Animals eat food for both the
energy and the organic
molecules used to assemble new
molecules.
An animals diet must supply chemical
energy, organic molecules, and
essential nutrients.
Cells, tissues, organs and whole animals
depend on chemical energy in the diet.
 This energy must be converted in ATP to
power life processes
 Animals ingest and digest nutrients, such as
carbohydrates, proteins, and lipids for use in
cellular respiration and energy storage.

Animal’s diet must allow provide raw
materials needed for biosynthesis of
complex molecules
There are four classes of essential nutrients:
 Essential amino acids; about half of the twenty
amino acids in proteins cannot be synthesized
and must be obtained from food.
 Essential fatty acids that animals cannot make,
example linoleic acid
 Vitamins, such as B and E vitamins
 Minerals, such as calcium and phosphorus

Ingestion is the act of taking in food
Suspension feeders sift small food
particles from water
 Substrate feeders are animals that live
in or on their food source.
 Fluid feeders suck nutrient rich fluids
from a living host.
 Bulk feeders eat relatively large pieces
of food and include most animals.

Digestion is the breakdown of
food into small molecules capable
of being absorbed by the cells of
the body.

Enzymatic hydrolysis, the breaking of
bonds with the addition of water, is the
reaction type by which macromolecules
are digested.
Mechanical digestion precedes
chemical digestion.
Breaks food into smaller pieces,
increasing the surface area for chemical
processes.
 Chewing is an example of mechanical
digestion.

Chemical digestion
Polysaccharides and dissacharides are
split into simple sugars.
 Proteins are broken down into amino
acids.
 Nucleic acids are broken down into
nucleotides
 Fats and phospholipids are broken down
into fatty acids and other components.

In the third stage of food
processing, absorption, the
animals cells take up (absorb)
small molecules, such as simple
sugars and amino acids.
Elimination completes the
process as undigested material
passes out of the digestive
system.
Digestive compartments prevent
animals from digesting their own
cells and tissues
Food is processed with specialized
compartments.
 Compartments may be intracellular,
such as food vacuoles.
 May be extracellular, as in digestive
organs or systems.

Intracellular Digestion
Cell engulfs solid food by phagocytosis
of liquid food by pinocytosis
 Newly formed food vacuoles fuse with
lysosomes.
 Fusion of food vacuoles and lysosomes
brings food together with hydrolytic
enzymes, allowing digestion to occur
safely with a compartment enclosed by
a protective membrane.

Sponges digest food entirely by
intracellular digestion
Extracellular digestion is the
breakdown of food in
compartments that are
continuous with the outside of
the animal’s body.

Enables an animal to devour much larger
sources of food that can be ingested by
phagocytosis.
Gastrovascular Cavities
Animals with a simple body plan have a
digestive compartment with a single
opening.
 Functions in digestion and distribution of
nutrients.

Cnidarians and many flatworms have
gastrovascular cavities in which food
and waste products of digestion enter
and exit through same opening.
Alimentary canals
Most animals have a digestive tube that
extends between two openings, a mouth
and an anus.
 Food moves along the alimentary canal
in one direction.
 Tube can be organized into specialized
compartments that carry out digestion
and absorption in steps.
 Can ingest food while digesting earlier
meals.

Earthworm: mouth sucks in food,
passed through esophagus, stored and
moistened in crop, crushed in gizzard,
then digestion and absorption in intestine.
Grasshopper:
Digestive tube divided into
foregut (esophagus and crop), midgut (gastric
caeca and intestine) and hindgut.
Gastric caecae are pouches where most
digestion and absorption takes place
Birds:
Three separate chambers, crop,
stomach, and gizzard. Crop and gizzard
moisten and crush food. Chemical
digestion and absorption takes place in
the intestine.
Mammalian Digestion
Organs specialized for successive
stages of food processing form the
mammalian digestive system.
 Movement of food through the digestive
system is controlled by peristalsis, the
rhythmic contraction of the smooth
muscle walls of the alimentary canal.
 Spincters, muscular ringlike valves
regulate the passage of material
between digestive compartments.

