1. 6.1 Digestion & Absorption D.2Digestion
The Digestive System
6.1 Digestion & Absorption
D.2Digestion

2. Accessory Digestive Organs
These are organs located outside of the GI tract
These organs create, or store and secrete enzymes, hormones and chemicals that are required for digestion.

4. Pancreas

5. Pancreas Long flat gland, nestled between the stomach and duodenum.
When chyme from the stomach enters the duodenum, the hormone CCK (cholecystokinin) is secreted (by the walls of the duodenum) and causes the pancreas to release its pancreatic juices and the liver to release bile
The pancreas does 3 things to aide in digestion

6. 1. Releases pancreatic juice
Travels from the pancreas to the duodenum via the pancreatic duct
Contains 28 different enzymes including:
Amylases: convert big, complex carbs into simple sugars
Lipases: split lipids into glycerol and fatty acids
Trypsin/peptidase – breaks proteins into amino acids

7. 2. Neutralizes the acidic chyme
The pancreatic juices are alkaline (basic) because of its high concentration of bicarbonate ions (HCO3-)
Changes the chyme from pH 2 (which is the pH in the stomach) to pH 8
If the acidic chyme was not neutralized, it could cause ulcers in the intestinal walls

8. Secretin Secretin is a hormone produced by the duodenal walls.
The optimal pH of the duodenum is pH 8
When the pH in the duodenum is too low, secretin is released into the blood stream, and will
stimulates the production of bicarbonate ions from the pancreas
Inhibit gastric juice production in the stomach

9. 3. Releases Hormones
INSULIN – controls blood sugar levels by allowing glucose to enter the cells allowing the glucose to be stored as glycogen.
GLUCAGON – when more glucose is needed in the blood, glucagon will allow glycogen to be broken down into glucose again

12. Exocrine vs Endocrine Glands
A gland is an organ that secretes a substance such as a hormone or an enzyme.
Endocrine Glands: secrete hormones and release them directly into the blood stream.
Ex: the pituitary glands, adrenal glands, the ovaries, the testes……

13. Exocrine vs Endocrine Glands
Exocrine Glands: secrete their products into a duct.
Ex: sweat glands, digestive glands

14. EXOCRINE GLANDS
- Secrete products into a duct
ENDOCRINE GLAND
- Secretes products (hormones) directly into the blood stream
Acinar cell
Makes products
Ex: pancreatic lipase
Acinus (plr acini)
- Where product accumulates
Duct

15. Exocrine Gland - Hollow space surrounded by secretory cells.
- Collects products and sends it to the duct
Produce the products and secrete them into the acinus
Can either lead to a lumen (space between cells, ex: the lumen of the small intestine) or out of the body

16. Identification of exocrine glands from EMs
See data base questions on page 674

17. Examples of Exocrine Glands
Place Released pH
Secretion
Enzyme
Substrate
Product
Salivary glands
Mouth 7
Saliva
Salivary Amylase
Gastric glands
Stomach 2
Gastric juice
Pepsin/
pepsinogen
pancreas
Small intestine
(duodenum) 8
Pancreatic Juice
Bicarbonate ions
Pancreatic amylase
Trypsin
Lipase
Phospholipase
Not an enzyme
Starch
Protein
Lipid
Phosopholipids
Maltose
Smaller Polypeptides
Glcerol & Fatty acids
Phosphate, glycerol & fatty acids
Examples of Exocrine Glands
Neutralizes stomach acid

18. Trypsin
Trypsin is released as the inactive precursor trypsinogen (made by the pancreas) because in it’s active form it would destroy the proteins and cells that make it.
The duodenum secretes the enzyme enternokinase which will catalyse the conversion of trypsinogen to trypsin in the duodenum.
enterokinase
Trypsinogen Trypsin

19. HCl
Pepsinogen pepsin
enterokinase
Trypsinogen trypsin

20. Lipid Digestion Not easy because they are not water soluble
Lipid molecules group together to from droplets of fat
Lipase acts on the surface of lipid spheres

21. Lipid Digestion - BILE BILE – produced by the liver
Aides in lipid digestion by emulsifying fats (dividing fat droplets into smaller droplets)
Creates a larger surface area for lipase to work

22. The emulsification of fat by bile

23. Bile is continuously produced in the liver but is stored in the gall bladder
When food enters the duodenum, the gall bladder contracts and bile is squeezed out of the gall bladder and travels to the duodenum via the bile duct.

25. Digestion in Small Intestine
As mentioned before, the majority of digestion occurs in the small intestine.
Macromolecules are broken down into monomers.
The pancreas add pancreatic juice and the liver/gallbladder add bile to aide in digestion.
The walls of the small intestine also produce a variety of enzymes to digest ingested materials.

