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What does a cell "eat"? Is it possible for objects larger than a small molecule to be engulfed by a cell? Of course it is. This image depicts a cancer cell being attacked by a cell of the immune system. Cells of the immune system consistently destroy pathogens by essentially "eating" them.
Some molecules or particles are just too large to pass through the plasma membrane or to move through a transport protein. So cells use two other active transport processes to move these macromolecules (large molecules) into or out of the cell. Vesicles or other bodies in the cytoplasm move macromolecules or large particles across the plasma membrane. There are two types of vesicle transport, endocytosis and exocytosis (illustrated in Figure below). Both processes are active transport processes, requiring energy. Illustration of the two types of vesicle transport, exocytosis and endocytosis.
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Endocytosis and exocytosis are the processes by which cells move materials into or out of the cell that are too large to directly pass through the lipid bilayer of the cell membrane. Large molecules, microorganisms and waste products are some of the substances moved through the cell membrane via exocytosis and endocytosis. Why is bulk transport important for cells?Cell membranes are semi-permeable, meaning they allow certain small molecules and ions to passively diffuse through them. Other small molecules are able to make their way into or out of the cell through carrier proteins or channels. Endocytosis involves cells taking in substances from outside the cell by engulfing them in a vesicle derived from the cell membrane. Exocytosis is where cells shift materials, such as waste products, from inside the cell to the extracellular space. The materials are engulfed in a vesicle, again derived from the cell membrane. 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. Vesicle function in endocytosis and exocytosisDuring bulk transport, larger substances or large packages of small molecules are transported through the cell membrane, also known as the plasma membrane, by way of vesicles – think of vesicles as little membrane sacs that can fuse with the cell membrane. 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. What is endocytosis? Endocytosis definition and purposesEndocytosis 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:
Types of endocytosisThere are two types of endocytosis: phagocytosis and pinocytosis. PhagocytosisPhagocytosis, 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. Macrophages and neutrophils, types of white blood cells, are the two primary phagocytes. These white blood cells are responsible for clearing out aged and damaged cells, as well as disposing of infectious microorganisms. PinocytosisPinocytosis, also known as cell drinking, is common in plant and animal cells. During pinocytosis, the cell takes in substances from the extracellular fluid that it needs to function. These include things like water and nutrients. Receptor-mediated endocytosis is a specialized type of pinocytosis. During receptor-mediated endocytosis, macromolecules bind to receptors along the surface of the cell’s plasma membrane. Cholesterol uptake is an example of receptor-mediated endocytosis. The steps of endocytosisThe following is an outline of the basic steps of the two types of endocytosis. Two types of endocytosis: phagocytosis and pinocytosis. Phagocytosis:
Pinocytosis:
Endocytosis exampleMacrophages are a type of white blood cell that play a central role in protecting mammals against pathogens like bacteria and viruses. When a macrophage comes into contact with a virus, say a cold virus in the bloodstream, it can bind to the virus’s cell surface. Next, the macrophage will form a vesicle around the virus, completely ingesting it. The vesicle then travels to the cytosol and fuses with the lysosome, where the virus is broken down. Some viruses replicate by “tricking” host cells into endocytosing them, at which point the cell is hijacked by the virus and is instructed to replicate the virus genome and capsid. What is exocytosis? Exocytosis definition and purposesExocytosis is the process by which cells move materials from within the cell into the extracellular fluid. Exocytosis occurs when a vesicle fuses with the plasma membrane, allowing its contents to be released outside the cell. Exocytosis serves the following purposes:
Types of exocytosisRegulated ExocytosisMost exocytotic vesicles contain substances created within the endoplasmic reticulum for use elsewhere in the body, such as neurotransmitters or hormones. These molecules are then packaged within a layer of membrane called a vesicle.Once excreted from the endoplasmic reticulum, these vesicles are transported to the Golgi apparatus (also known as the Golgi complex) for further modification. The molecules are then packaged once again in a vesicle that makes its way to the plasma membrane. The release of these molecules from the cell is termed regulated exocytosis because the expulsion of the materials is controlled, or regulated, by extracellular signals that cause membrane depolarization. Constitutive exocytosis, in contrast, doesn’t require any extracellular signals. The majority of molecules traveling to the plasma membrane do so using this pathway. After exocytosis, some exocytotic vesicles are incorporated into the plasma membrane (full vesicle fusion), while others return to the interior of the cell after their contents have been released (this is termed the “kiss-and-run” pathway). Others remain docked to the membrane, where they can be used multiple times (the “kiss-and-stay” pathway). Exocytosis involves the passage of a vesicle from the endoplasmic reticulum or Golgi apparatus, through the cytoplasm to the cell membrane, where it fuses and releases its contents. The steps of exocytosisBelow is an outline of the basic steps of exocytosis.
Exocytosis exampleLet’s take the macrophage that we discussed in our endocytosis example. Once the white blood cell has engulfed a foreign pathogen eliminate it, certain parts of the pathogen are no longer needed. The macrophage gets rid of this waste material through exocytosis, during which vesicles carry out the unwanted pathogen material. Endocytosis vs exocytosis: a comparison
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What is it called when a cell engulfs a large particle?
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