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Intracellular Digestion Intracellular Digestion By 0. ROGERANDERSON Downloaded from http://online.ucpress.edu/abt/article-pdf/32/8/461/27403/4443206.pdf by guest on 25 September 2021 It is now widely recognized that intracellular di- Thus intracellular digestion includes (i) the degrada- gestion is a process occurring in many different kinds tion of ingested food to produce energy-rich mole- of plant and animal cells. However, intracellular cules to support life-the process called phagotro- digestion is seldom discussed with much depth in phy; and (ii) the turnover of cellular structures to secondary-school biology classes. Some modern biol- provide reusable structural molecules in the syn- ogy curricula give only brief mention of it. Sometimes thesis of new organelles-the process called auto- the concept is presented as a specific protozoan pro- phagy. Cellular digestion of all kinds is accom- cess without further discussion of its general proper- plished, in part, by the digestive action of hydrolyt- ties. In view of the widespread occurrence of intra- ic enzymes called hydrolases. A list of hydrolases cellular digestion in living systems, secondary-school usually associated with cellular digestion is given, students should be introduced to the topic in its most along with their substrates, in the table. It should be general form. This paper (i) contains a discussion of obvious from examining this table that a wide range a general concept of intracellular digestion, (ii) pre- of substances found in living material can be de- sents evidence for its widespread occurrence in liv- graded by these enzymes. ing things, and (iii) makes some suggestions toward Extracellular digestion is known to occur in bac- application of these ideas in planning biology lessons. teria and fungi. They secrete hydrolases into the surrounding environment, where food is degraded. A Concept of IntracellularDigestion The resultant smaller molecules are absorbed by the cell. In intracellular digestion, however, the hy- All metabolically active organisms are dependent drolases are bound within digestive vacuoles, where on an external source of energy. Heterotrophic or- food is degraded. The potentially destructive en- ganisms obtain energy through ingestion and ab- zymes are regulated and directed toward the food sorption of energy-rich food. The release of some of substrates by compartmentalization within mem- the chemically bound food energy requires intracel- brane-bound vacuoles. Thus, vital cytoplasmic or- lular digestion of macromolecules to yield smaller, ganelles are protected from hydrolytic action by the more readily assimilated chemical compounds. Pro- barrier function of the vacuolar membrane. The di- teins are degraded to yield oligopeptides, and car- gested food products formed within the digestive bohydrates are cleaved to yield a sugar residue such vacuole are absorbed into the cytoplasm. Through as glucose. Moreover, the constant demand for re- the careful and often insightful research of Christian newal of cellular organelles requires that some ex- deDuve and his co-workers (1955) we know that the tant cellular structures, such as senescent mito- vacuolar system of many living cells performs a di- chondria, be degraded and their breakdown prod- gestive function. Vacuoles involved in intracellular ucts reincorporated into newly organized organelles. digestion are called lysosomes. Lysosomes are classi- fied according to their morphology and function. Author's address: Teachers College, Columbia University, Primary lysosomes are small (0.5 to 1.0 ,u in di- New York, N.Y. 10027. ameter) granular vacuoles whose enzymes have not 461 to i lie Golgi complex, where the peripheral ves- icles concentrate the enzymes and pinch off to form jRV primary lysosomes, as shown in fig. 1. The primary lysosomes migrate to food vacuoles, where by fusion they produce secondary lysosomes. When digestion DV is completed, the remaining vacuole is called a dense body. Some dense bodies may migrate to newly formed food vacuoles, where, again by fusion, a new secondary lysosome is produced. Thus there is a cel- .?Ls LY lular economy in recycling digestive enzymes. In some cases the remaining exhausted digestive vacuoles form residual bodies. These debris-filled vacuoles are emptied outside the cell by a defecation process called exocytosis. The residual body fuses with the plasma membrane and empties its undiges- table waste into the environment. Novikoff (1962) has suggested that lysosomes may form directly Downloaded from http://online.ucpress.edu/abt/article-pdf/32/8/461/27403/4443206.pdf by guest on 25 September 2021 Fig. 1. Events in intracellular digestion. Phagotrophy (at left) from the endoplasmic reticulum membranes (in ad- is initiated by ingestion of