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.