1. The Digestive System and Process
Biology 11

2. The Components of the Digestive System
Figure 24.1

3. Functions of the digestive system
Ingestion
Mechanical processing
Digestion
Secretion
Absorption
Excretion

4. Explain why digestion of large food molecules is essential
Most food is solid and in the form of large complex molecules which are insoluble and chemically inert (not readily usable)
As food was synthesized by other organisms, it contains materials not suitable for human tissue - these need to be separated and removed
Large molecules need to be broken down into smaller molecules that can be readily absorbed across membranes and into cells
Small molecules can be reassembled into new products (e.g. amino acids can be reassembled to make new proteins)

5. Digestive enzymes
Enzymes are globular proteins that control biological reactions.
Digestive enzymes speed up the breakdown (hydrolysis) of food molecules into their ‘building block’ components.
These reactions occur outside of the cells lining the gut.

6. Explain the need for enzymes in digestion
Enzymes are biological catalysts which speed up the rate of a chemical reaction (e.g. digestion) by lowering the activation energy
Enzymes allow digestive processes to occur at body temperature and at sufficient speed to meet the organism's survival requirements
Enzymes are specific for a given substrate and so can allow digestion of certain molecules to occur independently of others

7. Naming and classification of enzymes
There are 2 systems used for naming enzymes:
The suffix ‘-ase’ is used with the root name of the substance being acted upon, for example, when sucrose (sugar) is digested, it is acted upon by an enzyme called sucrase.
The type of chemical reaction involved as the enzyme functions, for example, when sucrase acts on sucrose, it breaks it into a molecule of glucose and a molecule of fructose. This reaction involves adding a water molecule to break a chemical bond and so the enzyme is a hydrolase. All digestive enzymes belong to this hydrolase class.
Enzymes are classified according to the type of chemical reaction catalysed.
All digestive enzymes are hydrolases, whereas most of the enzymes involved in energy release for muscular contraction are oxidation-reduction enzymes such as oxidases, hydrogenases and dehydrogenases.

8. Digestive Juices and Enzymes
Substance Digested   
Product Formed
Saliva
Gastric juice
Pancreatic juice
Intestinal enzymes
Bile from the liver

9. The mouth opens into the oral or buccal cavity
Its functions include:
Analysis of material before swallowing
Mechanical processing by the teeth, tongue, and palatal surfaces
Lubrication
Limited digestion

10. The tongue primary functions include: Mechanical processing
Assistance in chewing and swallowing
Sensory analysis by touch, temperature, and taste receptors

11. The pharynx Common passageway for food, liquids, and air
Lined with stratified squamous epithelium
Pharyngeal muscles assist in swallowing
Pharyngeal constrictor muscles
Palatal muscles

12. Histology of the esophagus
Distinctive features of the esophageal wall include
Non-keratinized, stratified squamous epithelium
Folded mucosa and submucosa
Mucous secretions by esophageal glands
A muscularis with both smooth and skeletal muscle portions
Lacks serosa
Anchored by an adventitia

13. The Esophagus
Figure 24.10a-c

14. Peristalsis
Figure 24.4

15. Control of the digestive system
Movement of materials along the digestive tract is controlled by:
Neural mechanisms
Parasympathetic and local reflexes
Hormonal mechanisms
Enhance or inhibit smooth muscle contraction
Local mechanisms
Coordinate response to changes in pH or chemical stimuli

16. The Swallowing Process
Figure 24.11a-h

17. The Regulation of Digestive Activities
Figure 24.5

18. Movement of digestive materials
Visceral smooth muscle shows rhythmic cycles of activity
Pacemaker cells
Peristalsis
Waves that move a bolus
Segmentation
Churn and fragment a bolus

19. Functions of the stomach
Bulk storage of undigested food
Mechanical breakdown of food
Disruption of chemical bonds via acids and enzymes
Production of intrinsic factor

20. Digestion and absorption in the stomach
Preliminary digestion of proteins
Pepsin
Permits digestion of carbohydrates
Very little absorption of nutrients
Some drugs, however, are absorbed
Mucous secretion containing several hormones
Entero-endocrine cells
G cells secrete gastrin
D cells secrete somatostatin

21. The Stomach
Figure 24.12b

22. The Stomach Lining
Figure 24.13a, b

23. The Stomach Lining
Figure 24.13c, d

24. Histology of the stomach
Gastric glands
Parietal cells
Intrinsic factor, and HCl
Chief cells
Pepsinogen
Pyloric glands

