Endocytosis is an active transport mechanism in which a cell takes in substances from its outside environment by engulfing them in a vesicle. Substances from the external environment that get transported into the cell include proteins, microorganisms, electrolytes, fluids, and some macromolecules. Inside the cell these substances undergo various processes and are broken down into smaller elements either for use as nutrients to support the cell or for eliminating potential pathogens.

There are three types of endocytosis- phagocytosis, pinocytosis, and receptor-mediated endocytosis.  

1. Phagocytosis refers to the process in which the cells bring in solid particles such as nutrients, proteins, and even bacterial. It is also known as cellular eating.

2. Pinocytosis involves the transportation of liquid particles into the cell. It is also known as cellular drinking.

3. Receptor-mediated endocytosis takes place with the help of special receptors that are located outside of the cell wall. These receptors are designed to identify and bind with specific molecules. After the interaction is complete, the cell membrane transports the new substance inside either through phagocytosis or pinocytosis depending on whether it is solid or liquid.

Endocytosis serves three main purposes. The process helps cells obtain nutrients such as proteins and lipids, which are essential for the growth, function, and repair. It also enables white blood cells of the immune system to capture microbial pathogens such as bacteria and other harmful substances, which are then broken down and destroyed. Endocytosis also facilitates the elimination of old and damaged cells that stop functioning as they should before they cause any damage to the other healthy cells.