Essential Cell Biology 5th edition

Transporters and Their Functions
395
(A)
water
PROTOZOAN
discharging
contractile
vacuole
(B)
cell wall
PLANT CELL
vacuole
Osmosis, if it occurs without constraint, can make a cell swell. Different
cells cope with this osmotic challenge in different ways. Some freshwater
protozoans, such as amoebae, eliminate excess water using contractile
vacuoles that periodically discharge their contents to the exterior (Figure
12–7A). Plant cells are prevented from swelling by their tough cell walls
and so can tolerate a large osmotic difference across their plasma membrane
(Figure 12–7B); indeed, plant cells make use of osmotic swelling
pressure, or turgor pressure, to keep their cell walls tense, so that the
stems of the plant are ECB5 rigid and E12.07/12.07
its leaves are extended. If turgor pressure
is lost, plants wilt. Animal cells maintain osmotic equilibrium by using
transmembrane pumps to expel solutes, such as the Na + ions that tend to
leak into the cell (Figure 12–7C).
(C)
nucleus
ANIMAL CELL
ions
Figure 12–7 Cells use different tactics to
avoid osmotic swelling. (A) A freshwater
amoeba avoids swelling by periodically
ejecting the water that moves into the cell
and accumulates in contractile vacuoles.
The contractile vacuole first accumulates
solutes, which cause water to follow by
osmosis; it then pumps most of the solutes
back into the cytosol before emptying its
contents at the cell surface. (B) The plant
cell’s tough cell wall prevents swelling.
(C) The animal cell reduces its intracellular
solute concentration by pumping out ions.
TRANSPORTERS AND THEIR FUNCTIONS
Transporters are responsible for the movement of most small, watersoluble,
organic molecules and a handful of inorganic ions across cell
membranes. Each transporter is highly selective, often transferring just
one type of solute. To guide and propel the complex traffic of substances
into and out of the cell, and between the cytosol and the different membrane-enclosed
organelles, each cell membrane contains a characteristic
set of different transporters appropriate to that particular membrane.
For example, the plasma membrane contains transporters that import
nutrients such as sugars, amino acids, and nucleotides; the lysosome
membrane contains an H + transporter that imports H + to acidify the lysosome
interior and other transporters that move digestion products out
of the lysosome into the cytosol; the inner membrane of mitochondria
contains transporters for importing the pyruvate that mitochondria use
as fuel for generating ATP, as well as transporters for exporting ATP once
it is synthesized (Figure 12–8).
In this section, we describe the general principles that govern the function
of transporters, and we present a more detailed view of the molecular
mechanisms that drive the movement of a few key solutes.
nucleotide
sugar
amino acid
Na +
H +
K +
pyruvate
ATP
lysosome
mitochondrion
plasma membrane
ADP
inner mitochondrial
membrane
Figure 12–8 Each cell membrane has its
own characteristic set of transporters.
These transporters allow each membrane to
carry out its unique functions. Only a few of
these transporters are shown here.