• Digestion is the chemical breakdown of nutrition refers to nourishment and the characteristics of essential nutrients.
• Extracellular digestion refers to digestion outside of cells as seen in bacteria and fungi. Intracellular digestion occurs in cells, generally in digestive vacuoles.

• Sponges sort food particles out of sea water brought by choanocytes (collar cells).  Particles caught in the mucus lined collar cells are phagocytized by the cells bellow for intracellular digestion and transported about by amebocytes.
• Coelenterates and flatworms begin digestion extracellularly in the gastrovascular cavity, with food phagocytized ands digestion completed in intracellularly.

Feeding and Digestive Structures in Higher Invertebrates

• Higher invertebrates have the tube-within-a-tube body plan digestion is mainly extracellular.  Intracellular digestion occurs in the invertebrate liver, which is lined by phagocytic cells.
• The earthworm gut contains specialized regions.  Food is swallowed by the pharynx and passes through the esophagus to the crop for storage. Grinding occurs in the gizzard, and digestion and absorption occur in the long intestine.  The typhlosole, a deep fold in the intestinal wall, increases surface area.  Chloragen cells convert glucose to glycogen and deaminate amino acids to be used as fuels.
• The grasshopper's chewing mouth parts include sensory palps, shearing mandibles and maxillae, the liplike labrum and labium, and salivary glands that secrete saliva.  The digestive system includes a foregut consisting of a pharynx, esophagus, and crop for swallowing and grinding, a midgut for digestion and absorption, gastric cecae for enzyme secretion, and a water-absorbing hindgut.  Insect mouth parts are also adapted for piercing and sucking.

Feeding and Digestive Structures in Vertebrates

• Vertebrate jaw and tooth structure varies with feeding specializations.  Carnivorous sharks have continuously growing rows of teeth.  In the gut, a winding flap the spiral valve, increases the surface area for digestion and absorption.  Teeth in bony fish vary from numerous sharp teeth to patches on the roof of the mouth.  Herbivorous fishes generally have lengthy coiled intestines suitable for the time consuming cellulose digestion process.
• Some amphibians have a protruding tongue for capturing insects.  The jaw, like that of the reptile is not suitable for chewing, so prey is swallowed whole.
• Crocodiles and alligators have a socket teeth, while some venomous snakes have hollow or grooved retractable fangs for injecting venom.  Snakes and lizards capture air molecules with tongue and analyze them with olfactory Jacobson's organ.  Pit vipers use their heat sensitive pits to orient themselves to their prey.
• The beak and (and feet) of birds are often specialized for their source of food.  Digestive specialization seed eaters include a storage crop and a two part stomach composed of the glandular, enzyme secreting proventriculus and the gravel filled and muscular ventriculus, or gizzard.
• Mammalian teeth are socketed and include temporary, or milk teeth, and permanent teeth.  Four types of teeth include incisors, canines, premolars, and  molars.  The shape and size of each group commonly relates to diet.
• Ruminants are grazing mammals whose four part stomach includes a rumen, where microorganisms digest cellulose.  The products and microorganism are themselves digested by the reticulum, omasum,  and abomasum (true stomach).

The Oral Cavity and Esophagus

• The lips and the tongue assist in eating and swallowing.  Taste buds located on the tongue detect salt, sour, sweet, and bitter flavors.  Saliva, a watery solution of ions, mucus, a nd amylase, a starch digesting enzyme, are produced in the parotid, submaxillary, and sublingual salivary glands.
• The pharynx divides to form the larynx and laryngopharynx.  Some of the structures involved in swallowing are the tongue, esophagus, soft palate, epiglottis, glottis, and larynx. The larynx is raised during swallowing, preventing food from entering the air passage.
• The esophagus, a food conducting tube, is made up of an innermost, secretory mucosa, a vascular and glandular submucosa, and a circular and longitudinal layer of smooth muscle, the muscularis all of which are surrounded by a fibrous serosa.  Wavelike contractions called peristalsis, coordinated by the autonomic nervous system, move the food bolus to the stomach.

The Stomach

• The stomach stores and mixes food, and its acidity helps destroy microorganisms.  The cardiac and pyloric sphincters close off the stomach during its churning peristalsis.  A third, oblique muscle layer aids the churning by producing a twisting action.
• The glandular lining contains pepsinogen secreting chief cells and parietal cells, whose hydrochloric acid secretions activate pepsinogen to the active form pepsin.  Other enzymes include gastric lipase and milk digesting rennin.

The Small Intestine

• The small intestine consists of the duodenum, jejunum, and ileum.  Its role is digestion and absorption.  In addition to what is provided by length, the surface area of the intestine is increased by folding, by projections called villi, and by cellular projections called microvilli.  Each villus contains a capillary bed and a lacteal, and is capable of movement.
• The small intestine surface contains bound enzymes forming the glycocalyx.  New cells continually replace those lost by abrasion.

