Intracellular digestion

Last updated April 09, 2019

Process of how macroautophagy works Macroautophagy.gif — Process of how macroautophagy works

Every organism requires energy to be active.^[1] However, to obtain energy from its outside environment, cells must not only retrieve molecules from their surroundings but also break them down.^[1] This process is known as intracellular digestion.^[1] In its broadest sense, intracellular digestion is the breakdown of substances within the cytoplasm of a cell. In detail, a phagocyte's duty is obtaining food particles and digesting it in a vacuole.^[2] For example, following phagocytosis, the ingested particle (or phagosome) fuses with a lysosome containing hydrolytic enzymes to form a phagolysosome; the pathogens or food particles within the phagosome are then digested by the lysosome's enzymes.

In cell biology, the cytoplasm is all of the material within a cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The main components of the cytoplasm are cytosol – a gel-like substance, the organelles – the cell's internal sub-structures, and various cytoplasmic inclusions. The cytoplasm is about 80% water and usually colorless.

Cell (biology) the basic structural and functional unit of all organisms. Includes the plasma membrane and any external encapsulating structures such as the cell wall and cell envelope.

The cell is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are often called the "building blocks of life". The study of cells is called cell biology or cellular biology.

Phagocytosis is the process by which a cell uses its plasma membrane to engulf a large particle, giving rise to an internal compartment called the phagosome. It is one type of endocytosis.

Intracellular digestion can also refer to the process in which animals that lack a digestive tract bring food items into the cell for the purposes of digestion for nutritional needs. This kind of intracellular digestion occurs in many unicellular protozoans, in Pycnogonida, in some molluscs, Cnidaria and Porifera. There is another type of digestion, called extracellular digestion. In amphioxus, digestion is both extracellular and intracellular.

Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. In chemical digestion, enzymes break down food into the small molecules the body can use.

Cnidaria is a phylum under Kingdom Animalia containing over 11,000 species of animals found exclusively in aquatic environments: they are predominantly marine.

Extracellular digestion is a process in which saprobionts feed by secreting enzymes through the cell membrane onto the food. The enzymes catalyze the digestion of the food into molecules small enough to be taken up by passive diffusion, transport, or phagocytosis. Since digestion occurs outside the cell, it is said to be extracellular. It takes place either in the lumen of the digestive system, in a gastric cavity or other digestive organ, or completely outside the body.

Function

Intracellular digestion is divided into heterophagic digestion and autophagic digestion.^[3] These two types take place in the lysosome and they both have very specific functions.^[3] Heterophagic intracellular digestion has an important job which is to break down all molecules that are brought into a cell by endocytosis.^[3] The degraded molecules need to be delivered to the cytoplasm; however, this will not be possible if the molecules are not hydrolyzed in the lysosome.^[3] Autophagic intracellular digestion is processed in the cell, which means it digests the internal molecules.^[3]

Autophagy

Generally, autophagy includes three small branches, which are macroautophagy, microautophagy, and chaperone-mediated autophagy.^[4]

Autophagy is the natural, regulated mechanism of the cell that disassembles unnecessary or dysfunctional components. It allows the orderly degradation and recycling of cellular components.

Microautophagy is the type of autophagic pathway which is mediated by direct lysosomal (mammals) or vacuolar engulfment of the cytoplasmic cargo. Cytoplasmic material is trapped in the lysosome/vacuole by the random process of membrane invagination. Microautophagic pathway is especially important for survival of cells under starvation, nitrogen deprivation or after treatment with rapamycin. Microautophagy generally is a non-selective process but there are 3 special cases of selective microautophagic pathway which are activated under a specific conditions.

Chaperone-mediated autophagy (CMA) refers to the chaperone-dependent selection of soluble cytosolic proteins that are then targeted to lysosomes and directly translocated across the lysosome membrane for degradation. The unique features of this type of autophagy are the selectivity on the proteins that are degraded by this pathway and the direct shuttling of these proteins across the lysosomal membrane without the requirement for the formation of additional vesicles.

