Dear Sophomores...

Diffusion, Osmosis, and Active Transport can be hard topics to remember. So, I am here to help.  First of all diffusion is the tendency for molecules of any substance to spread out evenly in an available space.  Diffusion is a result of thermal motion or heat.  A simple rule to remember about diffusion is thast a substance will move from an area of high concertration to an area of low concentration. This is simple diffusion.  Another type of diffusion is called, facilitated diffusion.  This is the transport of molecules across a membrane by carrier or channel proteins.  Channel proteins simply allow a way for a specific molecule to cross the cell membrane.  Carrier proteins bind substances together in order to move them through the membrane. One way to remember the general rule is that the letter "D" is higher up in the alphabet than the "O" in osmosis, so it goes high to low.

 Now on to.... Osmosis!

Osmosis is pretty similar to diffusion, but now you add water.  The definition is water that has moved across the membrane from an area of low solute concentration to an area of high solute concentration until the concentrations on both sides are equal.  The balancing of the two sides are crucial to organisms.  This process does not involve the input of energy.  Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the solution side becomes is equal. Those are the basic facts of Osmosis.  To remember this just think that "O" comes later in the alphabet, hence it going from low to high concentration and then add the aspect of water.

Finally, Active Transport!  To move a molecule across the membrane, requires work, so there must be extended energy, this "traffic" is called active transport.  The energy needed is generally referred to as ATP or Adenosine triphosphate.  This is all done with carrier proteins, rather than channel proteins. Channel proteins will not pick up the molecules and transport them the way carrier proteins will.  The materials are moved in the opposite direction, against the concentration gradient. In other words, this time the substances are moved from an area of low concentration to an area of high concentration.  This usually happens with ions, glucose, and amino acids. Active transport is a good example of a process for which cells require energy. This process is opposite of diffusion, in which the substances move from an area of low concentration to high concentration.  This is a good way of remembering the difference!