THE BIOLOGY CHASERS IN:
THE INTERESTING CASE OF THE
DIGESTIVE SYSTEM: WHERE DOES THE
FOOD THAT WE EAT TRAVEL?
BY: NIHAL KALA, NATHAN O’ BRIEN,
AND DAN MERINO
WHAT DO YOU KNOW
ABOUT THE DIGESTIVE
SYSTEM?
THE DIGESTIVE SYSTEM IN GENERAL
All organisms need to survive. But where do the nutrients come from?
As you recall, it depends on whether an organism is an autotroph (makes food through
photosynthesis) or a heterotroph (can’t make own food and must acquire their energy from
outside sources).
When we talk about digestion, we are talking about the breakdown of large food molecules
into simpler compounds, which are then absorbed by the body to carry out cell activities.
So keep in mind of these 3 questions.
1.
What do organisms need from the outside world in order to survive?
2.
How do they get those things?
3.
What do they do with them once they get them?
THE DIGESTIVE SYSTEM IN GENERAL
CONT.
 The
digestion of large food molecules is vital.
 The
food we eat is made up of many compounds (solid, large
molecules) made by other organisms which are not all suitable for
human tissues and therefore these large molecules have to be
broken down and reassembled (e.g. amino acids can be
reassembled to make new proteins) so that our bodies can use
them and the smaller molecules would be absorbed across
membranes and into cells.
 The
food molecules have to be small enough to be absorbed by
the villi in the intestine through diffusion, facilitated diffusion or
active transport and so large food molecules need to be broken
down into smaller ones for absorption to occur.
DIGESTIVE TRACTS

Digestive tract: where food is digested into small nutrient
molecules that can cross plasma membranes

1. Digest Food

2. Break food down into small molecules that can cross plasma
membranes

3. Absorbs these nutrient molecules

4. Eliminates indigestible remains
MULTICELLULAR ORGANISM
DIGESTIVE SYSTEMS

In simple animals, food is digested through intracellular digestion (digestion occurs
within food vacuoles)

Ex: hydra encloses food it captures in food vacuole

Lysosomes containing digestive enzymes then fuse with the vacuole and break
down the food.

In more complex animals, they have evolved a digestive tract and digest food
through extracellular digestion (food is digested in a gastrovascular cavity)

Ex: grasshopper (food passes through specialized regions)
DIGESTIVE SYSTEM DIAGRAM

Alimentary Canal: Contains organs through which the food actually passes
(esophagus, stomach, small intestine, large intestine, etc.)

Accessory Organs: Organs that assist in digestion but no food passes through
them (liver, pancreas, gall bladder, salivary glands, etc.)
THE MOUTH

4 groups of molecules must be broken down by the digestive tract: starch, proteins,
fats, and nucleic acids

The mouth or oral cavity is the first step in the digestive process (ingestion and
chewing).

When food enters the mouth, mechanical and chemical digestion begins.

The chewing, softening and breaking up of food into smaller particles is mechanical
digestion.

The mouth contains saliva, which is secreted by the salivary glands and contains an
important enzyme called salivary amylase, which begins the chemical digestion of
starch into maltose (fully digested further down into intestines) and helps dissolve some
of the food.

Saliva acts as a lubricant to help food move along digestive pathway.
ALL THE WAY DOWN TO ESOPHAGUS

The food needs to avoid the tongue, which functions to move food around while we chew and helps to arrange it into
a ball that we swallow called a bolus. It moves through the pharynx and later…

The food travels to the esophagus, which connects the throat to the stomach.

The force created by the rhythmical contraction of smooth muscle of the esophagus is called peristalsis. These waves
of contraction push the food toward the stomach along the alimentary canal.

Contraction of circular muscle behind food constricts the gut to prevent food from being pushed back towards the
mouth.

Contraction of longitudinal muscle where the food is located moves it on along the gut. Contraction of both layers of
muscle mixes food with enzymes in the small intestine.

Esophageal Sphincter: Food enters into stomach where more digestion will occur. Usually closed in order to keep food
from returning back up the esophagus to the mouth.
THE GASSY STOMACH

The stomach is an important part of the digestive system that breaks food into
smaller pieces.

