1.14 Passive Transport
An intravenous (IV) drip is a common sight in a hospital's
intensive care unit (ICU) (Figure 1). This piece of equipment
allows liquids to flow directly into a patient's veins. In most
cases, the liquids are aqueous solutions containing a variety of
different solutes. Common solutions used in IV drips include
aqueous sodium chloride, NaCl SUBSCRIPT (aq), and aqueous
glucose, C SUBSCRIPT 6 H SUBSCRIPT 12 O SUBSCRIPT 6(aq). An
aqueous solution of sodium chloride is administered when a
patient's blood and other body fluids (extracellular fluids)
have low concentrations of water and minerals. Minerals include
the positive and negative ions of common salts such as sodium
ions, Na SUPERSCRIPT + SUBSCRIPT (aq), potassium ions, K
SUPERSCRIPT + SUBSCRIPT (aq), calcium ions, Ca SUPERSCRIPT 2+
SUBSCRIPT (aq), chloride ions, Cl SUPERSCRIPT - SUBSCRIPT (aq),
and phosphate ions, PO SUBSCRIPT 4 SUPERSCRIPT 3- SUBSCRIPT
(aq). Ions dissoved in aqueous solutions are called
electrolytes.
Careful inspection of labels on typical IV solutions reveals
that the concentration of solute in the solution is always
stated (Figure 2). Doctors and nurses are careful to administer
a solution with a suitable solute concentration to their
patients. Giving an IV solution with the wrong concentration
could be fatal. Why is it important for patients to receive a
solution of a particular concentration in an IV drip? The
reasons have much to do with the structure of cell membranes and
how substances move across these membranes.
Simple Diffusion
Cell membranes are selectively permeable--only certain
substances are able to pass through them. As mentioned in
section 1.2, cell membranes are largely composed of a
phospholipid bilayer and proteins. Many small, uncharged
molecules, such as water, oxygen, and carbon dioxide, pass
through the cell membrane freely (Figure 3). These substances
either go through the phospholipid bilayer directly or through
channels formed by proteins in the membrane. Since the cell
membrane has a hydrophobic middle section, small lipid molecules
such as fatty acids are also able to pass through.
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However, ions, small charged molecules, and large molecules such
as amino acids, carbohydrates, nucleic acids, and large lipids
(triglycerides) cannot pass through easily (Figure 3, on the
previous page).
Substances that can pass through the membrane do so by a process
called simple diffusion. Simple diffusion is the movement of
particles from an area where they are more highly concentrated
to an area where they are less highly concentrated (Figure 4). A
difference in concentration between two areas is called a
concentration gradient, and diffusion always occurs down a
concentration gradient (from high concentration to low
concentration).
Diffusion is a natural process that occurs because particles are
in constant random motion and have a tendency to spread
throughout a given volume. Simple diffusion does not use any of
a cell's energy, and ends when the concentration of particles
becomes equal everywhere within the given volume. However, this
does not mean that the particles stop moving. In fact, particles
continue moving randomly in all directions all the time. During
diffusion, molecules move more in one direction than any other.
When diffusion ends, we say that a state of dynamic equilibrium
has been reached. Dynamic equilibrium is a state of balance,
where particles move at equal rates in all directions.
When diffusion occurs through a cell membrane, dynamic
equilibrium is reached when particles move through the membrane
in both directions at equal rates, and the concentration of
particles remains the same on both sides of the membrane (Figure
5).
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The rate (speed) of diffusion depends on temperature and the
concentration of solute molecules in solution. Diffusion occurs
faster at higher temperatures because molecules move faster.
Faster-moving molecules spread out faster and reach dynamic
equilibrium more quickly. Dynamic equilibrium will also be reached
more quickly if there are a greater number of solute molecules in
solution.
Facilitated Diffusion
Glucose, sodium ions, and chloride ions are three chemicals most
cells need to survive. These substances must be able to get
across the cell membrane in an efficient manner. However, large
polar molecules such as glucose and large ions such as sodium
and chloride cannot go through a membrane by simple diffusion
because they cannot easily pass through the hydrophobic middle
section of the phospholipid bilayer. To compensate, membranes
have protein molecules that help some substances through. In
general, there are three types of membrane proteins (Figure 6).
Some membrane proteins go part way through the phospholipid
bilayer, some are attached to the outside surface, and others go
all the way through. Those that span the bilayer are called
transmembrane proteins. Some transmembrane proteins act as
carrier proteins that assist certain substances through a
membrane. This method of transporting materials across a
membrane is called facilitated diffusion, or assisted diffusion.
As its name implies, facilitated diffusion is a form of
diffusion. This means that particles move down a concentration
gradient until dynamic equilibrium is reached. The key
difference between simple diffusion and facilitated diffusion is
that, in facilitated diffusion, the diffusing particles are
assisted through the membrane by transmembrane carrier proteins,
whereas in simple diffusion they pass directly through the
membrane's phospholipid bilayer or through protein channels.
