Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr

Chapter 36Digestion and Human Nutrition

(Sections 36.1 - 36.6)

36.1 The Battle Against Bulge

• When you eat more food than you need, you store the excess energy – mostly as fat in adipose tissue

• In the US about two-thirds of adults are overweight or obese, which increases risk of type 2 diabetes, cardiovascular disease, some cancers, and other disorders

• Increasing portion sizes and fast-food meals contribute to the obesity trend in adults and children

Battle Against Bulge (cont.)

• Genetics contribute to weight control

• The ob gene encodes a hormone (leptin) made by adipose cells that suppresses appetite – people who can’t make leptin are severely obese

• Another obesity-related gene, fto, affects food intake – people with the high-risk allele eat more food before they feel full

Effects of a Mutant ob Gene

ABC Video: Diet Soda and Weight Gain

36.2 Animal Digestive Systems

• Animals are heterotrophs that typically digest food inside their body, but outside of their cells

• The digestive system mechanically and chemically breaks food down to molecules that are absorbed into the internal environment, and expels unabsorbed residues

• The digestive system interacts with other organ systems to maintain homeostasis for the body as a whole

Incomplete and Complete Systems

• Some invertebrates have an incomplete digestive system, but most invertebrates and all vertebrates have a complete digestive system

• incomplete digestive system • Saclike digestive system; food enters and leaves through

the same opening

• complete digestive system • Tubelike digestive system; food enters through one

opening and wastes leave through another

Incomplete Digestive System

Fig. 36.2a, p. 596

single opening takes infood, expels waste

branching saclike gut

A Incomplete digestive system of a flatworm.

pharynx

Incomplete Digestive System

Complete Digestive System

Fig. 36.2b, p. 596

B Complete digestive system of a frog.

cloaca (opening through which digestive wastes, urinary wastes, and gametes exit the body)

pharynx stomach

small intestine

large intestine

mouth withtongue

liver

esophagus

gallbladder

pancreas

Complete Digestive System

Five Tasks of the Digestive System

1. Mechanical processing and motility:• Movements that break up, mix, and directionally propel

food material

2. Secretion:• Release of substances, especially digestive enzymes, into

the lumen (the space inside the tube)

3. Digestion:• Breakdown of food into particles, then to nutrient

molecules small enough to be absorbed

Five Tasks of the Digestive System

4. Absorption:• Uptake of digested nutrients and water across the gut wall,

into extracellular fluid

5. Elimination:• Expulsion of undigested or unabsorbed solid residues

Diet-Related Structural Adaptations

• Features of an animal digestive systems are shaped by natural selection and adapt the animal to a particular diet

• A bird’s beak-size and shape determines what kinds of food it can process

• Teeth of mammals differ, depending on their diet:• Carnivores have long, sharp canine teeth• Herbivores (e.g. antelopes) have prominent incisors for

grazing, and flat molars for grinding food

Herbivore and Human Molars

Fig. 36.3a, p. 597

human molar

crown

antelope molar

gumline

root

crown

root

Herbivore and Human Molars

Diet-Related Adaptations (cont.)

• Animal guts also are specialized to their diets

• Seed-eating birds have a large crop for storing food; and a large, tough gizzard for grinding food

Fig. 36.2c, p. 596

C Complete digestive system of a bird. The crop is an expandable organ for storing food. The muscular gizzard grinds up food.

gizzard

esophagus

beak

mouth

crop glandular part of stomach

cloaca

intestines

Diet-Related Adaptations (cont.)

ANIMATION: Examples of digestive systems

To play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to play

Mac Users: CLICK HERE

Diet-Related Adaptations (cont.)

• Ruminants have multiple stomach chambers that maximize nutrients extracted from plant foods rich in cellulose• Microbes ferment cellulose in the first two chambers• Solids in the second chamber form “cud” that is

regurgitated for a second chewing

• ruminant • Hoofed mammal with a multiple-chamber stomach that

adapts it to a cellulose-rich diet

A Ruminant Stomach

Fig. 36.3b, p. 597

stomach chamber 4

to smallintestine

stomachchamber 1

ingestion, regurgitation, reswallowingof food through esophagus

stomach chamber 2

stomach chamber 3

B

A Ruminant Stomach

ANIMATION: Antelope stomach function

To play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to play

Mac Users: CLICK HERE

Diet-Related Adaptations (cont.)