Layers of muscle and thick walls

Partially digests proteins and temporarily stores ingested food

Secretes gastric juices, which include digestive enzymes and HCL, which kills bacteria, pathogens, and other
harmful organisms preventing food poisoning and it also provides the optimum conditions for the enzyme
pepsin to work in (pH 1.5 - 2) that denatures proteins. It lowers the PH of the stomach.

Stomach releases hormone called gastrin, which regulates stomach secretions/stimulates stomach cells to
produce gastric juice.

Stomach secrets mucus, which protects stomach lining and wall from acidic juices and being digested along
with food.

In addition, the stomach secretes pepsin (works best in acidic environment), a protease, which starts the
digestion and hydrolysing of proteins into polypeptides and amino acids. These can then be absorbed by the
villi in the small intestine.

Pepsinogen activated into pepsin by HCl. If pepsin was active all the time, then it would digest things it’s not
supposed to digest.

Chemical digestion: proteins converted into peptones---casein of milk

Food is mechanically broken down by the churning action of the stomach. The partially digest food, now
called chyme (creamy paste), is ready to enter the small intestine.
23 FT OF THE SMALL INTESTINE

The small intestine is where usable food substances (e.g. nutrients) are absorbed into the bloodstream and
is the final stage for chemical digestion.

Most of digestion and absorption of food processes

Very long (23 ft in average man)

3 regions: duodenum, jejunum, and ileum

Chyme moves into first part of small intestine, duodenum, through the pyloric sphincter

The small intestine is lined with smooth muscle to allow for the mixing and moving of digested food
products (via segmentation and peristalsis)

Chemical digestion sugars converted into monosaccharides; peptones, into amino acids;
fats, into fatty acids and glycerol

It also contains small pits that secrete intestinal juices and even secrete fat-emulsifying substances.

Small intestine walls secrete enzymes that break down proteins (peptidases) and carbohydrates (maltase,
lactase, sucrase, which are carbohydrate digesting enzymes of small intestines), and lipase. It also
receives enzymes from pancreas.

The small intestine contain infoldings called villi (composed of large amount of microvilli), to increase
surface area and optimize the rate of absorption
THE PANCREAS

The pancreas is a structure that produces most of the hydrolytic enzymes that are active in the small intestine.

Releases hormones that control blood sugar levels by stimulating glycogen formation or breakdown

Converts sugar into monosaccharides, peptones into amino acids, fats into fatty acids, and neutralizes acid pH of stomach/ secretes
NaHCO3.

Secretes enzymes such as trypsin, chymotrypsin, pancreatic lipase, and pancreatic amylase. (Note: lipase is the major fat-digesting
enzyme in the body)

Trypsin and chymotrypsin break down/digest protein into dipeptides and endopeptidases of small intestine.

Pancreatic lipase breaks down lipids into fatty acids and glycerol.

Lipases’ substrates are triglycerides and lipids such as fats and oils.

Pancreatic amylase breaks down starch into disaccharides (carbohydrates into simple sugars).

Ribonuclease and deoxyribonyclease break down nucleic acids into nucleotides.

Enzymes are secreted into small intestine via pancreatic duct.

Sodium bicarbonate is a substance secreted by pancreas to neutralize acidity (chyme) to prevent damage of duodenum walls.

Insulin stimulates uptake of glucose by cells and beta cells secrete insulin and alpha cells secrete glucagon.
VROOM VROOM.. ENZYMES!

Enzymes are needed in the process of digestion as they are the biological catalysts
which break down the large food molecules into smaller ones so that these can
eventually be absorbed.

Enzymes allow digestive processes to occur at body temperature and at sufficient
speed to meet the organism's survival requirements

Digestion can occur naturally at body temperature, however this process takes a
very long time as it happens at such a slow rate. Enzymes are specific for a given
substrate and so can allow digestion of certain molecules to occur independently
of others.

For digestion to increase in these circumstances, body temperature would have to
increase as well. However this is not possible as it would interfere with other body
functions.

This is why enzymes are vital as they speed up this process by lowering the activation
energy required for the reaction to occur and they do so at body temperature.
ENZYME FACTS
Proteas
Lipase
e
Salivary
Pancreati
Enzyme
Pepsin
Amylase
c Lipase
Chief
Salivary cells in
Source
Pancreas
Glands stomach
lining
Triglyceri
Starch
Substrat
des such
(Carbohy Proteins
e
as fats
drates)
and oils
Small
Fatty
Product Maltose
polypepti Acids and
s
(sugars)
des
Glycerol
pH
pH 1.5 –
pH 7
Optimu
7 (neutra 2 (highly (slightly
m pH
l)
acidic) alkaline)
Amylase
OOOOH… BILE, LIVER, AND THE
GALLBLADDER

Let’s get it clear…. bile is not a digestive enzyme. It’s an emulsifier.

