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Objectives
Relate diffusion and equilibrium.
Describe how passive transport occurs.
Relate osmosis to solute concentration.
Explain how active transport differs from passive transport.
Describe how large molecules move across a membrane.
Key Terms
diffusion
equilibrium
selectively permeable membrane
passive transport
facilitated diffusion
osmosis
hypertonic
hypotonic
isotonic
active transport
vesicle
exocytosis
endocytosis
Materials such as water, nutrients, dissolved gases, ions, and wastes must
constantly move in two-way traffic across a cell's plasma membrane.
Materials also must move across membranes within the cell. Cellular
membranes function like gatekeepers, letting some molecules through but
not others. And, while certain molecules pass freely through the "gates,"
others move only when the cell expends energy.
Diffusion
Molecules in a fluid are constantly in motion, colliding and bouncing as
they spread out into the available space. One result of this motion is
diffusion, the net movement of the particles of a substance from where
they are more concentrated to where they are less concentrated.
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Wednesday, October 19, 2011 9:14:28 AM CT
Suppose there is a container of water in which a membrane separates pure
water from a solution of dye and water. This membrane happens to be
permeable to both the dye and water molecules—that is, the molecules
can pass through the membrane freely (Figure 6-11). As the molecules of
water and dye move randomly, the dye eventually diffuses across the
membrane until the concentration of dye—the ratio of dye to water—on
each side is the same. At this point, the number of dye molecules moving
in one direction is equal to the number moving in the other direction, and
the system is said to be in equilibrium, or balance.
Figure 6-11
Dye molecules diffuse across a membrane. At equilibrium, the
concentration of dye is the same throughout the container.
Passive Transport
Cellular membranes are barriers to the diffusion of some substances. A
selectively permeable membrane allows some substances to cross the
membrane more easily than others and blocks the passage of some
substances altogether. (Think of a window screen that lets a breeze
through but blocks the entry of mosquitoes.) In a typical cell, a few
molecules (primarily oxygen and carbon dioxide) diffuse freely through
the plasma membrane (Figure 6-12, left). Water also diffuses through the
membrane, but mostly through protein channels. Other molecules pass
less easily or only under specific conditions. Diffusion across a
membrane is called passive transport because no energy is expended by
the cell in the process. Only the random motion of the molecules is
required to move them across the membrane.
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Figure 6-12
Both diffusion and facilitated diffusion are forms of
passive transport, as neither process requires the cell to
expend energy. In facilitated diffusion, solute particles
pass through a channel in a transport protein.
Though small molecules generally pass more readily by passive transport
than large molecules, most small molecules have restricted access. For
example, sugars do not pass easily through the hydrophobic region of the
plasma membrane. The traffic of such substances can only occur by way
of transport proteins (Figure 6-12, right). In this process, known as
facilitated diffusion, transport proteins provide a pathway for certain
molecules to pass. (The word facilitate means "to help.") Specific proteins
allow the passive transport of different substances. In this way,
substances including some ions and small polar molecules, such as water
and sugars, diffuse into or out of the cell.
Osmosis
The passive transport of water across a selectively permeable membrane
is called osmosis (ahs MOH sis). Consider a sealed bag of concentrated
sugar water placed in a container of less-concentrated sugar water.
Suppose that water can pass through the bag (the membrane) but the
sugar molecules cannot. The solution with a higher concentration of
solute is said to be hypertonic (hyper means "above"). The solution with
the lower solute concentration is said to be hypotonic (hypo means
"below"). Think now, which solution has the higher concentration of
water? By having less solute, the hypotonic solution has the higher water
concentration. What will happen?
As a result of osmosis, water from the container (hypotonic solution) will
diffuse across the membrane to the inside of the bag (hypertonic
solution). The sugar molecules, however, cannot cross the membrane. In
time, the volume of water increases inside the bag. If the volume of the
bag is large enough, the concentration of sugar will become the same in
the water on either side of the membrane. Solutions in which the
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concentrations of solute are equal are said to be isotonic (isos means
"equal").
Figure 6-13
A selectively permeable
membrane (the bag) separates
two solutions of different sugar
concentrations. Sugar
molecules cannot pass through
the membrane.
Water Balance in Animal Cells Although the solution in Figure 613 became isotonic, the bag got bigger as it took on water. What happens
to an animal cell in a hypotonic solution? The cell gains water, swells, and
may even pop like an overfilled balloon. A hypertonic environment is
also harsh on an animal cell. The cell loses water, shrivels, and may die.
Animals living in aquatic environments may encounter conditions that are
not isotonic with their body tissues. These animals depend on
mechanisms that make up for the gain or loss of water that results from
osmosis. For example, the body of a freshwater fish constantly gains
water from its hypotonic environment. One function of the fish's gills and
kidneys is to prevent an excessive buildup of water in the body.
Water Balance in Plant Cells Water balance problems are
somewhat different for plant cells because of their strong cell walls. A
plant cell is firm and healthiest in a hypotonic environment—when
bathed by rainwater, for example. The cell becomes firm as a result of the
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net flow of water inward. Although the cell wall expands a bit, it applies
pressure that prevents the cell from taking in too much water and
bursting, as an animal cell would. In contrast, a plant cell in an isotonic
environment has no net inward flow of water. It becomes limp. Nonwoody plants, such as most houseplants, wilt in this situation. In a
hypertonic environment, a plant cell is no better off than an animal cell.
As a plant cell loses water, it shrivels, and its plasma membrane pulls
away from the cell wall. This situation usually kills the cell.
Active Transport
When a cell expends energy to move molecules or ions across a
membrane, the process is known as active transport. During active
transport, a specific transport protein pumps a solute across a membrane,
usually in the opposite direction to the way it travels in diffusion (Figure
6-16). This action requires chemical energy supplied primarily by the
mitochondria, which you will read more about in Concept 6.5.
Figure 6-16
Like an enzyme, a transport protein recognizes a specific
solute, molecule or ion. During active transport, the protein
uses energy, usually moving the solute in a direction from
lesser concentration to greater concentration.
Active transport plays a part in maintaining the cell's chemical
environment. For example, an animal cell has a much higher
concentration of potassium ions (K+) and a much lower concentration of
sodium ions (Na+) than its fluid surroundings. The plasma membrane
helps maintain these differences by pumping K+ ions into the cell and Na+
ions out of the cell. This particular case of active transport is central to
how your nerve cells work, as you'll learn in Chapter 28.
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Figure 6-17
Exocytosis (above left) expels molecules from the cell
that are too large to pass through the plasma
membrane. Endocytosis (below left) brings large
molecules into the cell and packages them in vesicles.
Transport of Large Molecules
So far you've seen how water and small particles of solutes enter and
leave a cell by moving through the plasma membrane. The process is
different for large particles. Their movement depends on being packaged
in vesicles (VES i kuhlz), which are small membrane sacs that specialize
in moving products into, out of, and within a cell (Figure 6-17). For
example, in exporting protein products from a cell, a vesicle containing
the proteins fuses with the plasma membrane and spills its contents
outside the cell—a process called exocytosis. The reverse process,
endocytosis, takes material into the cell within vesicles that bud inward
from the plasma membrane. Larger membrane sacs are also formed by
endocytosis when food particles are ingested.
Concept Check 6.3
1. What is diffusion?
2. What role does a cellular membrane play in passive transport?
3. Distinguish between hypertonic, hypotonic, and isotonic solutions, and
give an example of how each affects an animal cell.
4. What role does active transport play in cell function?
5. How do vesicles transport large molecules out of a cell?
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Wednesday, October 19, 2011 9:14:28 AM CT
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