Occurrence

Most organisms that use intracellular digestion belong to Kingdom Protista, such as amoeba and paramecium.

An amoeba, often called amoeboid, is a type of cell or unicellular organism which has the ability to alter its shape, primarily by extending and retracting pseudopods. Amoebas do not form a single taxonomic group; instead, they are found in every major lineage of eukaryotic organisms. Amoeboid cells occur not only among the protozoa, but also in fungi, algae, and animals.

<i>Paramecium</i> genus of unicellular ciliates, commonly studied as a representative of the ciliate group

Paramecium is a genus of unicellular ciliates, commonly studied as a representative of the ciliate group. Paramecia are widespread in freshwater, brackish, and marine environments and are often very abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced to conjugate and divide, it has been widely used in classrooms and laboratories to study biological processes. Its usefulness as a model organism has caused one ciliate researcher to characterize it as the "white rat" of the phylum Ciliophora.

Amoeba

Amoeba uses pseudopodia to capture food for nutrition in a process called phagocytosis.

Paramecium

Paramecium uses cilia in the oral groove to bring food into the mouth pore which goes to the gullet. At the end of the gullet, a food vacuole forms. Undigested food is carried to the anal pore.

Euglena

Euglena is photosynthetic but also engulfs and digests microorganisms.

Related Research Articles

The endomembrane system is composed of the different membranes that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles. In eukaryotes the organelles of the endomembrane system include: the nuclear membrane, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, endosomes, and plasma (cell) membrane among others. The system is defined more accurately as the set of membranes that form a single functional and developmental unit, either being connected directly, or exchanging material through vesicle transport. Importantly, the endomembrane system does not include the membranes of chloroplasts or mitochondria, but might have evolved from the latter.

Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of plasma membrane, which then buds off inside the cell to form a vesicle containing the ingested material. Endocytosis includes pinocytosis and phagocytosis. It is a form of active transport.

A lysosome is a membrane-bound organelle found in many animal cells and most plant cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane proteins, and its lumenal proteins. The lumen's pH (~4.5–5.0) is optimal for the enzymes involved in hydrolysis, analogous to the activity of the stomach. Besides degradation of polymers, the lysosome is involved in various cell processes, including secretion, plasma membrane repair, cell signaling, and energy metabolism.

A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these. The organelle has no basic shape or size; its structure varies according to the requirements of the cell.

In cell biology, a vesicle is a large structure within a cell, or extracellular, consisting of liquid enclosed by a lipid bilayer. Vesicles form naturally during the processes of secretion (exocytosis), uptake (endocytosis) and transport of materials within the plasma membrane. Alternatively, they may be prepared artificially, in which case they are called liposomes. If there is only one phospholipid bilayer, they are called unilamellar liposome vesicles; otherwise they are called multilamellar. The membrane enclosing the vesicle is also a lamellar phase, similar to that of the plasma membrane and vesicles can fuse with the plasma membrane to release their contents outside the cell. Vesicles can also fuse with other organelles within the cell.

In cell biology, a phagosome is a vesicle formed around a particle engulfed by a phagocyte via phagocytosis. Professional phagocytes include macrophages, neutrophils, and dendritic cells (DCs). A phagosome is formed by the fusion of the cell membrane around a microorganism, a senescent cell or an apoptotic cell. Phagosomes have membrane-bound proteins to recruit and fuse with lysosomes to form mature phagolysosomes. The lysosomes contain hydrolytic enzymes and reactive oxygen species (ROS) which kill and digest the pathogens. Phagosomes can also form in non-professional phagocytes, but they can only engulf a smaller range of particles, and do not contain ROS. The useful materials from the digested particles are moved into the cytosol, and waste is removed by exocytosis. Phagosome formation is crucial for tissue homeostasis and both innate and adaptive host defense against pathogens.