Bile contains phospholipids, cholesterol, and bile salts (major emulsifier of fat).

An emulsifier mechanically breaks up fats into smaller fat droplets.

This process makes fat globules more accessible to pancreatic lipase.

Bile enters the small intestine by the bile duct, which merges with the pancreatic duct.

Remember this: Bile is produced in the liver and stored in the gallbladder.

Liver emulsifies fats and secretes bile salts. The liver is where digested nutrients absorbed in the small
intestine are dumped into various veins that merge to form the hepatic portal vessel.

Liver then gets first crack at newly absorbed nutrients before they’re sent to the rest of the body.

Liver is also where glycogen typically occurs in large quantities as a storage product (glycogen to
glucose and glucose to glycogen)

Overall, liver performs removal and storage of excess nutrients, deamination (converting amino acids
to compounds that can be used in energy metabolism), production of urea, release of nutritive
materials, destruction of old red blood cells, detoxification of substances, and production of blood
proteins.
LARGE INTESTINE: IRONICALLY NOT AS
LONG AS THE SMALL ONE

The large intestine moves the material that has not been digested from the small
intestine and absorbs water.

Much shorter and thicker than small intestine

Reabsorbs water, dissolved minerals, and salts from indigestible food residues

Harbors innocuous bacteria that are quite useful, which break down undigested food
and in the process provide us with essential vitamins such as Vitamin K.

This produces solid feces (leftover undigested food) first moves out of large intestine
and stored in rectum, and later are then eliminated through the anus.
ABSORPTION AND ASSIMILATION:
WHAT’S THE DIFFERENCE?

Absorption occurs when the food enters the body as the food
molecules pass through a layer of cells and into the bodies tissues.
This occurs in the small intestine which has many villi that are
specialized for absorption.

Assimilation occurs when the food molecules becomes part of the
bodies tissue. Therefore, absorption is followed by assimilation.

In Shorter Terms

Absorption: The movement of a fluid or dissolved substances across
a membrane

Assimilation: The conversion of nutrients into fluid or solid parts of an
organism
MORE ON ABSORPTION/VILLI

Absorption of the soluble nutrients occurs in the small intestine through villi (tiny fingerlike projections),
which increase the surface area of small intestine for absorption.

Absorption of amino acids and glucose occurs through active transport, while the absorption of fatty
acids occurs through passive transport/diffusion.

Within each of the villi is a capillary that absorbs digested food and carries it into bloodstream.

Microvilli: Greatly increase the surface area of the villus, allowing for a greater rate of absorption
They have protein channels and pumps in their membranes to allow the rapid absorption of food by
facilitated diffusion and active transport.

Rich capillary networks: Help to maintain a concentration gradient for absorption by rapidly
transporting absorbed products away. Distance for diffusion of food molecules is small.

Single epithelial layer: Ensures minimal diffusion distance between the intestinal lumen and capillary
network; Food can be passed through easily and be absorbed quickly

Lacteals: lymph vessels that absorb lipids/fatty acids from the intestine into the lymphatic system
(which are later reabsorbed back into normal circulation)

Intestinal crypts: Located between villi and release juices that act as a carrier fluid for nutrients

Membrane proteins / mitochondria: High amounts to enable active transport into cells by providing
ATP (contents then passively diffuse into bloodstream)
DIGESTIVE SYSTEM HORMONES

Gastrin: stimulates stomach cells to produce gastric juice (enzyme in
stomach)

Pancreozymin: stimulates pancrease to secrete digestive enzymes
(enzyme in small intestine)

Enterokinin: stimulates pancrease to secrete digestive enzymes

Secretin: alkaline secretion released by small intestine, which
stimulates pancreas to produce bicarbonate and digestive enzymes
and the flow of pancreatic juice and bile (enzyme in small intestine)

Cholecystokinin: stimulates secretion of pancreatic enzymes and
release of bile (enzyme in small intestine)
VIDEO

https://www.youtube.com/watch?v=s06XzaKqELk