CYTOGENETICS
Lesson
7
[TRANS] LESSON 07: THE CELL CYCLE
NA+ PUMP & K+ CHANNELS
● It is how membrane potential is maintained
●The Na pump uses the energy of ATP
hydrolysis to pump Na+ out of animal cells
and K+ in. In this way, the pump helps keep the
cytosolic concentrations of NA+ low and K+ high
→
●“Na is extracellular cation, and K is intracellular
cation”
NEURONAL COMMUNICATION
NA+ PUMP
● Accounts for 30% of ATP-use
● Pumps Na+ out, carries K+ in
● Also known as Na+-K+ ATPase or the Na+-K+ pump
● 3 Na+ out
● 2 K+ in
● Creates a steep concentration gradient of both ions
● Na+ gradient produces most energy
●“In this case Na is higher concentration outside.
Each time it pumps out 3 Na+ ions and carries in
2 K+ ions, so dili equal ang transport sa ion.
That is why the outside is more positive than the
inside of the cell”
TAKEAWAYS
● The lipid bilayer of cell membranes is highly
permeable to small, non- polar molecules such
as oxygen and carbon dioxide and, to a lesser
extent, to very small, polar molecules such as
water. It is highly impermeable to most large,
water-soluble molecules and to all 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 transporters or
channels, each responsible for the transfer of a
particular type of solute.
● Channel proteins form pores across the lipid
bilayer through which solutes can passively
diffuse.
● Both transporters and channels can mediate
passive transport, in which an uncharged solute
moves spontaneously down its concen- tration
gradient.
● For the passive transport of a charged solute, its
electrochemical gradient determines its direction
of movement, rather than its con- centration
gradient alone.
● Transporters can act as pumps to mediate active
transport, in which solutes are moved uphill
against their concentration or electrochemi- cal
gradients; this process requires energy that is
provided by ATP hydrolysis, a downhill flow of
Na+ or H+ ions, or sunlight.
● Transporters transfer specific solutes across a
membrane by under- going conformational
changes that expose the solute-binding site first
on one side of the membrane and then on the
other.
● The Na+ pump in the plasma membrane of
animal cells is an ATPase; it actively transports
Na+ out of the cell and K+ in, maintaining a
steep Na+ gradient across the plasma
membrane that is used to drive other active
transport processes and to convey electrical
signals.
● Ion channels allow inorganic ions of appropriate
size and charge to cross the membrane. Most
are gated and open transiently in response to a
specific stimulus.
● Even when activated by a specific stimulus, ion
channels do not remain continuously open: they
flicker randomly between open and closed
conformations. An activating stimulus increases
the propor- tion of time that the channel spends
in the open state.
● The membrane potential is determined by the
unequal distribution of charged ions on the two
sides of a cell membrane; it is altered when
these ions flow through open ion channels in the
membrane.
● In most animal cells, the negative value of the
resting membrane potential across the plasma
membrane depends mainly on the K+ gradient
and the operation of K+-selective leak channels;
at this rest- ing potential, the driving force for the
movement of K+ across the membrane is almost
zero.
● Neurons produce electrical impulses in the form
of action potentials, which can travel long
distances along an axon without weaken- ing.
Action potentials are propagated by
voltage-gated Na+ and K+ channels that open
sequentially in response to depolarization of the
plasma membrane.
● Voltage-gated Ca2+ channels in a nerve
terminal couple the arrival of an action potential
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