mediated, the rate of transport reaches a maximum when transport protein is
saturated
the 1/2 Vmax and Km for carrier mediated is similar to those values for
enzyme:substrate kinetics
3 ways to drive active transport
coupled
ATP driven
light driven pumps
light driven pumps
found in bacteria and use energy from light
3 types of carrier mediated transport
uniport
symport
antiport
glucose carrier is driven by
Na+ gradient
glucose symport carrier mechanism
The glucose carrier in the gut oscillates between 2 states (A and B).
Binding of Na+ and glucose is cooperative; ie. when one binds this facilitates the binding of the other.
In State A the extracellular Na+ concentration is much higher than the cytosol concentration and so when Na+ binds this induces glucose to bind
When both are bound this induces a conformational change that results in the release of glucose and Na+ into the cytosol
Na+ and K+ intracellular concentrations
intracellular K+ is high and intracellular Na+ is low
these concentrations are maintained by the
Na+/K+ pump
for evert molecule of ATP that is hydrozyled,
3 Na+ are pumped out and 2 K+ are pumped into the cell
importance of ATP in the Na+/K+ pump
phosphorylate the aspartic acid residue
leads to binding and conformational change of the pump
types of occluding junctions (2)
tight junctions (vertebrates) septate junctions (invertebrates)
anchoring junctions with actin filament attachment sites (2)
cell-cell junctions (adherens junction)
cell-matrix junctions (focal adhesions)
anchoring junctions with intermediate filament attachment sites
cell-cell junctions (desmosomes)
cell-matrix junctions (hemidesmosomes)
communicating junctions (2)
gap junctions
plasmodesmata (plants only)
signal-relaying junctions (1)
chemical synapses
the progeny of the founder cell are retained in the epithelium by the basal lamina and by cell-cell adhesion mechanisms, including the formation of
intracellular junctions
E-cadherin location
epithelia
N-cadherin locations (5)
neurons, heart, skeletal muscle, lens, fibroblasts
P-cadherin locations (3)
placenta, epidermis, breast epithelium
VE-cadherin location
vascular endothelial cells
tight junctions and adherens junctions crease.
barrier to the movement of molecules extracellularly bretweencells
this creates the need for
intracellular and/or transcellular transport
transcellular transport
the movement of a molecule through a vesicle
vesicle membrane fuses with the plasma membrane to form one side of the cell to the other
cadherins require
Ca+
low Ca+
high Ca+
cadherins dont do much, limp
cadherins stiffen and induce binding of cadherins
gap junctions are protein tubes composed of
connexin monomers
gap junctions connect two cells by
penetrating the cell membranes of two adjacent cells
this provides a fluid filled space through which materials of less than about —- molecular weight can pass from one cytoplasm to the next
1,000
examples of materials transported via gap junctions are (2)
calcium
ATP
gap junctions contribute to the electrical couples of the (3)
heart
neurons
retinal tissues
hereditary mutations in specific connexin genes can cause
cataracts in infants or at birth, and deafness
gap junctions can exist in the
closed or open form
connexins can be either
homomeric or heteromeric
the intracellular channels can be either
homotypic of heterotypic
humans have – distinct connexins
14
each connexin is encodes by its own
gene
most cells express more than
one type of connexin
2 different connexins can assemble into a
heteromeric connexon
connexin hemichannels
Osteocytes connect to each other only at the tips of their dendrites where gap junctions are formed.
Osteocytes have connexin 43 hemichannels (half of a gap junction) along their cell body and dendritic processes that all movement of small molecules in and out of the cell through the lacunar-canalicular system
