jimtrue.com : school : BSC2011 : CH 45: Animal Systems - Digestive

Posted by Jim True on November 4, 2004 6:35 PM. Last Updated October 22, 2006 9:23 PM

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CH 45: Animal Systems - Digestive

Function

• Intake of Food (Ingestion) - food is obtained (energy source).
• Reduction of Food - Initial fragmentation (mechanical reduction) and then digestion to molecular level (chemical reduction).
• Temporary Storage - Does not occur in all animals
• Absorption of Digested Food - Uptake by passive/active transport into cells, tissues.
• Release of Solid Waste - Waste may be either indigestible materials or undigested food.

Feeding Types

• All animals are heterotrophs, although some are symbiotic with photosynthetic or chemosynthetic autotrophs
• Heterotrophs take in and convert already synthesized compounds as their energy source.
• For initial intake of food, an animal may be:
• Herbivore - food on plants.
• Carnivore - Eat other animals. Also known as a predator, although this is usually applied a carnivore that hunts for its prey.
• Omnivore - Eats both animals and plants.
• Detritivore - An omnivore that feeds on decaying organic material. An actively foraging detritivore is often referred to as a scavenger.
• Many animals are specialized to feed on only specific things, e.g., flower nectar, blood, dead skin.
• Passively feeding aquatic animals are typically filter feeders, i.e., they capture food particles as they float by in the water.

Digestive Processes

• The digestion process itself may be mainly intra- or extracellular or a combination.
• In intracellular digestion, the food is absorbed into the cell and digested by enzymes.
• In extracellular digestion, the food is reduced to the molecular level in a chamber or series of chambers into which enzymes are secreted.
• Evolutionary trend is from intra- to extracellular digestion.
• Mechanical Digestion - Reduction of food particle size by fragmentation.
• Involves hard structures such as:
• Gizzard (a muscular chamber lined with hardened plates or filled with small pebbles) grinding structures in birds, annelids, and extinct reptiles.
• Radula (toothed tongue) shreds food in P.Mollusca.
• Beak (sharp edged tearing structure) in birds and cephalopod mollusks.
• Masticator (chewing structure or jaws) - Includes some chelicerae and the mandibles of arthropods, plus teeth found in most vertebrates.
• Chemical Digestion - Release of enzymes (intra- or extracellular) reducing food to molecular level.
• Enzyme categories include amylases (break down CHO's - carbohydrates), proteases (break down proteins), and lipases (break down lipids).
• Enzymes are secreted in various regions of digestive tract.
• In vertebrates, includes salivary glands in mouth, cells in wall of stomach and intestines, also enzymes flow in through ducts from pancreas.

Digestion by Phylum

• P. Porifera - Filter feeding by choanocytes. Phagocytosis and intracellular digestion by coanocytes and amebocytes.
• P. Cnidaria - Food capture by cnidocytes, nematocysts and tentacles.
• Food enters a gastrovascular cavity (GVC), which is a sac-like structure. Food enters and solid waste is expelled out the same opening.
• Digestion is partially extracellular, with enzymes secreted into GVC. Partially digested food is then phagocytized by gastrodermal cells and digestion is completed intracellularly.
• P.Platyhelminthes - Also possess a single opening and GVC, which may be highly branched.
• A muscular pharynx (expanded portion of anterior digestive tract) near the mouth to pull food into the GVC.
• Extra- and intracellular digestion as in P.Cnidaria except in C.Cestoidea (tapeworms), which completely lack a digestive system.
• Tapeworms absorb already digested nutrients from host through the tegument.
• P. Nematoda - The first "complete" digestive system (gut) with separate openings for food intake and waste release.
• Internally, regions of the gut begin to show differentiation of function as well.
• P.Mollusca - Complete, regionally specialized guts.
• Bivalves lack a radula - filter feed and move food to and through digestive tract by cilia.
• Gastropods use radula to shred food and pass into gut.
• Cephalopods use tentacles to seize food, beaks to tear food into chunks and radula to shred food. Considered "raptors", because they seize prety and it into bits with their beaks.
• All possess regional specialization of gut.
• P.Annelida - Polychaetes use jaws or tentacles to capture food, oligochaetes use suction created by muscular pharynx, leeches use oral sucker and slicing jaws, plus pharynx to draw body fluids from prey.
• P.Arthropoda - Variable feeding structures (chelicerae or mandibles).
• Also, variable number of accessory feeding appendages such as palps (chelicerates), maxillae and maxillipeds (crustaceans and uniramians)
.
• Regionally specialized guts in all.
• P.Echinodermata - Only in the Class Echinoidea are there hardened feeding structure (Aristotle's lantern).
• In C. Asteroidea, the stomach everts over the prey.
• Enzymes are secreted and stomach absorbs digested food and then is pulled back into the body.
• Members of the class Ophiuroidea lack and intestine and anus.
• Class Holothuroidea members possess highly modified tube feet around the mouth which serve as feeding tentacles for trapping food.
• P. Chordata - In the urochordates and cephalochordates, food is captured by pharyngeal slits and passed into the digestive tract.
• All vertebrates except agnathans use jaws of varying shapes and sizes to bite down on and tear food.
• The evolution of the jaw and its variability in form and function in the class Osteichthyes is another characteristic that led to their high diversity.

Digestion in Vertebrates

• In vertebrate of animals, the digestive process is basically associated with three zones, basically:
  1. Acquisition and mechanical reduction, storage (some), predigestion. Food enters and is reduced to small particles in this zone.
  • This includes the mouth, jaws, teeth, and salivary glands. Also includes the pharynx (throat) in many fishes. Tooth shape and number often relate to diet.
  • The crop and gizzard in birds, which are food storage and grinding chambers in the esophageal region, also fall into this zone.
  2. Extracellular enzymatic digestion. The reduction of food particles to molecular size for absorption by cells and transport in the circulatory system occurs here.
  • In vertebrates, the stomach, pancreas, liver, gall bladder and small intestines are all associated with this zone, although not all vertebrates have all of these structures.
  • The overall length of the gut is a good indicator of the type of diet. Carnivores tend to have much shorter gut lengths than herbivores.
  • Absorptive surfaces are highly folded into finger-like projections called villi to increase surface area for absorption.
  3. Absorption of digestion products and H₂O plus elimination of solid wastes (defecation).
  • Includes the large intestine (also known as the colon)
  • In marine fishes, this part of the digestive tract may also assist in regulation of the body salt balance by eliminating excess salts (osmoregulation).

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