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1.7
Cells in Their Environment
Imagine if you had to live inside a sealed plastic bag. How long would
you survive? Well, not long without holes so oxygen could enter.
Soon, you would need a way to get water and food through the plastic.
Even this would not be enough. You would also need a way of removing
wastes, such as carbon dioxide and urine.
In some ways, the cell membrane is like that plastic bag. However, the
cell membrane is also much more complex, as you can see in Figure 1.
Cell Membranes
Figure 1
Cells allow some materials to enter
or leave, but not others. They are
said to be permeable to some
materials and impermeable to
others. Permeable means permitting
passage, and impermeable means
not permitting passage.
In general, small molecules pass
easily through the cell membrane,
medium-sized molecules move
through less easily, and large ones
cannot pass through without help
from the cell. Because it allows
certain substances to enter or leave,
but not others, the cell membrane
is said to be selectively permeable.
The cell membrane has two layers of fat (lipid). Embedded in
the fat layers are protein molecules (coloured blobs) and pores
made of protein. There are pores of several different sizes.
protein
pore
lipids
Diffusion
In Figure 2 a blob of ink gradually spreads
out and colours the whole beaker of water.
Why doesn’t the ink remain as a small blob?
What causes it to move outward?
The molecules of the ink blob are
constantly moving and colliding with other
ink molecules and with the molecules of
the water. When they collide, they bounce
off each other. This causes molecules that
are concentrated in one area to gradually
spread outward. Diffusion is the movement of
molecules from an area of high concentration
to an area of lower concentration.
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Unit 1
Figure 2
Ink diffusing in water.
Diffusion and Cells
Diffusion is one of the ways substances move into and
out of cells. A substance that a cell uses up, such as
oxygen, will be in low concentration inside the cell.
Outside the cell, the concentration of the substance will
be higher. The molecules of the substance will diffuse
across the cell membrane into the cell. Diffusion will
continue until the concentration of the substance is the
same inside and outside the cell.
Waste products, such as carbon dioxide, tend to
become more concentrated inside the cell than outside,
so they will diffuse out of the cell.
Understanding Concepts
1. Explain in your own words the
process of diffusion.
2. Explain what is meant by
impermeable, permeable, and
selectively permeable materials.
3. What type of membrane do cells
have? Explain why.
4. Hypothesize why the pores
in the cell membrane are of
different sizes.
Making Connections
Models of Membranes
Look at Figure 3. Compare the permeability of the
three materials—glass, mesh, and cloth—covering
the jars.
1. Which covering is impermeable to all three
substances?
2. Which covering is permeable to all three
substances?
3. Which covering is impermeable to some
substances, but permeable to others?
4. Name two other materials that are permeable to
some of the substances shown, but impermeable
to the others. Test the permeability of the
substances for yourself.
Figure 3
Three “membranes” — glass, wire or plastic mesh, and cloth.
5. The pores of cells aren’t always
open. Sometimes they are locked.
In humans and many other
animals, insulin acts like a key to
a lock. Insulin is a protein released
by the pancreas into the blood.
Once in the blood, insulin
molecules can reach all the cells
of the body. An insulin molecule
is able to attach itself to the cell
membrane of cells. When it does,
one kind of pore opens. Sugar
from the blood can then enter
the cell through the open pore.
(a) Imagine a mechanism that
would allow insulin to open
a pore. Draw a diagram
showing how insulin would
open the pore.
(b) People who are unable to
produce insulin have diabetes.
What problem would be
caused by not producing
insulin?
(c) What organelles within the
cell would be most affected
by a lack of insulin? Explain
your choice.
sugar-water
solution
water
sugar
Cells, Tissues, Organs, and Systems
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