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Need Food for:
• Energy
• Getting raw materials to make macromolecules
needed by the cells
• Getting nutrients :cofactors or co enzymes for
ensuring proper functioning of chemical reactions
by proving help to enzymes
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The Course of Starvation
Undernourishment- lack of nourishment
to meet energy needs
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Nutrient Requirements
• Humans are able to synthesize almost all of their
required lipids using acetyl groups obtained from
food. They also can synthesize some amino acids
but for others they depend on food to provide
nutrients.
• Essential fatty acids such as linoleic acid and
essential amino acids such as tryptophan must
be obtained through a dietary source.
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Nutrient Requirements
• Elements required in large amounts are called
macronutrients; elements required in only tiny
amounts are called micronutrients.
• Calcium is an example of a macronutrient. A 70-kg
person contains about 1.2 kg of calcium.
• Humans require 800–1,000 mg of calcium per day
in their diet.
• Iron is an example of a micronutrient.
• Although humans require only about 15 mg of iron
per day in their food, insufficient iron is the most
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Nutrient Requirements
• A chronic shortage of any nutrient produces a state
of deficiency called malnutrition.
• Chronic malnutrition leads to a characteristic
deficiency disease. Scurvy is a condition caused
by a vitamin C deficiency in primates. Other
mammals can make their own.
• Deficiency diseases can also result from an
inability to absorb or process an essential nutrient.
• For example, pernicious anemia (the failure of
red blood cells to mature), is caused by B12
deficiency, usually brought on by the person’s
inability to absorb the vitamin.
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Adaptations for Feeding
• Filter feeders, such as clams and blue whales,
prey on small organisms by filtering them from the
aquatic environment.
• Fluid feeders obtain food from the fluids
produced by various organisms, including blood
and nectar.
•Substrate Feeders are animals that live in or
on their food source
•Fluid feeders suck nutrient-rich fluid from a
living host
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Fig. 41-6a
Baleen
Humpback whale, a suspension feeder
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Fig. 41-6b
Leaf miner caterpillar,
a substrate feeder
Caterpillar
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Feces
Fig. 41-6c
Mosquito, a fluid feeder
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Fig. 41-6d
Rock python, a bulk feeder
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Digestion
• Digestion is usually carried out extracellularly in
gastrovascular cavities. (enzymes secreted into
cavity)
• The cavities may be just a simple tube with one
end (cnidarians) or a tube with two ends
(earthworm) or more specialized (birds, mammals)
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Fig. 41-8
Tentacles
Food
Mouth
Epidermis
Gastrodermis
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Gastrovascular
cavity
Fig. 41-9
Crop
Esophagus
Gizzard
Intestine
Pharynx
Anus
Mouth
Typhlosole
Lumen of intestine
(a) Earthworm
Foregut
Midgut
Hindgut
Esophagus
Rectum
Anus
Crop
Mouth
Gastric cecae
(b) Grasshopper
Stomach
Gizzard
Intestine
Mouth
Esophagus
Crop
Anus
(c) Bird
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Figure 50.10 The Human Digestive System
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Figure 50.11 Tissue Layers of the Vertebrate Gut
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Figure 50.9 Greater Intestinal Surface Area Means More Nutrient Absorption (Part 2)
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• The four stages of food processing
Small
molecules
Pieces
of food
Mechanical
digestion
Chemical digestion
(enzymatic hydrolysis)
Nutrient
molecules
enter body
cells
Undigested
material
Food
1 INGESTION
2 DIGESTION
3 ABSORPTION
Figure 41.12
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4
ELIMINATION
Digestion
• Hydrolytic enzymes break down protein,
carbohydrate, and fat macromolecules into their
simplest monomeric units.
• Digestive enzymes are classified according to the
substances they hydrolyze:
– Carbohydrases hydrolyze carbohydrates.
– Proteases hydrolyze proteins.
– Lipases hydrolyze fats.
– Nucleases hydrolyze nucleic acids.
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Protein digestion
Carbohydrate digestion
Oral cavity,
pharynx,
esophagus
Polysaccharides
(starch, glycogen)
Nucleic acid digestion
Fat digestion
Disaccharides
(sucrose, lactose)
Salivary amylase
Smaller polysaccharides,
maltose
Stomach
Proteins
Pepsin
Small polypeptides
Lumen of
small intestine
Polysaccharides
Pancreatic amylases
Maltose and other
disaccharides
Polypeptides
Pancreatic trypsin and
chymotrypsin (These proteases
cleave bonds adjacent to certain
amino acids.)
Smaller
polypeptides
DNA, RNA
Pancreatic
nucleases
Nucleotides
Pancreatic carboxypeptidase
Small peptides
Disaccharidases
Monosaccharides
Fat droplets (A coating of
bile salts prevents small droplets from coalescing into
larger globules, increasing
exposure to lipase.)
Glycerol, fatty
acids, glycerides
Nucleotidases
Dipeptidases, carboxypeptidase, and
aminopeptidase (These proteases split
off one amino acid at a time, working from
opposite ends of a polypeptide.)
Nucleosides
Amino acids
Nitrogenous bases,
sugars, phosphates
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Bile salts
Pancreatic lipase
Amino acids
Epithelium
of small
intestine
(brush
border)
Fat globules (Insoluble in
water, fats aggregate as
globules.)
Nucleosidases
and
phosphatases
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The Stomach
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Activating a Zymogen
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Figure 50.15 The Ducts of the Gallbladder and
Pancreas
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Structure and Function of the Vertebrate Gut
• The pancreas is a large gland that lies just beneath
the stomach and functions as both an endocrine and
exocrine gland.
