what process where fluid and dissolved substances and molecules are taken up by the cell?
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Transport into the Cell from the Plasma Membrane: Endocytosis
The routes that lead inward from the cell surface to lysosomes start with the process of endocytosis, by which cells take up macromolecules, particulate substances, and, in specialized cases, even other cells. In this process, the material to be ingested is progressively enclosed by a small portion of the plasma membrane, which first invaginates and then pinches off to form an endocytic vesicle containing the ingested substance or particle. Two main types of endocytosis are distinguished on the basis of the size of the endocytic vesicles formed. One type is called phagocytosis (“cellular eating”), which involves the ingestion of large particles, such as microorganisms or dead cells via large vesicles called phagosomes (generally >250 nm in diameter). The other type is pinocytosis (“cellular drinking”), which involves the ingestion of fluid and solutes via small pinocytic vesicles (about 100 nm in diameter). Most eucaryotic cells are continually ingesting fluid and solutes by pinocytosis; large particles are most efficiently ingested by specialized phagocytic cells.
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Molecular Biology of the Cell. 4th edition.
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Transport into the Cell from the Plasma Membrane: Endocytosis
The routes that lead inward from the cell surface to lysosomes start with the process of endocytosis, by which cells take up macromolecules, particulate substances, and, in specialized cases, even other cells. In this process, the material to be ingested is progressively enclosed by a small portion of the plasma membrane, which first invaginates and then pinches off to form an containing the ingested substance or particle. Two main types of endocytosis are distinguished on the basis of the size of the endocytic vesicles formed. One type is called (“cellular eating”), which involves the ingestion of large particles, such as microorganisms or dead cells via large vesicles called (generally >250 nm in diameter). The other type is (“cellular drinking”), which involves the ingestion of fluid and solutes via small pinocytic vesicles (about 100 nm in diameter). Most eucaryotic cells are continually ingesting fluid and solutes by pinocytosis; large particles are most efficiently ingested by specialized phagocytic cells.
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Specialized Phagocytic Cells Can Ingest Large Particles
Phagocytosis is a special form of endocytosis in which large particles such as microorganisms and dead cells are ingested via large endocytic vesicles called phagosomes. In protozoa, phagocytosis is a form of feeding: large particles taken up into phagosomes end up in lysosomes, and the products of the subsequent digestive processes pass into the cytosol to be utilized as food. However, few cells in multicellular organisms are able to ingest such large particles efficiently. In the gut of animals, for example, the particles of food are broken down extracellularly and their hydrolysis products are imported into cells.
Phagocytosis is important in most animals for purposes other than nutrition, and it is mainly carried out by specialized cells—so-called In mammals, three classes of white blood cells act as professional phagocytes—macrophages, neutrophils, and dendritic cells. These cells all develop from hemopoietic stem cells (discussed in Chapter 22), and they defend us against infection by ingesting invading microorganisms. Macrophages also have an important role in scavenging senescent cells and cells that have died by apoptosis (discussed in Chapter 17). In quantitative terms, the latter function is by far the most important: our macrophages phagocytose more than 1011 senescent red blood cells in each of us every day, for example.
Whereas the endocytic vesicles involved in pinocytosis are small and uniform, phagosomes have diameters that are determined by the size of the ingested particle, and they can be almost as large as the phagocytic cell itself (Figure 13-39). The phagosomes fuse with lysosomes inside the cell, and the ingested material is then degraded. Any indigestible substances will remain in lysosomes, forming Some of the internalized plasma membrane components never reach the lysosome, because they are retrieved from the phagosome in transport vesicles and returned to the plasma membrane.
Figure 13-39
Phagocytosis by a macrophage. A scanning electron micrograph of a mouse macrophage phagocytosing two chemically altered red blood cells. The point to edges of thin processes (pseudopods) of the macrophage that are extending as collars to engulf (more...)
To be phagocytosed, particles must first bind to the surface of the phagocyte. However, not all particles that bind are ingested. Phagocytes have a variety of specialized surface receptors that are functionally linked to the phagocytic machinery of the cell. Unlike pinocytosis, which is a constitutive process that occurs continuously, phagocytosis is a triggered process, requiring that receptors be activated that transmit signals to the cell interior and initiate the response. The best-characterized triggers are antibodies, which protect us by binding to the surface of infectious microorganisms to form a coat in which the tail region of each antibody molecule, called the Fc region, is exposed on the exterior (discussed in Chapter 24). This antibody coat is recognized by specific on the surface of macrophages and neutrophils, whose binding induces the phagocytic cell to extend pseudopods that engulf the particle and fuse at their tips to form a phagosome (Figure 13-40).
Figure 13-40
Phagocytosis by a neutrophil. An electron micrograph of a neutrophil phagocytosing a bacterium, which is in the process of dividing. (Courtesy of Dorothy F. Bainton, Phagocytic Mechanisms in Health and Disease. New York: Intercontinental Book Corporation, (more...)
Several other classes of receptors that promote phagocytosis have been characterized. Some recognize components, which collaborate with antibodies in targeting microbes for destruction (discussed in Chapter 25). Others directly recognize oligosaccharides on the surface of certain microorganisms. Still others recognize cells that have died by apoptosis. Apoptotic cells lose the asymmetric distribution of phospholipids in their plasma membrane. As a consequence, negatively charged phosphatidylserine, which is normally confined to the cytosolic leaflet of the lipid bilayer, is now exposed on the outside of the cell, where it triggers the phagocytosis of the dead cell.
