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Chapter Four: Cell Nutrition and Respiration
In Chapter Three, we discussed how small particles can diffuse across the cell membrane through either passive transport or active transport. However, large food particles, whether they be grains of sugar or other organisms, cannot simply diffuse across the membrane; they are just too big. Instead, they are engulfed by the cell membrane through a process known as endocytosis.
As a cell approaches a food particle, either the food particle pushes into the cell membrane forming an indentation, or pseudopodia are extended from the cell around the particle. When the two extensions of the cell membrane meet on the other side of the particle, they close and form a vacuole around the food inside the cell. This process of called endocytosis.
Exocytosis is a very similar process. In fact, it's just endocytosis run in reverse order. A vacuole within the cell moves toward and fuses with the cell membrane. In this manner, the contents of the vacuole are expelled into the external environment. This may occur, for example, after a cell has taken in a large particle through endocytosis, digested it using the enzymes in the lysosome, and then needs to expel the waste products.
Endocytosis and exocytosis are general terms which refer to the process by which anything is taken into or expelled from the cell through the action of vacuoles. Phagocytosis refers to endocytosis of a solid particle. Pinocytosis indicates the taking in of liquid and dissolved molecules through endocytosis.