In
some ways, this question is asking about an overall process and then naming specific types of
that process. Both osmosis and diffusion are types of passive transport. This means that both
processes do not require the cell to use ATP energy to make them happen. Both osmosis and
diffusion work because of a difference in the concentration gradient across the semi-permeable
cell membrane. Materials want to move from an area of high concentration to an area of lower
concentration until the concentration levels are at equilibrium. Osmosis is a specific type of
diffusion: the diffusion of water.
Active transport requires a cell to use
energy to move materials. This could be because the cell needs to move something in or out of
the cell membrane against the existing concentration gradient. This can happen at special
"pump" locations within the cell membrane. The sodium-potassium pump is a common
example. You can think of this pump like an airlock on a space craft. One side of pump is open,
and a material is placed inside. The pump will then close that side and open the other side. ATP
energy is required to activate the shape change of the pump, so it is active transport. Bulk
transport occurs when the cell needs to take in or get rid of large materials. The cell will do
this by manipulating the cell membrane to form membrane-bound sacs that pinch off from the cell
membrane. Endocytosis and exocytosis are examples of this kind of active transport
mechanism.
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