1. Structure and Function of the Human Digestive System

2. (15 video lessons, about 5 to 7 minutes each)
Screenshot of website with lesson outline
Chapter 8 on Digestion
(15 video lessons, about 5 to 7 minutes each)

3. Introduction to Digestion
food is made of macromolecules which are too large to move across cell membranes
the purpose of digestion is to break food macromolecules into their subunits so they can be absorbed by cells

4. The digestive process involves:
Ingestion: the process of taking food into the body
Digestion: the process of breaking large pieces of food into smaller subunits
Absorption: the transfer of nutrients from the digestive system into the bloodstream
Egestion: the process of eliminating undigested material from the body

5. The Digestive System
digestion takes place in the digestive tract, which is also known as the alimentary canal or gastrointestinal tract (gastro = stomach + intestine)
food enters the mouth and exits the anus
the human digestive system is about meters (30 feet) long. It is longer in herbivores and shorter in carnivores

6. There are two types of digestion:
Mechanical digestion is the physical break down of large food particles into smaller pieces, for example, by chewing or grinding
food particles are often mixed with liquid secretions at the same time
smaller food particles have a larger surface area which makes chemical digestion more efficient

7. There are two types of digestion:
2. Chemical digestion is the chemical break down (hydrolysis) of large molecules into smaller molecules, for example, by the action of digestive enzymes or acid
chemical digestion takes place in steps, breaking food into smaller and smaller molecules

8. Chemical Digestion of Carbohydrates

9. Chemical Digestion of Proteins

10. Chemical Digestion of Lipids

12. Digestive Enzymes
there are many types of digestive enzymes, each works on one type of substrate
are produced by specialized cells or groups of cells called glands
can be re-used
work best at a certain pH and temperature (changing pH or temperature or adding harsh chemicals can alter the shape of the enzyme’s active site and denature it so it no longer works)

13. The Mouth: Mechanical Digestion
chewing food increases the surface area of the food particles to increase the rate of digestion
chewing also mixes the food with saliva
the tongue rolls the food into a ball called a bolus and pushes it to the back of the mouth (pharynx) to be swallowed

14. The Mouth: Saliva and Chemical Digestion
Salivary glands secrete saliva which:
moistens and lubricates food
contains salivary amylase which begins the chemical digestion of starch into sugars
contains lysozyme, an enzyme that helps to kill bacteria by breaking down the peptidoglycan in their cell walls
Summary video, introduction to Digestion

15. Parts of the Mouth and Throat
Soft palate covers the opening to the nasal cavity during swallowing so food doesn’t go up into the nasal cavity (nose)
Pharynx (back of mouth, top of throat); it is directly behind the uvula and shared by the digestive and respiratory systems
Larynx (top of the trachea); it contains the ‘voice box’.
Epiglottis is a cartilage flap which covers the larynx during swallowing and prevents food from moving into the respiratory tract
Esophagus carries the bolus of food from the pharynx to the stomach. It has mucus to lubricate it

16. Parts of the Mouth (Oral Cavity)
Pharynx

18. Swallowing, Peristalsis and the Esophagus
Bolus of food
Uvula
Pharynx
Epiglottis
Larynx
Trachea
Tongue
Swallowing, Peristalsis and the Esophagus
After food has been chewed and moistened, the bolus of food is swallowed and pushed down the esophagus by waves of peristalsis (contractions of the smooth muscle)
The cardiac sphincter at the top of the stomach relaxes and the bolus of food enters the stomach.
The cardiac sphincter closes to prevent stomach contents from being squeezed back up into the esophagus
Video summary:

19. Parts of the Stomach the stomach can expand to hold up to 2 L
food (chyme) stays in the stomach for 2 – 6 hours before emptying into the duodenum
the pyloric sphincter controls the movement of chyme into the duodenum
Rugae are muscular ridges and folds in the lining of the stomach that allow the stomach to expand. They provide a rough surface which helps to break down food particles when the stomach contracts.

