General Idea about absorption and digestion of various food nutrients. overview of digestion and absorption

1. Unit: II
Energy – Units, Metabolisms, Energy expenditure,
and Energy imbalance. Digestion, absorption and
transport of Food Proteins and eye
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2. Unit: II
Digestion, absorption and transport of Food
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3. Overview of Digestive System
• Digestion: The process of breaking down food into
individual molecules small enough to be absorbed through
the intestinal wall
• Absorption: The process of moving nutrients from the
gastrointestinal (GI) tract into the bloodstream
• Transport: The process of moving absorbed nutrients
throughout the body through the circulatory and lymph
systems
• Elimination: The excretion of undigested and unabsorbed
food through the feces

4. OverviewofDigestiveSystem

5. Overview of Digestive System

6. OverviewofDigestiveSystem

7. Overview of Digestive System

8. Overview of Digestive System
• Most digestion occurs in the small intestines
• Extends from the pyloric sphincter to the ileocecal valve
• Contains three sections
• Duodenum
• Jejunum
• Ileum
• Takes 3 to 10 hours for food to traverse the small intestine
Small Intestine

9. Overview of Digestive System

10. Overview of Digestive System

11. Overview of Digestive System
• Site of water, sodium, potassium, and chloride absorption
• Bacteria produces vitamin K, thiamin, riboflavin, biotin, and vitamin
B12
• Only biotin and vitamin K can be absorbed
• In the large intestine, 1 liter of fluid material is gradually reduced to
200 grams of brown fecal material
• Brown color is due to unabsorbed iron mixed with yellowish-orange
substance called bilirubin
• Greater the iron content, the darker the feces
Large Intestine

12. OverviewofDigestiveSystem

13. Overview of Digestive System
The major functions of each region are:
Mouth
• Starch hydrolysis catalysed by amylase, secreted by the salivary glands;
• Fat hydrolysis catalysed by lingual lipase, secreted by the tongue;
• Absorption of small amounts of vitamin C and a variety of non-nutrients
(including nicotine).

14. Overview of Digestive System
Stomach
• Denaturation of dietary proteins and the release of vitamin B12, iron and other
minerals from protein binding, for which gastric acid is important;
• Protein hydrolysis catalysed by pepsin;
• Fat hydrolysis catalysed by lipase.
• Secretion of intrinsic factor, required for the absorption of vitamin B12

15. Overview of Digestive System
Small intestine (duodenum, jejunum and ileum)
• starch hydrolysis catalysed by amylase secreted by the pancreas;
• hydrolysis of disaccharides within the brush border of the intestinal mucosa;
• fat hydrolysis catalysed by lipase secreted by the pancreas;
• protein hydrolysis catalysed by a variety of exo- and endopeptidases secreted by the
pancreas and small intestinal mucosa;
• hydrolysis of di- and tripeptides within the brush border of the intestinal mucosa;
• absorption of the products of digestion;
• absorption of water (failure of water absorption, as in diarrhoea, can lead to serious
dehydration).

16. Overview of Digestive System
Large intestine (caecum and colon)
– bacterial metabolism of undigested carbohydrates and shed intestinal mucosal
cells
– absorption of some of the products of bacterial metabolism
– absorption of water.
Rectum
– storage of undigested gut contents prior to evacuation as faeces

17. Digestion and absorption of carbohydrates
Nutritionally, it is useful to consider sugars (both
monosaccharides and disaccharides) in two groups:
• Sugars contained within plant cell walls in foods. These are
known as intrinsic sugars.
• Sugars that are in free solution in foods, and therefore
provide a substrate for oral bacteria, leading to the
formation of dental plaque and caries. These are known as
extrinsic sugars.
Classification of carbohydrates

18. Digestion and absorption of carbohydrates
Classification of carbohydrates

19. Digestion of carbohydrates
• When food is chewed, it is mixed with saliva, which
contains the digestive enzyme ptyalin (an α -amylase)
secreted mainly by the parotid glands.
• This enzyme hydrolyzes starch into the disaccharide
maltose and other small polymers of glucose that contain
three to nine glucose molecule
• Only about 5% of starch is digested because food remains
in the mouth only for a short time
Digestion of Carbohydrates in the Mouth and Stomach

20. Digestion of carbohydrates
• At stomach activity of the salivary amylase is blocked by acid
of the gastric secretions ( amylase is nonactive at pH < 4.0)
• 30 to 40 per cent of the starches will have been hydrolyzed
mainly to form maltose at stomach
Digestion of Carbohydrates in the Mouth and Stomach

21. Digestion of carbohydrates
• Pancreatic secretion contains a large quantity of α-amylase
• within 15 to 30 minutes after the chyme empties from the
stomach into the duodenum and mixes with pancreatic juice,
virtually all the carbohydrates will have become digested.
• carbohydrates are almost totally converted into maltose
and/or other very small glucose polymers before passing
beyond the duodenum or upper jejunum.
Digestion of Carbohydrates in the Small Intestine

22. Digestion of carbohydrates
• The enterocytes lining the villi of the small intestine contain
four enzymes (lactase, sucrase, maltase, and α-dextrinase),
which split the disaccharides lactose, sucrose, and maltose
into their constituent monosaccharides.
• These enzymes are located in the enterocytes covering the
intestinal microvilli brush border
Digestion of Carbohydrates in the Small Intestine

