Endocytosis is a general term for a group of processes that bring macromolecules, large particles, small molecules, and even small cells into the eukaryotic cell. There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. In all three, the plasma membrane invaginates folds inward around materials from the environment, forming a small pocket. The pocket deepens, forming a vesicle. Exocytosis is the process by which materials packaged in vesicles are secreted from a cell when the vesicle membrane fuses with the plasma membrane.
The initial event in this process is the binding of a membrane protein protruding from the cytoplasmic side of the vesicle with a membrane protein on the cytoplasmic side of the target site on the plasma membrane. The phospholipid regions of the two membranes merge, and an opening to the outside of the cell develops.
Other small molecules are able to make their way into or out of the cell through carrier proteins or channels. But there are materials that are too large to pass through the cell membrane using these methods.
There are times when a cell will need to engulf a bacterium or release a hormone. It is during these instances that bulk transport mechanisms are needed. Endocytosis and exocytosis are the bulk transport mechanisms used in eukaryotes. As these transport processes require energy, they are known as active transport processes.
Cell membranes are comprised of a lipid bilayer. The walls of vesicles are also made up of a lipid bilayer, which is why they are capable of fusing with the cell membrane. This fusion between vesicles and the plasma membrane facilitates bulk transport both into and out of the cell.
Endocytosis is the process by which cells take in substances from outside of the cell by engulfing them in a vesicle. These can include things like nutrients to support the cell or pathogens that immune cells engulf and destroy. Endocytosis occurs when a portion of the cell membrane folds in on itself, encircling extracellular fluid and various molecules or microorganisms.
The resulting vesicle breaks off and is transported within the cell. Endocytosis serves many purposes, including:. There are two types of endocytosis: phagocytosis and pinocytosis. Phagocytosis Phagocytosis, also known as cell eating, is the process by which cells internalize large particles or cells, like damaged cells and bacteria. Within the human body, and in other mammals, phagocytosis is how immune cells engulf and destroy dangerous microorganisms or toxic compounds.
Exocytosis occurs when a cell produces substances for export, such as a protein, or when the cell is getting rid of a waste product or a toxin. Newly made membrane proteins and membrane lipids are moved on top the plasma membrane by exocytosis. Vesicle Transport Some molecules or particles are just too large to pass through the plasma membrane or to move through a transport protein.
Endocytosis and Exocytosis Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane. There are two main kinds of endocytosis: Phagocytosis , or cellular eating, occurs when the dissolved materials enter the cell. The plasma membrane engulfs the solid material, forming a phagocytic vesicle.
Pinocytosis , or cellular drinking, occurs when the plasma membrane folds inward to form a channel allowing dissolved substances to enter the cell, as shown in Figure below.
When the channel is closed, the liquid is encircled within a pinocytic vesicle. Illustration of an axon releasing dopamine by exocytosis.
Summary Active transport is the energy-requiring process of pumping molecules and ions across membranes against a concentration gradient. Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane , and bringing it into the cell. Exocytosis describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell.
Both endocytosis and exocytosis are active transport processes.
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