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Passive Transport EnBio
∗
David Cole
Based on Passive Transport† by
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0‡
Abstract
By the end of this section, you will be able to:
• Explain why and how passive transport occurs
• Understand the processes of osmosis and diusion
• Dene tonicity and describe its relevance to passive transport
Plasma membranes must allow certain substances to enter and leave a cell, while preventing harmful material from entering and essential material from leaving. In other words, plasma membranes are selectively
permeablethey allow some substances through but not others. If they were to lose this selectivity, the
cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts
of specic substances than do other cells; they must have a way of obtaining these materials from the extracellular uids. This may happen passively, as certain materials move back and forth, or the cell may have
special mechanisms that ensure transport.
The most direct forms of membrane transport are passive. Passive transport is a naturally occurring
phenomenon and does not require the cell to expend energy to accomplish the movement. In passive transport, substances move from an area of higher concentration to an area of lower concentration in a process
called diusion. A physical space in which there is a dierent concentration of a single substance is said to
have a concentration gradient.
1 Diusion
Diusion is a passive process of transport.
A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. You are familiar
with diusion of substances through the air. For example, think about someone opening a bottle of perfume
in a room lled with people. The perfume is at its highest concentration in the bottle and is at its lowest at
the edges of the room. The perfume vapor will diuse, or spread away, from the bottle, and gradually, more
and more people will smell the perfume as it spreads. Materials move within the cell's cytosol by diusion,
and certain materials move through the plasma membrane by diusion (Figure 1). Diusion expends no
energy. Rather the dierent concentrations of materials in dierent areas are a form of potential energy, and
∗ Version
1.1: May 4, 2015 8:10 pm -0500
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‡ http://creativecommons.org/licenses/by/4.0/
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diusion is the dissipation of that potential energy as materials move down their concentration gradients,
from high to low.
Figure 1: Diusion through a permeable membrane follows the concentration gradient of a substance,
moving the substance from an area of high concentration to one of low concentration. (credit: modication of work by Mariana Ruiz Villarreal)
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For an animation of the diusion process in action, view this short video on cell membrane transport.
:
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2 Osmosis
Osmosis is the diusion of water through a semipermeable membrane according to the concentration gra-
dient of water across the membrane. Whereas diusion transports material across membranes and within
cells, osmosis transports only water across a membrane and the membrane limits the diusion of solutes in
the water. Osmosis is a special case of diusion. Water, like other substances, moves from an area of higher
concentration to one of lower concentration. Imagine a beaker with a semipermeable membrane, separating
the two sides or halves (Figure 2). On both sides of the membrane, the water level is the same, but there are
dierent concentrations on each side of a dissolved substance, or solute, that cannot cross the membrane.
1 http://openstaxcollege.org/l/passive_trnsprt
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If the volume of the water is the same, but the concentrations of solute are dierent, then there are also
dierent concentrations of water, the solvent, on either side of the membrane.
Figure 2: In osmosis, water always moves from an area of higher concentration (of water) to one of
lower concentration (of water). In this system, the solute cannot pass through the selectively permeable
membrane.
A principle of diusion is that the molecules move around and will spread evenly throughout the medium
if they can. However, only the material capable of getting through the membrane will diuse through
it. In this example, the solute cannot diuse through the membrane, but the water can. Water has a
concentration gradient in this system. Therefore, water will diuse down its concentration gradient, crossing
the membrane to the side where it is less concentrated. This diusion of water through the membrane
osmosiswill continue until the concentration gradient of water goes to zero. Osmosis proceeds constantly
in living systems.
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Watch this video that illustrates diusion in hot versus cold solutions.
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3 Section Summary
The passive forms of transport, diusion and osmosis, move material of small molecular weight. Substances
diuse from areas of high concentration to areas of low concentration, and this process continues until the
substance is evenly distributed in a system. In solutions of more than one substance, each type of molecule
diuses according to its own concentration gradient. Many factors can aect the rate of diusion, including
concentration gradient, the sizes of the particles that are diusing, and the temperature of the system.
In living systems, diusion of substances into and out of cells is mediated by the plasma membrane.
Some materials diuse readily through the membrane, but others are hindered, and their passage is only
made possible by protein channels and carriers. The chemistry of living things occurs in aqueous solutions,
2 http://openstaxcollege.org/l/passive_trnsprt
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and balancing the concentrations of those solutions is an ongoing problem. In living systems, diusion of
some substances would be slow or dicult without membrane proteins.
Glossary
Denition 1: concentration gradient
an area of high concentration across from an area of low concentration
Denition 2: diusion
a passive process of transport of low-molecular weight material down its concentration gradient
Denition 3: facilitated transport
a process by which material moves down a concentration gradient (from high to low concentration)
using integral membrane proteins
Denition 4: hypertonic
describes a solution in which extracellular uid has higher osmolarity than the uid inside the cell
Denition 5: hypotonic
describes a solution in which extracellular uid has lower osmolarity than the uid inside the cell
Denition 6: isotonic
describes a solution in which the extracellular uid has the same osmolarity as the uid inside the
cell
Denition 7: osmolarity
the total amount of substances dissolved in a specic amount of solution
Denition 8: osmosis
the transport of water through a semipermeable membrane from an area of high water concentration
to an area of low water concentration across a membrane
Denition 9: passive transport
a method of transporting material that does not require energy
Denition 10: selectively permeable
the characteristic of a membrane that allows some substances through but not others
Denition 11: solute
a substance dissolved in another to form a solution
Denition 12: tonicity
the amount of solute in a solution.
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