The small intestine is the longest (6m) section of the alimentary canal.
The beginning of the small intestine is the site of most of the hydrolysis of macromolecules, and the rest of the small intestine is responsible for the absorption of nutrients into the blood.
The first section of the small intestine is known as the duodenum.
In the duodenum, the acid chyme mixes with secretions from the pancreas (bicarbonate, which acts as a buffer against acid chyme), the liver (bile, which contains bile salts-detergents that aid in digestion and help stabilize fat emulsions), the gall bladder, and the intestinal wall itself.
* Gall Bladder has no digestive secretions.
Here is how particular macromolecules are broken down in the small intestine:
1. Carbohydrates- The breakdown of starch and glycogen begins with salivary amylase in the mouth. In the small intestine, pancreatic amylases break starch, glycogen, and small polysaccharides into disaccharides.
The breakdown of these disaccharides occurs at the wall of the intestinal epithelium, and then the monosaccharides are quickly absorbed (they directly enter the blood)
2. Proteins- Pepsin begins the breakdown of proteins in the stomach and in the small intestine, trypsin and chymotrypsin break polypeptides into smaller chains. Dipeptidases, carboxypeptidase, and aminopeptidase break apart into amino acids. Considered the last source of energy.
3. Nucleic Acids- The breakdown of nucleic acids is similar to that of proteins. In the small intestine, nucleases break then down into nucleotides, nitrogenous bases, sugars, and phosphate groups.
4. Fats- Digestion of fats starts in the small intestine. Bile salts coat the fat droplets and keep then from coalescing (in emulsification), and lipase hydrolyzes them. Most absorbed fat first enters the lymphatic system.
Most absorbtion of nutrients occurs in the small intestine, and the epithelial lining of the small intestine has folds called villi, which in turn bear projections called microvilli-both of which radically increase the surface area available for absorption.
In each villus is a set of tiny blood vessels called capillaries and a lymph vessel called a lacteal.
Monosaccharides, such as glucose, cross via passive diffusion, whereas amino acids and dipeptides are pumped across in active transport.
The lacteal will absorb small fatty acids.
The capillaries and veins that drain the nutrients away from the villi all join the hepatic portal vessel, which brings them to the liver. The liver metabolizes the organic molecules in various ways.
Some hormones involved in digestion are gastrin, which stimulates the secretion of gastric juice; and enterogastrones, such as secretin and cholescystolcinin (CCK), that are secreted by the walls of the duodenum and that prompts the digestion of various macromolecules.
1. How long is the small intestine?
a. 15 m
b. 8 m
c. 6 m
d. 4 m
2. In the duodenum, the acid chyme mixes with secretion from what?
a. Gall Bladder
3. The breakdown of starch and glycogen begins with what in the mouth?
a. pacreatic amylase
c. salivary amylase
4. Where does the digestion of fats start?
b. small intestine
c. large intestine
5. All of these hormones are involved in digestion, except:
Answers: 1. C 2. B 3. C 4. B 5. B