Transcript
Chapter 41
Animal Nutrition
Overview: The Need to Feed
Every mealtime is a reminder that we are heterotrophs
Dependent on a regular supply of food
In general, animals fall into one of three dietary categories
Herbivores eat mainly autotrophs (plants and algae)
Carnivores eat other animals
Omnivores regularly consume animals as well as plants or algal matter
Regardless of what an animal eats, an adequate diet must satisfy three nutritional needs
Fuel for all cellular work
The organic raw materials for biosynthesis
Essential nutrients, substances such as vitamins that the animal cannot make for itself
Animals feed by four main mechanisms
Concept 41.1: Homeostatic mechanisms manage an animal’s energy budget
Nearly all of an animal’s ATP generation
Is based on the oxidation of energy-rich molecules: carbohydrates, proteins, and fats
Glucose Regulation as an Example of Homeostasis
Animals store excess calories
As glycogen in the liver and muscles and as fat
Glucose is a major fuel for cells
Its metabolism, regulated by hormone action, is an important example of homeostasis
When fewer calories are taken in than are expended
Fuel is taken out of storage and oxidized
Caloric Imbalance
Undernourishment
Occurs in animals when their diets are chronically deficient in calories
Can have detrimental effects on an animal
Overnourishment
Results from excessive food intake
Leads to the storage of excess calories as fat
Obesity as a Human Health Problem
The World Health Organization
Now recognizes obesity as a major global health problem
Obesity contributes to a number of health problems, including
Diabetes, cardiovascular disease, and colon and breast cancer
Researchers have discovered
Several of the mechanisms that help regulate body weight
Over the long term, homeostatic mechanisms
Are feedback circuits that control the body’s storage and metabolism of fat
Several chemical signals called hormones
Regulate both long-term and short-term appetite by affecting a “satiety center” in 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
Obesity and Evolution
The problem of maintaining weight partly stems from our evolutionary past
When fat hoarding was a means of survival
A species of birds called petrels
Become obese as chicks due to the need to consume more calories than they burn
Concept 41.2: An animal’s diet must supply carbon skeletons and essential nutrients
To build the complex molecules it needs to grow, maintain itself, and reproduce
An animal must obtain organic precursors (carbon skeletons) from its food
Besides fuel and carbon skeletons
An animal’s diet must also supply essential nutrients in preassembled form
An animal that is malnourished
Is missing one or more essential nutrients in its diet
Herbivorous animals
May suffer mineral deficiencies if they graze on plants in soil lacking key minerals
Malnutrition
Is much more common than undernutrition in human populations
Essential Amino Acids
Animals require 20 amino acids
And can synthesize about half of them from the other molecules they obtain from their diet
The remaining amino acids, the essential amino acids
Must be obtained from food in preassembled form
A diet that provides insufficient amounts of one or more essential amino acids
Causes a form of malnutrition called protein deficiency
Most plant proteins are incomplete in amino acid makeup
So individuals who must eat only plant proteins need to eat a variety to ensure that they get all the essential amino acids
Some animals have adaptations
That help them through periods when their bodies demand extraordinary amounts of protein
Essential Fatty Acids
Animals can synthesize most of the fatty acids they need
The essential fatty acids
Are certain unsaturated fatty acids
Deficiencies in fatty acids are rare
Vitamins
Vitamins are organic molecules
Required in the diet in small amounts
To date, 13 vitamins essential to humans
Have been identified
Vitamins are grouped into two categories
Fat-soluble and
water-soluble
Minerals
Minerals are simple inorganic nutrients
Usually required in small amounts
Mineral requirements of humans
Concept 41.3: The main stages of food processing are ingestion, digestion, absorption, and elimination
Ingestion, the act of eating
Is the first stage of food processing
Digestion, the second stage of food processing
Is the process of breaking food down into molecules small enough to absorb
Involves enzymatic hydrolysis of polymers into their monomers
Absorption, the third stage of food processing
Is the uptake of nutrients by body cells
Elimination, the fourth stage of food processing
Occurs as undigested material passes out of the digestive compartment
The four stages of food processing
Digestive Compartments
Most animals process food
In specialized compartments
Intracellular Digestion
In intracellular digestion
Food particles are engulfed by endocytosis and digested within food vacuoles
Extracellular Digestion
Extracellular digestion
Is the breakdown of food particles outside cells
Animals with simple body plans
Have a gastrovascular cavity that functions in both digestion and distribution of nutrients
Animals with a more complex body plan
Have a digestive tube with two openings, a mouth and an anus
This digestive tube
