Nutrition and Digestion

Human nutrition involves the following requirements and resources:

  • Energy is required, and the primary sources are carbohydrates and fats.
    • Lipids include fats, but also phospholipids and cholesterol which can have structural roles.
    • Carbohydrates include sugars and starches, which are sources for quick energy.
  • Proteins are required as sources for amino acids.
    • Dietary proteins are broken down into amino acids by digestion.
    • Amino acids are used in the body to build up the new proteins needed.
    • Body proteins play a vast number of roles, involved in accomplishing most of the body's tasks.
  • Minerals are required for specific molecular roles in the body.
  • Vitamins are required in small amounts for many specific roles in metabolism.

Providing the elements of nutrition in a usable form for the body's purposes requires digestion.

  • Chewing and swallowing are the first mechanical phase of digestion, but the brain is involved with the smell, taste and sight of food to stimulate secretioin of saliva and initiate the secretion of acid in the stomach and the hormone gastrin which stimulates further acid secretion.
    • Food is broken up mechanically
    • Saliva contains the digestive enzyme amylase, which begins the breakdown of starches into sugar.
  • The stomach holds the food, begins chemical digestion, and controls forward movement.
    • Glands in the lining of the stomach secrete enzymes to facilitate chemical breakdown of the food.
    • The hormone gastrin stimulates the secretion of hydrochloric acid by specialized stomach cells. It is limited by a negative feedback loop which inhibits the gastrin production at a certain level of acidity.
    • Other cells release pepsinogen, which serves as a precursor for the next step
    • In the acidic interior of the stomach, pepsinogen is converted to pepsin, an active protease that begins the breakdown of proteins into shorter chains of amino acids.
    • The multiple processes in the stomach convert the food and digestive fluids into an acidic fluid called chyme.
    • Under the control of the muscular pyloric sphincter at lower opening of the stomach, the fluid is allowed to move in small (~ teaspoon) quantities about every 20 seconds into the small intestine.
  • The small intestine accomplishes the main part of the food digestion
    • The liver produces bile, a critical component in the digestive process
      • Bile is a complex mixture of bile salts, other salts, water, and cholesterol
      • The bile is transported to and stored in the gallbladder, and released into the small intestine through the small bile duct.
      • The bile salts act as detergents to disperse the fats into microscopic particles.
    • The pancreas provides a supply of lipases, lipid-digesting enzymes.
      • The microscopic lipid particles produced by the bile are quickly digested by the lipases
    • The pancreas injects a sizable volume of "pancreatic juice" into the small intestine.
      • The injection of perhaps a liter per day of water, sodium bicarbonate and several enzymes acts to neutralize the acidic chyme to a slightly basic pH.
      • The additional enzymes are amylase, peptidases, trypsin and chymotrypsin, which need to work in a basic environment.
      • Amylase breaks down carbohydrates to constituent sugars
      • The pancreatic proteases trypsin and chymotrypsin break proteins and peptides in to shorter peptide chains.
      • Carboxypeptidase completes protein digestion by separating the individual amino acids from the ends of the peptide chains.
    • The wall of the small intestine completes the digestive process.
      • Specialized cells form the lining of the wall of the small intestine to complete the digestive process
      • Proteases complete the breakdown of the remaining peptides into amino acids.
      • Sucrase, lactase and maltase break down any disaccharides into simple sugars.
      • Small amounts of lipase complete the digestion of remaining lipids.
      • These process occur while these resulting small molecules are being absorbed into the cells of the intestinal wall, which is protected by a thick mucous from being digested itself.
    • The wall of the small intestine completes the absorption process.
      • Numerous foldings and projections of the small intestine wall give it an absorbing area some 600 times that of a smooth tube.
      • The total absorbing area of the small intestine including the villi is about 250 square meters!
      • Minute villi cover the wall and move back and forth to efficiently absorb the small molecules and place them into the blood supply.

    This material is part of a brief overview of the topics studied in biology with the intent to highlight the connections to basic ideas in physics and physical science.

Audesirk & Audesirk
Ch 29
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