Food Processing and Digestion

Introduction, Overview

 We ingest foods containing macronutrients (carbs, fats and proteins) in order to break

them down, either for energy, or for chemical parts for building new molecules.

 The macronutrient molecules are not used intact, they are always disassembled. In fact

they can’t even be absorbed until they are at least partly broken down.

 The general sequence is to 1) ingest food; 2) mechanically break it down: 3) chemically

break it down; 4) absorb the useful nutrient components; 5) reclaim much of the water

content; and 6) eliminate unusable wastes.

 This all takes place in various places in the gastrointestinal (GI) tract, assisted by the

accessory organs: the liver, gall bladder, and pancreas.

Passage through the GI Tract (= Alimentary Canal)

1. Mouth

 Chewing mechanically disrupts food. Results in greater surface area for more

efficient attack by digestive enzymes.

 Saliva, secreted by salivary glands.

o Wets and lubricates food.

o Mixes food with amylase, which breaks down a small amount of starch.

o (Mixes food with lingual lipase, which breaks down a little fat—although

only an insignificant amount in adults.)

 Some glucose can be absorbed through the mouth, but the amount absorbed is

inconsequential compared to that in the small intestine.

2. Esophagus

 A bolus of chewed food is swallowed and moves by peristalsis (coordinated

muscle contraction) down the esophagus to the stomach.

 Once food is in the stomach, the lower esophageal sphincter (muscular valve)

closes, preventing the acidic contents of the stomach from entering the

esophagus. (When this does happen through sphincter malfunction, you get

heartburn, or acid reflux.)

3. Stomach

 Can be closed tightly at both ends, by the lower esophageal and pyloric

sphincters.

 Food mixes with gastric juices to become acid chyme. Gastric secretions include:

o Water. Food becomes a wet slurry.

o Acid. Hydrochloric acid lowers pH to between pH 1 and 2. The acidity

attacks many carbon based molecules, especially proteins, “loosening

them up” for further attack by enzymes.

o Pepsin, an enzyme that catalyzes the partial breakdown of proteins.

– Secreted in an inactive form called pepsinogen.

– Pepsinogen is activated by the acid environment of the stomach

interior (the lumen).

o Mucus. Lubricates, and provides a protective coating for the stomach

lining.

o (Gastric lipase, a fat digesting enzyme, breaks down a very small,

insignificant amount of fat.)

 Food is churned by stomach contractions with sphincters closed. Furthers

mechanical breakdown, mixes food thoroughly with gastric juices.

4. Small Intestine

 Acid chyme from stomach enters the duodenum (1st ~12” of the small intestine)

one small squirt at a time.

o It is neutralized by bicarbonate coming from the pancreas. pH returns to

near neutral or slightly basic.

o It mixes with enzymes made by the pancreas and by the intestinal lining

itself:

– Amylases. Digest carbohydrates.

– Proteases. Digest proteins.

– Lipases. Digest fats/oils.

o It mixes with bile, made in the liver, stored in the gall bladder until

needed. Bile:

– Is made of bile acids, modified from cholesterol. Partly

hydrophilic, partly hydrophobic.

– Emulsifies fats—keeps small droplets from coalescing into larger

ones (very similar to how soaps/detergents disperse fats in

water). Maintains large surface area for lipases to attack.

 Nearly all chemical breakdown occurs in the small intestine.

 Nearly all absorption of the breakdown products also occurs in the small

intestine.

o 3 levels of topography increase surface area of the intestinal mucosa:

– Large ridges.

– Villi (singular villus): small, finger-like projections. Multicellular.

– Microvilli: cytoplasmic projections from each surface cell of the

villi.

o Nutrients cross the intestinal mucosa by diffusion, facilitated diffusion or

active transport.

o Each villus is supplied by

– Blood capillaries.

 Absorb and transport hydrophilic substances. Gather into

the hepatic portal vein, which goes to the liver for

processing.

– Lymph capillaries (lacteals).

 Absorb and transport most hydrophobic substances.

Circulate through body tissues before joining the blood

circulatory system. Bypasses the liver initially.

 Some water reabsorption also takes place.

5. Large Intestine (= Colon)

 Additional water reabsorption.

 Wastes consolidated in rectum (final ~12 inches) before elimination through

anus.

 Home to large population of resident bacteria (= colonic flora, gut flora).

o 100s of species, trillions of individual cells.

o Neutral or beneficial.

– Get their nutrition from what we can’t use.

– Digest some fiber, producing energy-rich wastes that we can

absorb.

– Produce some vitamins we can use.