Diffusion and Transport
This movement of particles back and forth across the cell membrane may be passive (passive transport; which requires no energy) or active (active transport; which requires energy or ATP. This transport of particles (atoms and/or molecules) happens because of differences in concentration. Concentration is defined as "how many particles are in a specific area". The differences in the concentration of particles forms a concentration gradient (hill) with more particles on one side than the other. Where the particles are more numerous we have a high concentration (top of the hill). Where particles are less numerous we have a low concentration (bottom of the hill). Particles are always moving (Brownian Motion) and trying to come to equilibrium- balance. This tendancy to try and achieve balance or equilbrium is what drives the particle movement from high concentrated to lower concentrated areas.
Passive Transport: When the movement of particles goes from a high concentration to a low concentration or "with the gradient"- down the hill. This does not take any energy at all and is typically called DIFFUSION. When you put food coloring in a pitcher of water and let it set, the coloring will go from high concentration (the middle of the pitcher) to lower concentration (the rest of the water) until equilibrium is reached (the water is all colored). You can speed this process up by adding energy or mixing with a spoon but given time the color will diffuse all the same. Another example of diffusion is smells. In the mall, the smell of perfume will be highest at the perfume counter and be diffusing throughout the store.
When small, neutral particles simply go through the spaces between the lipid heads it is simple diffusion. When the large or charged particles go through a protein it is called facilitated diffusion. IF the diffusion is of water (through a protein channel) it is called osmosis. For example, if you spend a lot of time in a pool (or the shower) the concentration of "fresh" water is higher in YOUR CELLS (in the cytoplasm) than it is in the pool or the shower water. Water from your cells will diffuse from your cells into the pool or the shower water. This causes your finger tips, toes, etc. to become wrinkled! Too much time in the water (like the ocean) will cause you to become dehydrated!
Active Transport: When the movement of particles goes from low concentration to high concentration "against the gradient". Like climbing a hill, this process takes energy in the form of ATP and always uses a protein channel. The ATP opens the channel and allows bigger or charged stuff to enter and/or exit the cell.