Ch. 21. Nutrition and Digestion

All animals must eat to provide

– energy and

– the building blocks used to assemble new molecules.

Animals also need essential

– vitamins and

– minerals.

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21.2 Overview: Food processing occurs in four stages

Food is processed in four stages.

1. Ingestion is the act of eating.

2. Digestion is the breaking down of food into molecules small enough for the body to absorb.

3. Absorption is the take-up of the products of digestion, usually by the cells lining the digestive tract.

4. Elimination is the removal of undigested materials out of the digestive tract.

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Figure 21.2A

Mechanicaldigestion

Piecesof food

Smallmolecules

Chemicaldigestion(hydrolysis)

Nutrientmoleculesenter bodycells

Undigestedmaterial

Ingestion Digestion Absorption Elimination3 421

Figure 21.2BFood Molecules Components

Protein

Protein-digestingenzymes

Carbohydrate-digestingenzymes

Nucleic-acid-digestingenzymes

Fat-digestingenzymes

Amino acids

Polysaccharide

Disaccharide Monosaccharides

Nucleic acid Nucleotides

Fat Glycerol Fatty acids

21.3 Digestion occurs in specialized compartments

Sponges digest food in vacuoles.

Most animals digest food in compartments.

Cnidarians and flatworms have a gastrovascular cavity with a single opening, the mouth.

– Food enters the mouth.

– Enzymes break down the food.

– Food particles move into cells lining the compartment.

– Undigested materials are expelled back out the mouth.

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Most animals have an alimentary canal with

– a mouth,

– an anus, and

– specialized regions associated with one-way flow of food.

21.3 Digestion occurs in specialized compartments

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The normal one-way flow moves food

– into the pharynx or throat,

– down the esophagus to a

– crop where food is softened and stored,

– gizzard, where food is ground and stored, and/or

– stomach where food is ground and stored,

– to the intestines, where chemical digestion and nutrient absorption occur, and finally

– undigested materials are expelled through the anus.

21.3 Digestion occurs in specialized compartments

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Figure 21.3A

Tentacles

3

1

2

4

Mouth

Food(a water flea)

Gastrovascularcavity

Digestive enzymesbeing released froma gland cell

Food digested tosmall particles

A food particlebeing engulfedA food particledigested in afood vacuole

Figure 21.3BEarthworm

Mouth

Pharynx

Esophagus

Crop

Gizzard

Intestine

Anus

Grasshopper

Esophagus

Esophagus

Bird

Mouth

Mouth

Crop

Crop

Gizzard

Stomach

Intestine

Anus

Anus

Midgut

HindgutGastricpouches

THE HUMAN DIGESTIVE SYSTEM

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21.4 The human digestive system consists of an alimentary canal and accessory glands

In humans, food is

– ingested and chewed in the mouth or oral cavity,

– pushed by the tongue into the pharynx,

– Saliva begins breakdown of di- and polysaccharides

– moved along by alternating waves of contraction and relaxation by smooth muscle in the walls of the canal in a process called peristalsis, and

– moved in and out of the stomach by muscular ring-like valves called sphincters.

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21.4 The human digestive system consists of an alimentary canal and accessory glands

Food is pushed by the pharynx into the esophagus, which connects to the stomach. In the stomach, enzymes begin digestion of proteins. Enzymes from the pancreas and bile (stored in gall bladder) from the liver are added to the small intestine where digestion is completed and nutrient absorption occurs.

Undigested materials move through the large intestine, feces are stored in the rectum, and then expelled out the anus.

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Figure 21.4

Esophagus

Pharynx

Esophagus

Sphincters

Stomach

Small intestine

Nasal cavity

Oral cavity(mouth)

Tongue

Smallintestine

Largeintestine

Rectum

Anus

KeyAlimentary canalAccessory digestiveglands

Rectum

Anus

Smallintestine

Largeintestine

Stomach

Esophagus

Oralcavity

A schematic diagram of thehuman digestive system

Salivaryglands

Gall-bladder

Liver

Pancreas

Salivaryglands

Gall-bladder

Liver

Pancreas

Figure 21.UN02

a.

b.

c.

d.e.

f.

g.

h.

i.

j.

k.

l.