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Site: Westsyde Secondary
Course: Biology 12 Filek
Book: Digestion
Printed by: Guest user
Date: Monday, 20 November 2017, 7:35 AM


Background Information

Digestion: Includes both the physical and chemical processes that reduce food to small soluble molecules that can be absorbed.

Absorption: The entrance of these molecules across the gut lining (especially across microvilli which are the tiny finger-like projections of the small intestine).

- Only certain substances can be absorbed
a. Amino acids of proteins
b. Sugars of carbohydrates
c. Glycerol and fatty acids of fats
d. Some minerals and water

Elimination (defecation): All components except those mentioned above exit the body.

Chemical Digestion: Chemical breakdown of food

- enzymes are used

Physical Digestion: - Physical breakdown of good

- Increases surface area so enzymes can work on them

For review ....

The process of digestion begins as soon as food—let’s say, a muffin—enters the mouth. Chewing breaks down the food into pieces small enough to swallow, and the mouth begins to water.


Structures of The Digestive System

  1. Oral Cavity (mouth) - Storage place for food while itmouthis being chewed. Place where saliva is mixed with food.
  2. Lips - Hold food in oral cavity and help direct food onto teeth.
  3. Teeth - Physical Digestion (breaks food into smaller pieces to create more surface area for enzymes to work on)
  4. Salivary Glands - Produce saliva for moistening and chemically digesting food. Saliva contain the enzyme salivary amylase which begins the chemical digestion of starch to maltose.
  5. Tongue - Directs food onto teeth and pushes chewed food to the pharynx.
  6. Pharynx - Back of the throat where oral and nasal cavities join. This is where swallowing occurs.
  7. Epiglottis - Flap of tissue, which closes off the trachea keeping food from entering the air passage.
  8. Esophagus - Tube leading from the pharynx to the stomach. Food is transported through this tube from the mouth to the stomach by peristalsis.

Swallowing - A reflex action - Usually performedperistalsisautomatically. Food enters the esophagus because the air passages are blocked. The opening to the nose is covered by the soft palate. The glottis (opening to the larynx and windpipe) is covered by the epiglottis. No breathing occurs. If it does, choking may occur. 

Peristalsis - Rhythmical contraction of the esophageal wall, pushing food along. Smooth muscle cause the contraction. Peristalsis also occurs all along the intestinal tract.

Swallowing Animation

Structures of Digestive System con't


  1. Cardiac Sphincter (esophageal sphincter) - Band of muscle which closes off the top part of the stomach to prevent food from being regurgitate into the esophagous.
  2. Stomach - J-shaped organ which stores and churns food. The churning helps physically digest food, which creates more surface area and results in a mushy liquid called acid chyme. The chemical digestion of proteins beings here.

    (Especially after eating a protein-rich meal)

    Gastrin - Is a hormone produced by the lower part of the stomach which enters the blood stream and later stimulate the upper part of the stomach to stimulate gastric glands to produce Pepsinogen and HCl. HCl and pepsinogen react with each other to produce Pepsin.

    Pepsin chemically digests proteins to peptides.

    HCL can burn the gut lining so a mucous layer is produced to prevent this from happening. If a portion of the gut is burned it is called an ulcer.


Structures Continued

  1. Pyloric Sphincter - Band of muscle which closes off the lower part of the stomach and only allows small amounts (~1 teaspoon) of chyme to enter the small intestine.
  2. Duodenum - First 10 inches of the small intestine. Enzymes from both the pancreas and the small intestine digest all 3 food groups here. (chemical digestions) Bile, which is made in the liver and stored in the gall bladder, is also secreted into the small intestine to emulsifies fat.
  3. Pancreas - Produces enzymes and sodium bicarbonate (the sodium bicarbonate is required to neutralizes acid chyme (pH of about 2) and bring it to a pH of about 8.5 so that the enzymes in the small intestine are not denatured.
  4. Gall Bladder - Stores Bile which is secreted into the small intestine from here.
  5. Liver - Produces bile to emulsify fats which is then stored in the gall bladder.liver



6. Small Intestine - Final digestion and absorption of nutrients through the villi.


The duodenal wall produces hormones, the most important of which are secretin and CCK (Cholocytokinin) in response to the presence of acidic chyme. Secretin stimulates release of Pancreatic Juices from the pancreas. CCK stimulates the release of Bile from the gall bladder.

Acid, especially HCl, stimulates the release of secretin, while partially digested protein and fat stimulate the release of CCK.
The Small Intestine is about 7m long - first 25 cm (10¨) is the duodenum

a) Duodenum: Sodium bicarbonate is secreted from the pancreas which turns acid chyme entering the small intestine basic (from a pH of about 2 to a pH of about 8.5)

Other Pancreatic Juices:
i) Pancreatic amylase breaks down starch to maltose.
ii) Trypsin breaks down protein to peptides.
iii) Lypase breaks down fat droplets, after bile has emulsified the fat, to fatty acids and glycerol.

b) Remainder of Small Intestine is responsible for the final digestion of protein and carbohydrates from intestinal juices made from millions of digestive glands along the intestinal wall.

i) Peptidase breaks down peptides to amino acids
ii) Maltase breaks down maltose to 2 glucose molecules.
Main function of the small intestine though, is the absorption of nutrient molecules. (A.A., glucose, fatty acids, and glycerol)
Other dissacharides (similar to maltose) are digested in the small intestine by their own enzymes. (e.g., lactase aids in lactose digestion, which is sugar in milk)

Absorption in the Small Intestine

The small intestine is specialized for absorption in that:
-- The small intestine is long with convoluted wall (folded walls)
-- The walls of the small intestine have villi (finger-like projections along the walls). The villi themselves have tiny microvilli on columnar epithelial cells.

