chap12 Flashcards
(43 cards)
The lipid bilayer of cell membranes is highly permeable to _, _ molecules such as _ and _ and, to a lesser
extent, to _, _ molecules such as _. It is highly impermeable to most _, _ molecules and to all _.
small
nonpolar
oxygen
carbon dioxide
very small
polar
water
large
water-soluble
ions
Transfer of nutrients, metabolites, and inorganic ions across cell membranes depends on _.
membrane transport proteins
Cell membranes contain a variety of transport proteins that function either as _ or _, each responsible for the transfer of a _ of solute.
transporters
channels
particular type
Channel proteins form _ across the lipid bilayer through which solutes can _.
pores
passively diffuse
Both transporters and channels can mediate passive transport, in which an _ solute moves _
uncharged
spontaneously down its concentration gradient.
For the passive transport of a charged solute, its _ determines its direction of movement, rather than its _ alone.
electrochemical gradient
concentration gradient
An electrochemical gradient is the combined effect of two forces across a membrane:
_
_
Concentration Gradient
Electrical Gradient
Electrical Gradient:
Difference in charge across the membrane.
Concentration Gradient:
Difference in the amount of ions on each side.
Transporters can act as _ to mediate _ transport, in which solutes are moved _; this process requires _ that is provided by _ hydrolysis, a _, or _.
pumps
_
active
_
uphill against their concentration or electrochemical gradients
_
energy
_
ATP
_
downhill flow of Na+ or H+ ions
_
sunlight
Transporters transfer _ solutes across a membrane by _ that _ the solute-_ first on one side of the membrane and then _
specific
undergoing conformational changes
expose
binding site
on the other.
The Na+ pump in the plasma membrane of animal cells is an ATPase; it actively transports Na+ _ of the cell and K+ _, maintaining a _ Na+ gradient across the plasma membrane that is used to _ and to convey _.
out
in
steep
drive other active transport processes
electrical signals
Ion channels allow _ of _ size and charge to cross the membrane. Most are _ and open _ in response to a _.
inorganic ions
appropriate
gated
transiently
specific stimulus
Even when activated by a specific stimulus, ion channels _: they _ . An activating stimulus increases the _.
do not remain continuously open
_
flicker randomly between open and closed conformations
_
proportion of time that the channel spends in the open state
_
The membrane potential is determined by the _ distribution of _ on the two sides of a cell membrane; it is altered when these ions _ in the membrane.
unequal
charged ions
flow through open ion channels
In most animal cells, the _ value of the resting membrane potential across the plasma membrane depends mainly on the _ and the operation of _; at this resting potential, the driving force for the movement of K+ across the membrane is _.
negative
K+ gradient
K+-selective leak channels
almost zero (the concentration gradient pushing K+ out and the electrical gradient pulling K+ back in= balanced forces)
Higher Concentration Outside the Cell:
_
_
_
Higher Concentration Inside the Cell:
_
_
Na⁺ (Sodium)
_
Ca²⁺ (Calcium)
_
Cl⁻ (Chloride)
_
K⁺ (Potassium)
_
Various organic ions and proteins (contributing to a negative charge)
Neurons produce electrical impulses in the form of action potentials, which can travel long distances along an axon without weakening. Action potentials are propagated by _ that open _ in response to _ of the plasma membrane.
voltage-gated Na+ and K+ channels
sequentially
depolarization
_ in a nerve terminal couple the arrival of an action potential to neurotransmitter release at a synapse. _ convert this chemical signal back into an electrical one in the postsynaptic target cell.
Voltage-gated Ca2+ channels
Transmitter-gated ion channels
Excitatory neurotransmitters open _ that allow the _, which _ the postsynaptic cell’s plasma membrane and encourages the cell to fire an action potential. Inhibitory neurotransmitters open _ in the postsynaptic cell’s plasma membrane, making it harder for the membrane to _ and fire an action potential.
transmitter-gated cation channels
influx of Na+
depolarizes
transmitter-gated Cl– channels
depolarize
_ in the human brain exploit all of the above mechanisms to make human behaviors possible.
Complex sets of nerve cells
The rate at which a solute
crosses a protein-free, artificial lipid
bilayer by simple diffusion depends on
its _ and _.
size
solubility
which molecules pass the most without membrane proteins?
and then?
and then?
and then?
SMALL, NONPOLAR MOLECULES (02, Co2)
_
SMALL, UNCHARGED POLAR MOLECULES (H2O)
_
LARGER UNCHARGED POLAR MOLECULES (glucose)
_
IONS
Ion channels can exist in _
either an open or a closed conformation