23. Digestion of carbohydrates
• Maltase catalyses the hydrolysis of maltose to two molecules of
glucose.
• Sucrase–isomaltase is a bifunctional enzyme that catalyses the
hydrolysis of sucrose to glucose and fructose, and of isomaltose to
two molecules of glucose.
• Lactase catalyses the hydrolysis of lactose to glucose and galactose.
• Trehalase catalyses the hydrolysis of trehalose to two molecules of
glucose.
Digestion of Carbohydrates in the Small Intestine

24. Digestion of carbohydrates
• Lactose splits into a molecule of galactose and a molecule of
glucose.
• Sucrose splits into a molecule of fructose and a molecule of
glucose
Digestion of Carbohydrates in the Small Intestine

25. Digestion of Proteins
1. Endopeptidases cleave proteins by hydrolysing peptide
bonds between specific amino acids throughout the
molecule.
2. Exopeptidases remove amino acids one at a time from
either the amino or carboxyl end of the molecule, again by
the hydrolysis of the peptide bond.

26. Digestion of Proteins
1. The first enzymes to act on dietary proteins are the
endopeptidases: pepsin in the gastric juice
2. trypsin, chymotrypsin and elastase secreted by the
pancreas into the small intestine.

27. Digestion of Proteins
• The result of the combined action of the endopeptidases is that the large
protein molecules are broken down into a number of smaller polypeptides
with a large number of amino and carboxy terminals for the exopeptidases to
act on
• There are two classes of exopeptidase:
1. Carboxypeptidases, secreted in the pancreatic juice, release amino acids from
the free carboxyl terminal of peptides.
2. Aminopeptidases, secreted by the intestinal mucosal cells, release amino acids
from the amino terminal of peptides.

28. Digestion of Proteins
• Pepsin, the important peptic enzyme of the stomach (active at a pH
• of 2.0 to 3.0)
• Pepsin can digest collagen fiber.
• only 10 to 20 per cent of the total protein digestion to convert the protein to
proteoses, peptones, and a few polypeptides
Digestion of Proteins in the Stomach

29. Digestion of Proteins
• Most protein digestion occurs in the upper small intestine, in the duodenum
and jejunum
• major proteolytic pancreatic enzymes: trypsin, chymotrypsin,
carboxypolypeptidase, and proelastase
• trypsin and chymotrypsin split protein molecules into small polypeptides
• Carboxypolypeptidase then cleaves individual amino acids from the carboxyl
ends of the polypeptides.
• Proelastase, in turn, is converted into elastase, which then digests elastin fibers
that partially hold meats together.
Digestion of Proteins y pancreatic secretion

30. Digestion of Proteins
• Only a small percentage of the proteins are digested all the way to their
constituent amino acids by the pancreatic juices. Most remain as dipeptides
and tripeptides
Digestion of Proteins y pancreatic secretion

31. Digestion of Proteins
• Enterocytes have a brush border that consists of hundreds of microvilli
projecting from the surface of each cell.
• In the membrane of each of these microvilli are multiple peptidases that
protrude through the membranes to the exterior
• Two types of peptidase enzymes are especially important, aminopolypeptidase
and several dipeptidases.
• inside the cytosol of the enterocyte are multiple other peptidases
Digestion of Proteins by intestine secretions

32. Digestion of fat
• A small amount of triglycerides is digested in the stomach by lingual lipase that
is secreted by lingual glands in the mouth and swallowed with the saliva.
•
• The first step in fat digestion is physically to break the fat globules into very
small sizes so that the water-soluble digestive enzymes can act on the globule
surfaces. This process is called emulsification of the fat,
• most of the emulsification occurs in the duodenum under the influence of bile
Digestion of Proteins by intestine secretions

33. Digestion of fat
• bile salts and the phospholipid lecithin are extremely important for
emulsification of the fat.
• The lipase enzymes are water-soluble compounds and can attack the fat
globules only on their surfaces
Digestion of Proteins by intestine secretions

34. Digestion of fat
• The most important enzyme for digestion of the triglycerides is pancreatic
lipase, present in enormous quantities in pancreatic juice
• In addition, the enterocytes of the small intestine contain still more lipase,
known as enteric lipase, but this is usually not needed
Digestion of Proteins by pancreatic secretions

35. Digestion

36. Absorption
• Majority of absorption takes place in the small intestine
• Nutrients are absorbed via
• Passive diffusion – nutrients move from high concentration to low
concentration; no energy is required
• Facilitated diffusion – nutrients move from high concentration to low
concentration with the help of a carrier protein; no energy is required
• Active transport – nutrients move from low concentration to high
concentration with the help of a carrier protein, energy is required
• Endocytosis – cell forms a vesicle to surround and engulf a nutrient

37. Absorption

38. Absorption

39. Absorption
Absorption of Carbohydrates

40. Absorption
Absorption of
Carbohydrates

41. Absorption

42. Absorption

43. Absorption

44. Absorption

45. Absorption
• Nutrients are absorbed into the circulatory or lymphatic system
• Water-soluble nutrients are absorbed into the circulatory system
• Carbohydrates, amino acids, and water-soluble vitamins
• Water-soluble
nutrients
GI tract
Capillaries
• Water-
soluble
nutrients
Hepatic
Portal Vein
• Water-
soluble
nutrients
Liver

46. Absorption
• Fat-soluble nutrients are absorbed into the lymphatic system
• Fat-soluble vitamins, long-chain fatty acids, and proteins too large
to be transported via the capillaries
• Fat-
soluble
nutrients
Lymph
Capillaries
• Fat-
soluble
nutrients
Lymphatic
vessels • Fat-
soluble
nutrients
Thoracic
Duct