The plasma membrane is an oily layer of fat molecules that prevents water and salts from passing through. So how do water, salts, and large molecules like sugars get into cells? These molecules are essential for living things. The membrane controls what goes in and out by having protein channels that act like funnels in some cases and pumps in other cases. Passive transport does not require energy molecules and happens when a funnel opens in the membrane, letting molecules flow through. Active transport requires energy, because protein machines actively grab molecules on one side of the membrane and push them through to the other side.
Passive Transport Through Channels
The simplest way that a cell membrane can control what goes in and out is to have a protein channel that fits only one type of molecule. In this way, the cell can control the flow of just water, salts, or the hydrogen ions that make a liquid acidic or not acidic. Aquaporins are protein channels that allow water to pass freely through the cell membrane. Since water does not mix with oil, and the cell membrane is oily, water cannot freely pass in or out of a cell. Aquaporins allow water molecules to flow into cells as a single-file line.
Symport and Antiport
Diffusion is the random but directional movement of molecules from a place where there are many of them to a place where there are few of them. The flow of molecules down this gradient, or difference in concentration, is like the flow of water down a waterfall. It is a form of energy that can be used to do other things. Protein pumps in the membrane can exploit the natural flow of salt ions across a membrane to pump in other types of ions or molecules. This is like hitchhiking. The pumping of a molecule in the same direction as the diffusing molecule is called symport. The pumping of a molecule in the opposite direction of the diffusing molecule is called antiport.
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Letting molecules diffuse down their gradient does not require energy, but pumping these molecules in other directions to make the gradient in the first place does require energy. Active transport describes the movement of molecules against their concentration gradients, like stuffing more people into a room that is already overcrowded, and requires pumps that are powered by an energy molecule called ATP (adenosine triphosphate). ATP is like a rechargeable battery. Each use releases a jolt of energy that turns one ATP into its uncharged state called ADP. ADP can be recharged into ATP. Proteins that pump molecules against their gradient have a pocket into which ATP fits.
Exocytosis and Endocytosis
Cells can move large molecules or large mixtures of molecules across their membrane. This type of cargo is too big to be pumped or too diverse to be controlled by just one channel. The movement of this type of material across a membrane requires the process of pinching or fusion of membrane pouches. Endocytosis is the process in which the cell membrane pinches inward to swallow a molecule that is outside the cell. Exocytosis is the transport process in which a membrane pouch inside the cell runs into the cell's surface membrane. This collision connects the pouch with the surface membrane, causing the pouch to break and release its contents outside the cell. The contents end up on the outside because the broken membrane of the pouch becomes part of the surface membrane -- like two droplets of olive oil that fuse to form a larger droplet on top of water.