Digestion and Nutrition
Chapter 32
Obtaining Energy
 All organisms require energy to maintain their complex
structure.
 The ultimate source of energy is the sun.
 Green plants utilize energy in sunlight to make
glucose.
 Autotrophs (phototrophs)
 A few autotrophs are chemotrophs, gaining
energy from inorganic chemical reactions.
Animals are Heterotrophs!
 Animals are heterotrophs, depending on other
organisms for food.
 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.
 Saprophagous animals feed on decaying organic
matter.
Why We Eat
 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.
The Main Stages of Food
Processing
 Ingestion is the act of eating.
 Digestion is the process of breaking food
down into molecules small enough to absorb.
 Involves enzymatic hydrolysis of polymers
into their monomers.
The Main Stages of Food
Processing
 Absorption is the uptake of nutrients by body
cells.
 Elimination occurs as undigested material
passes out of the digestive system.
Intracellular Digestion
 In intracellular digestion,
food particles are engulfed
by endocytosis and
digested within food
vacuoles.
 Protozoa, sponges.
Extracellular Digestion
 Extracellular digestion is the breakdown of
food particles outside cells.
 Digestion occurs in the alimentary canal.
 Cells lining the lumen of the alimentary canal are
specialized for secreting enzymes or absorbing
nutrients.
Digestive Systems
 Animals with simple body
plans have a
gastrovascular cavity that
functions in both
digestion and distribution
of nutrients.
Digestive Systems
 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.
Digestive Systems
 The digestive tube can
be organized into
specialized regions that
carry out digestion and
nutrient absorption in a
stepwise fashion.
Mammalian Digestive System
 The mammalian digestive system consists of
the alimentary canal and various accessory
glands that secrete digestive juices through
ducts.
Digestive Enzymes
 Enzymes are essential in the breakdown of
food into small, absorbable units.
 Digestive enzymes are hydrolytic enzymes.
 Food molecules are split by hydrolysis.
 R-R + H2O digestive enzyme R-OH + H-R
Digestive Enzymes
 Proteins are broken down into individual amino acids.
 Complex carbohydrates are broken down into simple
sugars.
 Fats are reduced to glycerol, fatty acids, and
monoglycerides.
Motility in Alimentary Canal
 Food moves through the alimentary canal by cilia,
specialized musculature, or both.
 Gut musculature is present in coelomates.
Motility in Alimentary Canal
 The gut is lined
with opposing
layers of smooth
muscle: a
circular layer and
a longitudinal
layer.
Motility in Alimentary Canal
 Two types of gut movement:
 Segmentation involves
alternate constriction of
rings of smooth muscle that
move the contents around,
mixing with enzymes.
 Peristalsis involves waves
of contraction behind the
food mass that move it
through the gut.
Organization - Five Major Regions
 Reception
 Conduction & Storage
 Grinding & early
digestion
 Terminal digestion and
absorption
 Water absorption and
concentration of solids.
Receiving Region
 The receiving region consists of devices for
feeding and swallowing.




Mouthparts – mandibles, jaws, teeth, radula, bills.
Buccal cavity – mouth
Muscular pharynx – throat
Salivary glands – produce lubricating secretions
that may also contain toxic enzymes or salivary
enzymes to begin digestion.
 Amylase begins hydrolysis of starches.
Receiving Region
 The vertebrate tongue assists in food
manipulation and swallowing.
 Also used as a chemosensor.
Conduction and Storage
Region
 The esophagus transfers food to the digestive region.
 In many invertebrates (annelids, insects, octopods) the
esophagus is expanded into a crop used for storage.
 Birds also have a crop that serves to store and soften
food.
Region of Grinding & Early Digestion
 The stomach provides initial digestion as well as
storing and mixing food with gastric juice.
 For further grinding of food, terrestrial oligochaetes and
birds have a muscular gizzard that is assisted by
stones or grit swallowed with food.
The Stomach
 The lining of the
stomach is coated
with mucus, which
prevents the
gastric juice from
destroying the
cells.
 Pepsin is a
protease that
splits specific
peptide bonds.
