The Digestive System
The Digestive System
The Digestive System
By: Natalie Jacquez
An animals requirements for survival
Food for fuel (chemical energy)
Organic raw materials used in biosynthesis
ATP’s importance in homeostasis
It powers resting metabolism and regulates temperature in
It comes from the oxidation of organic fuel molecules-
carbohydrates, proteins, and fats= in cellular respiration.
Fats are especially rich in ATP; 2x the amount of energy
compared to carbohydrates and proteins
If the animal isn’t growing or reproducing, the body tends to
store surplus energy in energy depots.
In humans, the liver and muscle cells store energy in the
form of glycogen (composed of many glucose units which
fuel cell’s metabolism and regulate hormone action).
If not enough calories are consumed, fuel is taken out of
storage depots and is oxidized.
Undernourishment- If the diet of a person is chronically
deficient in calories in which the stores of glycogen and fat
are used up and the body results in the breakdown of its own
proteins for fuel.
Consequences: brain can become protein-deficient &
muscles decrease in size. (This can result in death)
Overnourishment- Results from excessive food intake and
Complex feedback mechanisms regulate fat storage and use.
An increase in adipose tissue increase leptin levels in the
blood which cues the brain to depress appetite and to
increase energy- consuming muscular activity and body-heat
production result in weight gain.
Loss of body fat subsequently decreases leptin levels.
Essentials for Biosynthesis
To build the complex molecules it needs to grown and
maintain itself, an animal must obtain organic precursors
(carbon skeletons) from its food.
Essential Nutrients- materials that must be obtained in
preassembled form because the animal’s cells cannot make
them from any raw material. There are 4 tours of these
nutrients: Essential Amino Acids, Fatty Acids, Vitamins, and
Malnourished- An animal whose diet is missing one or more
Essential Amino Acids
E.A.A- Must be obtained from food in prefabricated form.
Eight amino acids are essential in the adult human diet.
A diet that provides insufficient amounts of one or more of
these amino acids, causing a form of malnutrition known as
protein deficiency. In children it can hurt their physical and
mental development (most common form of malnutrition)
Reliable sources: meat, eggs, cheese
Proteins in animal products are “complete”, which means
they provide all of the essential amino acids.
Essential Fatty Acids
E.F.A- Certain unsaturated fatty acids in which animals are
incapable of creating themselves. Ex) Linoliec acid
Vitamins- Organic molecules required in the diet in amounts that
are quiet small compared with the relatively large quantities of
essential amino acids and fatty acids animals need.
13 Vitamins are essential to humans.
They are grouped as: water soluble and fat soluble
Water Soluble: Includes the B complex and Vitamin C which
function as coenzymes in key metabolic processes. (Excess are
excreted in urine)
Fat Soluble: Vitamins A, D, E, K (Excess are deposited as fat)
Minerals- Simple inorganic nutrients required in small
Ex) Large quantities of calcium and phosphorus are needed
for the construction and maintenance of bones.
Ingesting an excess of minerals can upset homeostatic
balance and cause toxic side effects. Ex) Too much sodium
leads to high blood pressure/ excess iron results in liver
Types of Feeders
Herbivores- Eat mainly autotrophs (plants, algae)
Carnivores- Eat other animals
Omnivores- Consume animals & plant matter
Most animals are opportunistic, eating foods that are outside
their main dietary category when these foods are available.
Suspension Feeders- They swift small food particles from the water. Ex)
Substrate Feeders- They live in or on their food source, eating their way
through the food. Ex) maggots/caterpillar
Deposit Feeders- They eat their way though the dirt and salvage partially
decayed organic material consumed along with soil. Ex) earthworms
Fluid Feeders- They make their living by sucking nutrient rich fluids from
a living host. (considered parasites) Ex) mosquitos/leeches/aphids
Bulk Feeders- They eat relatively large pieces of food. Contain tentacles,
pincers, claws, poisonous fangs, and jaws and teeth to tear off the meat
easily. Ex) python
Food Processing: The Four Stages
1) Ingestion- The act of eating in which complex arrays of molecules,
including large polymers are consumed. Organic material in food consists
mainly of proteins, fats, starches, and other polysaccharides that can’t be
used due to their inability to pass through membranes/ their unidentical
2) Digestion- The process of breaking down food into their component
monomers which are small enough to absorb and can be used to make
molecules and fuel for ATP producion.
Polysaccharides break down to simple sugars, fats to glycerol, proteins to
amino acids, and nucleic acids to nucleotides.
Enzymatic hydrolysis- In which digestion breaks macromolecule’s bonds
with the enzymatic addition of water.
Food Processing: The Four Stages Contd.
Chemical Digestion is usually followed by mechanical
fragmentation of the food by chewing.
3) Absorption- After the food is digested, the animal’s cells
absorb small molecules such as amino acids and simple
sugars from the digestive compartment.