Pinocytosis Overview, Process & Examples
Learn what pinocytosis is and how cells use this function. See a pinocytosis diagram shown and explained, and review a description of the...
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Pinocytosis Overview, Process & Examples
Ranaa Aboumosallam Arafat, Jeremy Battista
Learn what pinocytosis is and how cells use this function. See a pinocytosis diagram shown and explained, and review a description of the pinocytosis process. Updated: 05/20/2022
Table of Contents
What is Pinocytosis?
How Does Cell Drinking Work?
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What is Pinocytosis?
Some people think that respiration, digestion, and excretion are the most important processes that occur in the living organism's body, but in fact, there are many processes that are as important as these processes such as the pinocytosis process? So, what is pinocytosis? Pinocytosis literally means "cell drinking" Pino in Greek means drinking. Pinocytosis definition is the process where fluid and dissolved substances and molecules are taken up by the cell.
The cell consists of many organelles surrounded by the Plasma membrane. A plasma membrane is defined as the membrane surrounding the cell which separates the cell content from the outside environment. The cell membrane or plasma membrane is characterized by being Semi permeable. Semi-permeable plasma membrane means that the cell allows certain molecules and substances to enter the cell while preventing other molecules and substances from entering the cell. The entrance of molecules to the cell mainly depends on the cell's needs and it may also depend on the nature of the molecule as well. The plasma membrane structure is complex as it is made up of a lipid bilayer that contains the hydrophobic head (water-hating) and hydrophilic tail (water-loving). This specific structure aid the plasma membrane in its function. Inside the cell, there is a gel-like liquid called the cytoplasm. The cytoplasm is found around organelles and is surrounded by a plasma membrane. The composition of cytoplasm is proteins, salts, and water.
The structure of plasma membrane
The cytoplasm is the blue-colored structure surrounding the organelles circled by red.
In the pinocytosis process, the fluid and dissolved solutes, and nutrients are absorbed from the outside environment and that is why it is called cell drinking. The amount of fluid and dissolved particles enter the cell according to the cell's needs. The energy-carrying molecule ATP which stands for Adenosine Triphosphate is important in this process. The fuel and energy generated from ATP are necessary for transporting the substance from outside to inside the cell which is then stored in an organelle called vesicle. The vesicle has a circle shape and is found in most living organisms. This organelle is surrounded by a bilayer lipid of the plasma membrane and it has an important function in the storage and transport of essential elements and particles.
ATP structure
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How Does Cell Drinking Work?
How the process of cell drinking, aka pinocytosis, works?
The initiation of the pinocytosis process begins when the extracellular fluid along with molecules and substances binds with the plasma membrane's receptors.
After binding to receptors, the plasma membrane responds by folding or forming a structure-like pocket around the extracellular fluid.
The cell membrane completely folds around the extracellular absorbed fluid to form a complete circle around it.
The pocket formed in the cell membrane completely separates from it to form the vesicle and is now inside the cell itself. As mentioned before vesicle contains fluids and is surrounded by the lipid bilayer of the plasma membrane.
The vesicle then deals with the substances inside it according to the cell's needs; if the cell needs the substances, the vesicle will bind with the endosome. The endosome is an organelle that function to transport important substances inside the cell.
If the cell wants to get rid of the substances inside the vesicle, it will bind with lysosome. The lysosome is an organelle that contains digestive acidic enzymes that break down waste materials and food particles and has a role in killing viruses and bacteria as well.
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What is Pinocytosis?
All cells need to carry out basic functions, like 'eating' and 'drinking.' Pinocytosis is literally translated from Greek as 'cell drinking.' It is one mechanism by which cells will take in fluids.
Inside of cells we have the liquid portion called the cytoplasm, and each cell is surrounded by a semi-permeable cell membrane (which allows some things in and out) made of lipids (fats).
Through pinocytosis, a cell will suck in everything that is in the extra-cellular fluid outside of the cell, including solutes (dissolved particles). Only small amounts of material will enter the cell during this process, and a minimal amount of adenosine triphosphate (ATP, an energy molecule used by the cell) is necessary.
Endocytosis and Exocytosis
Endocytosis and Exocytosis
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. Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that the uptake and release of large particles by the cell requires energy. A large particle, however, cannot pass through the membrane, even with energy supplied by the cell.
There are two primary mechanisms that transport these large particles: endocytosis and exocytosis.
LEARNING OBJECTIVES
Describe endocytosis and identify different varieties of import, including phagocytosis, pinocytosis, and receptor-mediated endocytosis
Identify the steps of exocytosis
Endocytosis
Endocytosis is a type of active transport 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 invaginates, forming 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
Figure 1. In phagocytosis, the cell membrane surrounds the particle and engulfs it. (credit: Mariana Ruiz Villareal)
Phagocytosis (the condition of “cell eating”) is the process by which large particles, such as cells or relatively large particles, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil (Figure 1).
In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly formed compartment (endosome). When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.
Pinocytosis
Figure 2. In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off. (credit: Mariana Ruiz Villareal)
A variation of endocytosis is called pinocytosis. This literally 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 does phagocytosis, and the vesicle does not need to merge with a lysosome (Figure 2).
A variation of pinocytosis is called potocytosis. This process uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin.
The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.
Receptor-Mediated Endocytosis
Figure 3. In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane. (credit: modification of work by Mariana Ruiz Villareal)
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. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by 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.
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