food to form a phagosome (PV). dition to their production by the Golgi complex). Primary lysosomes (LY) secreted by the Golgi complex (G) fuse with the phagosomes to produce secondary lysosomes The cellular control mechanism governing the called phagolysosomes, or digestive vacuoles (DV). Autophagy formation of primary lysosomes-their enzyme com- (at right) begins with the sequestration of cytoplasmic struc- plement, the regulation of enzyme activity therein, tures within an autophagic vacuole (AV) formed from the and their fusion with other vacuoles-remains un- smooth membranes of the endoplasmic reticulum (SER). known. Clearly, the discovery of this fundamental Fusion of these vacuoles with primary lysosomes produces an autolysosome (AL). Food substances and reusable structural cellular control mechanism will be a significant con- molecules are absorbed by the cytoplasm. When digestion is tribution to our knowledge about regulation of cel- near completion, dense bodies (DB) containing residual di- lular metabolism. The structure of the Golgi com- gestive enzymes pinch off and fuse with newly formed food plex and indicators of its activity in producing pri- vacuoles. Sometimes the last stages of digestion produce a mary lysosomes is shown in fig. 2 and 3. These ultra- residual body (RV)whose wastes are defecated. (After deDuve, 1966.) thin sections were taken near the nuclear region of a cell of the golden-brown alga Ochromonas mal- hamensis. The Golgi complex as seen in cross-sec- tion (fig. 2) presents a stack of thin central cisternae, whose peripheral segments are dilated in the form of saccules. Saccule formation is most active on one yet acted on substrates. As the name implies, pri- surface of the Golgi system, in this case on the side mary lysosomes are newly formed and have not farthest from the nucleus. There appears to be a undergone digestive action. When a primary lyso- developmental sequence wherein new cisternae are some fuses with another vacuole containing sub- continuously being formed on one side and pushed strate the resultant organelle is called a secondary outward into the cytoplasm as they mature. As the lysosome. Secondary lysosomes, therefore, are formed from the fusion of primary lysosomes with existing food vacuoles. A vacuole containing substrate without digestive enzymes is called a phagosome. When engulfed ma- terial in a food vacuole is a host cell structure, the vacuole is called an autophagic vacuole. There are two kinds of secondary lysosomes: phagolysosomes, which are produced from phagosomes, and autoly- sosomes, which are produced from autophagic vacu- oles. Their names indicate respectively that dives- tive action is directed against extracellular engulfed food or against intracellular sequestered cytopiasmic structures. Fig. 1 shows the process of phagolysosome and autolysosome formation during intracellular diges- tion. Thanks largely to the early cytochemical re- search of Novikoff (1962) and others, we know that the Golgi apparatus is one site of primary lysosome Fig. 2. A Golgi complex appears in cross-section near the production. Hydrolases synthesized in the rough nucleus and chloroplasts of Ochromonasmalhamensis, a endoplasmic reticulum are believed to be transported flagellated protozoon.40,000 X. 462 THE AMERICANBIOLOGY TEACHER, NOVEMBER 1970 Golgi cisternae are displaced they form saccules, Some intracellular digestive which pinch off as primary lysosomes. Several of enzymes and their substrates. these free vesicles can be seen in the Golgi region in fig. 2. Enzymes Evidence for the presence of hydrolases in cellu- (acid hydrolases) Substrates lar structures is obtained through cytochemical techniques. The specimen is incubated in a substrate Lipases Triglycerides (fats) that deposits electron-opaque material at the site of Phospholipases Lecithins (membrane enzyme action (Gomori, 1952). A substrate such as lipids) beta-glycerophosphate is adjusted to the proper pH Acid phosphatases Glycerophosphates and (for example, pH 5.0 for lysosomal acid hydrolases) phenyl phosphates and an acetate buffer at the same pH is added. This Ribonuclease Ribonucleic acid (RNA) solution is mixed with lead nitrate to produce an in- Deoxyribonuclease Deoxvribonucleic acid cubation medium. When the substrate and the lead (DNA) ions diffuse into an organelle of the fixed specimen Glycosidases Mucopolysaccharides and where hydrolases are present, the phosphate is glycogen cleaved off from the substrate. The free phosphate Cathepsins Proteins Downloaded from http://online.ucpress.edu/abt/article-pdf/32/8/461/27403/4443206.pdf by guest on 25 September 2021 reacts with the lead to form an insoluble precipitate at the site of enzyme action. The enzymic reaction product appears densely black when viewed
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