Passive transport is the transfer of the particles of a substance to an area of low concentration from an area of higher
concentration by random motion, meaning no energy was required from the cell. The movement of substances accross the cell
membrane is an active process. Often the concentration of these substances will vary across the membrane. This variance is
know as the concentration gradient. When one area of the membrane has a higher concentration, diffusion will occur. Diffusion
is the movement of particles from an area of higher concentration to an area of lower concentration due to the random movement
of particles. This random movement can be pictured as bouncing rubber balls that will persist in motion without slowing. If
there is a concentration gradient along the membrane, these particles will naturally move into the area with the least density.
If diffusion occurs uninhibited, a state known as equilibrium will be acheived, meaning the concentration across the entire
membrane is equal. Diffusion is the simplest of the forms of passive transport.
Yet there are other forms, such as osmosis. Omosis is the diffusion of water through a membrane with selective permeability.
Instead of taking place across the cell membrane, this form of passive transport means the movement of particles through the
membrane. This movement can occur either way. In a hypertonic solution, the concentration of free water molecules is greater
in the cell (in the cytoplasm) than in the liquid outside of the cell. So the water molecules, naturally wanting to move into
the area of lower concentration, will go through the membrane and out of the cell. This will cause the cell to shrink from
the loss of the water molecules. However, in a hypertonic solution, the cell will swell because the water molecules will enter
the cell from the outside liquid, which has the higher concentration. In an isotonic solution, the concentration of free water
molecules in the the cytoplasm and in the outside liquid is equal. As with equalibrium, the molecules in an isotonic solution
transfer back and forth at the same rates.
Other substances need to enter through the membrane in order for the cell to continue opperating. This is done through the
use of carrier proteins. Carry proteins are embedded throughout the membrane, and they act as a gate of sorts. These substances,
such as amino acids and sugars, bind to the carrier protein and are guided through it and released into the cell on the other
side. This process is known as facilitated diffusion. Although the molecules are carried into the cell, this act requires
no energy from the cell, therefore facilitated diffusion is a form of passive transport.
Movement of the particles of a substance required the use of energy by the cell is reffered to as Active Transport. More
specifically, active transport is the movement of particles against their concentration gradient. This is what requires the
energy, which is generally supplied by ATP. Naturally, the cells move from high to low concentrations, but through active
transport the particles in the area of lower concentration are transfered to an area of higher concentration.
This form of transport can be done with the use of carrier proteins as well. These proteins work the opposite way, carrying
particles from low to high concentrations, and a reffered to as "pumps". The sodium-potassium pump, for example, maintains
the needed amounts of the two ions, sodium and potassium. Sodium is found in higher concentrations outside the cell. Going
against the concentration gradient, the sodium-potassium pump would therefore transfer more particles out of the cell into
the surrounding liquid. Potassium, however, has it's higher concentration within the cell, so the sodium-potassium pump transfers
more potassium ions into the cell. In a continual cycle, this pump transfers three sodium ions out, and two potassium ions
in. The sodium-potassium pump is one of the cells most important pumps.
Vesicles, which you can remember as being defined as small membrane-bound sacs, are involved in two more forms of active
transport, endocytosis and exocytosis. Vesicles move through the cytoplasm, carrying substances such as proteins and polysaccharides,
which are too big to be moved by carrier proteins. These substances will cause a pouch to form in the membrane. This pouch
will severe from the membrane and travel through the cytoplasm, possibly to the lysosomes or other organelles. This is called
exocytosis. Exocytosis, however, acts to carry the proteins modified by the Golgi apparatus out of the cell, such as with
nerve cells and other glands. These vesicles will travel through the cytoplasm out toward the membrane, where they will fuse
and release the modified proteins.