L. digestion and absorption

1
L. digestion and absorption

• Use figure 16-18 on page 541 of the textbook to
  follow along as we look at what happens to the
  materials being processed in the small intestine.

2

• 1. carbohydrates
• salivary amylase works during chewing to break
  starch and glycogen down into disaccharides
  (mostly maltose)
• salivary amylase is inactivated once the food
  gets to the stomach
• when food arrives in the duodenum, pancreatic
  amylase continues the breakdown of starch and
  glycogen into disaccharides
• when the disaccharides are pushed up against the
  intestinal mucosa epithelial cells, the
  intestinal epithelial enzymes break all
  disaccharides (maltose, sucrose and lactose) down
  into monosaccharides (glucose, fructose and
  galactose)
• the monosaccharides are absorbed through
  intestinal epithelial cells into vascular
  capillaries

3

• 2. proteins
• when food reaches the stomach, the stomach
  secretes the enzyme pepsin
• pepsin breaks proteins and peptides in the chyme
  down into smaller peptides (chains of amino
  acids)
• when chyme reaches the duodenum, pancreatic
  proteases continue to break the peptides down
  into smaller and smaller sections (mostly
  dipeptides)
• when the dipeptides are pushed up against the
  intestinal mucosa epithelial cells, the
  intestinal epithelial enzymes break all
  dipeptides down into amino acids
• the amino acids are absorbed through intestinal
  epithelial cells into vascular capillaries

4

• 3. lipids
• the major type of lipid in ingested food is
  triglycerides
• before any enzymes can begin to break down these
  triglycerides, they have to be emulsified by bile
  (this is not chemical digestion)
• bile makes smaller drops of fat so that more
  enzymes can work on digesting the lipids
• the only enzyme that digests lipids is pancreatic
  lipase
• it breaks triglycerides down into fatty acids and
  monoglycerides
• these are absorbed into the intestinal epithelial
  cells
• cholesterol does not need to be digested before
  it is absorbed along with fatty acids and
  monoglycerides
• then they are converted back into triglycerides
  and combined with proteins to make chylomicrons
• chylomicrons are leave the intestinal epithelial
  cells and are absorbed into lacteals (lymphatic
  capillaries inside the villi)

5

• 4. water and electrolytes are absorbed along
  with food
• (there are no diagrams showing this process)
• a. sodium and chloride are transported from the
  lumen into the intestinal epithelial cells and
  then go into the vascular capillaries
• b. water follows by osmosis
• c. iron absorption is carefully controlled to
  prevent toxicity the intestinal epithelial cells
  have a locking mechanism that keeps too much
  iron from entering the blood

6

• 5. vitamins are absorbed along with food
• a. most water soluble vitamins (B, C) diffuse
  through the intestinal epithelial cells and then
  go into the vascular capillaries
• b. the fat soluble vitamins (A, D, E, K) are
  absorbed along with lipids from food
• c. vitamin B12 is a water soluble vitamin that
  can only be absorbed if it has first been bound
  to intrinsic factor, a protein made in the
  stomach