• Some meat-eaters have stomachs that can stretch enormously – a python may eat only once or twice a year

Key Concepts

• Animal Digestive Systems• Some animal digestive systems are saclike, but most are a

tube with two openings• The structure of an animal’s digestive system often reveals

adaptations that allow the animal to eat a specific diet• For example, antelopes have adaptations that allow them

to eat grasses

36.3 The Human Digestive System

• Humans have a complete digestive system with specialized regions for digesting food, absorbing nutrients, and concentrating and storing unabsorbed waste

• Salivary glands, the pancreas, and the liver are accessory organs that secrete enzymes and other substances into the tubular portion of this system

Overview of the Digestive System

• Food taken into the human mouth is swallowed and moves into the pharynx, which opens onto the esophagus

• Peristalsis of the esophagus conveys food to the stomach

• From the stomach, food enters the small intestine, then the large intestine (colon)

• Wastes are stored in the rectum until they are eliminated through the anus

Key Terms

• esophagus • Muscular tube between the throat and stomach

• peristalsis • Wavelike smooth muscle contractions that propel food

through the digestive tract

• stomach • Muscular organ that mixes food with acid and digestive

enzymes (gastric fluid) that it secretes

Key Terms

• small intestine • Longest portion of the digestive tract• Receives secretions from the liver and pancreas• Site of most digestion and absorption

• colon (large intestine)• Receives digestive waste from the small intestine and

concentrates it as feces• Absorbs water and ions

Key Terms

• rectum • Region where feces are stored prior to excretion

• anus • Opening through which digestive waste is expelled from a

complete digestive system

Major Organs and Accessory Organs

Fig. 36.5, p. 598

Anus Opening through which feces are expelled from the body.

Rectum Expandable sac that stores feces.

Large Intestine (colon) Wider than the small intestine, but shorter. It absorbs most remaining water, thus concentrating any undigested waste and forming the feces.

Small Intestine Longest tube of the gut. Its first part receives secretions from the liver, gallbladder, and pancreas. These secretions help complete the process of digestion. Most water and products of digestion are absorbed across the highly folded wall of this organ.

Stomach J-shaped muscular sac that receives food and mixes it with gastric fluid secreted by cells in its lining.

Esophagus Muscular tube through which food moves to the stomach.

Pharynx (throat) Entrance to the gut and respiratory system. Action of the epiglottis keeps food from entering the trachea.

Mouth Oral cavity. Teeth break food into bits. Tongue mixes food with saliva.

A Major Organs B Accessory Organs

Pancreas Secretes enzymes and bicarbonate (a buffer) into the small intestine.

Gallbladder Stores and concentrates bile, then secretes it into the small intestine.

Liver Produces bile, which aids digestion and absorption of fats.

Salivary Glands Produce and secrete saliva, which moistens food and begins the process of carbohydrate digestion.

Major Organs and Accessory Organs

ANIMATION: Human digestive system

To play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to play

Mac Users: CLICK HERE

Key Concepts

• Human Digestive System• In humans, food taken into the mouth is mechanically

broken down by teeth• Saliva starts the process of chemical digestion• The stomach stores and digests food• Digestion is completed and absorption begins in the small

intestine• The large intestine concentrates wastes

ANIMATION: Digestion

To play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to play

Mac Users: CLICK HERE

36.4 Digestion in the Mouth

• Mechanical digestion begins in the mouth when teeth break food into smaller pieces

• Human adults have four types of teeth: incisors, canines, premolars, and molars

• Each tooth consists of bonelike dentin, with a crown covered by enamel, the hardest material in the body

Human Teeth

Fig. 36.6a, p. 599

Human Teeth

Fig. 36.6a, p. 599

A Adult teeth. Incisors shear off bits of food. Canines tear meats. Premolars and molars have broad, bumpy crowns that are platforms for grinding and crushing food.

incisors (8)

molars (12)

lower jaw upper jaw

canines (4)

premolars (8)

Human Teeth

Fig. 36.6b, p. 599

Human Teeth

Fig. 36.6b, p. 599

B Cross-section of one human tooth. The crown is the portion extending above the gum; the root is embedded in the jaw.

periodontal membrane

root canal

ligaments

pulp cavity (contains nerves and blood vessels)

dentin

enamel

bone

root

gingiva (gum)

crown

Human Teeth

Digestion in the Mouth (cont.)

