Biology, Answering the Big Questions of Life/cell transport
The plasma membrane forms the border of a cell. It is a barrier made of phospholipids that separates the inside and outside of the cell. Most molecules can not pass through the barrier without help. There are many structures floating within the plasma membrane that aid the entry and exit of substances into the cell. The process by which items enter and exit the cell is called cellular transport.
The main part of all membranes is a lipid bilayer
The lipid molecules keep separate from the water by forming a liquid barrier that hydrophobic proteins can dissolve in.
Endocytosis and Exocytosis
Transport across a membrane can be done by endocytosis where the membrane wraps around an object and takes it into a cell as a vesicle. This is how many protists eat their food. The membrane flows around the food and merges forming a food vacuole that will merge with a lysosome to digest the food.
When the food has been digested, it will merge back with the membrane and eject the undigested wastes is a process called exocytosis. Many cells without cell walls eat and drink this way. Cell eating is called phagocytosis and cell drinking is called pinocytosis.
Receptor mediated endocytosis requires membrane proteins called receptors to recognize and bind to a molecule outside of the cell before it is enveloped and brought into the cell in a vesicle.
Proteins have many functions, but many of them are involved in cellular transport as pumps and channels. Pumps and channels are transmembrane proteins (they cross the membrane) that allow objects to pass through. Channels usually allow specific molecules to travel passively through the membrane by diffusion. Pumps carry objects through the membrane but require payment in the form of energy from molecules such as ATP.