A contractile vacuole (CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole

Endoplasm generally refers to the inner, dense part of a cell's cytoplasm. This is opposed to the ectoplasm which is the outer (non-granulated) layer of the cytoplasm, which is typically watery and immediately adjacent to the plasma membrane. These two terms are mainly used to describe the cytoplasm of the amoeba, a protozoan, eukaryotic cell. The nucleus is separated from the endoplasm by the nuclear envelope. The different makeups/viscosities of the endoplasm and ectoplasm contribute to the amoeba's locomotion through the formation of a pseudopod. However, other types of cells have cytoplasm divided into endo- and ectoplasm. The endoplasm, along with its granules, contains water, nucleic acids amino acids, carbohydrates, inorganic ions, lipids, enzymes, and other molecular compounds. It is the site of most cellular processes as it houses the organelles that make up the endomembrane system, as well as those that stand alone. The endoplasm is necessary for most metabolic activities, including cell division.

The bafilomycins are a family of macrolide antibiotics produced from a variety of Streptomycetes. Their chemical structure is defined by a 16-membered lactone ring scaffold. Bafilomycins exhibit a wide range of biological activity, including anti-tumor, anti-parasitic, immunosuppressant and anti-fungal activity. The most used bafilomycin is bafilomycin A1, a potent inhibitor of cellular autophagy. Bafilomycins have also been found to act as ionophores, transporting potassium K+ across biological membranes and leading to mitochondrial damage and cell death.

Phagolysosome A membrane-bounded intracellular vesicle formed by maturation of an early phagosome following the ingestion of particulate material by phagocytosis; during maturation, phagosomes acquire markers of late endosomes and lysosomes.

In biology, a phagolysosome, or endolysosome, is a cytoplasmic body formed by the fusion of a phagosome with a lysosome in a process that occurs during phagocytosis. Formation of phagolysosomes is essential for the intracellular destruction of microorganisms and pathogens. It takes place when the phagosome's and lysosome's membranes 'collide', at which point the lysosomal contents—including hydrolytic enzymes—are discharged into the phagosome in an explosive manner and digest the particles that the phagosome had ingested. Some products of the digestion are useful materials and are moved into the cytoplasm; others are exported by exocytosis.

<i>Paramecium aurelia</i> species of unicellular ciliate

Paramecium aurelia are unicellular organisms belonging to the genus Paramecium of the phylum Ciliophora. They are covered in cilia which help in movement and feeding.Paramecium can reproduce sexually, asexually, or by the process of endomixis. Paramecium aurelia demonstrate a strong “sex reaction” whereby groups of individuals will cluster together, and emerge in conjugant pairs. This pairing can last up to 12 hours, during which the micronucleus of each organism will be exchanged. In Paramecium aurelia, a cryptic species complex was discovered by observation. Since then, some have tried to decode this complex using genetic data.

An autophagosome is a spherical structure with double layer membranes. It is the key structure in macroautophagy, the intracellular degradation system for cytoplasmic contents. After formation, autophagosomes deliver cytoplasmic components to the lysosomes. The outer membrane of an autophagosome fuses with a lysosome to form an autolysosome. The lysosome's hydrolases degrade the autophagosome-delivered contents and its inner membrane.

Omegasome Omega-shaped (as in the Greek capital letter) intracellular membrane-bounded organelle enriched in phosphatidylinositol 3-phosphate and dynamically connected to the endoplasmic reticulum. Omegasomes are the first step of the formation of autophagosom

Omegasome is a cell compartment consisting of lipid bilayer membranes enriched for phosphatidylinositol 3-phosphate and related to a process of autophagy. It is a subdomain of the endoplasmic reticulum (ER) membrane and has a morphology resembling Greek capital letter omega (Ω). Omegasomes are the sites from which phagophores form. Phagophores are sack-like structures that mature into autophagosomes that fuse with lysosomes in order to degrade the contents of the autophagosomes. The formation of omegasomes is increased as a response to starvation.

