142 Chapter 6
6.3 Carrier-Mediated Transport
Molecules such as glucose are transported across plasma
membranes by carrier proteins. Carrier-mediated transport
in which the net movement is down a concentration gra-
dient, and which is therefore passive, is called facilitated
diffusion. Carrier-mediated transport that occurs against a
concentration gradient, and which therefore requires met-
abolic energy, is called active transport.this causes the posterior pituitary to release antidiuretic
hormone (ADH), also known as vasopressin, into
the blood.
3. ADH acts on the kidneys to promote water retention,
so that a lower volume of more concentrated urine is
excreted.
A person who is dehydrated, therefore, drinks more and uri-
nates less. This represents a negative feedback loop ( fig. 6.14 ),
which acts to maintain homeostasis of the plasma concentra-
tion (osmolality) and, in the process, helps to maintain a proper
blood volume.
A person with a normal blood volume who eats salty
food will also get thirsty, and more ADH will be released
from the posterior pituitary. By drinking more and excreting
less water in the urine, the salt from the food will become
diluted to restore the normal blood concentration, but at a
higher blood volume. The opposite occurs in salt deprivation.
With a lower plasma osmolality, the osmoreceptors are not
stimulated as much, and the posterior pituitary releases less
ADH. Consequently, more water is excreted in the urine to
again restore the proper range of plasma concentration, but at
a lower blood volume. Low blood volume and pressure as a
result of prolonged salt deprivation can be fatal (chapter 14,
section 14.2).
Clinical Investigation CLUES
Jessica’s plasma osmolality was higher than normal, and
she complained of being constantly thirsty.- How does her high plasma osmolality cause her
thirst? - How does her high plasma osmolality relate to her
taking a diuretic drug and having glucose in her
urine?
| CHECKPOINT
- Explain what is meant by simple diffusion and list the
factors that influence the diffusion rate. - Define the terms osmosis, osmolality, and osmotic
pressure, and state the conditions that are needed
for osmosis to occur. - Define the terms isotonic, hypotonic, and hypertonic,
and explain why hospitals use 5% dextrose and
normal saline as intravenous infusions. - Explain how the body detects changes in the
osmolality of plasma and describe the regulatory
mechanisms by which a proper range of plasma
osmolality is maintained.
LEARNING OUTCOMESAfter studying this section, you should be able to:- Describe the characteristics of carrier-mediated
transport, and distinguish between simple diffusion,
facilitated diffusion, and active transport. - Explain the action and significance of the Ca^2 1 pump
and the Na^1 /K^1 pumps.
In order to sustain metabolism, cells must take up glucose,
amino acids, and other organic molecules from the extracel-
lular environment. Molecules such as these, however, are too
large and polar to pass through the lipid barrier of the plasma
membrane by a process of simple diffusion. The transport of
such molecules is mediated by carrier proteins within the
membrane. Although the action of carrier proteins cannot be
directly observed, carrier-mediated transport can be inferred
by characteristics it shares with enzyme activity. The common
characteristics of enzymes and carrier proteins are (1) specific-
ity, (2) competition, and (3) saturation.
Like enzyme proteins, carrier proteins interact only with
specific molecules. Glucose carriers, for example, can interact
only with glucose and not with closely related monosaccha-
rides. As a further example of specificity, particular carriers for
amino acids transport some types of amino acids but not oth-
ers. Two amino acids that are transported by the same carrier
compete with each other, so that the rate of transport for each
is lower when they are present together than it would be if each
were present alone ( fig. 6.15 ).
As the concentration of a transported molecule is increased,
its rate of transport will also be increased—but only up to a
transport maximum ( Tm ). Concentrations greater than the
transport maximum do not produce further increase in the
transport rate, indicating that the carrier transport is saturated
( fig. 6.15 ).
As an example of saturation, imagine a bus stop that is ser-
viced once an hour by a bus that can hold a maximum of 40
people (its “transport maximum”). If there are 10 people waiting
at the bus stop, 10 will be transported each hour. If 20 people are
waiting, 20 will be transported each hour. This linear relation-
ship will hold up to a maximum of 40 people; if there are 80
people at the bus stop, the transport rate will still be 40 per hour.