Typically, a given carrier protein transports only one type of
substance, or a small group of chemically related substances. An
example of carrier protein facilitated diffusion is the movement
of glucose into cells of the liver (Figure 7, on the next page).
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Osmosis
Water molecules move freely through cell membranes. Under normal
conditions, large quantities of water molecules move into and
out of a cell by simple diffusion. The cell remains the same
size because equal amounts of water go into and out of the cell.
There are, however, many cases in which a net amount of water
flows into or out of a cell. This means that more water may
enter the cell than leaves the cell, so that the cell gains
water, or more water may leave the cell than enters it, so that
the cell loses water. In such situations, water still moves
through the cell membrane by simple diffusion, but the process
is important enough to warrant a special name--osmosis.
Osmosis is the net movement of water across a selectively
permeable membrane from the side where water is more
concentrated to the side where it is less concentrated. Note
that solutions have a high concentration of water when they have
a low concentration of solute, and vice versa. Osmosis occurs
because more water molecules strike the membrane on the side
with a higher concentration of water molecules (i.e., a lower
solute concentration) than on the side with a lower
concentration of water molecules (i.e., a higher solute
concentration). More strikes result in more water molecules
passing through the membrane and a net diffusion of water from
one side to the other.
The key point to remember about osmosis is that water moves
through a membrane from the side with a lower solute
concentration to the side with a higher solute concentration.
Dynamic equilibrium is reached when sufficient water has moved
to equalize the solute concentrations on both sides of the
membrane, and at that point, net movement of water (osmosis)
ceases (Figure 8).
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Osmosis occurs whenever there is a difference in solute
concentration across a selectively permeable membrane. A number
of special terms are commonly used to describe differences in
solute concentration (Figure 9). Isotonic solutions are
solutions that have equal solute concentrations. When a
selectively permeable membrane separates isotonic solutions,
osmosis does not occur. A hypertonic solution is one with a
higher concentration of solutes than another solution. The
solution with the lower concentration of solutes is called a
hypotonic solution.
We may now understand why it is important for patients to
receive a solution of a particular concentration in an IV drip.
Blood serum (the liquid part of blood) is normally isotonic with
respect to red blood cell cytoplasm. Under normal conditions,
osmosis does not occur into or out of red blood cells. The cells
maintain their normal size and shape (Figure 10(a)).
When a patient receives an IV drip, the IV solution mixes
directly with blood serum. If the solution contains a lower
solute concentration than blood serum (i.e., a hypotonic
solution), it may dilute the blood serum until it is hypotonic
to blood cell cytoplasm. If so, osmosis will occur into the red
blood cells, causing the cells to swell, and maybe burst (Figure
10(b)). This condition is called hemolysis, and may be fatal
because the blood cells will be unable to transport oxygen to
body tissues efficiently.
If a patient receives an IV solution that is hypertonic to blood
serum, it may concentrate blood serum until it is hypertonic to
blood cell cytoplasm. Osmosis will occur out of the blood cells.
The cells will lose water and become small and scallop-shaped
(Figure 10(c)). Scallop-shaped cells have a tendency to stick to
one another and clog small veins and arteries, preventing oxygen
from reaching body tissues. This condition, called crenation,
may also be fatal.
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Any solution injected directly into veins and arteries must be
isotonic with blood serum. Typical isotonic IV solutions include
5% glucose (also called 5% dextrose, or D5W) and 0.9% sodium
chloride (also called isotonic saline).
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BEGIN PRODUCER'S NOTE:
Investigation is not reproduced.
END PRODUCER'S NOTE.
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BEGIN PRODUCER'S NOTE:
Investigation is not reproduced.
END PRODUCER'S NOTE.
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1.16 Active Transport and Bulk
Transport
Simple diffusion (including osmosis) and facilitated diffusion
are methods used by cells to move substances through membranes
from areas of high concentration to areas of low concentration
until concentrations are equal. These methods for moving
materials are useful in many situations, but may be wasteful in
others. The primary purpose of eating is to absorb nutrient
molecules into the cells of your body. Nutrients include amino
acids from proteins, fatty acids from fats, nucleotides from
nucleic acids, and glucose from complex carbohydrates such as
starch. Glucose is a particularly important nutrient because it
is used as a source of energy by all the cells of your body. To
ensure that the body absorbs the maximum amount of nutrients
from food, energy may be used to "pump" nutrients across cell
membranes. A compound called adenosine triphosphate (ATP)
provides the energy needed in this process, and in many of the
other energy-requiring processes of living cells.
Energy, Cells, and ATP
Living organisms need a continuous supply of energy to power the
energy-requiring processes of life. Movement, reproduction,
protein synthesis, and certain forms of transport across cell
membranes all require energy. This energy is provided by the
breakdown of ATP. The molecular structure of ATP is similar to
that of the DNA nucleotide containing adenine, except that ATP
contains three linked phosphate groups instead of just one
(Figure 1). When ATP reacts with certain compounds in a cell,
the reactions release energy that the cell can use to power
energy-requiring activities (Figure 2).
