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Movement Into and Out of a Cell
Drew, 2005
Because a cell is a living thing it must do all the processes that any organism must do:
respire, metabolize, excrete and reproduce. In order to accomplish these things the cell
must be able to get molecules of food, water, waste into and out of the cell. Water is
usually involved due to the fact that a cell is mostly composed of water and that cells
typically exist where they are surrounded by water. Consider the amoeba, an organism
that lives in the water of a pond, or a muscle cell that is surrounded by the fluid that
moves between the cells in a body (called interstitial fluid). It really does not matter what
kind of cell: every cell needs to get water and the things that are dissolved in water into
and out of it.
There are a variety of ways that cells accomplish this movement of substances. One
method is diffusion. Diffusion is the movement of molecules from an area of high
concentration to an area of lower concentration. In other words, molecules move from
where there are a lot of them (concentrated) to where there are fewer (dilute). This
happens in nature all the time. The smell of a skunk will spread through the air so that it
is smelled miles away from the source. A drop of dye placed into a bottle of water will
spread—without stirring—to become evenly mixed throughout the bottle. Water melting
from an ice cube in a glass of soda will spread into the soda to make it watery. The
common feature of each of these examples is the movement of the molecules that make up
the substance from where there are a lot of them to where there are not as many of
them: the molecules spread out. How do molecules move like this?
Atoms and molecules are constantly moving, some quickly and some slowly. This constant
motion is called Brownian motion. If we were able see individual molecules we would see
that they are constantly bouncing off other molecules. It’s almost as though molecules
were ping pong balls, constantly bouncing in different directions. Of course, how fast they
move depends on the size and temperature of the molecules. Usually smaller or warmer
molecules will travel faster than larger or colder molecules. The smell of a skunk travels
because the molecules of skunk scent bounce off other air molecules, spreading out
through the air each time a molecule is bumped by another. Of course, if it is a hot day,
this process will happen a good deal faster than it will on a cold day!!
= skunk scent molecules
= air molecules
skunk picture from www.campain.org/hello/ 98/1018/640/skunk.jpg
Because of their motion, molecules tend to spread out across what is called a
concentration gradient. The word gradient refers to a gradual change from where there
is a lot of a substance to where there is a little of it: it might help to think of this as a
hill. The bigger the difference between the two ends of the scale the faster the
movement of the concentrated molecules (where there are a lot) to the dilute area (where
there are few). This movement will generally continue until an equilibrium is reached.
Equilibrium is reached when the concentration of a substance is the same throughout a
given space. Consider the example of the dye being added to a container of water.
a. drop of dye added to
beaker of water; dye
molecules concentrated
in one area
= water molecule
b. dye molecules beginning
spread throughout via
random motion of
molecules
= dye molecule
c. equilibrium reached
Beaker diagram (not including molecules) comes from:
www.chemistry.eku.edu/GODBEY/Homepage_files/CHE_112_Lab_Manual/Experiments/Kinetics.htm
Cells must move substances across their membranes in order to survive. In some cases a
cell must use energy to accomplish this; in others, the cell does not need to use energy.
Passive transport is a type of movement across a cell membrane that relies on the
ordinary movement of molecules: the cell does not need to use its energy to make it
happen. Because molecules are in constant motion some molecules will encounter the cell
membrane and, if the molecule is small enough, it will be able to pass through the semipermeable cell membrane. Permeable means that substances can get through: since cell
membranes are semi-permeable, they only let certain things through. Water is one of the
molecules that can pass freely through a cell membrane. The process of diffusion through
a semi-permeable membrane is called osmosis. An example of this can be seen in plants
that have wilted from lack of water: water moves into the cells of the plant after it
moves through the plant because there is a higher concentration of water outside the cell
than in. Once the water moves into the cells, the cells expand and the plant expands to its
normal size and shape. This example of passive transport happens due to the ordinary
movement of molecules and their tendency to down the concentration gradient: a cell does
not have work at—or use energy—to get water into the cell because water molecules will
continue to move through the semi-permeable cell membrane until there is an equal
concentration of water molecules both outside and inside the cell. Of course, once
equilibrium is reached the movement of water across the membrane continues, but the
movement into and out of the cell happen at an equal rate.
Whether water diffuses into or out of a cell is dependent on the liquid that surrounds the
cell. There are three different relationships that can exist between the concentration of
the cytoplasm and the liquid outside a cell: hypertonic, hypotonic and isotonic. The
following table, which comes from Biology: Principles and Explorations (2001, p. 77,
Johnson, G.B., and Raven, P.H.), summarizes what each state is and the effect the
relationship will have on a cell.
If the fluid outside
Then outside
Water diffuses….
Effect on cell
the cell has….
fluid is……
…lower water molecule
….hypertonic
…..out of cell.
Cell shrinks.
concentration than
cytoplasm
…higher water
molecule concentration
than cytoplasm
….same water
concentration as
cytoplasm.
…hypotonic
…isotonic
….into cell
Cell swells.
….into and out of cell at
equal rates.
Cell stays the same
size.
In addition to water, cells need to be able to move other substances, such as sugar, across
their membranes. Some substances are composed of very small molecules that easily move
through the semi-permeable membrane along with water. The diffusion of substances
other than water across a cell membrane is known as dialysis. Things that are dissolved in
water are called solutes. Hemodialysis is an example of the type of dialysis that patients
with kidney failure need to do several times per week. During hemodialysis blood from the
patient is run through a machine that has a semi-permeable membrane between the blood
and another liquid—a washing fluid. Things that are dissolved in the blood can cross the
membrane into the washing fluid, but blood cells cannot. The washing fluid is discarded,
and the filtered blood is returned to the patient. In this way waste products in the blood
are eliminated, hopefully before they can cause damage to other parts of the patient’s
body. (See diagram below, from www.kidneyetn.org/images/simpledialysisimage.gif).