Every organism requires energy to be active.Anderson, O. Roger (1 January 1970). "Intracellular Digestion". ''The American Biology Teacher''. 32 (8): 461–467. doi:10.2307/4443206. However, to obtain energy from its outside environment, cells must not only retrieve molecules from their surroundings but also break them down. This process is known as intracellular digestion. In its broadest sense, intracellular digestion is the breakdown of substances within the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic 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. ...

of a

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

. In detail, a phagocyte's duty is obtaining food particles and digesting it in a vacuole.Roberts, M. B. V. ''Biology: A Functional Approach''. Nelson Thornes. . For example, following

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

, the ingested particle (or phagosome) fuses with a

lysosome A lysosome () is a membrane-bound organelle found in many animal 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 pr ...

containing hydrolytic enzymes to form a

phagolysosome 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 mic ...

; the pathogens or food particles within the phagosome are then digested by the lysosome's enzymes. Intracellular digestion can also refer to the process in which animals that ''lack'' a

digestive tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...

bring food items into the cell for the purposes of

digestion 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 intest ...

for nutritional needs. This kind of intracellular digestion occurs in many unicellular protozoans, in

Pycnogonida Sea spiders are marine arthropods of the order Pantopoda ( ‘all feet’), belonging to the class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). They are cosmopolitan, foun ...

, in some

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

and Porifera. There is another type of digestion, called

extracellular digestion Extracellular phototropic 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 ie diffusion, transport, osmotrophy or phagocytosis. Since dige ...

. In

amphioxus The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochorda ...

, digestion is both extracellular and intracellular.

Function

Intracellular digestion is divided into heterophagic digestion and autophagic digestion.Jamieson, G. A.; Robinson, D. M. ''Mammalian Cell Membranes: Volume 2: The Diversity of Membranes''. Elsevier. . These two types take place in the lysosome and they both have very specific functions. Heterophagic intracellular digestion has an important job which is to break down all molecules that are brought into a cell by endocytosis. 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. Autophagic intracellular digestion is processed in the cell, which means it digests the internal molecules. Autophagy's function

Autophagy

Generally, autophagy includes three small branches, which are

macroautophagy Autophagy (or autophagocytosis; from the Ancient Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent re ...

microautophagy Microautophagy is one of the three common forms of autophagic pathway, but unlike macroautophagy and chaperone-mediated autophagy, it is mediated—in mammals by lysosomal action or in plants and fungi by vacuolar action—by direct engulfment ...

, and

chaperone-mediated autophagy 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 typ ...

Occurrence

Most organisms that use intracellular digestion belong to Kingdom Protista, such as

amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...

and

paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...

. Amoeba

Amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...

uses pseudopodia to capture food for nutrition in a process called

. Paramecium

Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...

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 ''Euglena'' is a genus of single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 200 species. Species of ''Euglena'' are found in f ...

is photosynthetic but also engulfs and digests microorganisms.

References

1 Roberts, M. B. V. ''Biology: A Functional Approach''. Nelson Thornes. . 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 An International Standard Serial Number (ISSN) is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is especially helpful in distinguishing between serials with the same title. ISSNs ...

br>0022-3417
5 Mizushima, Noboru (15 November 2007)
"Autophagy: process and function"
''Genes & Development''. 21(22): 2861–2873. doi: 10.1101/gad.1599207.

br>0890-9369
Cell biology es:Digestión intracelular