Is called a complete digestive tract or an alimentary canal
The digestive tube can be organized into specialized regions
That carry out digestion and nutrient absorption in a stepwise fashion
Concept 41.4: Each organ of the mammalian digestive system has specialized food-processing functions
The mammalian digestive system consists of the alimentary canal
And various accessory glands that secrete digestive juices through ducts
Food is pushed along the digestive tract by peristalsis
Rhythmic waves of contraction of smooth muscles in the wall of the canal
The Oral Cavity, Pharynx, and Esophagus
In the oral cavity, food is lubricated and digestion begins
And teeth chew food into smaller particles that are exposed to salivary amylase, initiating the breakdown of glucose polymers
The region we call our throat is the pharynx
A junction that opens to both the esophagus and the windpipe (trachea)
The esophagus
Conducts food from the pharynx down to the stomach by peristalsis
From mouth to stomach
The Stomach
The stomach stores food
And secretes gastric juice, which converts a meal to acid chyme
Gastric juice
Is made up of hydrochloric acid and the enzyme pepsin
The lining of the stomach
Is coated with mucus, which prevents the gastric juice from destroying the cells
Gastric ulcers, lesions in the lining
Are caused mainly by the bacterium Helicobacter pylori
The Small Intestine
The small intestine
Is the longest section of the alimentary canal
Is the major organ of digestion and absorption
Enzymatic Action in the Small Intestine
The first portion of the small intestine is the duodenum
Where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and intestine itself
The pancreas produces proteases, protein-digesting enzymes
That are activated once they enter the duodenum
Enzymatic digestion is completed
As peristalsis moves the mixture of chyme and digestive juices along the small intestine
Hormones help coordinate the secretion of digestive juices into the alimentary canal
Absorption of Nutrients
The small intestine has a huge surface area
Due to the presence of villi and microvilli that are exposed to the intestinal lumen
The enormous microvillar surface
Is an adaptation that greatly increases the rate of nutrient absorption
The core of each villus
Contains a network of blood vessels and a small vessel of the lymphatic system called a lacteal
Amino acids and sugars
Pass through the epithelium of the small intestine and enter the bloodstream
After glycerol and fatty acids are absorbed by epithelial cells
They are recombined into fats within these cells
These fats are then mixed with cholesterol and coated with proteins
Forming small molecules called chylomicrons, which are transported into lacteals
The Large Intestine
The large intestine, or colon
Is connected to the small intestine
A major function of the colon
Is to recover water that has entered the alimentary canal
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 colon houses various strains of the bacterium Escherichia coli
Some of which produce various vitamins
Concept 41.5: Evolutionary adaptations of vertebrate digestive systems are often associated with diet
Some Dental Adaptations
Dentition, an animal’s assortment of teeth
Is one example of structural variation reflecting diet
Mammals have specialized dentition
That best enables them to ingest their usual diet
Stomach and Intestinal Adaptations
Herbivores generally have longer alimentary canals than carnivores
Reflecting the longer time needed to digest vegetation
Symbiotic Adaptations
Many herbivorous animals have fermentation chambers
Where symbiotic microorganisms digest cellulose
The most elaborate adaptations for an herbivorous diet
Have evolved in the animals called ruminants
Click to edit Master subtitle style
Copyright © 2005 Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Click to edit Master title style
Click to edit Master subtitle style
Copyright © 2005 Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Click to edit Master title style
Figure 41.1
Figure 41.2
Baleen
SUSPENSION FEEDERS
Feces
SUBSTRATE FEEDERS
BULK FEEDERS
FLUID FEEDERS
Caterpillar
Figure 41.3
1
When blood glucose
level rises, a gland called
the pancreas secretes insulin,
a hormone, into the blood.
Insulin enhances the
transport of glucose into body
cells and stimulates the liver
and muscle cells to store
glucose as glycogen. As a
result, blood glucose level
drops.
2
STIMULUS:
Blood glucose
level rises
after eating.
Homeostasis:
90 mg glucose/
100 mL blood
STIMULUS:
Blood glucose
level drops
below set point.
Glucagon promotes
the breakdown of
glycogen in the
liver and the
release of glucose
into the blood,
increasing blood
glucose level.
4
When blood glucose
level drops, the pancreas
secretes the hormone
glucagon, which opposes
the effect of insulin.
3
Figure 41.4
100 µm
Figure 41.5
Produced by adipose (fat)
tissue, leptin suppresses
appetite as its level increases.
When body fat decreases,
leptin levels fall, and appetite
increases.
Leptin
PYY
Insulin
Ghrelin
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.
The hormone PYY,
secreted by the small
intestine after meals,
acts as an appetite
suppressant that
counters the appetite
stimulant ghrelin.
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.
