Digestion And Absorption

Digestion: The enzymatic breakdown of food to components that can be absorbed
Absorption: The movement of nutrients, salts, and water across the GI epithelium into blood and lymph.

Goals

To describe the action of the digestive enzymes and bile salts.
To describe tranepithelial transport of monosaccharides and amino acids
To explain the role of bile salts as emulsifiers essential todigestion and absorption of fat

What you need to know

The chemical structure of major nutrients
How pH affects enzyme activity
How molecules diffuse across membranes
How carriers transport molecules across membranes

Ingested food is first broken down mechanically into pieces that can be swallowed, then into small particles that can be pass out of the stomach into the small intestine. Smallparticles contain the major nutrients in our diet. Enzymes digest small particles into components that can be absorbed: Fat, protein, carbohydrates.
Carbohydrates: The most abundant dietary carbohydrates are starch and three disaccharides: sucrose, lactose, and maltose.
Starch is digested to glucose. Sucrose is digested to glucose and fructose; lactose is digested to glucose and galactose.Maltose is digested to glucose. Carbohydrates are built from monosaccharides and must be digested to their component monosaccharides so they can be absorbed. Proteins are digested to amino acids and small peptide chains of two or three amino acids.
Most dietary fat consists of triglycerides. Trigliceydes are digested to monoglycerides and free fatty acids. Nonploar product of digestion can beabsorbed across the intestinal epithelium by simple diffusion. In general, polar substances are absorbed by carrier mediated transport. Plant starch and glycogen are the most abundant dietary carbohydrates. They are long polymers of glucose. Digestion of starch begins in the mouth whit salivary amylase, which is maximally active at about pH 7.

.Amylase continues to work in the stomach until food ismixed with gastric juice and acidified. Acid denatures amylase.
In the stomach, pepsin begins the digestion of protein. Proteins are long polymers of amino acids. Pepsin is the only major digestive enzyme that is maximally active at acidic pH. Pepsin cleaves protein into peptide fragments and small amount of individual amino acids.
Pepsin is active only in the stomach. When chyme is neutralized inthe duodenum, pepsin is denatured.
Most digestion and almost all absorption occur in the small intestine. The pancreas supplies the digestive enzymes for most foodstuffs.
Human lack digestive enzymes for plant, polysaccharide, cellulose and some other complex carbohydrates.
Vegetables and fruits are good sources of vitamins, minerals, and other nutrients. The indigestible polysaccharidescontained in vegetables and fruits make up dietary fibre. Fibre increases bulk of the stool and promotes its timely movement through the colon. Most of the salt and water that enter the GI tract are absorbed in the small intestine. Salt and water may be absorbed by transcellular transport through intestinal epithelial cells or by paracellular transport through leaky tight junctions between cells. Thesmall intestine has the capacity to absorb more food than is usually ingested at one time. Increased intake causes increased absorptive capacity. It is almost impossible to exceed the absorptive capacity of the small intestine.
The carbohydrates that enter the small intestine for digestion include: Starch, breakdown products of starch from salivary amylase digestion. Dietary disaccharides:sucrose (table sugar; Lactose (milk sugar); Maltose (grain sugar).
Pancreatic amylase continues the breakdown of starch in the lumen. It is identical to salivary amylase and maximally active at about pH 7. Since only monosaccharides must be digested, further. Brush border enzymes accomplish this job, a brush border enzyme called glucoamylase that lies close to the NA+ glucose cotransporter breaks...