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Cell Biology
The World of Cells
Endo- and Exocytosis
Getting Substances
In and Out of Cells
Endocystosis, is a general term for the process whereby very large particles of material are wrapped with plasma membrane and moved into the cell in the form of vesicles or vacuoles. None of the trapped material actually moves through the membrane, but remains on the other side of the original membrane, even while the vacuole is inside the cell.
Exocytosis Exocytosis is the reverse of endocytosis. Quatities of material are expelled from the cell without ever passing through the membrane as individual molecules. By using the processes of endocytosis and exocytosis, some specialized types of cells move large amounts of bulk material into and out of themselves.
Phagocytosis Solid particles are engulfed by phagocytosis ("cell eating"), a process that begins when solids make contact with the outer cell surface, triggering the movement of the membrane.

The desired particles are then enclosed within a small piece of the plasma membrane which forms a sac called a vacuole (or vesicle), with the food particle inside it. This vacuole is then moved to the interior of the cell. Strictly speaking, the food particles are not yet part of the cell as it is still surrounded by membrane.

Before food can be used, it must be broken down to smaller pieces and those pieces moved into the cytoplasm. Digestion occurs when the food vacuole is fused with a second vacuole, called a lysosome, that contains powerful digestive enzymes.

Food is degraded, its nutrients are absorbed by the cell and its waste products are left in the digestive vacuole, which may then leave the cell by exocytosis.

Phagocytosis occurs in the scavenging white blood cells of our body. They prowl around looking for invading bacteria and viruses which they engulf and destroy.

Pinocytosis Pinocytosis ("cell drinking") is almost the same process as phagocytosis, except it involves liquids instead of solids.

During exocytosis a vacuole containing material to be excreated from the cell moves to the plasma membrane and fuses with it. The vacuole membrane becomes part of the plasma membrane and the contents are released to the outside.

Cells use this method to eliminate the wastes left after digestion and metabolism and also to release a whole variety of materials that have been synthesized inside the cell but which are needed outside the cell. Release of hormones and digestive enzymes, found in multicellular animals, are two examples of this process.

Figure legend: Large particles of organic matter become enclosed in a portion of the plasma membrane, forming vacuoles inside the cytoplasm of the cell. This mechanism enables cells to engulf food and digest the nutrients internally. Waste material, or specially synthesized molecules enclosed in vacuoles, move to the plasma membrane and are released when the membrane of the vacuole fuses with the outer membrane of the cell.

© 2001, Professor John Blamire