Region of Terminal Digestion and
Absorption
 The small intestine is the longest section of
the alimentary canal.
 It is the major organ of digestion and
absorption.
Region of Terminal Digestion and
Absorption
 Increasing the surface
area of the intestine
increases the area
available for absorption.
 Longer intestine
 Villi – fingerlike
projections of
intestinal tissue in
birds and mammals
 Microvilli – tiny
processes on
intestinal cells.
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 Small Intestine
 The pancreas
produces:
 Proteases,
protein-digesting
enzymes.
 Lipases for
breaking up fat.
 Amylase for
hydrolyzing
starches.
 Nucleases which
degrade RNA &
DNA into
nucleotides.
The Small Intestine
 The liver secretes bile into the bile duct which
drains into the duodenum.
 Bile is stored in the gallbladder between
meals.
 Bile salts are important for digestion of fats.
The Small Intestine
 Enzymatic digestion is completed as peristalsis
moves the mixture of chyme and digestive
juices along the small intestine.
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.
Absorption of Nutrients
 The enormous
microvillar
surface is an
adaptation that
greatly
increases the
rate of nutrient
absorption.
Absorption of Nutrients
 The core of each villus
contains a network of
blood vessels and a
small vessel of the
lymphatic system
called a lacteal.
Absorption of Nutrients
 Amino acids and simple sugars pass through
the epithelium of the small intestine and enter
the bloodstream.
 Initial absorption occurs by facilitated
transport, later by active transport.
Absorption of Nutrients
 Fats are emulsified by bile salts.
 Micelles are tiny droplets consisting
of fatty acids and monoglycerides
complexed with bile salts.
 Micelles diffuse into epithelial cells.
 Resynthesized into triglycerides and
pass into the lacteals of the lymphatic
system.
Region of Water Absorption
 The large intestine,
or colon is
connected to the
small intestine.
Region of Water Absorption
 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.
 The terminal portion of the colon is the rectum
where feces are stored until they can be
eliminated.
Region of Water Absorption
 The colon houses various strains of the
bacterium Escherichia coli.
 Some produce various vitamins.
Regulation of Food Intake
 Hunger centers in the brain regulate food
intake.
 A drop in blood glucose level stimulates a
craving for food.
 Homeostatic mechanisms control the body’s
storage and metabolism of fat.
Regulation of Food Intake
 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.
Regulation of Digestion
 Hormones help coordinate the secretion of
digestive juices into the alimentary canal.
Glucose Regulation as an Example
of Homeostasis
 Animals store excess calories as glycogen in
the liver and muscle.
 Glycogen is made up of many glucose
subunits.
 Glucose is a major fuel for cells.
Glucose Regulation as an Example
of Homeostasis
 Blood glucose levels rise,
the pancreas produces
insulin.
 Insulin enhances
transport of glucose
into body cells and
stimulates storage of
glucose as glycogen.
 Results in lower blood
glucose levels.
Glucose Regulation as an Example
of Homeostasis
 Lower blood glucose
levels stimulates the
pancreas to secrete
glucagon.
 Glucagon promotes
breakdown of glycogen
in the liver back into
glucose which is
released into the blood.
Nutritional Requirements
 An animal must obtain organic carbon (from
glucose) and organic nitrogen (from amino
acids obtained during digestion of protein) in
order to build organic molecules such as
carbohydrates, lipids and proteins.
Nutritional Requirements
 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.
Nutritional Requirements
 Herbivorous animals may suffer mineral
deficiencies if they graze on plants in soil
lacking key minerals.
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
 Water-soluble
Minerals
 Minerals are simple inorganic nutrients that are
usually required in small amounts.
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.
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.
Essential Amino Acids
 A diet that provides insufficient amounts of one
or more essential amino acids causes a form of
malnutrition called protein deficiency.
 Malnutrition is much more common than
undernutrition in human populations.
Essential Amino Acids
 Most plant proteins
are incomplete in
amino acid makeup.
 Individuals who eat
only plant proteins
need to eat a variety
to ensure that they
get all the essential
amino acids.