20. The Stomach: Chemical Digestion
Gastric glands in the stomach secrete 2 – 3 litres of gastric juice each day. It contains:
hydrochloric acid (HCℓ) which digests proteins and kills bacteria
pepsinogen which is converted to the active enzyme pepsin by hydrochloric acid. Pepsin is a protease which begins to digest proteins into short peptide chains
gastric lipase is secreted, but it only becomes active in the duodenum when pH is no longer acidic
mucus (mucin) which lines and protects the stomach from being digested
Video summary:

21. The Stomach: Mechanical Digestion
the stomach has three layers of smooth muscle which run in different directions: longitudinal (long-ways) muscles, transverse (circular or side-ways) muscles and oblique (diagonal) muscles
the muscles contract to churn and mix food with gastric juices, making semi-liquid ‘chyme’
the churning action breaks down large pieces of food and contributes to mechanical digestion

22. The Stomach: Absorption
water, alcohol and some drugs (such as aspirin and caffeine) can be absorbed directly from the stomach
the majority of absorption happens in the small intestine

23. The Stomach and Accessory Organs
Video Summary:

24. Accessory Organs: The Liver and Gall Bladder
the liver makes bile (among the many other jobs it does)
the gall bladder stores the bile and releases it when chyme is in the duodenum. Bile emulsifies fat into tiny droplets to increase surface area for digestion
bile also helps to neutralize stomach acid and this activates the enzyme lipase
Summary Video

25. Accessory Organs: The Pancreas
The pancreas secretes “pancreatic juice” into the duodenum. It contains:
sodium carbonate and sodium bicarbonate to neutralize stomach acid
28 different types of hydrolytic enzymes including lipases, carbohydrases to digest disaccharides, and proteases such as trypsin
Summary Video

26. The Small Intestine: Structure
The small intestine has 3 parts:
the duodenum is the first 20 – 25 cm of the small intestine where chyme, pancreatic juice and bile are mixed. It is the major site of chemical digestion. The cells lining the duodenum secrete enzymes including lipases, proteases, sucrase, maltase and lactase
the jejunum is about 2.5 meters long. Here digestion is finished and almost all absorption takes place. The folds, villi and microvilli increase surface area for absorption
the ileum is the last 3 meters of the small intestine. Absorption is completed here. Bacteria in the ileum make vitamins B12 and K

27. The small intestine has an enormous surface area for absorption:

28. The Small Intestine: Absorption
chyme is pushed through the small intestine by peristalsis
it takes about 3 – 5 hours for food to pass from the duodenum to the end of the ileum and during this time, food is digested and nutrients are absorbed
monosaccharides and amino acids pass through intestinal cells and directly into the blood capillaries found in the villi. This blood goes to the liver.
fatty acids and monoglycerides are taken into intestinal cells, packaged and transported into lacteals found in the villi. Lacteals feed into the lymphatic system which joins the blood stream close to the heart
the ileocaecal valve is a sphincter at the end of the ileum which controls the movement of undigested material into the large intestine
Video summary:

29. Video Summaries of Lipid, Protein and Carbohydrate Digestion
Digestion and absorption of fats:
and
Digestion and absorption of protein:
Digestion and absorption of carbohydrates:
(same video, different link at

30. The Large Intestine
undigested material enters the large intestine (large bowel or colon) through the ileocaecal valve
the large intestine is about 1.5 meters long. It takes approximately 24 – 48 hours for feces to pass through the large intestine
in the large intestine, water is re-absorbed from the waste material
bacteria in the ileum and large intestine digest some of the wastes and produce vitamins K and vitamin B12 which are absorbed here
the remains of the food (mainly cellulose, bacteria and water) become more solid and are compacted into feces

31. Egestion
muscular contractions of the large intestine (which are much slower than peristalsis) move feces to the rectum for temporary storage
when feces stretch the walls of the rectum, it signals the need to defecate
the external anal sphincter gives us some control over when wastes are expelled

32. The Large Intestine

33. Video Summary of Functions of the Large Intestine
Two different videos on structure and function:
Overall summary video: (5 minutes)
Crash course Biology: the Digestive System

34. Test Responsibilities:
Be able to answer the questions on the Review of the Digestive System