Accessory Organs:  The Liver and Pancreas

• The liver secretes bile, an alkaline secretion that emulsifies fats.  Its contents include hemoglobin breakdown pigments (bilirubin and biliverdin).
• Bile is stored in the gall bladder, and secreted through the bile duct, which joins the pancreatic duct before entering the intestine.  Bile secretion is a response to the hormone cholecystekinin-pancreozymin, which is released when fat are present in the gut.
• The glandular pancreas secretes sodium bicarbonate, which neutralizes stomach acids in the intestine.  It also secretes a variety of enzymes that act on carbohydrates, fats, proteins, and nucleic acids.

The Large Intestine

• The large intestine or colon, begins with the ileocecal valve, which opens into a pouch like cecum (to which the appendix is attached).  Included are the ascending colon, transverse colon, descending colon, and sigmoid colon.  Further along are the rectum, anal canal, and anus.
• The colon absorbs water, concentrates the feces, and provides a suitable environment for bacteria that secrete useful vitamin K, biotin, and folic acid.  Within the rectum, rectal valves help support the feces, and the anus controls defecation.

THE CHEMISTRY OF DIGESTION

• The bonds connecting subunits of complex foods are broken through hydrolysis, releasing the simple subunits that can be absorbed or actively transported into cells lining the intestine.

• Starches are hydrolyzed into maltose by salivary amylase in the mouth and by pancreatic amylase in the small intestine.  Maltase hydrolyzes maltose into glucose, which is then actively transported by a sodium carrier into the blood.
• Lactase hydrolyzes milk sugar, bu adults in some races fail to produce the enzyme.  Upon taking milk they experience the symptoms of lactose intolerance.
• Glucose is transported to the liver, where it is converted to glycogen and gradually released again as glucose.

Fat Digestion

• Fats are hydrolyzed by lipase into fatty acids, glycerol, monoglycerides, and diglycerides.  These diffuse into the lining cells, reform as triglycerides, and join cholesterol to form chylomicrons, which pass into the lacteals.

Protein Digestion

• Protein digestion begins in the stomach, where pepsin, an endopeptidase, hydrolyzes inner peptide bonds forming shorter peptides.
• In the small intestine, pancreatic trypsin and chymotrypsin, also endopeptidases, hydrolyze specific amino acid linkages, and finally, intestinal exopeptidases break the remaining peptide bonds.  The latter include carboxypeptidases, from the pancreas and aminopeptidases and dipeptidases from the intestine.
• Amino acids enter the blood for transport to the liver where some are deaminated and used for energy, while others form serum proteins for transport to needy cells.

Nucleic Acid Digestion

• Nucleases, nucleic acid hydrolyzing enzymes, include exonucleases that work on the ends and endonucleases that work within the chain.  Phosphodiesterase breaks sugar-phosphate linkages.

Integration and Control of Digestive Enzymes

• Digestive enzymes are released through mechanical (physical presence of food), neural (thoughts and detection of food), and hormonal mechanisms.  Neural messages travel to the stomach via the vagus nerve when the gut senses the chemical presence of food.  The cells react by releasing a hormone such as gastrin into the blood.  When hormones reach their specific target cells or tissues, they stimulate them to release enzymes into the gut.
• Hormonal systems generally work through negative feedback, in which a stimulus evokes the release of a hormone, which removes the stimulus and thereby stops its own release.

• Carbohydrate is a vital energy source, but in excess it is converted to body fat.

• Fats are excellent energy providing foods and sources of fat-soluble vitamins.  Unsaturated fats are needed for plasma membranes.  Although unsaturated fats must come form the diet, most foods can be converted to unsaturated fats.

• Protein must be provided daily because of its constant turnover.  Excess amino acids are deaminated and used for energy or converted to fats and carbohydrate.
• Eight of the amino acids - essential amino acids - must be provided by the diet, while the remaining 12 are interconvertible by the body.  High quality dietary protein contains sufficient amounts and kinds of amino acids to provide what is needed.

Vitamins

• Fat-soluble vitamins are generally essential in forming coenzymes (NAD and FAD), and water soluble vitamins as antioxidants (removing freee radiclas) and other uses.  The former are harmful in overdoses.

• Major essential minerals, inorganic ions, include calcium, magnesium, iron, sodium, potassium, and chloride, used in structure, enzyme action, respiration and osmotic regulation.
• Trace elements include iodine, cobalt, and silicon, A shortage of iodine can cause goiter, an overgrowth of the thyroid.