4) Elimination- Undigested material passes out of the
Food Vacuoles- organelles in which hydrolytic enzymes
break down food without digesting the cell’s own cytoplasm
are the simplest digestive compartments
Intracellular Digestion- Newly formed vacuoles fuse with
lysosomes which mix the food with enzymes allowing
digestion to occur safely within a compartment enclosed by a
Ex) Sponges/ heterotrophic protists solely rely on this process
to digest their food entirely
Extracellular Digestion- In which hydrolysis occurs and food is broken down
outside of cells. Having extracellular cavities for digestion therefore enables an
animal to devour larger prey than can be ingested by phagocytosis and
Gastrovascular Cavities- function in both digestion and the distribution of
nutrients throughout the body. The gastrodermis, tissue layer that lines the
cavity, secrete digestive enzymes that break the soft tissues of the prey into tiny
pieces. Ex) hydra, sponges, flatworms, cnidarians
Complete Digestive Tracts (Alimentary Canals)- Possessing digestive tubes
extending between two openings, the mouth and the anus. Ex) nematodes,
annelids, mollusks, echinoderms, chordates
Food ingested through the mouth and pharynx passed through the esophagus
that leads to a crop, gizzard, or stomach and next enters the intestine where
digestive enzymes hydrolyze the food molecules and absorbs nutrients across
the lining of the tube into the blood.
Mammalian Digestive System
Consists of: alimentary canal and glands that secrete
digestive juices into the canal
Peristalsis- rhythmic waves of contraction by smooth
muscles in the wall of the canal that push food along the tract
Sphincters- Ring-like valves that close off the tube to
regulate the passage of material between chambers of the
Accessory Glands- Salivary glands, pancreas, liver, and the
gallbladder (stores digestive juice)
Both physical and chemical digestion begins in the mouth
The presence of food in the oral cavity triggers a nervous reflex that causes
the salivary glands to deliver saliva through ducts to the oral cavity.
Saliva contains glycoprotein (carbodhyrdrate-protein complex) which
protects the soft lining of the mouth from damage, lubricating the food &
preventing tooth decay
Chemical digestion of carbohydrates begins in the oral cavity
Salivary Amylase- an enzyme that hydrolyzes starch and glycogen which
produces the disaccharide maltose and smaller polysaccharides
Bolus- Is the ball the tongue shapes the food into to push into the pharynx
Pharynx- a junction that opens to both the esophagus and
the windpipe (trachea).
When we swallow, the top of the windpipe moves up so that
its opening, the glottis, is blocked by a cartilaginous flap, the
It ensures that a bolus will be guided into the entrance of the
Esophagus- conducts food from the pharynx down to the
stomach by peristalsis. The muscles at the very top are
voluntary however, the contraction by smooth muscles are
Stomach- Is located in the upper abdominal cavity, just below the
diaphragm. It can stretch to store food and perform digestive
functions by secreting a digestive fluid called gastric juice which
mixes with the food by the churning action of the smooth muscles
in the stomach wall.
Gastric juice contains a pH of 2 to disrupt the extracellular matrix
that binds cells together in meat and plant material all while killing
bacteria swallowed with food.
It also contains pepsin, an enzyme that begins the hydrolysis of
proteins. It breaks peptide bonds adjacent to specific amino acids,
making proteins into smaller polypeptides. (works in acidic env.)
How it prevents self-digestion
Pepsinogen- Pepsin’s inactive form by specialized cells called chief
cells located in gastric pits. It’s activation in the lumen of the digestive
tract is an example of positive feedback.
Parietal cells- secrete hydrochloric acid which converts pepsinogen to
active pepsin by removing a small portion of the molecule and
exposing its active site
Epithelial cells- secrete a coating of mucus that help protect the
stomach lining. Mitosis generates enough cells to completely replace
the stomach lining every 3 days.
Acid chyme- As a a result of mixing and enzyme action what was once
a swallowed meal becomes nutrient-rich broth.
Pyloric Sphincter- At the opening from the stomach to the small
intestine that helps regulate the passage of chyme into the intestine.
Small intestine- longest section of the alimentary canal in which
most of the enzymatic hydrolysis of food macromolecules and
absorption of nutrients into the blood occurs.
Duodenum- the first 25cm of it where acid chyme mixes with
digestive juices from the pancreas, liver, gallbladder, and glad cells
of the intestinal wall itself
Pancreas- makes hydrolytic enzymes & an alkaline solution full of
bicarbonate that acts as a buffer to offset the chyme’s acidity
Liver- makes bile, a mixture of substances that is stored in the
gallbladder until needed. It contains bile salts which aid in the
digestion and absorption of fat.
The digestion of starch and glycogen begins by salivary
amylase in the oral cavity to the small intestine.
