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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.
© 2012 Pearson Education, Inc.
Figure 21.2A
Pieces
of food
Small
molecules
Mechanical
digestion
1
Ingestion
2
Digestion
Undigested
material
Nutrient
molecules
enter body
cells
Chemical
digestion
(hydrolysis)
3
Absorption
4
Elimination
Figure 21.2B
Components
Food Molecules
Proteindigesting
enzymes
Amino acids
Protein
Polysaccharide
Carbohydratedigesting
enzymes
Disaccharide
Monosaccharides
Nucleic-aciddigesting
enzymes
Nucleotides
Nucleic acid
Fat-digesting
enzymes
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.
© 2012 Pearson Education, Inc.
21.3 Digestion occurs in specialized compartments
 Most animals have an alimentary canal with
– a mouth,
– an anus, and
– specialized regions associated with one-way flow
of food.
© 2012 Pearson Education, Inc.
21.3 Digestion occurs in specialized compartments
 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.
© 2012 Pearson Education, Inc.
Figure 21.3A
Tentacles
1
Digestive enzymes
being released from
a gland cell
2
Food digested to
small particles
3
A food particle
being engulfed
A food particle
digested in a
food vacuole
Mouth
Food
(a water flea)
Gastrovascular
cavity
4
Figure 21.3B
Earthworm
Mouth
Pharynx
Intestine
Esophagus
Anus
Crop
Gizzard
Grasshopper
Midgut
Esophagus
Anus
Mouth
Crop
Hindgut
Gastric
pouches
Bird
Stomach
Gizzard
Mouth
Intestine
Esophagus
Crop
Anus
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.
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
Figure 21.4
Nasal cavity
A schematic diagram of the
human digestive system
Oral cavity
(mouth)
Tongue
Oral
cavity
Salivary
glands
Salivary
glands
Pharynx
Esophagus
Esophagus
Liver
Gallbladder
Liver
Pancreas
Esophagus
Stomach
Small
intestine
Sphincters
Gallbladder
Large
intestine
Pancreas
Rectum
Anus
Small
intestine
Large
intestine
Key
Alimentary canal
Accessory digestive
glands
Rectum
Anus
Stomach
Small intestine
Figure 21.UN02
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j.
f.
k.
l.