Science Cell Transport Diffusion and Osmosis

Cells constantly need to move materials back and forth across their membranes. These substances may include salts, water, glucose, and waste products. Cells always want to maintain a constant internal environment. However, at times, there is an accumulation of materials on one side or the other of the cell membrane that needs to equal out. When this happens, a concentration gradient is formed. Usually materials flow down their concentration gradients, from high to low. This is a passive process that does not use energy. Water and oxygen are two substances that can flow into and out of a cell whenever they need to.

There are a few instances when molecules need to move up their concentration gradients. Like going up a steep hill, this process takes energy. Active transport, as this process is known, often involves ions moving back and forth across the cell membrane (i.e. the Sodium/Potassium pump of nerve cells).

Facilitated diffusion is when molecules are still traveling down their concentration gradients, but they need some help. Like regular diffusion, this process does not require any energy, but needs the involvement of some other kind of molecule to help move the gradient to equilibrium. A lot of times these "helper molecules" are called carrier proteins, which are found engrained within the cell membrane.

Other times, particles are too large to fit through the openings in the cell membrane but they still need to get inside. When this happens, cells use a transport mechanism called endocytosis (taking in) or exocytosis (pushing out) to move the materials across the membrane (see the diagram for a model of endocytosis).