• The exocrine tissues of the pancreas produce a
number of digestive enzymes, released as
zymogens.
• Trypsinogen is activated in the duodenum
• The pancreas also produces a secretion rich in
bicarbonate ions, which neutralize the pH of the
chyme from the stomach.
• This process is essential because intestinal enzymes
function best at a neutral or slightly alkaline pH.
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Figure 50.16 The Digestion and Absorption of Fats
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Control and Regulation of Fuel Metabolism
• Lipoproteins transport fats in the aqueous
circulatory system.
• They consist of a core of fat and cholesterol
covered by a protein that makes them watersoluble. The largest are the chylomicrons.
• Lipoproteins are classified according to their
density—the more fat, the lower its density.
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Control and Regulation of Fuel Metabolism
• High-density lipoproteins (HDL) are about 25%
cholesterol. They carry cholesterol to the liver where it
is used to make bile.
• Low-density lipoproteins (LDL) are 50–60%
cholesterol. They transport cholesterol to tissues
around the body for biosysthesis and storage.
• Very-low-density lipoproteins (VLDL) contain mostly
triglyceride fats, which are transported to fat cells.
• A high ratio of LDL to HDL is a risk factor for
atherosclerotic heart disease.
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Structure and Function of the Vertebrate Gut
• Bile does not get absorbed into the cells, but
shuttles back and forth between gut contents and
microvilli.
• Bile is synthesized from cholesterol.
• One major way that cholesterol leaves the body is
through unresorbed bile in the feces.
• Some kinds of fiber will bind with bile, decreasing
its absortion by the ileum and helping to lower
cholesterol levels.
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Structure and Function of the Vertebrate Gut
• Peristalsis pushes the contents of the small
intestine into the large intestine, or colon.
• The colon absorbs water and ions, producing
semisolid feces from indigestible material.
• Too much water absorption results in constipation
and too little water absorption results in diarrhea.
• Large populations of bacteria live in the colon,
including Escherichia coli, which synthesizes
vitamin K and biotin that are absorbed across
the wall of the colon.
• Prolonged intake of antibiotics can lead to vitamin
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Fig. 41-UN1
Bloodstream
Veins to heart
Lymphatic
system
Hepatic portal vein
Liver
Lipids
Stomach
Mouth
Esophagus
Secretions from
the gastric glands
of the stomach
Absorbed food Absorbed
(except lipids) water
Small intestine
Anus
Large Rectum
intestine
Secretions from the
pancreas and the liver
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Control and Regulation of Digestion
• The digestive tract in addition to being controlled
by the autonomic nervous system also has an
intrinsic nervous system that allows neural
messages to travel from one region of the
digestive tract to another without being processed
by the central nervous system.
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Figure 50.18 Hormones Control Digestion
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The Regulation of Food Intake
• The amount of food that an animal eats is
governed by sensations of hunger and satiety,
which are influenced by the hypothalamus.
• When the medial hypothalamus of rats is
damaged, they will increase their food intake and
become obese.
• If the lateral hypothalamus is damaged, they will
decrease their food intake and become skinny.
• Regulation of body weight involves feedback
information, and there is evidence that levels of
blood glucose and insulin, as well as signals from
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Several hormones regulate both long-term and short-term appetite
by affecting a “satiety center” in the brain
Secreted by the stomach
wall, ghrelin is one of the
signals that triggers feelings
of hunger as mealtimes
approach. In dieters who lose
weight, ghrelin levels increase,
which may be one reason
it’s so hard to stay on a diet.
Produced by adipose (fat)
tissue, leptin suppresses
appetite as its level increases.
When body fat decreases,
leptin levels fall, and appetite
increases.
Ghrelin
Insulin
The hormone PYY,
secreted by the small
intestine after meals,
acts as an appetite
suppressant that
counters the appetite
stimulant ghrelin.
Leptin
PYY
Figure 41.5
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A rise in blood sugar level
after a meal stimulates
the pancreas to secrete
insulin (see Figure 41.3).
In addition to its other
functions, insulin suppresses
appetite by acting on the brain.
• The complexity of weight control in humans
– Is evident from studies of the hormone leptin
• Mice that inherit a defect in the gene for leptin
– Become very obese
Figure 41.6
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Concept 41.4: Evolutionary adaptations of
vertebrate digestive systems correlate with diet
• Digestive systems of vertebrates are variations
on a common plan
• However, there are intriguing adaptations, often
related to diet
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Fig. 41-19
Small intestine
Stomach
Small
intestine
Cecum
Colon
(large
intestine)
Carnivore
Herbivore
Mutualistic Adaptations
• Many herbivores have fermentation chambers,
where symbiotic microorganisms digest
cellulose
• The most elaborate adaptations for an
herbivorous diet have evolved in the animals
called ruminants
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Fig. 41-20
1
Rumen
2
Reticulum
Intestine
Esophagus
4
Abomasum
3
Omasum
Fig. 41-UN2
You should now be able to:
1. Name the three nutritional needs that must be
met by an animal’s diet
2. Describe the four classes of essential
nutrients
3. Distinguish among undernourishment,
overnourishment, and malnourishment
4. Describe the four main stages of food
processing
5. Distinguish between a complete digestive
tract and a gastrovascular cavity
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6. Follow a meal through the mammalian
digestive system:
– List important enzymes and describe their
roles
– Compare where and how the major types of
macromolecules are digested and absorbed
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