Endocytosis and Exocytosis - Podcast Version 0:00 / 0:00 1x 0.25x 0.5x 0.75x 1x 1.25x 1.5x 1.75x 2x Endocytosis and exocytosis are the names given to the active, bulk transport of products across the cell membrane. These processes allow larger molecules that cannot diffuse through the lipid bilayer to cross the membrane. Endocytosis is the process by which substances are engulfed into the cell. Exocytosis is the reverse; the process by which substances are released from the cell. In this article, we will discuss the processes of endocytosis and exocytosis, and we will consider clinical conditions which result from defects in these processes. Pro Feature - 3D Model You've Discovered a Pro Feature Access our 3D Model Library Explore, cut, dissect, annotate and manipulate our 3D models to visualise anatomy in a dynamic, interactive way. Learn More Endocytosis Endocytosis, the process by which large molecules are internalised into the cell, occurs when foreign material is engulfed within the cell membrane, which then forms a vesicle containing the ingested material. There are three main subtypes of endocytosis, as seen in Figure 1. OpenStax (https://commons.wikimedia.org/wiki/File:0309_Three_Forms_of_Endocytosis.jpg), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0), via Wikimedia Commons Fig 1Diagram showing the three forms of endocytosis Phagocytosis – This is the process of engulfing large, solid particles such as bacteria into the cell for immune purposes. Extensions of the cytoplasm, termed pseudopodia (‘false feet’), sense, surround and enclose the target, creating a vacuole or phagosome on the inside of the cell membrane. This allows the process of phagocytosis to be highly specific. Pincytosis – This describes the non-specific uptake of fluid surrounding the cell, allowing it to take in nutrients such as ions, enzymes and hormones. In this process, the cell membrane invaginates, before budding off to create a vesicle known as a pinosome. Receptor-mediated endocytosis – uptake of specific target substances, such as iron, via their receptor. Receptors cluster in regions termed coated pits, as they are coated with proteins such as clathrin. Clathrin causes the coated pit to invaginate and become a vesicle, bringing the desired ligand into the cell. This process can be hijacked to allow for toxins to enter the cell, such as cholera. Exocytosis Exocytosis is a form of active transport through which large molecules are moved from the interior to the exterior of the cell. Vesicles are packaged within the cell and transported to the cell membrane, where their phospholipid bilayers fuse. This allows the contents to be released outside the cell. In some cases, the vesicle will only fuse temporarily before reforming on the interior of the cell. Alternatively, the vesicle may fuse completely, becoming a permanent part of the cell membrane. Exocytosis is used in many areas of the body, including neurotransmitter release at synapses or release of secretions in the sweat glands. Glands which secrete their products via exocytosis are termed merocrine. OpenStax (https://commons.wikimedia.org/wiki/File:0310_Exocytosis.jpg), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0), via Wikimedia Commons Fig 2Diagram showing exocytosis Clinical Relevance Familial hypercholesterolaemia Low-Density Lipoproteins (LDLs) are taken into the cell via receptor-mediated endocytosis, following binding with the LDL receptor. Genetic mutations in the gene which codes for the LDL receptor can prevent successful binding to the receptor. This prevents LDLs from being taken into the cell, causing high cholesterol concentration in the blood. If left untreated, men with familial hypercholesterolaemia are at a 50% risk for a fatal or nonfatal coronary event by age 50 years and women are at a 30% risk by age 60. Do you think you’re ready? Take the quiz below Pro Feature - Quiz Endocytosis and Exocytosis Question 1 of 3 Submitting... Skip Next Rate question: You scored 0% Skipped: 0/3 More Questions Available Upgrade to TeachMePhysiology Pro Challenge yourself with over 2100 multiple-choice questions to reinforce learning Learn More Frequent questions What is endocytosis? Endocytosis is the active process by which cells engulf large molecules or particles by forming vesicles from the cell membrane. This mechanism allows substances that cannot pass through the lipid bilayer to enter the cell. What are the main types of endocytosis? The three primary types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis involves the engulfing of solid particles, pinocytosis refers to the uptake of fluid and nutrients, and receptor-mediated endocytosis targets specific substances using cell surface receptors. How does exocytosis function in cells? Exocytosis is the process by which cells transport large molecules from their interior to the outside environment. This occurs when vesicles containing these molecules fuse with the cell membrane, releasing their contents outside the cell. What clinical condition is associated with defects in endocytosis? Familial hypercholesterolaemia is a condition linked to defects in receptor-mediated endocytosis, specifically involving Low-Density Lipoproteins (LDLs). Genetic mutations affecting the LDL receptor prevent the uptake of LDLs, leading to elevated cholesterol levels in the bloodstream. What role does clathrin play in receptor-mediated endocytosis? Clathrin is a protein that coats regions of the cell membrane known as coated pits, facilitating the invagination and formation of vesicles during receptor-mediated endocytosis. This process is crucial for the selective uptake of specific ligands, such as iron. Rate This Article