MCAT Content / Plasma Membrane / Exocytosis And Endocytosis

Exocytosis and endocytosis

Topic: Plasma Membrane

Exocytosis and Endocytosis are methods for expelling and taking in a material using the cells membrane.

Endocytosis is a type of active transport (requires energy) that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: the plasma membrane of the cell forms a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly-created intracellular vesicle formed from the plasma membrane.

Phagocytosis is the process by which large particles, such as cells, large particles or viral particles, are taken in by a cell. The membrane from the body of the cell and surrounds the particle, eventually enclosing it creating a vesicle. Once the vesicle containing the particle is enclosed within the cell, the vesicle merges with a lysosome for the breakdown of the material in the newly formed compartment (endosome).

Pinocytosis means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than phagocytosis.

Receptor-mediated endocytosis employs receptor proteins in the plasma membrane that has a specific binding affinity for certain substances and works similarly to phagocytosis.

Exocytosis expels material from the cell into the extracellular fluid; this is the opposite of what occurs in endocytosis. In exocytosis, waste material is enveloped in a vesicle membrane and fuses with the interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell and the waste material is expelled into the extracellular space.

Practice Questions

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Key Points

• Endocytosis takes particles into the cell that are too large to passively cross the cell membrane.

• Phagocytosis is the taking in of large food particles, while pinocytosis takes in liquid particles.

• Receptor-mediated endocytosis uses special receptor proteins to help carry large particles across the cell membrane

• Exocytosis is the opposite of endocytosis as it involves releasing materials from the cell.

Key Terms

Endocytosis: process that takes particles into the cell that are too large to passively cross the cell membrane

Phagocytosis: “cell-eating”, takes up large molecules from the environment

Pinocytosis: “cell-drinking”, takes up smaller molecules from the environment

Receptor-mediated endocytosis: works like phagocytosis, relies on receptors to attract certain chemicals

Exocytosis: process that removes particles from the cell

Extracellular: outside the cell

Active transport: the movement of substances against a concentration gradient using energy

Vesicle: a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer

Plasma membrane: the semipermeable barrier that surrounds the cytoplasm (inside contents) of a cell

Lysosomes: a membrane-bound cell organelle that contains digestive enzymes

Endosome is a membrane-bound compartment inside a eukaryotic cell

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