Overview of Digestion

Eating provides us access to a variety of substances needed to build, repair, regulate and maintain our own tissues and bodily functions. In order to take advantage of the needed chemicals, or "nutrients" in food, we must employ a specialized system capable of breaking down complex foods into simpler, smaller molecules that can then be absorbed into the bloodstream. The system of tissues and organs that work to accomplish this end are collectively known as the "digestive system."

Simply stated, the digestive system is a tube running from the mouth to the anus that functions like a "dis-assembly line" for foods (the tube may be called the "gut"). The gut's main job is to break down large macromolecules of protein, fat and starch into tiny molecules of amino acids, fatty acids and glucose that can be absorbed across the wall of the tube into the blood stream for use around the body.

Food breakdown is accomplished through a combination of mechanical and enzymatic processes. Attached to the gut at key points are a variety of accessory digestive organs such as the salivary glands, the liver and pancreas. These provide secretions that are essential to the digestive process.

The digestive system function something like a factory where three main processes are taking place:

• Secretion: The delivery of mucus, enzymes and ions into the gut tube and hormones into the blood
• Absorption: The transport of water, ions and nutrients across the lining f the gut into the blood
• Motility: Contractions of the smooth muscle in the wall of the gut to crush, mix and propel the contents along

Digestion begins in the mouth where food is first mechanically broken down by chewing (called "mastication"). Secretions of the salivary glands ("saliva") contain water to solubilize food, mucus to lubricate and enzymes to begin the breakdown of starch.

Swallowing propels food through the throat ("pharynx") and into the esophagus. The esophagus performs essentially no absorption, but is the conduit through which food gets to the stomach. In humans, the strong peristaltic contractions of the esophagus send most boluses of food into the stomach within 3-5 seconds. If the first contraction does not get the bolus there, secondary waves of muscle activity will get the job done.

The stomach has several functions:

• A storage reservoir for large meals
• Beginning the chemical and enzymatic digestion of protein
• Liquefaction of food for presentation to the intestine

The stomach lining secretes mucus for lubrication and for protection of the lining itself from the hydrochloric acid secreted by special, "parietal' cells. Acid inactivates bacteria eaten with the food, and helps to activate the enzyme "pepsin" to begin the enzymatic breakdown of proteins.

Entry of the now liquid foodstuff into the "duodenum," or first part of the small intestine,prompts the pancreas to deliver its enzyme-laiden secretion into the gut and the gallbladder to inject its concentrated, liver-made "bile". Proteases and lipases from the pancreas continue and complete the breakdown of proteins and fats, and pancreatic amylase completes the breakdown of starch. "Bile salts" in the golden bile fluid manufactured in the liver, are admixed here as well, and are essential to the solubilization and absorption of fats and fat-soluble vitamins. The small intestine itself is a long structure lined by a complex membrane with tiny blood vessels just beneath the surface. By the end of its trip down the small intestine, foodstuff has given up most all its nutrient content, and this molecular mixture has crossed into the individual's bloodstream for use throughout the body.

When foodstuff leaves the small intestine to enter the large intestine ("colon"), it has already given up most of its salts and about 90% of its water. However, reabsorption of much of the remaining water and salt is critical to the health of the individual. Under normal circumstances, the colon retrieves enough of the remaining water to create a relatively dry excrement, preventing dehydration.

Anatomy

The digestive system can be followed from the mouth, through which foods enter the system, to the anus, from which the wastes of the process are expelled. (See anatomical diagram).

• Mouth: Foodstuffs are mechanically broken down by chewing
• Salivary glands inject mucous and serous fluid for lubrication and the enzyme amylase to begin the breakdown of starches.
• Esophagus: The essential conduit of food from the mouth, through the thorax, into the stomach residing in the abdomen.
• Stomach: Here the major action begins with the enzymatic degradation of proteins begun and the food converted to a liquid form
• Liver: This metabolic powerhouse organ provides a variety of essential functions to the body, but in digestion functions to create the bile salts whose injection into the small intestine will make possible the absorption of fats and key vitamins.
• Pancreas: An organ of importance in the "endocrine," or hormone system, the pancreas is also the major producer of digestive enzymes that make possible the complete breakdown and assimilation of proteins, carbohydrates and fats.
• Small Intestine: This is the "center stage" organ in the process. It is here where the final stages of chemical enzymatic digestion occur and where nearly all the nutrients are absorbed.
• Large Intestine: Water absorption is completed by this organ leading to a "neater" and less resource-wasteful waste product.

