extracellular digestion

Examples of extracellular digestion in the following topics:

• Invertebrate Digestive Systems

  • Invertebrates can be classified as those that use intracellular digestion and those with extracellular digestion.
  • The alimentary canal is a more advanced digestive system than a gastrovascular cavity and carries out extracellular digestion.
  • Because the food has been broken down exterior to the cells, this type of digestion is called extracellular digestion.
  • Most invertebrates use some form of extracellular digestion to break down their food.
  • Their food is broken down in their digestive tract (extracellular digestion), rather than inside their individual cells (intracellular digestion).

• Phylum Cnidaria

  • Cnidarians are diploblastic, have organized tissue, undergo extracellular digestion, and use cnidocytes for protection and to capture prey.
  • Polyp forms are sessile as adults, with a single opening to the digestive system (the mouth) facing up with tentacles surrounding it.
  • Despite the simplicity of the nervous system, it coordinates the movement of tentacles, the drawing of captured prey to the mouth, the digestion of food, and the expulsion of waste.
  • The cnidarians perform extracellular digestion in which the food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity absorb nutrients.
  • The gastrovascular cavity has only one opening that serves as both a mouth and an anus; this is termed an incomplete digestive system.

• Phylum Platyhelminthes

  • The flatworms are acoelomates: their bodies are solid between the outer surface and the cavity of the digestive system.
  • Most flatworms have a gastrovascular cavity rather than a complete digestive system; in such animals, the "mouth" is also used to expel waste materials from the digestive system.
  • Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis.
  • One group, the cestodes, lacks a digestive system.
  • The worms may produce enzymes that digest the host tissues or simply graze on surface mucus and skin particles.

• Complex Tissue Structure

  • As multicellular organisms, animals differ from plants and fungi because their cells don't have cell walls; their cells may be embedded in an extracellular matrix (such as bone, skin, or connective tissue); and their cells have unique structures for intercellular communication (such as gap junctions).
  • This connective tissue constitutes the extracellular surroundings of cells and is made up of organic and inorganic materials.
  • Epithelial tissues cover, line, protect, and secrete; these tissues include the epidermis of the integument: the lining of the digestive tract and trachea.

• Extracellular Matrix of Animal Cells

  • Most animal cells release materials into the extracellular space.
  • Collectively, these materials are called the extracellular matrix .
  • Cells have protein receptors on the extracellular surfaces of their plasma membranes.
  • An example of the role of the extracellular matrix in cell communication can be seen in blood clotting.
  • The extracellular matrix consists of a network of proteins and carbohydrates.

• Selective Permeability

  • There are peripheral proteins on the exterior of the membrane that bind elements of the extracellular matrix.
  • These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid.
  • Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues.

• Fertilization

  • In mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida.
  • In placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane .

• Programmed Cell Death

  • For example, most normal animal cells have receptors that interact with the extracellular matrix, a network of glycoproteins that provides structural support for cells in an organism.
  • The binding of cellular receptors to the extracellular matrix initiates a signaling cascade within the cell.
  • However, if the cell moves away from the extracellular matrix, the signaling ceases, and the cell undergoes apoptosis.
  • Macroautophagy, often referred to as autophagy, is a type of programmed cell death accomplished through self-digestion.

• Parathyroid Glands

  • Another function of the parathyroid glands is to secrete parathyroid hormone, which causes the release of the calcium present in bone to extracellular fluid.
  • Finally, PTH stimulates synthesis and secretion of calcitriol by the kidneys, which enhances Ca2+ absorption by the digestive system.

• Cell-Mediated Immunity

  • First, an antigen-presenting cell (APC, such as a dendritic cell or a macrophage) detects, engulfs (via phagocytosis in the case of macrophages or by entry of the pathogen of its own accord in the case of dendritic cells), and digests pathogens into hundreds or thousands of antigen fragments.
  • TC cells are particularly important in protecting against viral infections because viruses replicate within cells where they are shielded from extracellular contact with circulating antibodies.
  • An antigen-presenting cell (APC), such as a macrophage, engulfs a foreign antigen, partially digests it in a lysosome, and then embeds it in an MHC class II molecule for presentation at the cell surface.