Digestion begins in the Mouth
Food in the mouth stimulates the
secretion of saliva.
 Saliva lubricates food for swallowing.
 Saliva contains enzyme amylase, which
hydrolyzes starch and smaller
polysaccharides into the disaccharide
maltose.
 Chemical digestion begins in mouth.
 Chewing is mechanical digestion.

During chewing food is shaped
into a ball called a Bolus
During swallowing, the bolus enters the
pharynx (the throat), a junction that
opens to the esophagus and the trachea
(windpipe).
 During swallowing a flap of cartilage –
the epiglottis - covers the opening to the
trachea and keeps food from going
down the airway.
 Esophagus moves food to the stomach.

The Stomach
The stomach’s functions include storing
food and secreting digestive fluid called
gastric juice.
 Gastric juice is composed of

 Hydrochloric Acid: pH of 2; begins
breakdown of meat and plant materials; also
kills most bacteria
 Pepsin: enzyme that attack peptide bonds
and breaks down proteins.
Gastric Glands contain mucous cells that secrete
mucus which lubricates and protects the cells
lining the stomach, chief cells which secrete
pepsinogen, and parietal cells that secrete HCL.
The result of digestion in the
stomach is a substance called
acid chyme.

Acid chyme is shunted from the stomach to
the small intestine through the pyloric
sphincter.
Small Intestine
The first section of the small intestine is the
duodenum.
 Duodenum is the major site of chemical
digestion.
 The intestine does not have mucus
protection. The pancreas releases a
bicarbonate fluid that buffers the acidic
contents from the stomach.
 In the duodenum, acid chyme mixes with
secretions from the pancreas and the liver.

Bile
Bile is made in the liver and stored in the
gall bladder.
 Bile emulsifies fat – bile coats fat
droplets, turning large fat droplets into
small fat droplets, which are easier to
digest.
 Bile enters the intestine at the
duodenum.

Chemical digestion in the
duodenum
Carbohydrates: pancreatic amylase
breaks down starch, glycogen, and small
polysaccharides into maltose. Maltose and
other disaccharides are broken down by
enzymes in the wall of the duodenum.
 Proteins: Trypsin and chymotrypsin break
polypeptides into smaller chains.
Dipeptidases, carboxypeptidase and
aminopeptidase break polypeptides into
amino acids.

Duodenal digestion continued
Nucleic acids: nuclei acids are
hydrolyzed into nucleotides by
nucleases that enter the duodenum from
the pancreas.
 Fats: digestion of fats starts in the small
intestine. Bile coats the fat droplets and
keeps them from clumping. Lipase,
produced in the pancreas hydrolyzes
small fat droplets.

Villi
Lining of the small intestine has folds called
villi, which are divided into microvilli.
Together they greatly increase the surface
area available for absorption.
 Each villus contains capillaries for
absorption of monomers and a lymph
vessel, called a lacteal which absorb fatty
acids.
 Passive, facilitated diffusion, and active
transport move monomers across the
intestinal membrane into blood vessels.

Hepatic portal vessel
Capillaries and veins that carry nutrients
away from the villi form the hepatic
portal vessel.
 Hepatic portal vessel carries nutrients to
the liver.
 Liver regulates the distribution of
nutrients to the body.

The remaining regions of the
small intestine, the jejunum and
the ileum function mainly in the
absorption of water and
nutrients.
Large Intestine

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
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Large intestine, also called the colon is
connected to the small intestine by a sphincter.
The point of this connection is the site of the
caecum, and the extension called the
appendix.
Functions of the large intestine are to compact
waste and recover water.
Harmless bacteria in the colon produce some
vitamins.
At the end of the colon is the rectum, where
feces is stored until elimination.
Evolutionary adaptations of
vertebrate digestive systems
correlate with diet.
Mammals have specialized teeth that allow
them to ingest their food.
 Herbivores have longer alimentary canals
than carnivores.
 Many herbivores harbor large populations
of symbiotic bacteria and protists than
enable the digestion of cellulose.
