There are four types of transportation of things across a membrane: Diffusion, active transport, osmosis, and endocytosis. Diffusion is when the molecules bump into each other and spread out, and sometimes one of the molecules gets bumped into a cell. The places that do not have a lot of molecules in them have a low concentration, the places that have a lot of molecules have a high concentration, and the difference between them is called the concentration gradient. An example of diffusion is: if you put a drop of food coloring in a cylinder of water, you will see that the food coloring spreads out to the places where there is a low concentration. After a couple hours the food coloring will have completely diffused through all of the water in the cylinder.
A molecule in active transport, however, can go either way
with the concentration gradient. It uses proteins to bring molecules in and out,
and only one kind of molecule can fit into a certain protein. Active transport uses
ATP to function, unlike osmosis.
Osmosis is when water flows freely in and out of the cell.
If sugar or salt is present outside the cell, the water will go towards it,
therefore lowering the concentration gradient of the cell. Because they are
both polar, the water will be attracted to the sugar or salt.
Endocytosis is when a big molecule is surrounded by the cell
membrane. It then pinches off the membrane and forms a package called a vesicle
or a vacuole. White blood cells do this with viruses and bacteria, and amoeba do
this also with food. A similar type of transportation is exocytosis, which is
when the cell pushes large molecules out. Both use ATP.
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| Photo courtesy of www.hartnell.edu |
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