• Chemical digestion (enzymatic breakdown of food into molecular subunits) also begins in the mouth

• Saliva, secreted by salivary glands, contains enzymes, bicarbonate, and mucins

• Salivary amylase begins breakdown of starch; salivary lipase begins breakdown of fats

• salivary gland • Exocrine gland that secretes saliva into the mouth

3D Animation: Digestive Motility

36.5 Food Storage and Digestion in the Stomach

• The stomach is a muscular, highly-folded, stretchable sac with a sphincter at either end

• It stores food and continues the process of digestion that began in the mouth

• sphincter • Ring of muscle that controls passage through a tubular

organ or body opening

Structure of the Stomach

Fig. 36.7, p. 600

esophagus

gastroesophageal sphincter

serosa

longitudinal muscle

circular muscle

oblique muscle

submucosa

mucosasmall intestine

pyloric sphincter

Structure of the Stomach

Functions of the Stomach

• The stomach has three functions:

1. It stores food and controls the rate of passage to the small intestine

2. It mechanically breaks down food

3. It secretes substances that aid in chemical digestion

Digestion in the Stomach

• Cells of the glandular epithelium (mucosa) lining the stomach secrete gastric fluid, which includes mucus, hydrochloric acid, and pepsinogen, an inactive form of the protein-digesting enzyme pepsin

• gastric fluid• Fluid secreted by the stomach lining• Contains digestive enzymes, acid, and mucus

Digestion in the Stomach (cont.)

• Action potentials generated in the upper portion of the stomach cause rhythmic stomach contractions that mix gastric fluid with food to form chyme

• chyme • A semiliquid mixture of food and gastric fluid

Chemical Digestion of Protein

• Arrival of food in the stomach triggers endocrine cells to secrete gastrin into the blood, which stimulates acid secretion by cells of the stomach lining

• Acid in denatures proteins and exposes their peptide bonds

• Acids also converts pepsinogen into pepsin, which breaks peptide bonds, cutting proteins into smaller polypeptides

Stomach Disorders

• Gastroesophageal reflux:• When the gastroesophageal sphincter does not close

properly, acidic chyme splashes back into the esophagus, causing heartburn or acid indigestion

• Stomach ulcers:• When the stomach’s protective mucus layer is disrupted,

gastric fluid and enzymes can erode the stomach lining• Most ulcers result from a bacterial infection (H. pylori)

ANIMATION: Peristalsis

36.6 Structure of the Small Intestine

• Chyme leaves the stomach through the pyloric sphincter and enters the duodenum, the first part of the small intestine

• The small intestine is the longest segment of the gut, and has a highly folded lining with many projections that increase surface area for digestion and absorption

Lining of the Small Intestine

• Longitudinal cross section through the small intestine showing its highly folded lining

Increasing Surface Area

• Millions of hairlike villi containing blood and lymph vessels project from the intestinal lining

• Epithelial cells called brush border cells at the surface of a villus have even smaller projections called microvilli

• Collectively, folds and projections of the small intestinal lining increase its surface area to about the size of a tennis court

Key Terms

• villi • Multicelled projections at the surface of each fold in the

small intestine

• brush border cell• In the lining of the small intestine, an epithelial cell with

microvilli at its surface

• microvilli • Thin projections that increase the surface area of some

epithelial cells

Wall of the Small Intestine

Fig. 36.8b-f, p. 601

B Structure of the wall of the small intestine.

C One intestinal fold with hair-like villi at its surface.

D One villus with brush border cells at its surface.

E A brush border cell with microvilli at its free surface.

F Micrograph of microvilli at the surface of a brush border cell.

lymph vessel

villi

circular muscle

longitudinal muscle

autonomic nerves

blood vessels

Wall of the Small Intestine