To log in and use all the features of Khan Academy, please enable JavaScript in your browser. Donate Login Sign up Search for courses, skills, and videos. Introduction to passive and active transport. Passive transport and selective permeability. Passive transport review. Active transport review. Endocytosis, phagocytosis, and pinocytosis. Practice: Membrane transport. Next lesson. Current timeTotal duration Google Classroom Facebook Twitter. 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. Phagocytosis is the process by which cells ingest large particles, including other cells, by enclosing the particles in an extension of the cell membrane and budding off a new vacuole. During pinocytosis, cells take in molecules such as water from the extracellular fluid.
Finally, receptor-mediated endocytosis is a targeted version of endocytosis where receptor proteins in the plasma membrane ensure only specific, targeted substances are brought into the cell.
Exocytosis in many ways is the reverse process from endocytosis. Here cells expel material through the fusion of vesicles with the plasma membrane and subsequent dumping of their content into the extracellular fluid. Answer the question s below to see how well you understand the topics covered in the previous section.
This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section. Skip to main content. Module 5: Cell Membranes.
Search for:. Endocytosis and Exocytosis Describe the primary mechanisms by which cells import and export macromolecules In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles. Learning Objectives Describe endocytosis and identify different varieties of import, including phagocytosis, pinocytosis, and receptor-mediated endocytosis Identify the steps of exocytosis. In Summary: Endocytosis and Exocytosis Cells perform three main types of endocytosis.
Licenses and Attributions. A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances Figure 3. In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane.
If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration.
Some human diseases are caused by the failure of receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely.
People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles from their blood. Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells.
Cells perform three main types of endocytosis. Phagocytosis is the process by which cells ingest large particles, including other cells, by enclosing the particles in an extension of the cell membrane and budding off a new vesicle.
During pinocytosis, cells take in molecules such as water from the extracellular fluid. Finally, receptor-mediated endocytosis is a targeted version of endocytosis where receptor proteins in the plasma membrane ensure only specific, targeted substances are brought into the cell.
The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes discussed in the last section in that its purpose is to expel material from the cell into the extracellular fluid.
Waste material is enveloped in a membrane and fuses with the interior of the plasma membrane.
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