E X P E C TAT I O N S
Explain the processes of
endocytosis and exocytosis.
Figure 1.39 By binding to two
cell membrane receptors, a
human immunodeficiency
virus (HIV) tricks a cell into
transporting it inside (through
a process called receptorassisted endocytosis).
Very few materials that the cell must take in or
expel are too big or too polar to cross through the
cell membrane via passive transport (diffusion,
osmosis, and facilitated diffusion) or active transport.
The cell uses a specialized method of moving these
substances in and out so that they do not have to
pass through the lipid bilayer. The cell membrane
can fold in on itself to create a membrane-enclosed,
bubble-like sac, or vesicle. The cell uses these
vesicles to “swallow” or expel various materials.
Receptor-assisted endocytosis involves the
intake of specific molecules that attach to special
proteins in the cell membrane that serve as
receptors. These membrane receptors possess a
uniquely shaped projection or cavity that fits the
shape of only one specific molecule.
Endocytosis
When the cell membrane folds inward, trapping
and enclosing a small amount of matter from the
extracellular fluid, the process is called endocytosis
(endo means within). There are three main forms of
endocytosis: pinocytosis, phagocytosis, and
receptor-assisted endocytosis.
Pinocytosis, or cell “drinking,” involves the
intake of a small droplet of extracellular fluid,
together with any dissolved substances or very
small particles that it may contain. This process
occurs in nearly all cell types nearly all of the time.
Phagocytosis, or cell “eating,” involves the
intake of a large droplet of extracellular fluid, often
including particulate matter such as bacteria or bits
of organic matter. This process occurs only in
specialized cells, such as a single-celled amoeba or
the bacteria-eating cells of our own immune system
(macrophages) — and only when they encounter
something “suitable for engulfing.”
Phagocytosis
Pinocytosis
Figure 1.40 (A) Animal macrophages take in worn-out red
blood cells as well as bacteria by phagocytosis. (B) In
pinocytosis, the cell takes in solute particles along with fluids.
Exploring the Micro-universe of the Cell • MHR
35
The Mystery of the Frozen Frogs
help save human lives by allowing organs that have been
donated for transplant to be frozen. Doctors now have to
race against time once a heart or a liver has been taken
from a donor. Human livers taken from donors for
transplant generally last only six to eight hours after
death, a heart and lung only four to five. Freezing would
give doctors time to wait for the perfect match. During
the Second World War, scientists did experiments to
learn how to freeze blood. After the war, they tried to
same approach with organs, but failed to freeze them
without destroying them. That is when they began
looking for a model in the natural world. When a scientist
in Minnesota accidentally left frogs in his car trunk
overnight and found they survived freezing, Dr. Storey
was intrigued.
One Mystery at a Time
Dr. Kenneth Storey with a graduate student
Humans die if they are frozen. Ice crystals form inside our
cells, irreparably damaging the cell membrane. But some
species of frogs freeze solid every winter in Canada.
Frozen frogs have no heartbeat. They stop breathing.
They appear to be dead, but come spring, they thaw out
and hop away. Award-winning cryobiologist Kenneth
Storey has spent much of his career trying to understand
this survival mechanism. At his lab at Carleton University
in Ottawa, he began freezing thumbnail-sized wood frogs
and spring peepers in cozy moss-filled boxes, carrying
out experiments to try to discover the molecular secrets
that allow them to survive being frozen.
A Natural Cryoprotectant
Dr. Storey suspected that the frogs have a mystery
molecule that allows their cells to withstand the rigours
of freezing. He soon discovered that the molecule was
glucose, the same blood sugar that your cells use for
fuel. As soon as ice starts forming on their smooth, green
skin, frogs start packing their cells with glucose (released
from glycogen stored in the liver). This prevents ice
crystals from forming within the cells. If humans had the
same glucose levels in their blood as freezing frogs, they
would be diabetic and extremely sick. So Dr. Storey’s
next challenge is to find out what makes frog cells pull
the glucose out of the blood.
But Would It Work for Individual
Human Organs?
From the beginning of his frog research, Dr. Storey
believed that the secret of the icy frogs might one day
36
MHR • Cellular Functions
Dr. Storey was recognized in 1984 with a Steacie Award
as one of Canada’s most promising young scientists.
Discovering that glucose was the cryoprotectant in frogs
was only the first step. He has found more than 20 genes
(out of the 2 000 that make up a frog’s chromosomes)
that are turned on when the animal starts to freeze.
It appears that these genes shut down the frog’s
metabolism and then pack its cells with sugar. Dr. Storey
explains that once he and other scientists have identified
all the genes that are involved in cryoprotection, they then
have to discover how to turn the genes on and off.
Not Only Frogs Are Cool
Researchers in his lab now study ground squirrels, bats,
snails, and turtles. Some turtles can hibernate for three or
four months under water without breathing. A hibernating
ground squirrel does not freeze solid, but it does turn
off its metabolism and lives in a state of suspended
animation. Are the same genes involved in freeze-tolerant
frogs, turtles, and squirrels? No, says Dr. Storey, but the
processes and mechanisms seem to be similar. For
example, in both squirrels and frogs, enzymes known as
“stress-activated kinases” are turned on when an animal
starts to freeze, and these act to shut its metabolism
down. Dr. Storey says it will take years to figure out
exactly how it all works. If he cannot get organs to freeze
without damage (like those of a frog), the next best thing
would be to get them to stay in a squirrel-like state of
hibernation.
