Biology 10, Las Positas College, Fall 2004
Digestive System

Chapter 37

I-Introduction
II-Anatomy & Physiology
III-Activities of Digestive Tract
IV-Diet & Nutrition

A. Food Guide Pyramid
B. Disposition of Absorbed Organic Compounds

 

I-Introduction-Digestion involves breaking up of large food molecules into smaller ones that can be used by cells.

Digestive tract = mouth, pharynx, esophagus, stomach, intestines = tube within body from mouth to anus. Nutrients within this tube are still outside body until they are absorbed from the intestine.
Function of digestive system = breakdown ingested food to small enough particles to be absorbed into blood.
Function of metabolism = produce energy (ATP) in the cells for growth, maintenance and activity.

II-Anatomy & Physiology

Note: ase = enzyme

A. Mouth-breakdown of food begins with chewing & enzymatic action. Teeth break up food. Salivary glands -> saliva = amylase, mucous, H2O, various salts. Amylase - breaks down starches & glycogen -> sugars.

B. Pharynx-upper part of throat, performs swallowing, propels bolus from mouth into esophagus.

C. Food passes through mucus lined (for lubrication) esophagus to stomach. Food moves through digestive tract by peristalsis = rhythmic muscular contractions.

D. Longitudinal & circular muscle layers of gut, squeeze & mix food, then progressively move it through the tube.

E. Stomach-stores food & breaks it down with acid & enzymes.

Churns ingested nutrients, breaks them into small particles & mixes them with gastric juices. Stomach secretes hydrochloric acid-> breaks down large proteins, kills bacteria which enter with food, activates enzymes-> protein breakdown = protease. Regulates even flow of food into intestine.

Control-feedback loops, i.e. stretch, pH, product of earlier step -> appropriate next step, i.e. opening of sphincter, push food along tract, release enzyme or hormone, etc.
Protein fragments, distention, caffeine, alcohol -> increased gastric secretions. As protein leaves stomach & less distention, gastric secretion decreases. If stomach acid increases, gastric secretion goes down. If fat, acid or duodenum full, gastric emptying down.
Food in stomach about 1-3 hours.

F. Small Intestine-further digestion, mixing & absorption. Alkaline mucus secreted for lubrication & neutralization of stomach acids. 1-6 hours for chyme (mixture of partly digested food, gastric juice & mucus, having the consistency of pea soup) to move through 20 foot long intestine.

Protease breaks down proteins.
Lipases break down fats.

Small intestine assisted by secretions form liver & pancreas. Liver -> bile -> emulsifies fats. Pancreas -> enzymes -> breakdown starch, proteins, fats, nucleic acids.
Small intestine absorbs almost all digested food.

Amino acids (originally proteins) & sugars absorbed by blood through capillaries in villi by active transport.

Fats emulsified by bile salts from liver & absorbed by lymphatic system in villi. Fats broken down by lipases -> liver.

Inside of small intestine modified to facilitate absorption by having villi with microvilli.

Folds-villi & microvilli increase surface. Inside villi are capillaries & lymph vessels that absorb nutrients & lymphoid tissue that helps destroy microorganisms absorbed from the small intestine.

G. Large intestine-resorbs H2O & some salts. Accumulates & excretes undigested substances as feces. Many bacteria -> Vitamins K & B12, make up 25-50% of dry weight of feces. The anal sphincters relax & peristaltic waves in the colon & rectum push feces past sphincters & out of body through anus.

H. Associated structures assist in breakdown & conversion of food.

1. Pancreas

islet cells (about 1%) -> glucagon & insulin-hormone control blood sugar
acini cells (about 99%) -> 1.5 L fluid & digestive enzymes. Fluid mostly H2O, is alkaline. Enzymes break down fats, proteins, carbohydrates

2. Liver

receives blood from gastrointestinal tract. Stores glucose as glycogen when blood sugar. high. Breaks down glycogen, proteins, fats -> glucose, releases to blood when blood sugar low.

Hepatic portal system- nutrient-rich blood goes to liver before general circulation

Forms bile which breaks down fats. Goes to gall bladder for storage, Breaks amino acids & synthesizes other proteins.
Stores fat-soluble vitamins (A, D, E, K), B12 & minerals.
Detoxification
Synthesizes essential forms of fats.
Synthesizes blood clotting proteins

3. Gall Bladder

Stores bile which is continuously secreted by liver. When chyme enters duodenum (beginning of small intestine), gall bladder squirts bile into duodenum.

Enzyme overview-Speed chemical reactions. Are specific for compound they act on. Proteins- proteases> amino acids Carbohydrates- amylases> simple sugars. Fats- lipases> glycerol & fatty acids.

III-Activities of digestive tract:

1. Mechanical Breakdown = food physically broken down into smaller pieces. Physical processes like chewing, peristalsis, churning movements of stomach & small intestines to mix food with enzymes & digestive juices.

2. Secretion - enzymes, fluid, bile, mucus, hormones, acid, bases.

3. Digestion or Chemical breakdown = conversion of large nutrient molecules into small molecules. Breaks down food particles through series of chemical reactions involving enzymes.

4. Absorption = passage of usable nutrient molecules from intestine into bloodstream & lymphatic system for the final passage into body cells. Small molecules used for energy & rebuilding, repairing & reproduction of cells.

5. Elimination of undigested & unabsorbed materials from the body as solid wastes.

IV-Diet & Nutrition

A. Food Guide Pyramid, p. 624, Fig. 37.14.

Need to eat amino acids that body cannot make = essential amino acids.
Combine legumes with any grain, nut or seed OR any grain, legume, nut or seed with small amount of milk cheese, yogurt, egg, meat, poultry, fish to absorb proteins.

B. Disposition of Absorbed Organic Compounds-Fig 37.12, p 622

Carbohydrates & fats -> energy + CO2; structural components of cells; storage forms; specialized chemicals
Amino Acids -> structural proteins; enzymes; specialized chemicals
Note that carbohydrates are interchangeable, +/- NH3