What materials are transported by endocytosis?

The movement of macromolecules such as proteins or polysaccharides into or out of the cell is called bulk transport. There are two types of bulk transport, exocytosis and endocytosis, and both require the expenditure of energy (ATP).

In exocytosis, materials are exported out of the cell via secretory vesicles. In this process, the Golgi complex packages macromolecules into transport vesicles that travel to and fuse with the plasma membrane. This fusion causes the vesicle to spill its contents out of the cell. Exocytosis is important in expulsion of waste materials out of the cell and in the secretion of cellular products such as digestive enzymes or hormones.

Endocytosis, on the other hand, is the process by which materials move into the cell. There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. In phagocytosis or “cellular eating,” the cell’s plasma membrane surrounds a macromolecule or even an entire cell from the extracellular environment and buds off to form a food vacuole or phagosome. The newly-formed phagosome then fuses with a lysosome whose hydrolytic enzymes digest the “food” inside.

In pinocytosis or “cellular drinking,” the cell engulfs drops of fluid by pinching in and forming vesicles that are smaller than the phagosomes formed in phagocytosis. Like phagocytosis, pinocytosis is a non-specific process in which the cell takes in whatever solutes that are dissolved in the liquid it envelops.

Unlike phagocytosis and pinocytosis, receptor-mediated endocytosis is an extremely selective process of importing materials into the cell. This specificity is mediated by receptor proteins located on depressed areas of the cell membrane called coated pits. The cytosolic surface of coated pits is covered by coat proteins. In receptor-mediated endocytosis, the cell will only take in an extracellular molecule if it binds to its specific receptor protein on the cell’s surface. Once bound, the coated pit on which the bound receptor protein is located then invaginates, or pinches in, to form a coated vesicle. Similar to the digestive process in non-specific phagocytosis, this coated vesicle then fuses with a lysosome to digest the engulfed material and release it into the cytosol. Mammalian cells use receptor-mediated endocytosis to take cholesterol into cells. Cholesterol in the blood is usually found in lipid-protein complexes called low-density lipoproteins (LDLs). LDLs bind to specific receptor proteins on the cell surface, thereby triggering their uptake by receptor-mediated endocytosis.

Endocytosis and Exocytosis Quiz

Understanding:

•  The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis 

    
The membrane is principally held together by weak hydrophobic associations between the fatty acid tails of phospholipids

This weak association allows for membrane fluidity and flexibility, as the phospholipids can move around to some extent

This allows for the spontaneous breaking and reforming of the bilayer, allowing larger materials to enter or leave the cell without having to cross the membrane (this is an active process and requires ATP hydrolysis)

Endocytosis

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

• An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
• The invagination is then sealed off to form an intracellular vesicle containing the material

There are two main types of endocytosis:

• Phagocytosis – The process by which solid substances are ingested (usually to be transported to the lysosome)
• Pinocytosis – The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels) 

Endocytosis via an Electron Microscope

Exocytosis

The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane

• Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment
• The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis

Process of Exocytosis

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.

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.

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1. Phagocytosis - This is the process of engulfing large, solid particles such as bacteria into the cell for immune purposes. Extensions of the cytoplasm, termed psuedopodia ('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.
2. 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.
3. 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.

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Clinical correlation - 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.

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