“These animals are living in a state of suspended
animation. If you think about suspended animation,
that is exactly what you want for organs for transplant.
Squirrels aren’t dead at five degrees. We want
mammalian organs to act like that in fridges everywhere.”
Figure 1.41 The transport of cholesterol molecules from the extracellular fluid into
the cell interior is an example of receptor-mediated endocytosis.
Animal cells bring in cholesterol using receptorassisted endocytosis. To ensure that the membranes
of cells and of structures within cells have enough
cholesterol, your liver manufactures the cholesterol
that your cells need from natural lipids in your diet
— even if you eat only “cholesterol-free” foods.
However, being a lipid, cholesterol cannot dissolve
directly in your blood or extracellular fluid (which
are water based). Thus, cholesterol cannot cross the
cell membrane by pinocytosis. Figure 1.41 shows
the method by which your cells take up
cholesterol.
Freezing Cells
or honey in 50 mL water). For comparison, fill model cell C
with a more concentrated sugar solution, such as undiluted
corn syrup, molasses, or table syrup. Make sure all three
containers are filled to the very top. Place them in a sealed
plastic bag, and leave them in a freezer overnight. Examine
each container the next day.
Many Canadian plants and animals must be able to survive
–40˚C temperatures for extended periods. What happens to
the cell membrane and the contents of the cell at these
temperatures? Freezing models of the cell will help you
appreciate the nature of the challenge organisms face in
cold climates.
Collect three small pill vials or film canisters. (A pharmacy or
photo shop may be able to provide containers.) These
containers will represent the cell membranes of three
individual cells. Use a permanent marker to label one “A,”
one “B,” and the third “C.” Because cells contain more
water than any other substance, fill model cell A with water
to represent the cell’s contents. Living cells run on sugar, so
fill model cell B with a dilute sugar solution (5 mL glucose
Analyze
1. What happened to model cell A? Suggest why this
happened.
2. What happened to model cells B and C? Suggest why.
3. Based on the behaviour of these models, what do you
think would happen if a real cell were frozen?
4. Explain how your own cells could survive in belowfreezing temperatures. What about plant cells?
Exploring the Micro-universe of the Cell • MHR
37
Cholesterol molecules are transported in the
blood and extracellular fluid inside droplets
covered with a single layer of phospholipids. These
phospholipids have their lipid-soluble tails pointed
inward (at the cholesterol) and their water-soluble
heads pointed outward (at the blood or fluid). Each
droplet has a protein “tag” on its surface that can
be recognized only by a matching receptor on the
cell membrane. Once the tag and receptor connect,
the surrounding membrane folds inward, forming a
vesicle filled with cholesterol molecules attached
to receptors. Figure 1.42 depicts this process.
The vesicle empties its contents inside the cell and
then returns to the cell membrane, where it turns
inside out so that the receptors face outward again.
This recycles both the receptors and the membrane.
List the three main forms of endocytosis and state what
they have in common. Develop a chart or set of diagrams to
show how they differ.
Exocytosis
The reverse of endocytosis is exocytosis. In
exocytosis, a vesicle from inside the cell moves to
the cell surface. There, the vesicle membrane fuses
with the cell membrane (thus restoring membrane
removed in endocytosis). The contents of the
outward-bound vesicle are secreted into the
extracellular fluid. Exocytosis is especially
important in cells that specialize in the secretion
of various cell products. For example, specialized
cells in the human pancreas below secrete the
hormone insulin by means of exocytosis.
Figure 1.42 How do these photographs of receptor-assisted
endocytosis provide clues to the way that the cell
membrane works?
To find out more about endocytosis, visit the web site shown
below. Go to Science Resources, then to BIOLOGY 11 to find
out where to visit next. Compare the images and describe what
you see.
www.school.mcgrawhill.ca/resources/
Figure 1.43 Exocytosis in pancreatic cells appears as
vesicles releasing their contents on the surface of the cells.
1.
Identify two examples of exocytosis in your
5.
What do membrane receptors and carrier
proteins have in common? How are they different?
6.
Compare and contrast endocytosis and active
transport, using a chart to organize your information.
7.
Suggest one way that specialized cells using
phagocytosis to bring in materials may replace the
pieces of cell membrane used to form vesicles.
8.
Explain how the development of the electron
microscope and research into cell membrane function
have helped scientists understand how a virus can
get into cells. How might this technology and its
applications have affected the field of medicine?
body.
38
2.
White blood cells move throughout the body to
engulf matter, including parts of dead cells. Use
labelled diagrams and words to explain this process.
3.
How does the structure of the cell membrane
facilitate endocytosis and exocytosis?
4.
Endocytosis involves the formation of vesicles
to bring matter into the cell. Once inside, what
happens to this matter?
MHR • Cellular Functions