Figure 41.6
Figure 41.7
Figure 41.8
Figure 41.9
Corn (maize)
and other grains
Beans
and other
legumes
Essential amino acids for adults
Methionine
Valine
Threonine
Phenylalanine
Leucine
Isoleucine
Lysine
Tryptophan
Figure 41.10
Figure 41.11
Table 41.1
Table 41.2
Figure 41.12
Pieces
of food
Small
molecules
Mechanical
digestion
Food
Chemical digestion
(enzymatic hydrolysis)
Nutrient
molecules
enter body
cells
Undigested
material
INGESTION
1
DIGESTION
2
ELIMINATION
4
ABSORPTION
3
Figure 41.13
Gastrovascular
cavity
Food
Epidermis
Mesenchyme
Gastrodermis
Mouth
Tentacles
Mesenchyme
Food vacuoles
Gland cells
Flagella
Nutritive
muscular
cells
Esophagus
Mouth
Pharynx
Crop
Gizzard
Intestine
Anus
Typhlosole
Lumen of intestine
Esophagus
Anus
Rectum
Mouth
Crop
Gastric ceca
Anus
Intestine
Gizzard
Crop
Stomach
Mouth
Esophagus
Foregut
Midgut
Hindgut
Earthworm. The digestive tract of
an earthworm includes a muscular
pharynx that sucks food in through the
mouth. Food passes through the
esophagus and is stored and moistened
in the crop. The muscular gizzard, which
contains small bits of sand and gravel,
pulverizes the food. Digestion and
absorption occur in the intestine,
which has a dorsal fold, the typhlosole,
that increases the surface area for
nutrient absorption.
(b) Grasshopper. A grasshopper has several
digestive chambers grouped into three
main regions: a foregut, with an esophagus
and crop; a midgut; and a hindgut. Food is
moistened and stored in the crop, but most
digestion occurs in the midgut. Gastric ceca,
pouches extending from the midgut,
absorb nutrients.
(c) Bird. Many birds have three separate chambers—
the crop, stomach, and gizzard—where food is
pulverized and churned before passing into the
intestine. A bird’s crop and gizzard function very
much like those of an earthworm. In most birds,
chemical digestion and absorption of nutrients
occur in the intestine.
Figure 41.14a–c
IIeum
of small
intestine
Duodenum of
small intestine
Appendix
Cecum
Ascending
portion of
large intestine
Anus
Small intestine
Large intestine
Rectum
Liver
Gall-
bladder
Tongue
Oral cavity
Pharynx
Esophagus
Stomach
Pyloric
sphincter
Cardiac
orifice
Mouth
Esophagus
Salivary
glands
Stomach
Liver
Pancreas
Gall-
bladder
Large
intestines
Small
intestines
Rectum
Anus
Parotid gland
Sublingual gland
Submandibular gland
Salivary
glands
A schematic diagram of
the human digestive system
Pancreas
Figure 41.15
Esophagus
Epiglottis
down
Tongue
Pharynx
Glottis
Larynx
Trachea
Bolus of food
Epiglottis
up
To lungs
To stomach
Esophageal
sphincter
contracted
Glottis up
and closed
Esophageal
sphincter
relaxed
Glottis
down
and open
Esophageal
sphincter
contracted
Epiglottis
up
Relaxed
muscles
Contracted
muscles
Relaxed
muscles
Stomach
Figure 41.16
1
When a person is not
swallowing, the esophageal
sphincter muscle is contracted,
the epiglottis is up, and the
glottis is open, allowing air
to flow through the trachea
to the lungs.
The swallowing
reflex is triggered
when a bolus of
food reaches the
pharynx.
2
The larynx, the
upper part of the
respiratory tract,
moves upward and
tips the epiglottis
over the glottis,
preventing food
from entering the
trachea.
3
The esophageal
sphincter relaxes,
allowing the
bolus to enter the
esophagus.
4
After the food
has entered the
esophagus, the
larynx moves
downward and
opens the
breathing
passage.
5
Waves of muscular
contraction
(peristalsis)
move the bolus
down the esophagus
to the stomach.
6
Figure 41.17
Pepsin (active enzyme)
HCl
Parietal cell
Chief cell
Stomach
Folds of
epithelial
tissue
Esophagus
Pyloric
sphincter
Epithelium
Pepsinogen
3
2
1
Interior surface of stomach.
The interior surface of the
stomach wall is highly folded
and dotted with pits leading
into tubular gastric glands.
Gastric gland. The gastric
glands have three types of cells
that secrete different components
of the gastric juice: mucus cells,
chief cells, and parietal cells.
Mucus cells secrete mucus,
which lubricates and protects
the cells lining the stomach.
Chief cells secrete pepsino-
gen, an inactive form of the
digestive enzyme pepsin.
Parietal cells secrete
hydrochloric acid (HCl).
1
Pepsinogen and HCI
are secreted into the
lumen of the stomach.
2
HCl converts
pepsinogen to pepsin.
3
Pepsin then activates
more pepsinogen,
starting a chain
reaction. Pepsin
begins the chemical
digestion of proteins.
