CHAPTER 4: Movement of Water/Solutes Flashcards
simple diffusion
movement of molecules from one location to another (requires no energy other than heat)
flux
amount of material crossing a surface in a unit of time
diffusion equilibrium
when 2 one-way fluxes are now equal in magnitude but opposite in direction
net flux always proceeds from _____ to _____ concentration
high to low (downhill)
factors that magnitude of net flux depends on
temperature, mass of the molecule, surface area, medium through which the molecules are moving
requirements for net flux to occur
1) concentration gradient of solute
2) surface must be permeable to solute
how can diffusion rate across plasma membrane be measured?
by analyzing the rate at which its IC concentration approaches diffusion equilibrium with its EC concentration
variables in fick’s law of diffusion
rate of diffusion, permeability constant, surface area, concentration in and out
increases lipid solubility increases _____? why?
increases flux, because # of polar/ionized groups are decreased, so # of molecules dissolved in membrane lipids will increase
how do ion channels show selectivity?
channel diameter, charged and polar surfaces of the subunits, and # of water molecules associated with the ions
true or false: the excess negative charges inside of the cell attract the positive charges outside of it so they align on membrane’s surface
true
electrochemical gradient
direction and magnitude of ion flux depends on concentration diff and mem potential
channel gating
process of opening and closing ion channels = total # of ions that passes through the channel depends on how often channel opens & how long it stays open
3 factors that alter channel protein conformations:
1) LIGAND GATED: binding of specific molecules to channel proteins may produce allosteric or covalent change in shape
2) VOLTAGE GATED: changes in potential can cause movement of certain charged regions on a channel protein
3) MECHANICALLY-GATED physically deforming (stretching) the membrane
transporters
integral membrane proteins that mediate passage of molecules + non diffusional movements of ions
true or false: transporters don’t have to change shape for each molecule transported across membrane
false
as the concentration of solute to be transported increases…
of occupied sites increases until transporters become saturated (all sites are occupied)
facilitated diffusion
net flux of a molecule across membrane always proceeds from higher to lower concentration
how long does facilitated diffusion continue?
using a transporter to move solute, it continues until the concentrations of solute on both sides of membrane become equal
active transport
energy is used to move a substance UPHILL across a membrane aka against its concentration gradient and requires a substance to bind to transporters in the membrane
what is used in primary active transport?
phosphorylation of transporter by ATP
what is used in secondary active transport?
use of electrochemical gradient via binding of ions
how does Na move from?
from intracellular to extracellular
how does K move from?
from extracellular to intracellular
primary active transport
hydrolysis of AYP by a transporter provides energy –> ATPase
secondary active transport
movement of an ion down its electrochemical gradient is coupled to transport of another molecule (ex: glucose, amino acid)
ultimately, energy is required indirectly via primary Na+ pumps for maintaining the concentration gradient
how is the electrochemical gradient for Na directed?
it is directed into the cell because of higher concentration of Na in the EC fluid + excess negative charges inside the cell
binding of Na increases _____ for transported solute, then transporter changes conformation, which decreases _____
affinity, affinity
what is the most important distinction between primary and secondary active transport?
secondary uses stored energy of an electrochemical gradient to move both an ion and second solute
both use ATP because the energy in secondary is technically derived from ATP
contransport
movement of actively transported solute in the cell (symport)
countertransport
movement of actively transported solute out of the cell (antiport)
how does water cross membranes?
1) diffusing through the lipid bilayer and 2) diffusing through
protein channels in the membrane
what is osmosis mediated by?
membrane proteins called aquaporins that form channels through which water can move
describe movement of solute and water during osmosis
solute: low to high solute
water: high to low water
osmolarity
total solute concentration in the solution – determines the water concentration
the higher the osmolarity of a solution, the lower the water concentration (and greater the osmotic pressure)
mormal intracellular osmolarity is 300 mOsm
tonicity
concentration of only the non-penetrating particles
semi-permeable membrane
a membrane permeable to water but not to solutes
what are substances that cannot cross the plasma membrane called?
non-penetrating solutes (ex: Na because it moves into cells and is also pumped back out)
hypertonic solution
solution containing greater than 3 mOsm of non-penetrating solutes causing cells to SHRINK as water diffuses out
isotonic solution
solution that doesn’t cause a change in cell size
endocytosis
when regions of the plasma membrane can often be seen to have folded into the cell, forming pockets that punch off to produce IC vesicles that enclose a small volume of EC fluid
hypotonic solution
solutions have a non-penetrating solute concentration lower than that found in cells that causes them to SWELL as water goes in
exocytosis
when membrane-bound vesicles in cytoplasm fuse w/ plasma membrane & release their contents to the outside of the cell
pinocytosis
an endocytotic vesicle encloses a small volume of EC fluid (engulf water in EC fluid and solutes)
phagocytosis
cells engulf bacteria or large particles like debris from damaged tissues, PM folds arounf surface to engulf entirely (psuedophobia –> phagosomes)
receptor-mediated endocytosis
certain molecules in EC fluid bind to specific proteins called receptors, and each one recognizes a ligand w/ high affinity (ex: clathrin!)
potocytosis
vesicles called caveolae pinch off
and deliver their contents (vitamins, lipoproteins) directly into the cytosol
active transport and facilitated diffusion within secondary active transport
AT: Na+ is then moved via active transport by the sodium pump to move uphill against the concentration gradient into the bloodstream
FD: in organic solute, x, hitches a ride
with the Na+ transporter via secondary
active transport against the
concentration gradient into the cell…it then uses facilitated diffusion to
move down the concentration gradient
into the bloodstream
epithelial transport
epithelial cells line hollow organs or tubes & regulate absorption/secretion of substances across the surfaces
apical membrane
PM on one surface of an e. cell that faces a hollow or fluid-filled tube or chamber
basolateral membrane
PM on opposite surface that rests upon a basement membrane and is usually adjacent to a network blood vessels
paracellular pathway
diffusion occurs in tight junctions BETWEEN adjacent cells which limits it
transcellular pathway
substance moves into an e. cell via either membrane, diffuses through cytosol, and exits across the opp. mem
movement of Na from lumen to e. cell occurs by diffusion vs Na channels in apical membrane and the concentration goes from _____ to _____
low to high
