Essential Cell Biology 5th edition

422 CHAPTER 12 Transport Across Cell Membranes
Figure 12–44 Light-gated ion channels
can control the activity of specific
neurons in a living animal. (A) In
this experiment, the gene encoding
channelrhodopsin was introduced
into a subset of neurons in the mouse
hypothalamus. (B) When the neurons
are exposed to blue light using a tiny
fiber-optic cable implanted into the
animal’s brain, channelrhodopsin opens,
depolarizing and stimulating the channelcontaining
neurons. (C) When the light
is switched on, the mouse immediately
becomes aggressive; when the light is
switched off, its behavior immediately
returns to normal. (C, from D. Lin et al.,
Nature 470:221–226, 2011.)
gene encoding
channelrhodopsin
CHANNELRHODOPSIN
GENE INTRODUCED
INTO SUBSET OF
NEURONS IN
THE HYPOTHALAMUS
OF MOUSE BRAIN
(A)
blue light
fiber-optic
cable
Na +
+
blue light
FLASHING THE HYPOTHALAMUS
WITH BLUE LIGHT OPENS THE
CHANNEL, DEPOLARIZING
NEURONS THAT EXPRESS IT
open channelrhodopsin
(B)
+ +
+
+ +
depolarized plasma
membrane
LIGHT OFF
LIGHT ON
LIGHT OFF
(C)
ESSENTIAL CONCEPTS
• The lipid bilayer of cell membranes is highly permeable to small, nonpolar
molecules such as oxygen and carbon dioxide and, to a lesser
extent, to very small, polar molecules such as water. It is highly
ECB5 e12.43/12.44
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 concentration
gradient.
• For the passive transport of a charged solute, its electrochemical
gradient determines its direction of movement, rather than its concentration
gradient alone.
• Transporters can act as pumps to mediate active transport, in which
solutes are moved uphill against their concentration or electrochemical
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 undergoing
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.