Active Transport
The scientific definition of active transport is the movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy. In active transport, the cell expends energy in the form of ATP to move a substance against its concentration gradient. It’s a lot more work than passive transport, and the proteins in the bilayer do most of the work during active transport. They pull the substances, usually ions, from areas of lower concentration into areas of higher concentration. This is one of the three membrane proteins used to do active transport, called a carrier protein. An example would be, like a very crowded amusement park, pushing through a lot of people to get from one place to another. Some transporters bind to ATP directly and use the energy of its hydrolysis to drive active transport. Other transporters use the energy already stored in the gradient of a directly –pumped ion. Direct active transport of the ion establishes a concentration gradient. When this is relieved by facilitated diffusion, the energy released can be harnessed to the pumping of some other ion, or molecule.
Passive Transport
Passive transport is a movement of biochemical and other atomic or molecular substances across cell membrane without need of energy input. Unlike active transport, it does not require an input of cellular energy because it is instead driven by the tendency of the system to grow entropy. Cell’s sophisticated membrane is semipermeable. It’s selective, but not permeable. Some examples are diffusion, osmosis, and facilitated diffusion. Passive transport move substances from higher concentration to lower concentration in the cell. Like walking down a flight of stairs, it takes little energy from the cell itself. Some molecules diffuse through transport proteins. It transports through a phospholipid bilayer, and there’s a hydrophobic (water-fearing) region in the middle. It is an important barrier between anything large, charged or hydrophilic (water-loving). So how do larger hydrophilic molecules get through? Facilitated diffusion, passive transport that uses integral membrane proteins to help larger, charged, hydrophilic, and polar molecules across a concentration gradient. This does take up a little energy from the cell. Passive transport is like water running down in a river, it doesn’t take the cell much energy and always moves from higher concentration to lower concentration.