Within each villus are blood vessels and small lymph vessel called a lacteal which absorbs fluids and returns it to the veins later on.

Absorption occurs across the walls of each villus by active transport (uses energy). Glucose and Amino acids enter the blood vessels and travel to the liver. Glycerol and fatty acids enter the lacteals, which will go back into the bloodstream later at the subclavian veins. 

sm intest
**Be sure you understand how the structure of the small intestine facilitates its function**Appendix - Found at junction of small intestine 
  1. digestive systemand large intestine.
    --unknown function in humans
  2. Large Intestine (colon) absorption of water from undigested food.
    -- The large intestine usually contains a large population of E. Coli bacteria. These organisms live off any substances that were not digested earlier. When they break down these substances, they give off odorous molecules that cause the characteristic odor of feces.

    Some vitamins, Amino Acids, and other growth substances required by the body ( Growth factors), are produced by these bacteria. The growth factors spill out into this gut and are absorbed by the gut lining. E. Coli helps us metabolize what our bodies were unable to thereby providing us with a vital service.

  3. Rectum - Enlarged portion of the colon. Undigested food is concentrated and stored here temporarily.

  4. Anal Sphincter (anus) - Bands of muscle which allow undigested wastes to exit the body.




Carbohydrate Digestion:

SALIVARY AMYLASE- Produced by the salivary glands and secreted into the mouth.
--Acts on starch to break it into many molecules of maltose.
--Maltose is later broken down in the system to glucose.

 - Also acts on starch to convert it to maltose.
--Occurs in the duodenum, but is produced by the pancreas.

 - Produced in the small intestine.
-- Converts maltose to glucose.
Maltose + Water -----------> 2 Glucose

Protein Digestion

PROTEASES - Break down proteins to peptides.
- There are two types of proteases:
a) Pepsin - Produced by the gastric glands of the stomach.
b) Trypsin - Produced by the pancreas.

Protein + Water ------------------> Peptides

PEPTIDASES - Break down peptides into A.A.
-- Produced by the small intestine.

Peptides + Water ---------------> Amino Acids 

Fat Digestion

BILE - Produced by the liver
- Stored in the gall bladder
- Breaks down fat in the duodenum into fat droplets
**not an enzyme, but rather an emulsifier**
EMULSIFICATION: The breaking down of fats to fat droplets by bile.

NOTE: A person who has had his/her gall bladder removed will have trouble digesting fatty foods. The gall bladder stores bile for use at the proper time during the digestive process
Fat -----> Fat Droplets

LIPASE - Produced by the pancreas
-- Breaks down fat droplets into glycerol and 3 fatty acids.
Fat droplets + Water --------> Glycerol + 3 Fatty Acids
NUCLEASE - produced by the Small intestine and Pancreas
- works in the small intestine
- breaks down RNA and DNA into nucleotides
NUCLEOSIDASES - produced by the Small intestine
- works in the small intestine
- breaks down nucleotides into base, sugar and phosphate components

VIDEO: Lipid Digestion

Digestive Enzymes Summary Table


Glandular Source

Site of Action and pH

Substrate or food acted upon


Salivary Amylase

Salivary Glands (Mouth)

Mouth neutral (7)




Gastric Glands (Stomach)

Stomach acidic (3.5)



Pancreatic Amylase


Small Intestine basic (7.5)





Small Intestine basic (7.5)





Small Intestine basic (7.5)

Fat Droplets

Glycerol and fatty acids


Pancreas and small intestine

Small Intestine basic (7.5)

Nucleic Acids (DNA & RNA)



Small Intestine

Small Intestine basic (7.5)


Amino Acids


Small Intestine

Small Intestine basic (7.5)




Small Intestine

Small Intestine basic (7.5)


Base, Sugar, Phosphate

Insulin and Glucagon


Insulin - Hormone
- Production by pancreas
- Secreted when blood sugar concentration is high.
- Causes liver and muscles to take up and store excess glucose as Glycogen.
- Also promotes synthesis of protein and fats.

Glucagon - Another pancreatic hormone.
- Secreted when blood sugar concentration is low.
- Causes liver and muscles to break down glycogen to glucose.
- Stops protein and fat synthesis.

Pancreas is called both an Exocrine and an Endocrine organ

Exocrine - Produces some enzymatic substances.
Endocrine - Produces hormones.


Six Major Functions of the Liver

  1. Destroys old red blood cells and converts hemoglobin to a product in bile.
  2. Produces Bile that is stored in the gall bladder before entering the Duodenum where it emulsifies fat.
  3. Store Glucose as Glycogen after eating, and breaks down glycogen to glucose between eating to maintain the glucose concentration of the blood.
  4. Produces Urea from the breakdown of amino acids (deamination)
  5. Makes Blood Proteins from amino acids.
  6. Detoxifies the blood by removing poisonous substances and metabolizing them (converting them to harmless substances).
**The liver is an essential organ**
**Be sure you know the structures and functions of each part of the digestive system, all the enzymes including where they are produced, the substrate they work on, and the product produced, how insulin and glucagon regulate blood sugar levels and finally, know all 6 functions of the liver**