SLIDE 2: Digestion
Digestion is animal-mediated, if you leave food somewhere and it decomposes without being
mediated by an animal, it’s not considered digestion.
o Breakdown of food molecules into smaller chemicals
Hydrolytic digestion uses water to break chemical bonds by enzymes produced by the animal
Fermentative digestion involves microbes for example cows harbor microbial symbionts in
several regions of their gut which aid the digestion of their food.
Extracellular digestion occurs in an extracellular body cavity (e.g., gut lumen) – most animals
o Outside the cells
Intracellular digestion involves taking food particles into specialized cells via phagocytosis prior
to digestion – sponges, coelenterates , flatworms, some molluscs
Example of intracellular digestion includes the sponge, which uses choanocyte to catch the
food, then passes it to amoebocyte where digestion takes place. This is rare in animals.
SLIDE 3: Digestion
An example that uses intracellular and extracellular digestion is the flatworm. Flatworms carry
out some amount of extracellular digestion. They have muscular pharynx that protrude from the
body and excrete digestive enzymes to liquify the prey.
The cells you see here are the epithelial cells lining the gut. You’ll find that these flatworms that
secrete the proteases to liquify the prey, they don’t allow that protein to be completely digested
outside the body, they phagocytose some of that protein and it will go to a large vacuole. Then
the process of digestion takes place within the vacuoles containing the partially digested protein
by enzymes. As the proteins become increasingly broken down, the vesicles (dark circles)
become smaller in size.
The clear vacuoles you see are filled with fat, they’re not empty. As the protein is being
absorbed in the flatworms, it’s used at the gut epithelial cells to generate fat. Some of the
protein is converted to fat and that fat is loaded into the vesicles. The fat is then used by the gut
epithelial cells to power the phagocytosis. So you’ll find that immediately after feeding, the fat
vesicles decrease in size because the fat is taken out of the vesicles and used as a source of
energy to power phagocytosis.
SLIDE 4: Digestion: Hydrolytic Digestion- Proteins
Proteins are digested by enzymes called peptidases, they are also called proteases. These
enzymes break the chemical bonds between adjacent amino acids.
Endopeptidases only break internal peptide bonds; they can’t cleave terminal peptide bonds
(the two ends of the protein). They produce oligopeptides, which means short proteins.
Exopeptidases can only hydrolyze terminal bonds. Endopeptidases and exopeptidases work hand in hand. Every time endopeptidases breaks a
bond, it liberates two new terminal ends in a protein. Twice as many terminal bonds leave more
substrate for exopeptidase to use.
The complete breakdown of a protein into its individual amino acids doesn’t have to happen in a
gut. Sometimes, di- and tri-peptides can be absorbed by the animal through the gut, and there
are peptidases in the gut epithelial cells that can finish the job of breaking down that protein
into amino acids where its shipped off into the blood and go where it needs to go.
o Di- and tri-peptides can be absorbed via endocytosis and further digested via
intracellular peptidases found in the cytosol of gut epithelial cells
Most peptidases have a marked specificity for a particular amino acid residue, but can still
cleave other sites more slowly. This means for example, Trypsin which is a common peptidase
found in animal, it has a preference for a basic amino acid (ex. arginine or lysine) on the N-
terminal side of where it’s going to make the cut.
Chymotrypsin prefers there to be an aromatic amino acid like tyrosine.
Carboxypeptidase B prefers there to be a basic amino acid to be at the carboxyl end.
Many of these peptidases don’t just break any peptide bonds; they have preferences for what
kind of amino acids make up that bond. This doesn’t mean they can’t cleave other bonds but
they can’t do is as efficiently.
SLIDE 5 Digestion: Hydrolytic Digestion- Proteins
How do you make an enzyme not go around and cut all the proteins in the cell.
The peptidases are first formed in an inactive form called a zymogen. It becomes activated only
when it gets to the site where it needs to be active (secreted)
Example in vertebrates, pepsins are endopeptidases and largely present in the foregut
(stomach) and their job is to break down proteins that you eat.
They’re first secreted in an inactive zymogen form called pepsinogen. Once they’re secreted into
the stomach, the acidic nature leads to the activation of these enzymes. Pepsin is the active
Pepsinogen has 2 parts to it. The red part which makes most of it is the active pepsin part of the
molecule. The blue strain is the part of the inactive form. The blue part has to be removed from
the protein before this protein becomes fully active.
Once you get rid of the blue piece you’re left with pepsin. How do you do this?
o There are 3 amino acids, two of them are aspartic acid, one of them is lysine (located at
the region where it says D215, D32, K37).
o The two aspartic acids are present on the active pepsin. They’re important in breaking
peptide bonds. The two aspartic acids are the active site.
o In the inactive pepsinogen state, the blue protein is blocking the active site. What’s
allowing that blue ribbon to sit and block the active sit is the lysine.
o At neutral pH such as the cytosol of the cell, aspartic acid is negatively charged and
lysine is positively charged, so the lysine is electrostatically attracted to the two aspartic
acids and that holds the blue part of the protein in place and blocks the active site. o When you secrete this protein into the acidic environment, a proton jumps on the
carboxylic site and it loses the negative charge, and the blue part of the pepsinogen
moves out of the way of the active site.
o Once the active site is free, it cleaves the blue part off the molecule and floats away and
can carry out hydrolysis of proteins.
