What is exocytosis and endocytosis in biology?

Endocytosis and exocytosis are the mechanisms by which cells transport items that are too big to pass through the lipid bilayer of the cell membrane directly into or out of the cell. Exocytosis and endocytosis are two processes that allow large molecules, bacteria, and waste materials to pass through the cell membrane.

Endocytosis and Exocytosis

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Endocytosis:

Endocytosis is a transport mechanism that allows materials from the cell's exterior side to enter the cell, such as large molecules, cell components, hormones, and impulses. Because it needs the expenditure of energy, endocytosis is active transport. When the target particle reaches the plasma membrane, it forms a pinching pocket around it, enclosing it in a newly generated intracellular vesicle composed of a plasma membrane.

Types of endocytosis

Phagocytosis and pinocytosis are the two types of endocytosis.

Phagocytosis

• The process through which phagocytes, or live cells, ingest or consume other cells or particles is known as phagocytosis.
• A phagocyte can be a single-celled creature, such as an amoeba, or a body cell, such as a white blood cell. Some species, such as amoebas and sponges, have a way of feeding called phagocytosis.
• Phagocytosis is primarily a defensive reaction to infection and foreign material invasion (antigens). in higher animals.

Pinocytosis

• Cell drinking, also known as pinocytosis, is widespread in plant and animal cells.
• During pinocytosis, the cell takes in the components it needs to operate from the extracellular fluid. Two examples are water and fertilizers.
• Pinocytosis with receptor-mediated endocytosis is a kind of pinocytosis. Macromolecules bind to receptors on the cell's plasma membrane surface during receptor-mediated endocytosis.
• Receptor-mediated endocytosis is exemplified by cholesterol absorption.

Purpose of Endocytosis

• Endocytosis serves several purposes, including:
• Getting nutrients into your body for cellular growth.
• When the immune system detects bacteria or other pathogens, immune cells engulf them and destroy them.
• To avoid causing damage to neighboring cells, when cells stop performing properly, they must be carefully disposed of away.
• To kill these cells, endocytosis is utilized.

Exocytosis:

Exocytosis is the transfer of materials from the inside of the cell to the outside of the cell. It is an active transport since it necessitates the expenditure of energy. Membrane-bound vesicles carrying biological molecules are transported across the cell membrane as transportation agents.

Vesicles are attached to the cell membrane and spew out their contents. This procedure is necessary for removing excess water from the cell, transferring chemical signals between cells, and reassembling the cell membrane.

Golgi apparatus, endosomes, and presynaptic neurons produce the vesicles used in transportation. Constitutive exocytosis, controlled exocytosis, and lysosome-mediated exocytosis are three methods that can be used to do this. The vesicles containing the cell's waste products are delivered to the cell membrane, which subsequently binds the vesicles to themselves, causing the vascular contents to be released outside the cell.

Purpose of Exocytosis

Exocytosis serves the following purposes:

• Cells create waste or toxins that must be removed to maintain homeostasis. For example, during aerobic respiration, cells produce the waste products carbon dioxide and water. Carbon dioxide and water are removed from these cells during exocytosis.
• Signaling molecules generated by cells include hormones and neurotransmitters. They are transported to other cells after escaping from the cell via exocytosis.
• Endocytosis is when cells acquire items from outside the cell by producing vesicles from the plasma membrane containing lipids and proteins. When certain exocytotic vesicles join with the cellular membrane, these components are replenished.

During endocytosis and exocytosis, transport occurs via the semipermeable plasma membrane. Aside from that, during endocytosis and endocytosis cells have a variety of transport mechanisms. The bulk transport mechanisms used by eukaryotes are endocytosis and exocytosis. They are known as active transport processes because they require energy. Hence both endocytosis and endocytosis have active transport processes and require energy.

Difference between endocytosis and exocytosis

The below table shows how are endocytosis and exocytosis different:

Conclusion:

• 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.
• The process of exocytosis involves vesicles fusing with plasma membranes and releasing their contents.
• Endocytosis and exocytosis are both active transport processes.
• Inactive transport, molecules, and ions are pumped across membranes against a gradient of concentration via an energy-consuming process.

FAQs:

1. What are endocytosis and exocytosis, both examples of?

In the cytoplasm, macromolecules or large particles are moved across the plasma membrane by vesicles. Endocytosis and exocytosis are examples of vesicle transport. They are both active, energy-consuming processes.

2. What is the function of exocytosis?

Cells create waste or toxins that must be removed in order to maintain homeostasis. For example, during aerobic respiration, cells produce the waste products carbon dioxide and water, and carbon dioxide and water are removed from these cells during exocytosis.

3. How does endocytosis differ from exocytosis?

The major distinction between endocytosis and exocytosis is that endocytosis relates to bringing matter into the cell from the outside, whereas exocytosis refers to the material being exported out of the Golgi complex via secretory vesicles into the outside world.

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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 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.

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.

Exocytosis

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. This fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into the extracellular space (Figure 4). Other examples of cells releasing molecules via exocytosis include the secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by synaptic vesicles.

Figure 4. In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents are then released to the exterior of the cell. (credit: modification of work by Mariana Ruiz Villareal)

A summary of the cellular transport methods discussed is contained in Table 1, which also includes the energy requirements and materials transported by each.

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 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.

Check Your Understanding

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