Food Chemistry

The human diet involves there ingestion of thousands of different molecules. Nearly all of these molecules are too big to be directly absorbed into the blood, and so they require breakdown from these large forms (called "macromolecules") into smaller, simpler forms. This breakdown is accomplished by the action of proteins called "enzymes" in the gut, and the most important enzymatic reaction used in the digestion of food is called "hydrolysis," or the breaking of chemical bonds by the addition of water.

Carbohydrates

Carbohydrates are a diverse group of macromolecules that range from simple sugars to huge, complex polymers. They are defined chemically as neutral compounds of carbon, hydrogen and oxygen, and act as primary energy sources for our bodies. The breakdown of carbohydrates provides small molecules of "glucose" that are used to feed cells throughout the body. The energy supplied by carbohydrates is 4 calories per gram. Carbohydrates eaten in excess can be converted by the body to lipids (fat) for storage.

Monosaccharides are simple sugars with six carbon atoms like glucose and fructose, (called "hexoses"), or they have five carbons like ribose (and are called "pentoses.") These are the products left after breakdown of more complex carbohydrates and are small enough to be absorbed across the wall of the intestine into the blood stream.

Disaccharides are made from two monosaccharides linked together by a bond. The disaccharides most important in nutrition are:

• lactose ("milk sugar")- glucose bound to galactose
• sucrose ("table sugar")- glucose bound to fructose
• maltose- glucose bound to glucose

Polysaccharides are the most abundant dietary carbohydrates. Three very important polysaccharides are polymers of glucose (i.e. made up of many, repeating glucose "building blocks"):

• Starch is a major plant storage for for glucose. Its two forms are called "alpha-amylose" (straight chains of glucose) and "amylopectin" (highly branched glucose chains). Most of the bonds in starch are hydrolyzed by enzymes called "amylases."
• Cellulose is another major plant carbohydrate. It is the main constituent of plant cell walls with more than half of the earth's organic carbon being found in cellulose molecules. Cellulose consists of lots of unbranched chains of D-glucose molecules linked together by what are called "beta glycosidic bonds." No vertebrate animals have the enzymatic capacity to digest this molecule, but animals called "herbivores" do digest it by virtue of having digestive tracts full of bacteria that make cellulase enzyme that hydrolyzes cellulose.
• Glycogen is the main animal storage carbohydrate. The glucose molecules in glycogen are linked by alpha-glycosidic bonds like those found in starch.

Proteins

Proteins are polymers of blocks called "amino acids" that are linked together by "peptide" bonds. The length of the chains is quite variable, and many of the proteins we eat have been modified to "glycoproteins" by the addition of carbohydrate segments, and to "lipoproteins" by the addition of lipid (fat).

Proteins have varied functions in our bodies that are determined by the amino acid sequence in their structure. They may function as building blocks of muscle and other tissue, they may function as hormones or enzymes, and can add to our bodies' defenses as "antibodies."

Although some limited absorption of very short chain polymers called "peptides" is possible, most all proteins need to be broken down into individual amino acids for absorption. The enzymes that breakdown the peptide bonds of proteins and peptides are called peptidases, or "proteases."

Lipids (Fats)

Lipids are essential to the body. They act as energy sources (yielding 9 calories per gram), and are components used by the body for a variety of tasks, including the formation of regulatory molecules called "prostaglandins," the binding and transportation of fat-soluble vitamins (Vit. A,D,E, and K), the maintenance of healthy hair and skin and the protection of vital organs.

The building blocks of complex lipids are "fatty acids." These building blocks contain a long chain of carbon and hydrogen that ends in a "carboxyl group." Nearly all fatty acids have an even number of carbons and have chains between 14 and 22 carbons long. The main difference between the fatty acids is the length of the chains and the position of unsaturated or double bonds.

The most important form for storage of fat in animals is as what is called "neutral fat," or "triglyceride." A triglyceride is formed from a molecule of glycerol that has each of its three carbons bound by an "ester" bond to a fatty acid. Because triglycerides cannot be effectively absorbed, we must employ enzyme called "lipase" from the pancreas to break down these fats into monoglycerides and free fatty acids that we can absorb.