Intracellular transport is the movement of vesicles and substances within the cell. Eukaryotic cells transport packets of components to particular intracellular locations by attaching them to molecular motors that haul them along microtubules and actin filaments. This method of transport is often confused with intercellular transport, which deals solely with the movement of cargo between cells not the net movement within a cell. Since intracellular transport heavily relies on microtubules for movement, the components of the cytoskeleton play a vital role in trafficking vesicles between organelles and the plasma membrane.

Vomocytosis is the cellular process by which live organisms that have previously been engulfed by a white blood cell are expelled without being destroyed. Vomocytosis was first reported in 2006 by two groups, working simultaneously in the UK and the USA, based on time-lapse microscopy footage characterising the interaction between macrophages and the human fungal pathogen Cryptococcus neoformans. Subsequently, this process has also been seen with other fungal pathogens such as Candida albicans and Candida krusei. It has also been speculated that the process may be related to the expulsion of bacterial pathogens such as Mycobacterium marinum from host cells. Vomocytosis has been observed in phagocytic cells from mice, humans and birds, as well as being directly observed in zebrafish and indirectly detected in mice. Amoebae exhibit a similar process to vomocytosis whereby phagosomal material that cannot be digested is exocytosed. Cryptococci are exocytosed from amoebae via this mechanism but inhibition of the constitutive pathway demonstrated that cryptococci could also be expelled via vomocytosis.

References

1 2 3 Anderson, O. Roger (1 January 1970). "Intracellular Digestion". The American Biology Teacher. 32 (8): 461–467. doi:10.2307/4443206.
↑ Roberts, M. B. V. Biology: A Functional Approach. Nelson Thornes. ISBN 9780174480198.
1 2 3 4 5 Jamieson, G. A.; Robinson, D. M. Mammalian Cell Membranes: Volume 2: The Diversity of Membranes. Elsevier. ISBN 9781483162782.
↑ Glick, Danielle; Barth, Sandra; Macleod, Kay F. (2016-11-21). "Autophagy: cellular and molecular mechanisms". The Journal of Pathology. 221 (1): 3–12. doi:10.1002/path.2697. ISSN 0022-3417. PMC 2990190  . PMID 20225336.

1 Roberts, M. B. V. Biology: A Functional Approach. Nelson Thornes. ISBN 9780174480198.

2 Jamieson, G. A.; Robinson, D. M. Mammalian Cell Membranes: Volume 2: The Diversity of Membranes. Elsevier. ISBN 9781483162782.

3 Anderson, O. Roger (1 January 1970). "Intracellular Digestion". The American Biology Teacher. 32 (8): 461–467. doi:10.2307/4443206.

4 Glick, Danielle; Barth, Sandra; Macleod, Kay F. (21 November 2016). "Autophagy: cellular and molecular mechanisms". The Journal of Pathology. 221 (1): 3–12. doi:10.1002/path.2697. ISSN 0022-3417.

5 Mizushima, Noboru (15 November 2007). "Autophagy: process and function". Genes & Development. 21(22): 2861–2873. doi:10.1101/gad.1599207. ISSN 0890-9369.

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[:3-1] 1 2 3 Anderson, O. Roger (1 January 1970). "Intracellular Digestion". The American Biology Teacher. 32 (8): 461–467. doi:10.2307/4443206.

[:1-2] Roberts, M. B. V. Biology: A Functional Approach. Nelson Thornes. ISBN 9780174480198.

[:2-3] 1 2 3 4 5 Jamieson, G. A.; Robinson, D. M. Mammalian Cell Membranes: Volume 2: The Diversity of Membranes. Elsevier. ISBN 9781483162782.

[4] Glick, Danielle; Barth, Sandra; Macleod, Kay F. (2016-11-21). "Autophagy: cellular and molecular mechanisms". The Journal of Pathology. 221 (1): 3–12. doi:10.1002/path.2697. ISSN 0022-3417. PMC 2990190  . PMID 20225336.