Download File

Cell Transport, Diffusion, and Osmosis Reading
Diffusion
Many times this year, you can walk down the hallway and already know that we are doing a
lab. I hear from the hallway, “Smells like science!”. The smells that come from my classroom
spread into the hallways quickly. This is an example of diffusion. Diffusion is when molecules
move from an area of high concentration to an area where there is low concentration. If there
are a lot of molecules in my classroom that have a strong scent, that means there is a strong
concentration. If the molecules in the hallway do not have a scent, that means there is a low
concentration. Concentration means the number of particles of a substance in a particular
volume. In many things in science, reach an equilibrium, or reach stability. That means that the
scent spreads until there is an even amount of it everywhere. As a result, in many real world
phenomena, things will move from a high to a low concentration. The bigger the difference in
concentration, the faster the molecules spread out. In science, when molecules move from high
to low concentrations it is referred to as molecules moving across the concentration gradient.
Diffusion occurs in cells too. Diffusion is essential in order for a cell to survive.
Important molecules needed for important processes and reactions that take place in cells must
enter and exit through the cell membrane. For example, oxygen that is needed for cells to go
through cellular respiration enters the cell through diffusion. On the “flip side”, carbon dioxide
leaves the cell through diffusion as a product of cellular respiration. As you know, cell
membranes are semipermeable; they let some things in and out. These membranes have tiny
gaps in them that materials can move through. Take a look at the diagram below to see one
process that uses diffusion:
Plant cells go
through
photosynthesis,
they produce
oxygen.
Now, there are
equal amounts of
O2 inside and
outside of the cell.
O2
O
O2 2 O2
O2
There are more O2
molecules inside
the cell than
outside the cell.
That means there is
a higher
concentration.
O2
O2
O2
O2 molecules start
to move from high
to low
concentrations.
They diffuse
through gaps in the
cell membrane out
of the cell.
O2
O2
O2
O2
Osmosis
Water makes up 2/3 of the cell. It is an extremely important molecule for cell function.
Thus, it is important that it can diffuse in and out of the cell when it needs to. Because water
is so important, scientists have given a special name for water diffusing in and out of a cell.
Osmosis is the name for water moving in and out of cells from high to low concentrations.
Passive Transport
Diffusion and osmosis are examples of passive transport. Passive transport occurs when
the cell does not need to use its own energy to move a molecule through the cell membrane.
Active Transport
Active transport is the process of using energy to move important molecules across a cell
membrane. This is much different than diffusion, osmosis, and passive transport. Cells require
many materials that cannot simply pass through the cell membrane. Molecules such as salt need
to be able to pass in and out of a cell but require energy in order to do so.
Endocytosis
One example of active transport is endocytosis. Endocytosis occurs when
molecules are too large to fit through the cell membrane and into the cell. Notice the
prefix “endo”. This means “in”. These molecules that are too large to fit through the
membrane are often called “packages”. When this occurs, the cell membrane wraps itself
around the “package” and breaks off from the cell membrane. The cell membrane reseals
itself and the package enters the cell completely wrapped up by the cell membrane. Look
at the diagram below to see what endocytosis looks like.
Cell membrane
Package
Exocytosis
Exocytosis begins with the prefix “exo”. This means out. Exocytosis is simply the
opposite of endocytosis. It is a form of active transport that allows things or “packages”
that are too large to fit through the membrane. Cells often use exocytosis when they
are trying to remove waste from the cell. Look at the diagram below to see what
exocytosis looks like.
Cell membrane
Package
Surface Area to Volume Ratio
Surface area is the amount of surface covering the outside of an object. As the cell
membrane is the organelle that surrounds the cell, when we talk about surface area, we are
often talking about how large the surface of the cell membrane is. Volume is the amount of
space an object takes up. In a cell, this often refers to how big the cell is. The cell’s ability to
transport objects across the membrane has to do with the surface area to volume ratio of the
cell. This means it is important to recognize the relationship between how large the cell
membrane is compared to how much space the cell takes up. The higher the surface to volume
ratio, the better the cell can be at transporting substances in and out of the cell, and, in turn,
the better the cell can perform its functions.
How do you calculate surface area to volume ratio?
Let’s say the surface area of a large cell is 96 cm2 and the volume is 64 cm3. To create
the ratio, we write the surface area, then a colon, and then the volume. The surface area
always has to go first! So, we would write 96:64. Then, we see if we can reduce. Are there
any numbers that go into both 96 and 64? Yes, 32! If we divide both numbers by 32, we get a
surface area to volume ratio of 3:2. In order for cells to be effective at transport, we want
the first number (the surface area) to be higher than the second number (the volume).
What affects surface area to volume ratio?
The size of a cell affects its surface area to volume ratio. In order to be effective at
transport, cells need to remain small. As a cell grows in size, the volume of the cell increases a
lot more than the surface area of the cell does. This means the surface area to volume ratio
goes down and transport of important materials becomes difficult. Smaller cells have a higher
surface area to volume ratio. If a cell grows and begins to get too big, the big cell divides into
two smaller cells to keep the surface area to volume ratio high! When the cell divides, it means
there is more membrane available for transport. It also means there is a greater surface
area. With more surface area to the membrane, important molecules that need to get in and
out of the cell to carry out cell functions have more places to enter and exit.
The shape of the cell affects its surface area to volume ratio. Some cells are shaped
differently than others. If a specialized cell requires a lot of transport, it is extremely
important that it has a higher surface area to volume ratio. For example, single-celled
organisms are frequently thin and flat to help with this. In our bodies, nerve cells and muscle
cells are long and skinny in order to have a higher ratio for better transport of signals and
molecules. Thus, the structure of the cells helps with its function!
Surface area to volume ratio does not only affect the entire cell, it affects organelles as
well. Organelles inside cells that require a lot of transport of materials have unique shapes
that increase their ratio. Think of the endoplasmic reticulum and golgi apparatus (pictured
below). These organelles are folded up and “smooshed” which increases their surface area to
volume ratio.