Functions:•Takes in food (ingests it)

•Breaks it down physically and chemically into nutrient molecules (digests it)

•Absorbs the nutrients into the bloodstream

• It rids the body of the indigestible remains (defecates)

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Chapter IV. The Digestive System

• The digestive tube can be organized into specialized regions

• That carry out digestion and nutrient absorption in a stepwise fashion

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Types of animal body plans

•Sac-like plans – are found in many invertebrates

– have a single opening for food intake and the discharge of wastes

•Tube-within-a-tube plan– found in vertebrates

–food entering through one opening (the mouth) and wastes leaving through another (the anus)

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Types of Digestion

• Intracellular digestion–occurs in sponges, coelenterates (corals, hydras and their relatives) and most protozoans

•Extracellular digestion–occurs in chordates, annelids, and crustaceans

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Stages in the Digestive Process

• movement: propels food through the digestive system

• secretion: release of digestive juices in response to a specific stimulus

• digestion: breakdown of food into molecular components small enough to cross the plasma membrane

• absorption: passage of the molecules into the body's interior and their passage throughout the body

• elimination: removal of undigested food and wastes 8

The Mammalian Digestive System• Each organ has specialized food-processing functions

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• Alimentary canal/ GI tract- mouth- pharynx- esophagus

- stomach- small intestine- large intestine

• Accessory organs- salivary glands

- teeth- pancreas- liver

- gall bladder

Components of the Digestive System

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Mechanics of Swallowing

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Peristalsis- rhythmic waves of contraction of smooth muscles in the wall

of the canal

• Liver- regulation of amino acids in blood, production of glycogen (storage molecule) and bile

• Pancreas- secretion of amylase and insulin (lack of insulin may cause diabetes)

• Gallbladder- storage of bile

Accessory Digestive Organs

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The Stomach• Large muscular storage organ; function in storage,

mixing, and secretion of gastric juice, which converts a meal to acid chyme

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Small intestine– is the major organ of digestion and absorption

• The enormous microvillar surface– Is an adaptation that greatly increases the rate of nutrient absorption

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• Hormones help coordinate the secretion of digestive

juices into the alimentary canal

• reabsoption of water

• Bacteria in colon produce vitamin K• The wastes of the digestive tract, the

feces

–Become more solid as they move through the colon–Pass through the rectum and exit via

the anus

The large intestine, or colon

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Chemistry of Digestion

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Hydrolysis- digests foods:

1. Carbohydrates (starches) - becomesimple sugars

2. Proteins - become amino acids

3. Fats - become fatty acids & glycerol4. Nucleic acids (RNA, DNA) - become

nucleotides

Carbohydrate Digestion

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• Begins in mouth - salivary amylase breaks downstarch

• Mostly occurs in small intestine aided by followingenzymes:

a. Pancreatic enzymes - breaks starch into maltoseb. Maltase - breaks maltose into 2 glucosesc. Sucrase - breaks sucrose into 1 glucose and 1

fructose

d. Lactase - breaks lactose into 1 glucose + 1 galactose

• Last 3 enzymes produced by small intestine. Followingabsorbtion, glucose, fructose, & galactose transportedto the liver where they are converted to glycogen.

Fat Digestion

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• Gastric lipase of stomach breaks down somefats.

• Most fat digestion occurs in small intestine.• bile emulsifies fat, exposing more fat to

enzymes• Bile salts link fat molecules to water

molecules; (normally fats are hydrophobic)• Pancreatic lipases continues fat digestion

• subunits now cross into microvilli• subunits are reassembled into triglycerides,

combined with cholesterol, and transported tothe circulatory system

Protein Digestion

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• Stomach - pepsinogen converted to pepsin inthe presence of HCL. Pepsin breaks some ofthe peptide bonds of some proteins

• Small intestine - Pancreatic enzymes trypsin& chymotrypsin break proteins into smaller andsmaller units. The pancreatic enzymecarboxypeptidase breaks peptides into freeamino acids. Several enzymes produced by thesmall intestine further break peptides intoamino acids.

• Amino acids- absorbed and transported to theliver.− used directly by the liver to make liver

proteins− others converted to acetyl coenzyme A

(used in citric acid cycle)− other amino acids sent to various parts of

the body for protein synthesis• Proteases must be activated before being used

− Prevents them from breaking down pancreas(where they’re made & stored)

− Activation occurs when they come in contactw/ certain chemicals found in the smallintestine 24

Nucleic Acid Digestion

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• Enzymes called nucleases break down

nucleic acids such as RNA (ribonucleicacid) and DNA (deoxyribonucleic acid)into nucleotide chains.

• The pancreas produces ribonuclease and deoxyribonuclease

• small intestine produces nucleases that break down nucleotides into smaller

subunits.

Integration & Regulation of Digestive Processes

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• Mechanical - act of chewing causes release ofsalivary amylase.

• Neural - food in stomach stimulates vagusnerve, signal sent to brain, brain sends anothersignal down vagus nerve for stomach to releasegastric secretions.

• Hormonal - e.g. the stomach releases thehormone gastrin when a protein concentration isdetected; gastrin circulates in blood, target cellsare gastric glands which release gastricenzymes; similar in small intestine and pancreas.

Importance of Nutrition

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• provides food for fuel

• provides food for fabrication

• provides food to obtain essentialnutrients, water, minerals