1. International School Bangkok
Digestion in Humans
IB Biology SL
International School Bangkok

2. It All Starts Here

3. What Happens to Food in the Mouth?
It gets chewed to small bits by the jaws, teeth and tongue.
It gets moistened.
It gets tasted.
It gets mixed with saliva and forms a “bolus” – ready for swallowing.
A little starch is digested.

4. Chemical Digestion in the Mouth
Enzyme
Where is it made?
Where does it work?
Substrate
Product
Special
Salivary glands
(3 pairs)
In the Mouth
Salivary Amylase
Maltose
(glucose + glucose)
Neutral pH required
Starch

5. Salivary Glands are “common” glands. Their secretions travel in ducts.

7. Features of the STOMACH
The esophagus carries food from mouth to stomach
This valve is also called the CARDIAC SPHINCTER
Notice the “expandable” RUGAE (folds) in the stomach wall.
There are muscle layers in the stomach wall which allow it to “churn” the food
This valve is also called the PYLORIC SPHINCTER
FACTOID: Some say the stomach is “J” shaped!

9. What Happens to Food in the Stomach?
It gets liquefied. (Carbs quickly; proteins take longer)
It gets warmed.
Bacteria are killed.
Very little gets absorbed – some salts, monosaccharides, water, alcohol.
It leaves as a liquid called CHYME.

10. Chemical Digestion in the Stomach
Enzyme
Where is it made?
Where does it work?
Substrate
Product
Special
Gastric glands of stomach
pH 2
HCl activated
Pepsin
In the Stomach
Proteins
Peptides

11. The Role of HCl
Turns pepsinogen into pepsin (or, it “activates” pepsin
Kills bacteria
Gives the enzyme pepsin its preferred environment (pH 2)
Denatures proteins -- it unfolds the polypeptide chains

12. Features of the Small Intestine
Is about 6 meters in length.
Consists of the duodenum, jejunum, and ileum.
Is lined with millions of villi giving it a “velvety” appearance.
Produces some of its own enzymes in the intestinal glands.
Receives some enzymes from the pancreas.
Receives bile from the liver.
Operates at a slightly alkaline pH

13. What Happens to Food in the Small Intestine?
All type of foods (proteins, carbohydrates, triglycerides, nucleic acids) are digested.
The products of digestion are absorbed.
The products of digestion enter circulation – some through the lymphatic system.

14. LIVER FEATURES The liver produces a secretion which is called “bile”.
Bile contains bile pigments and bile salts. Bile does NOT contain enzymes!
Bile salts help to emulsify (liquefy) fats – giving a larger surface area.
Bile pigments are recycled hemoglobin and most will be reasborbed in the intestines.
Bile is stored, temporarily, in the gall bladder. (36 mL capacity)
FACTOID: bile salts sometimes start to crystallize = GALLSTONES!

15. PANCREAS FEATURES The pancreas is a “compound” gland.
FACTOID: Compound glands are both “common” and endocrine”!
“Common” secretions -- enzymes. They enter a duct of some sort.
Endocrine secretions – hormones (insulin and glucagon). They enter directly into the bloodstream
The pancreas looks like a cluster of grapes and is located in the fold between the stomach and the duodenum

16. 2 Problems
1. How do the gall bladder and pancreas “know” when food is in the small intestine and their secretions are needed?
Answer: Secretagogues from the duodenum! Secretin stimulates the pancreas and CCK (cholecystokinin) stimulates the liver and gall bladder!
2. How do the secretions of the gall bladder and pancreas get to the small intestine?
Answer: Secretions enter into 2 ducts that join to become the “common bile duct” This enters the duodenum!

17. CARBOHYDRATE DIGESTION IN THE SMALL INTESTINE
Enzyme
Where is it made?
Where does it work?
Substrate
Product
Special
Pancreatic amylase
Pancreas
Small intestine
Starch
Maltose
Alkaline pH
Maltase
Small Intestine
Glucose (2)
Sucrase
Sucrose
Glucose and Fructose
Lactase
Lactose
Glucose and Galactose

18. PROTEIN DIGESTION IN THE SMALL INTESTINE
Enzyme
Substrate
Product
Where is it made?
Where does it work?
Special
Trypsin
Protein
Peptides
Pancreas
Small Intestine
Alkaline pH
Peptidase
Amino Acids

19. TRIGLYCERIDE DIGESTION IN THE SMALL INTESTINE
Enzyme
Substrate
Product
Where is it made?
Where does it work?
Special
Pancreatic Lipase
Triglyceride
Glycerol and Fatty Acids
Pancreas
Small Intestine
Alkaline pH, Fat Droplets

20. PROBLEM: How do all of those products get absorbed?
ANSWER: The villi that line the small intestine will absorb them!
FACTOID: The total surface area of the villi approach that of a tennis court!

21. Lacteal (a branch of the lymphatic system Arteriole from the heart
Features of a VILLUS
Lacteal (a branch of the lymphatic system
Capillaries
Arteriole from the heart
Venule to the liver

22. VILLUS – another view Notice that a villus is multicellular!
Arteriole from the heart
Lacteal – to bloodstream eventually!
Venule to the liver

24. The Lymphatic System – Your “other” circulatory system!
The lacteals in the villi empty into the Lymphatic System
Skeletal muscle contraction forces lymph through the system
Lymph enters the blood stream at the left subclavian vein
FACTOID: Lymph is the clear fluid that is forced from the capillaries – very similar to plasma

25. VILLUS – another view
Capillaries absorb monosaccharides, amino acids, A, T, G, C, phosphates, vitamins
Remember! These travel through the bloodstream to the liver.
Lacteals absorb glycerol and fatty acids – and reassemble them into small triglycerides called “chylomicrons”
Remember! These travel the lymphatic system and enter the bloodstream eventually

26. PROBLEMS: 1) How does the water get removed from the undigestable materials and 2) what if it doesn't?
1) The large intestine absorbs the water!
2) Diarrhea will occur and the dehydration could cause death!
FACTOID: There are NO villi in the large intestine!

27. The large intestine is about 1.5 meters long
Bacteria, particularly E. coli, are at work here
Fiber (undigestable cellulose) in the diet helps to keep things moving along!
FACTOID #1: E. coli provides us with histidine (one of the essential amino acids) and therefore has a MUTUALISTIC relationship with us!
FACTOID #2: Darwin identified the appendix as a vestigial structure in humans. It may, at one time in our past, been a place where cellulose was worked on by bacteria to help with its breakdown.

29. The End