The Digestive System

Nutrition• Diet = everything you eat• Proper diet

10% Protein______________

30% fat____________________

60% carbohydrates

• Carbohydrate–Primary energy source for cells–Cereals, grains, breads, fruits, vegetables

–4 calories per gram

• Protein–Use as building block for cell structures, enzymes, hormones, muscle, bones

–Dairy products, poultry, fish, meat, grains

–4 calories per gram

• Fat–Use to construct cell

membranes & other cell structures, insulate

nervous tissue, energy source, fat-soluble vitamins (A, D, E, K)

–Oils, margarine, butter–9 calories per gram

• Rule of balancing diet & activity–Energy obtained from food – energy used by body’s cells = energy stored in fat

                                                                           

Other compounds needed

• Only obtained through food eaten

Amino acids

• 8 of 20 amino acids the body cannot produce – must be eaten

Vitamins• Body requires in small

amounts for normal growth & activity

• Used as components of cellular enzymes or coenzymes

• Water soluble – soluble in water, body doesn’t store• ex: thiamin, niacin,

ascorbic acid, folic acid

• Fat soluble – soluble in fat –Excessive amounts accumulate in body’s fatty tissue & can reach toxic levels

–A, D, E, K

• Trace elements – minerals that are required by the body– present only in minute amounts–They are also required for plant growth so they are usually obtained by eating salads

Digestion

• Food molecules are broken down by enzymes (amylases)

Fats• Insoluble in water

–Aggregate into large globules consisting of many fat molecules

–Not easily attacked by enzymes

• Emulsification – breaks up fat globules into tiny fat droplets–bile salts emulsify fats–produced in liver

• Lipases – break fat molecules into fatty acids and glycerol

Proteins

• Folded into tight balls or wound together into tough fibers

–Enzymes cannot break down these balls & fibers because they cannot get at the individual protein chains

• Protein digestion – 2 steps:1.Hydrochloric acid (HCl) in

stomach unfolds large proteins

2.Enzymes (proteases) break down single polypeptide strands

Starch• Broken down into sugars

by enzymes (amylase)

Starch-binding domain of ß-amylase

Structures of the Digestive System

• Mouth–Teeth rip or chew food into shreds

–Tongue mixes it with watery solution called saliva

•saliva is secreted into mouth by 3 pairs of salivary glands

•Moistens and lubricates food so it is swallowed easier

•Contains amylase – begins breakdown of starch

• Pharynx–Food passes through

• Esophagus–Long tube that connects the mouth to the stomach

–No digestion takes place

–About 25 cm long

–Food is pushed down the tube•Peristaltic contractions – successive rhythmic waves of contraction of the smooth muscle wall of the esophagus

• Stomach –Passes from esophagus into stomach through a valve called a sphincter•Prevents acid soaked food from going back into esophagus

–When food enters stomach, gastric juice is secreted by stomach lining•Gastric juice – HCl and acid stable proteases

•Mixture of food and gastric juice is called chyme

–Stomach mixes contents by contraction and relaxation of muscular wall

• Small intestine–Only small amounts of

chyme pass into smallintestine so that acid

can be neutralize

–Some digestive enzymes secreted by cells of intestinal walls

–Most digestive enzymes produced in pancreas•Sends enzymes through duct into first part of small intestine (duodenum)

• Small intestine is around 6 m (19.8 ft) long

• Complete digestion of carbohydrates, fats, and proteins in duodenum occurs in first 25 cm

• Lining covered with fingerlike projections called villi– increase surface area for more absorption

• Fluid absorption is about 99%–Average human consumes about 0.85 liters of solid food and 1.2 liters of water/day

–With food, water, and digestive secretions, about 9 liters pass through/day

–Only about 0.05 liters of solid and 0.1 liters of liquid leave body as waste

• Large intestine (colon)–About 1 m long–No digestion takes place

•Small amount of fluid, sodium, and vitamin K are absorbed

–Primary function – trash compactor•Undigested food (plant fiber and cellulose) is compacted and stored

•Bacteria help process into feces

–Final segment of colon is called the rectum•Compacted material passes through by peristaltic contractions

•From rectum exits body through anus

Nutrients delivered to cells• Blood that leaves small intestine is rich with products of digestion–Collected in portal vein and goes to liver

1. Supplies quick energy by releasing sugar into bloodstream

2. Builds complex carbohydrate from sugar

3. Makes protein from amino acids

4. Stores vitamins and minerals

5. Produces bile salts

6. Regulates blood clotting

7. Monitors cholesterol production

8. Detoxifies alcohol and poisons