Many peptidases are actually activated when their active form physically cuts the inactive form.
Protein digestion continues in the intestines (midgut)
o Pancreas produces and releases several peptidases (e.g., chymotrypsin, trypsin,
carboxypeptidase A, etc.), usually in the form of zymogens that must be activated upon
Example chymotrypsin and trypsin are first secreted as zymogens; Trypsinogen for example is
activated when an active trypsin cuts off the trypsinogen’s inactive part and thereby makes it
In other words, an active enzyme can activate another one of its inactive form of enzyme.
SLIDE 6: Hydrolytic Digestion: Lipids
Lipids are not water-soluble, but the enzymes that break them down such as lipases are water –
So you have a water soluble protein trying to degrade something that isn’t water soluble.
The only place on a fat globule where these enzymes can break the fat down is along the
surface, it can’t go into the fat globule.
If you want to facilitate digestion of lipid, you need to increase surface area of the fat globule by
breaking down into many smaller fat globules. This creates space for lipases to carry out
digestion. This is done by using bile salts.
How is this done?
o Bile salts are derivatives of cholesterol.
o In the liver of most vertebrates, cholesterol is used to create bile salts. Bile salts are
amphipathic, meaning they have a polar region and the bulk of the molecule is the non-
o The bile salts are able to interact with the fat globule and the watery environment
surrounding the fat globule.
o The process of emulsification is the idea of taking the bile salt and use it to break the fat
globule down into smaller globules.
So you produce bile salt in the liver using cholesterol, these are then dumped in the intestines,
and work to take large fat globules that are digested and break them into smaller globules.
Bile salts can also be recycled through portal circulation. They can be reabsorbed from the gut
and returned to the liver.
Most of what’s absorbed from your gut first passes the liver because it helps with detoxification.
Along with that detoxification process, we can also take the bile salts, return them to liver, and
secrete them again At the same time, bile salts can be excreted. This is one way the body gets rid of excess
cholesterol. If you have too much cholesterol, the liver converts it to bile salt, and excretes the
bile salt. The excreted bile salt in your feces gives it the brown color.
SLIDE 7: Hydrolytic Digestion: Lipids
Animals store fats as triacylglycerides. Triacylglycerides can’t be absorbed across the intestinal
You have to break it down into its components which are readily absorbed. Example you can
break it down into free fatty acids and glycerol where all of the fatty acids are removed from the
You can also cut one of the fatty acids of producing a fatty acid, and a diacylglycerol which can
We can cut off two fatty acids which is done by pancreatic lipase which is the most common
lipase found in vertebrates. This leaves two free fatty acids and a monoacylglycerol.
As long as you cut at least one fatty acid off you can absorb the products.
SLIDE 8: Hydrolytic Digestion: Carbohydrates
Monosaccharides require no digestion: they can be absorbed directly
Disaccharides need to be hydrolyzed. Example hummingbird drinking nectar ingests sucrose
which contains a link between glucose and fructose. Sucrase breaks down sucrose. Only sucrase
can break down sucrose into its components
In milk, lactose is broken down by lactase. Lactase can’t break sucrose down and sucrase can’t
break lactose down.
Humans are the only adult mammals that have lactase expression. We drink milk as adults, but
no other mammal does that because no other mammal continues to express enzymes needed
to break down lactose.
Polysaccharides are hydrolyzed into disacchardies, which are subsequently hydrolyzed into
monosacchardies: amylase cannot hydrolyze terminal bonds but they’re broken down in a two
Amylase is capable of breaking down bonds in a polysaccharide but it can’t break the terminal
bonds of any polysaccharide. After amylase breaks a polysaccharide such as starch, glycogen or
cellulose which are made up of glucose, it liberates a disaccharide called maltose (2 glucose
linked together). Maltose is broken down by maltase.
SLIDE 9 Hydrolytic Digestion: Carbohydrates
Amylase can’t break branching bonds such as those found in starch and glycogen. It can break
linear bonds found in glucose.
If amylase breaks the bond which liberates an isomaltose, it needs to be broken down by
Glycogen is highly branched and starch is less branched. Cellulose is a glucose polysaccharide like starch and glycogen, but the majority of animals have
no capacity to break down cellulose even though it’s the most abundant molecule on Earth.
Why can’t we break down the bonds in glycogen and starch and not cellulose?
o The difference is the orientation of the bonds.
o The bond between glucose molecule in glycogen and starch is known as alpha glycosidic
bond. If you look at the CH2OH group in glycogen and starch, they’re all in the same
position, but in cellulose they are flipped between adjacent sugars which create the
beta glycosidic bonds.
o Amylase can break alpha glycosidc bonds but not beta glycosidic bonds so it can’t break
Most animals don’t produce cellulase, but they have a way around it. They harbor microbial
symbionts which have cellulose and break it down.
SLIDE 10 Hydrolytic Digestion: Carbohydrates
The ONLY exception of an animal that expresses its own cellulase is the silverfish. This is known
since the 1950’s.
They’re able to consume cellulose and break it down in the absence of any microbial symbionts.
SLIDE 11 Digestion: Fermentative Digestion