Maltose- completes the digestion of maltose, splitting it into
two molecules of the simple sugar glucose. (In the family:
disaccharides which cover the intestinal epithelium, also
known as the site of sugar absorption)
Digestion of proteins occurs in the small intestine where the process begun by
pepsin in the stomach is completed
Trypsin & Chymotrypsin- specific for peptide bonds adjacent to certain amino
acids, and like pepsin, break large polypeptides into shorter chains (secreted by
Dipeptidases- Attached to the intestinal lining, split small peptides
Carboxypeptidase- Splits off one amino acid at a time beginning at the end
that has a carboxyl group (secreted by pancreas)
Aminopeptidase- It is secreted by the intestinal epithelium and works in the
opp. direction as Carboxypeptidase. (team work between the two speed up the
Tetropeptidase- Directly/ Indirectly triggers activation of these enzymes
within the intestinal lumen
Nucleic Acid Digestion
Nucleases- enzymes that hydrolyze DNA and RNA into their
Other hydrolytic enzymes than break nucleotides down
further into nucleosides, nitrogenous bases, sugars, and
Fat molecules are insoluble in water.
Emulsification- Bile salts from the gallbladder secreted into
the duodenum coat fat droplets to prevent them from
Lipase- an enzyme that hydrolyzes the fat molecules
Peristalsis moves the mixture of chyme and digestive juices
along the small intestine
Jejunum & Ileum- Parts of the small intestine that function
mainly in the absorption of nutrients and water
Absorption of Nutrients
Most of the absorption of nutrients occurs in the small intestine
Villi- large fingerlike projections that contain appendages called microvilli, which are exposed to the
Penetrating the core of each villi are capillaries and lacteals- small vessel in the lymphatic system
Nutrients are absorbed across the intestinal epithelium and then across the unicellular epithelium of the
capillaries or lacteals. (these two layers separate nutrients in the lumen of the intestine from the
Active transport- allows the intestine to absorb high amounts of nutrients
Amino acids/sugars pass through the epithelium, enter the capillaries, and are carried away from the
intestine by the bloodstream. After the glycerol/fatty are absorbed by the epithelial cells, they are
recombined into fats and are mixed with cholesterol/ coated with proteins that form small globules called
chylomicrons. Most are transported out of the epithelial cells and into the lacteals which converge into
large vessels of the lymphatic system.
Hepatic Portal vessel- In which the capillaries and veins that drain the nutrients away from the villi all
converge and are lead directly to the liver. This ensures the livers access to amino acids and sugars.
Consequently, the liver regulates the level of glucose molecules in the blood and from there travels to the
Because digestion depends on peristalsis, enzyme secretion,
and active transport, there is a substantial energy cost to
It may require one to expend 3-30% of the chemical energy
available from the meal
Hormones help regulate digestion
Gastrin- Certain substances in food stimulate the stomach wall to secrete this
hormone. As it gradually recirculates in the bloodstream back to the stomach wall,
the hormone stimulates further secretion of gastric juice
If the pH of the stomach is too low, the acid will inhibit the release of gastrin,
decreasing the secretion of gastric juice. (ex: of negative feedback system)
Enterogastrones- Hormones secreted by the wall of the duodenum. This signals
the pancreas to release bicarbonate, which neutralizes the acid chyme.
Secretin- The acidic pH of the chyme that enters the duodenum stimulates cells in
the wall to release this hormone
Cholecystokinin (CKK)- A second enterogastrone which is secreted in response to
the presence of amino acids or fatty acids. This causes the gallbladder to contract
and release bile into the small intestine. It also triggers the release of pancreatic
Chyme- If rich in fats, causes the duodenum to release other enterogastrones that
inhibit peristalsis in the stomach.
Large Intestine (colon)- Is connected to the small intestine at a T-shaped Junction, where a sphincter
controls the movement of material
Cecum- One “arm” of the T is this pounch.
Appendix- A fingerlike extension of the cecum
The colon’s major function is to recover water in the lumen that has entered the alimentary canal as the
solvent of the various digestive juices
Together the small intestine and colon retain 90% of that water that entered
Feces- waste of the digestive tract that become more solid as they are moved along the colon by
peristalsis which contain masses of bacteria and undigested materials.
Escherichia coli- common inhabitant of the colon and the presence of it in lakes is an indication of
Intestinal bacteria live on unabsorbed organic material and produce gases as byproducts of their
metabolism. While some produce vitamins including biotin, folic acid, vitamin K, and vitamin B.
Rectum- terminal portion of the colon where feces are stored until they can be eliminated
Possessing large populations of symbiotic bacteria and
protists in special fermentaton chambers in their alimentary
canals helps herbivorous animas hydrolyze cellulose. They
can therefore digest it to simple sugars and other compounds
the animal can absorb. Most are held in the cecum.