Chemical Digestion and Absorption

Carbohydrates

  1. Starch

    1. Digested first to oligosaccharides → disaccharide maltose → glucose which is absorbed by the small intestine

    2. Begins in mouth

      1. Salivary amylase breaks starch into oligosaccharides

      2. Amylase takes longer the larger the meal

    3. 50% of dietary starch is digested before it reaches the small intestine

      1. Digestion in the small intestine involves chyme mixing with pancreatic amylase

      2. Chyme touches the brush border

      3. Brush border enzymes, dextrinase and glucoamylase, hydrolyze the oligosaccharides.

      4. Maltase hydrolyzes maltoses

      5. All turns into absorbable glucose

  2. Monosaccharides

    1. Transport proteins from enterocyte plasma membranes absorb monosaccharides as soon as they are produces

    2. Glucose (80% of absorbed sugar) is taken by a sodium-glucose transporter

    3. Glucose is moved out of the base cell into the extracellular fluid and into the villus core through tight junctions between epithelial cells

Protein

  1. Proteases/Peptidases

    1. Protein digesting enzymes

    2. None in saliva, all in stomach

    3. Pepsin hydrolyzes all peptide bonds between tyrosine and phenylalanine → digests 10% - 15% dietary proteins into smaller polypeptides

Lipids

  1. More complicated digestion than proteins and carbs due to hydrophobic qualities

  2. Lipases - enzymes that digest lipids

  3. Lingual Lipase - digests tiny amount of fat in mouth

    1. Secreted by intrinsic salivary glands

  4. Gastric Lipase - digests preduodenal fat in the stomach, 10%-15% digested before duodenum is reached

  5. Antral Pumping - breaks up fat into tiny pieces dispersed by fluid chyme

    1. Exposes more surface to enzymatic actions forming → emulsification droplets

    2. Droplets are moved into the duodenum and coated in bile acids and lecithin

    3. Intestinal segmentation agitations and breaks down the drops more

    4. Lecithin and bile acid coating keep droplets from merging back together

Water

  1. Digestive tract receives about 9L of water per day

    1. 8L absorbed by small intestine

    2. .8 L absorbed by large intestine

    3. .2 L in fecal matter

  2. Absorbed by osmosis

  3. Diarrhea

    1. Too little water absorbed by large intestine

    2. Large intestine is irritates by bacteria and feces moves too quickly for water absorption

  4. Constipation

    1. Feces moves too slow and too much water is absorbed, harding the stool

    2. Lack of dietary fiber, lack of exercise, emotional upset, long-term laxative abuse