5 µm
Small
intestine
Cardiac orifice
Figure 41.18
1 µm
Bacteria
Mucus
layer of
stomach
Figure 41.19
Liver
Bile
Acid chyme
Stomach
Pancreatic juice
Pancreas
Intestinal
juice
Duodenum of
small intestine
Gall-
bladder
Pancreas
Membrane-bound
enteropeptidase
Trypsin
Active
proteases
Lumen of duodenum
Inactive
trypsinogen
Other inactive
proteases
Figure 41.20
Figure 41.21
Oral cavity,
pharynx,
esophagus
Carbohydrate digestion
Polysaccharides
(starch, glycogen)
Disaccharides
(sucrose, lactose)
Salivary amylase
Smaller polysaccharides,
maltose
Stomach
Protein digestion
Nucleic acid digestion
Fat digestion
Proteins
Pepsin
Small polypeptides
Lumen of
small intes-
tine
Polysaccharides
Pancreatic amylases
Maltose and other
disaccharides
Epithelium
of small
intestine
(brush
border)
Disaccharidases
Monosaccharides
Polypeptides
Pancreatic trypsin and
chymotrypsin (These proteases
cleave bonds adjacent to certain
amino acids.)
Smaller
polypeptides
Pancreatic carboxypeptidase
Amino acids
Small peptides
Dipeptidases, carboxypeptidase, and aminopeptidase (These proteases split
off one amino acid at a time, working from opposite ends of a polypeptide.)
Amino acids
DNA, RNA
Pancreatic
nucleases
Nucleotides
Nucleotidases
Nucleosides
Nucleosidases
and
phosphatases
Nitrogenous bases,
sugars, phosphates
Fat globules (Insoluble in
water, fats aggregate as
globules.)
Bile salts
Fat droplets (A coating of
bile salts prevents small drop-
lets from coalescing into
larger globules, increasing
exposure to lipase.)
Pancreatic lipase
Glycerol, fatty
acids, glycerides
Figure 41.22
Amino acids or fatty acids in the duodenum trigger the release of cholecystokinin (CCK), which stimulates the release of digestive enzymes from the pancreas and bile from the gallbladder.
Liver
Gall-
bladder
CCK
Entero-
gastrone
Gastrin
Stomach
Pancreas
Secretin
CCK
Duodenum
Key
Stimulation
Inhibition
Enterogastrone secreted by the duodenum inhibits peristalsis and acid secretion by the stomach, thereby slowing digestion when acid chyme rich in fats enters the duodenum.
Secreted by the duodenum,
secretin stimulates the pancreas
to release sodium bicarbonate,
which neutralizes acid chyme
from the stomach.
Gastrin from the stomach
recirculates via the bloodstream
back to the stomach, where it
stimulates the production
of gastric juices.
Epithelial
cells
Key
Nutrient
absorption
Vein carrying blood to
hepatic portal vessel
Villi
Large
circular
folds
Intestinal wall
Villi
Epithelial cells
Lymph
vessel
Blood
capillaries
Lacteal
Microvilli
(brush border)
Muscle layers
Figure 41.23
Figure 41.24
Large fat globules are
emulsified by bile salts
in the duodenum.
1
Digestion of fat by the pancreatic
enzyme lipase yields free fatty
acids and monoglycerides, which
then form micelles.
2
Fatty acids and mono-
glycerides leave micelles
and enter epithelial cells
by diffusion.
3
Fat globule
Lacteal
Epithelial
cells of
small
intestine
Micelles made
up of fatty acids,
monoglycerides,
and bile salts
Fat droplets
coated with
bile salts
Bile salts
Chylomicrons containing fatty
substances are transported out
of the epithelial cells and into
lacteals, where they are carried
away from the intestine by lymph.
4
Figure 41.25
Figure 41.26a–c
(a) Carnivore
(b) Herbivore
(c) Omnivore
Incisors
Canines
Premolars
Molars
Figure 41.27
Carnivore
Herbivore
Colon
(large
intestine)
Cecum
Stomach
Small
intestine
Small intestine
Figure 41.28
Reticulum. Some boluses
also enter the reticulum. In
both the rumen and the
reticulum, symbiotic prokaryotes
and protists (mainly ciliates) go
to work on the cellulose-rich
meal. As by-products of their
metabolism, the microorganisms
secrete fatty acids. The cow
periodically regurgitates and
rechews the cud (red arrows),
which further breaks down the
fibers, making them more
accessible to further microbial action.
Rumen. When the cow first chews and
swallows a mouthful of grass, boluses
(green arrows) enter the rumen.
1
Intestine
2
Omasum. The cow then reswallows
the cud (blue arrows), which moves to
the omasum, where water is removed.
3
Abomasum. The cud, containing great numbers of microorganisms,
finally passes to the abomasum for digestion by the cow‘s own
enzymes (black arrows).
4
Esophagus