Passive Transport
Despite their differences in size and shape, all cells are enclosed by a cell membrane that consists of a double
layer of phospholipids interspersed with proteins. Its unique structure permits some substances to cross it
rapidly, while others are unable to cross it, or cross it slowly. Thus, the plasma membrane regulates the
substances entering and leaving the cell. In this plate we explore three methods for the passive transport of
molecules through the plasma membrane. Passive transport processes are ones that do not require cellular
energy to proceed.
Looking over the plate, notice that it is composed of three diagrams, each of which depicts a form of transport.
A plasma membrane that permits the passage of certain substances is said to be semi-permeable. For
example, a semi-permeable cell membrane might not be permeable to certain large molecules, but might be
permeable to oxygen and carbon dioxide (this means that they pass freely across the membrane). The force
that propels oxygen and carbon dioxide across the membrane is called diffusion. Diffusion is the net movement
of molecules from a region of high concentration to one of low concentration.
In the first diagram, we illustrate the process of diffusion in the absence of a membrane. A beaker contains
both water molecules (A) and a crystal (B) of colored material. Within the triangle are a number of crystal
molecules (C). You should use contrasting colors for the crystal molecules and water.
Molecules diffuse, or move, from areas where they are highly concentrated to areas where their concentration
is lower. In the second beaker, you can see the movement of crystal molecules (C,) away from the area of
the crystal, and a movement of water molecules (A,) into the crystal area.
In the third beaker, equilibrium has been reached, and you can see completely mixed water and crystal
molecules (D). Diffusion has taken place and no more net movement of the crystal molecules will occur.
We now turn to a special kind of diffusion called osmosis. This time we will use a cell and show movement
across its membrane. The movement is passive, meaning that it occurs without the input of any energy.
Continue coloring as you read the paragraphs below.
The net movement of water molecules across a membrane is a special kind of diffusion called osmosis. In the
first view, we see a red blood cell (E), which should be colored in a light color. The concentrations of salt and
water (F) inside and outside the cell are identical, so that the movement of water (G) occurs at equal rates,
into and out of the cell. The inside and outside environments are said to be isotonic.
In the second view, the red blood cell (E) is placed in a very salty solution. Water molecules begin to flow out of
the cell, as is shown by the movement of water (G), and the cell will shrink. The outside environment is said to
be hypertonic relative to the interior, which is hypotonic.
Now examine the third view, in which the red blood cell has been placed in a solution that contains no salt.
There is a higher salt concentration inside the cell, and water flows through the membrane into the cell to dilute
the salt; this causes the cell to swell. The inside environment is said to be hypertonic relative to the exterior cell
environment, which is hypotonic.
We will complete the plate by focusing on a third type of passive transport called facilitated diffusion, which
allows the movement of carbohydrate molecules across the plasma membrane. Continue your reading below
as you color the third diagram in the plate.
Facilitated diffusion is the movement of molecules across the membrane with the aid of a membrane protein.
This diagram shows a high concentration of carbohydrates (H) at the cell exterior; this is shown by the
concentration gradient (I). Facilitated diffusion takes place through transport proteins (J) embedded in the
cell membrane (K), and diffusion occurs with the concentration gradient.
In the second view, diffusion has begun. The carbohydrate molecules are moving from the area of high
concentration to an area of low concentration inside the cell. One carbohydrate molecule is presently being
transported. In the third view, the carbohydrate molecule is added to the one already in the cell. The remaining
carbohydrate molecules will follow it until the number of carbohydrates on either side of the membrane is
equal.