25. The Secretions of Hydrochloric Acid
Figure 24.14

26. The Phases of Gastric Secretion
Figure 24.15a

27. The Phases of Gastric Secretion
Figure 24.15b

28. The Phases of Gastric Secretion
Figure 24.15c

29. Small intestine Important digestive and absorptive functions
Secretions and buffers provided by pancreas, liver, gall bladder
Three subdivisions:
Duodenum
Jejunum
Ileum
Ileocecal sphincter
Transition between small and large intestine

30. Regions of the Small Intestine
Figure 24.16a

31. Histology of the small intestine
Plicae
Transverse folds of the intestinal lining
Villi
Fingerlike projections of the mucosa
Lacteals
Terminal lymphatic in villus
Intestinal glands
Lined by entero-endocrine, goblet and stem cells

32. The Intestinal Wall
Figure 24.17a

33. The Structure of the Digestive Tract
Figure 24.3

34. The Intestinal Wall
Figure 24.17b, c

35. The Intestinal Wall
Figure 24.17d, e

36. Intestinal juices Moisten chyme Help buffer acids
Maintain digestive material in solution

37. Small Intestine Duodenal glands (Brunner’s glands) Ileum
produce mucus, buffers, urogastrone
Ileum
aggregated lymphoid nodules (Peyer’s patches)

38. Intestinal movements Peristalsis Segmentation Gastroenteric reflexes
Initiated by stretch receptors in stomach
Gastroileal reflex
Triggers relaxation of ileocecal valve

39. The pancreas Pancreatic duct penetrates duodenal wall
Endocrine functions
Insulin and glucagons
Exocrine functions
Majority of pancreatic secretions
Pancreatic juice secreted into small intestine
Carbohydrases
Lipases
Nucleases
Proteolytic enzymes

40. The Pancreas
Figure 24.18a-c

41. The liver
Performs metabolic and hematological regulation and produces bile
Histological organization
Lobules containing single-cell thick plates of hepatocytes
Lobules unite to form common hepatic duct
Duct meets cystic duct to form common bile duct

42. The Anatomy of the Liver
Figure 24.19a

43. The Anatomy of the Liver
Figure 24.19b, c

44. Liver Histology
Figure 24.20a, b

45. The gallbladder Hollow, pear-shaped organ
Stores, modifies and concentrates bile
PLAY
Animation: Accessory Organ

46. The Gallbladder
Figure 24.21a, b

47. Coordination secretion and absorption
Neural and hormonal mechanisms coordinate glands
GI activity stimulated by parasympathetic innervation
Inhibited by sympathetic innervation
Enterogastric, gastroenteric and gastroileal reflexes coordinate stomach and intestines

48. The Activities of Major Digestive Tract Hormones
Figure 24.22

49. Functions of the large intestine
Reabsorb water and compact material into feces
Absorb vitamins produced by bacteria
Store fecal matter prior to defecation

50. The Large Intestine
Figure 24.23a

51. The Large Intestine
Figure 24.23b, c

52. Comparison of Small and Large Intestines

53. The rectum Last portion of the digestive tract
Terminates at the anal canal
Internal and external anal sphincters

54. Histology of the large intestine
Absence of villi
Presence of goblet cells
Deep intestinal glands

55. Physiology of the large intestine
Reabsorption in the large intestine includes:
Water
Vitamins – K, biotin, and B5
Organic wastes – urobilinogens and sterobilinogens
Bile salts
Toxins
Mass movements of material through colon and rectum
Defecation reflex triggered by distention of rectal walls

56. The Defecation Reflex
Figure 24.25

57. Processing and absorption of nutrients
Disassembles organic food into smaller fragments
Hydrolyzes carbohydrates, proteins, lipids and nucleic acids for absorption

58. Carbohydrate digestion and absorption
Begins in the mouth
Salivary and pancreatic enzymes
Disaccharides and trisaccharides
Brush border enzymes
Monosaccharides
Absorption of monosaccharides occurs across the intestinal epithelia

59. Lipid digestion and absorption
Lipid digestion utilizes lingual and pancreatic lipases
Bile salts improve chemical digestion by emulsifying lipid drops
Lipid-bile salt complexes called micelles are formed
Micelles diffuse into intestinal epithelia which release lipids into the blood as chylomicrons

60. Protein digestion and absorption
Low pH destroys tertiary and quaternary structure
Enzymes used include pepsin, trypsin, chymotrypsin, and elastase
Liberated amino acids are absorbed

61. Absorption Water Ions Vitamins
Nearly all that is ingested is reabsorbed via osmosis
Ions
Absorbed via diffusion, cotransport, and active transport
Vitamins
Water soluble vitamins are absorbed by diffusion
Fat soluble vitamins are absorbed as part of micelles
Vitamin B12 requires intrinsic factor

62. Digestive Secretion and Absorption of Water
Figure 24.27