26. Digestion in Small Intestine
While the gland cells produce some intestinal juice that will mix with the chyme, most of the enzymes remain immobilized in the plasma membrane of the epithelium cells that line the lumen of the small intestine

27. Digestion in Small Intestine
This includes:
Nucleases: digest DNA and RNA into nucleotides
Maltase: digests maltose into glucose
Lactase: digests lactose into glucose and galactose
Sucrase: digests sucrose into glucose and fructose
Exopeptidases: digest proteins by removing one amino acid at a time
Dipeptidases: digests dipeptides into 2 amino acids
Note: these are all hydrolysis reactions

28. Absorption of Digested Foods

29. Cross Section of ileum

30. Identify the tissue layers in a transverse section of the small intestine (EM) (page 281)
mucosa
serosa

31. Epithelial cell features (of small intestine)
Epithelial Cells are the cells that line cavities and surfaces
They are the cells that create the villi that line the small intestine lumen

32. Epithelial Cell Features
Form villi – increase surface area
Contain microvilli – increase surface area
Tight Junctions – connections between epithelial cells which fuse adjacent cells together at their membranes to form a seal
Rich in mitochondria to produce ATP for active transport
Pinocytotic vesicles –to aide in endocytosis

34. Ileum - Cross section Villus (plr villi) lumen Circular muscle
Longitudinal muscle
submucosa
Folds in small intestine

35. Absorption by Villi
The epithelium that covers the villi forms a barrier to harmful substances but also must be permeable to allow for useful nutrients to pass through into villi.
Villi will absorb:
Monosaccharides, amino acids, fatty acids, monoglycerides, glycerol, nitrogeneous bases, mineral ions (Ca2+, K+, Na+), vitamins (ex: vitamin C, ascorbic acid)
Some harmful and harmless but unwanted substances will be absorbed as well (ex: ethanol, food dyes)

36. How are nutrients absorbed into epithelial cells?
Passive Transport
Active Transport

37. Passive Transport
Transport across a membrane that does not require energy
Driven by the concentration gradient
SIMPLE DIFFUSION – the molecules move through the membrane moving from an area of high concentration to low concentration.
Ex: lipids

38. Simple Diffusion

40. Passive Transport
Facilitated Diffusion – the molecules are too large to pass through membrane on their own
They enter the membrane via a pore or a carrier protein/protein channel
Ex: amino acids, glucose

41. Facilitated Diffusion

42. Active Transport Requires ATP energy
Often moves molecules against their concentration gradient (from low concentration to high concentration)

43. Active Transport ACTIVE TRANSPORT via Transport Proteins/
Protein Pumps
These proteins embedded in the cell membrane require ATP to activate them

44. Endocytosis
The plasma membrane forms a vesicle around the nutrient molecule(s) to bring them into the cell
Pinocytosis: for liquids or small molecules
Phagocytosis: for larger molecules

47. Transport Animations

48. Animations

49. Triglycerides Absorption
Triglycerides are digested into monoglycerides and fatty acids
Monoglycerides are absorbed via simple diffusion
Fatty acids are absorbed by facilitated diffusion (by proteins on the membrane of microvilli)
Once inside the epithelium cells, the fatty acids combine with the monoglycerides to reform triglycerides

50. Triglyceride Absorption
Triglycerides then form small lipoprotein droplets by combining with cholesterol and being surrounded by phospholipids
The lipoproteins then exit the epithelium cells on the inner side of the villus by exocytosis.
Lipoproteins will either enter a lacteal and be carried away by lymph or a capillary and enter the blood stream.

52. Glucose Absorption
Glucose cannot pass through the plasma membrane by simple diffusion.
Sodium-potassium pumps will pump Na+ out of the villus epithelium cells (and K+ in)
=> low [Na+]

53. Glucose Absorption
A sodium-glucose co-transporter protein will move both Na+ and glucose together from the intestine lumen into the epithelium cells.
This is facilitated diffusion (moves with the Na+ concentration gradient)
Glucose protein channels will then move glucose into interstitial spaces inside the villi and into the capillaries

55. Unabsorbed Materials
The following molecules are not absorbed by the body (because there is no system of membrane proteins or receptors to absorb them or enzymes to break them down):
Cellulose
Lignin (large polysaccharide that strengthens wood)
Bile pigments
Bacterial and intestinal cells
These materials are egested in feces

56. Why isn’t Cellulose Digested?
Cellulose is a large polysaccharide (made of glucose molecules) that provides structure in plant cell walls.
The main difference between starch and cellulose are the linkages
Starch can be made of amylose which has α-1,4 glycosidic bonds and/or amylopectin has α-1,4 glycosidic bonds and α-1,6 glycosidic bonds
Cellulose has β-1,4 glycosidic bonds

58. Starch Digestion
Amylase can work on the 1,4 alpha glycosidic bonds to make maltose or maltotriose
Amylase cannot work on the 1,6 alpha bonds of amylopectin
The fragments with the 1,6 bonds are known as dextrins.
3 enzymes on the membranes of microvilli will complete the digestion of these three small sugars into glucose
Maltase: digests maltose to glucose
Gluodidase: digests maltotriose to glucose
Dextrinase: digests dextrin to glucose

60. Cellulose indigestion
Amylase only works on the 1,4 alpha linkages and cannot work on the 1, 4 beta linkages of cellulose
Cellulose does not dissolve in water
The enzyme cellulase can break down cellulose however mammals do not poses the enzyme

61. Ruminant Animals
Some forms of bacteria can produce cellulase and they can live symbiotically in the guts of herbivores (such as cattle, llamas, sheep, goat, giraffes, deer….)

62. Role of Dietary Fibre
The rate of movement of materials (feces)through the large intestine is correlated to their fibre content.
A healthy diet contains fibre.
Fibre increases the bulk of material passing through the intestines
It prevents constipation because it draws water into the intestines
The higher the water content, the faster the movement of feces.

63. Role of Dietary Fibre
The presence of bulky fibre also increase feelings of satiety, making you feel satisfied and preventing overeating.
Fiber slows down the absorption of sugars (preventing type II diabetes)
Fibre rich foods: whole grain breads and cereals, cabbage, celery