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Active Transport
After a meal, partially digested food travels from the stomach
into the small intestine, where most of the nutrient molecules
are absorbed into cells (Figure 3).
As the nutrients move through the small intestine, they must be
absorbed efficiently to avoid being excreted with waste products
that move through the digestive system. If glucose were absorbed
into intestinal cells by simple diffusion or facilitated
diffusion, only about half of the molecules would be absorbed,
since diffusion ends when solute concentrations are equal on
both sides of the cell membrane. To maximize the absorption of
such an important nutrient, cells of the small intestine may
"pump" glucose through their cell membranes against a
concentration gradient. This means that glucose continues to
enter intestinal cells even when the concentration inside the
cytoplasm is greater than the concentration outside the
cytoplasm (in the intestinal space). In order to accomplish
this, the cells have special protein carriers that use chemical
energy from ATP to transport substances such as glucose through
their cell membranes. This process is called active transport
(Figure 4). It is important
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to note that simple diffusion and facilitated diffusion (unlike
active transport) are processes that do not require a source of
cellular energy (ATP).
Various types of active-transport "pumps" are found in the
membranes of different cells. Potassium ions and sodium ions are
moved into and out of cells by a pump known as the sodiumpotassium pump (Figure 5). Without this pump, your nerve cells
and muscle cells could not function properly. Other substances,
such as vitamins, amino acids, and hydrogen ions, are also
pumped across membranes. All of these pumps require cellular
energy (ATP) to operate.
Bulk Transport
Diffusion and active transport are responsible for moving large
amounts of material through cell membranes. However, in both
cases, the substances move through the membranes as dissolved
particles (atoms, ions, or molecules). Sometimes cells need to
move large quantities of materials (bulk) into or out of their
cytoplasm all at once. A process called bulk transport may
accomplish this. As in active transport, all bulk-transport
mechanisms use energy in the form of ATP. There are two forms of
bulk transport, endocytosis and exocytosis.
Endocytosis
Endocytosis is the form of bulk transport used to bring large
amounts of material into the cell from the extracellular fluid.
There are two forms of endocytosis, phagocytosis and
pinocytosis. Phagocytosis (cell eating) is the bulk transport of
solids into the cell, and pinocytosis (cell drinking) is the
bulk transport of (liquid) extracellular fluid into the cell.
Phagocytosis
Phagocytosis begins when a solid particle comes in contact with
the plasma membrane of a cell (Figure 6, on the next page).
The cell membrane sends out fingerlike projections called
pseudopods that surround and eventually enclose the particle in
a vesicle that is within the cell's cytoplasm. Such a vesicle is
called a phagocytotic vesicle. Lysosomes containing digestive
enzymes may fuse with the phagocytotic vesicle to digest the
particles it contains. Nutrients formed by this digestion
process move through the vesicle's membrane into the cell's
cytoplasm. White blood cells called macrophages frequently
engulf invading harmful bacteria by phagocytosis, removing them
from the bloodstream and other body tissues (Figure 7, on the
next page).
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Pinocytosis
Pinocytosis occurs when a cell's plasma membrane engulfs a drop
of extracellular fluid in a process similar to phagocytosis
(Figure 8). This results in the formation of a pinocytotic
vesicle. Cells bring cholesterol molecules into their cytoplasm
by a special form of pinocytosis called receptor-mediated
pinocytosis. In this process, cholesterol molecules in the
extracellular fluid attach to receptor molecules on the external
surface of the cell membrane. A pinocytotic vesicle then forms
that brings the cholesterol molecules into the cell.
Exocytosis
In exocytosis, cells move large amounts of material out of their
cytoplasm by a process that is essentially the reverse of
endocytosis. In some cases, cells produce substances, such as
hormones or enzymes, that must be exported out of the cytoplasm.
In these cases, the material is enclosed in a membrane sac
called a secretory vesicle (Figure 9). The secretory vesicle
fuses with the cell membrane and spills its contents into the
extracellular fluid. In some cases, proteins produced in the
rough endoplasmic reticulum are packaged into secretory vesicles
by the Golgi apparatus and are subsequently transported out of
the cell by exocytosis.
QUESTIONS:
1. List 4 areas where a solute would move from an area with
lots of it to an area with little solute inside your body.
HINT: These things will NOT pass through a cell membrane.
2. List three things that your cells need that are moved by
facilitated diffusion. HINTS:
A. Main sugar for cell =
B. Main charged atom for nerves to work =
C. Main charged atom for nerves to work =
HINT#2: B and C are in table salt.
3. List 4 areas where water would move from an area with lots
of water to an area with little water inside your body.
4. List 3 items that are moved across a cell membrane using
active transport. HINTS:
A. People take supplements every morning that contain =
B. Break down of proteins into =
C.