1. Digestion and Nutrition
Chapter 32

2. Obtaining Energy
All organisms require energy to maintain their complex structure.
The ultimate source of energy is the sun.
Green plants utilize energy in sunlight to make glucose.
Autotrophs (phototrophs)
A few autotrophs are chemotrophs, gaining energy from inorganic chemical reactions.

3. Animals are Heterotrophs!
Animals are heterotrophs, depending on other organisms for food.
Animals fall into one of three dietary categories:
Herbivores eat mainly autotrophs (plants and algae).
Carnivores eat other animals.
Omnivores regularly consume animals as well as plants or algal matter.
Saprophagous animals feed on decaying organic matter.

4. Why We Eat
Regardless of what an animal eats, an adequate diet must satisfy three nutritional needs:
Fuel for all cellular work.
The organic raw materials for biosynthesis.
Essential nutrients, substances such as vitamins that the animal cannot make for itself.

5. The Main Stages of Food Processing
Ingestion is the act of eating.
Digestion is the process of breaking food down into molecules small enough to absorb.
Involves enzymatic hydrolysis of polymers into their monomers.

6. The Main Stages of Food Processing
Absorption is the uptake of nutrients by body cells.
Elimination occurs as undigested material passes out of the digestive system.

7. Intracellular Digestion
In intracellular digestion, food particles are engulfed by endocytosis and digested within food vacuoles.
Protozoa, sponges.

8. Extracellular Digestion
Extracellular digestion is the breakdown of food particles outside cells.
Digestion occurs in the alimentary canal.
Cells lining the lumen of the alimentary canal are specialized for secreting enzymes or absorbing nutrients.

9. Digestive Systems
Animals with simple body plans have a gastrovascular cavity that functions in both digestion and distribution of nutrients.

10. Digestive Systems
Animals with a more complex body plan have a digestive tube with two openings, a mouth and an anus.
This digestive tube is called a complete digestive tract or an alimentary canal.

11. Digestive Systems
The digestive tube can be organized into specialized regions that carry out digestion and nutrient absorption in a stepwise fashion.

12. Mammalian Digestive System
The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices through ducts.

13. Digestive Enzymes
Enzymes are essential in the breakdown of food into small, absorbable units.
Digestive enzymes are hydrolytic enzymes.
Food molecules are split by hydrolysis.
R-R + H2O digestive enzyme R-OH + H-R

14. Digestive Enzymes
Proteins are broken down into individual amino acids.
Complex carbohydrates are broken down into simple sugars.
Fats are reduced to glycerol, fatty acids, and monoglycerides.

15. Motility in Alimentary Canal
Food moves through the alimentary canal by cilia, specialized musculature, or both.
Gut musculature is present in coelomates.

16. Motility in Alimentary Canal
The gut is lined with opposing layers of smooth muscle: a circular layer and a longitudinal layer.

17. Motility in Alimentary Canal
Two types of gut movement:
Segmentation involves alternate constriction of rings of smooth muscle that move the contents around, mixing with enzymes.
Peristalsis involves waves of contraction behind the food mass that move it through the gut.

18. Organization - Five Major Regions
Reception
Conduction & Storage
Grinding & early digestion
Terminal digestion and absorption
Water absorption and concentration of solids.

19. Receiving Region
The receiving region consists of devices for feeding and swallowing.
Mouthparts – mandibles, jaws, teeth, radula, bills.
Buccal cavity – mouth
Muscular pharynx – throat
Salivary glands – produce lubricating secretions that may also contain toxic enzymes or salivary enzymes to begin digestion.
Amylase begins hydrolysis of starches.

20. Receiving Region
The vertebrate tongue assists in food manipulation and swallowing.
Also used as a chemosensor.

21. Conduction and Storage Region
The esophagus transfers food to the digestive region.
In many invertebrates (annelids, insects, octopods) the esophagus is expanded into a crop used for storage.
Birds also have a crop that serves to store and soften food.

22. Region of Grinding & Early Digestion
The stomach provides initial digestion as well as storing and mixing food with gastric juice.
For further grinding of food, terrestrial oligochaetes and birds have a muscular gizzard that is assisted by stones or grit swallowed with food.

23. The Stomach
The lining of the stomach is coated with mucus, which prevents the gastric juice from destroying the cells.
Pepsin is a protease that splits specific peptide bonds.

24. Region of Terminal Digestion and Absorption
The small intestine is the longest section of the alimentary canal.
It is the major organ of digestion and absorption.

25. Region of Terminal Digestion and Absorption
Increasing the surface area of the intestine increases the area available for absorption.
Longer intestine
Villi – fingerlike projections of intestinal tissue in birds and mammals
Microvilli – tiny processes on intestinal cells.

26. The Small Intestine
The first portion of the small intestine is the duodenum, where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and intestine itself.

27. The Small Intestine The pancreas produces:
Proteases, protein-digesting enzymes.
Lipases for breaking up fat.
Amylase for hydrolyzing starches.
Nucleases which degrade RNA & DNA into nucleotides.

28. The Small Intestine
The liver secretes bile into the bile duct which drains into the duodenum.
Bile is stored in the gallbladder between meals.
Bile salts are important for digestion of fats.

29. The Small Intestine
Enzymatic digestion is completed as peristalsis moves the mixture of chyme and digestive juices along the small intestine.

31. Absorption of Nutrients
The small intestine has a huge surface area due to the presence of villi and microvilli that are exposed to the intestinal lumen.

32. Absorption of Nutrients
The enormous microvillar surface is an adaptation that greatly increases the rate of nutrient absorption.

33. Absorption of Nutrients
The core of each villus contains a network of blood vessels and a small vessel of the lymphatic system called a lacteal.

34. Absorption of Nutrients
Amino acids and simple sugars pass through the epithelium of the small intestine and enter the bloodstream.
Initial absorption occurs by facilitated transport, later by active transport.

35. Absorption of Nutrients
Fats are emulsified by bile salts.
Micelles are tiny droplets consisting of fatty acids and monoglycerides complexed with bile salts.
Micelles diffuse into epithelial cells.
Resynthesized into triglycerides and pass into the lacteals of the lymphatic system.

36. Region of Water Absorption
The large intestine, or colon is connected to the small intestine.

37. Region of Water Absorption
A major function of the colon is to recover water that has entered the alimentary canal.
The wastes of the digestive tract, the feces, become more solid as they move through the colon.
The terminal portion of the colon is the rectum where feces are stored until they can be eliminated.

38. Region of Water Absorption
The colon houses various strains of the bacterium Escherichia coli.
Some produce various vitamins.

39. Regulation of Food Intake
Hunger centers in the brain regulate food intake.
A drop in blood glucose level stimulates a craving for food.
Homeostatic mechanisms control the body’s storage and metabolism of fat.

40. Regulation of Food Intake
Undernourishment occurs in animals when their diets are chronically deficient in calories.
Can have detrimental effects on an animal.
Overnourishment results from excessive food intake.
Leads to the storage of excess calories as fat.

41. Regulation of Digestion
Hormones help coordinate the secretion of digestive juices into the alimentary canal.

42. Glucose Regulation as an Example of Homeostasis
Animals store excess calories as glycogen in the liver and muscle.
Glycogen is made up of many glucose subunits.
Glucose is a major fuel for cells.

43. Glucose Regulation as an Example of Homeostasis
Blood glucose levels rise, the pancreas produces insulin.
Insulin enhances transport of glucose into body cells and stimulates storage of glucose as glycogen.
Results in lower blood glucose levels.

44. Glucose Regulation as an Example of Homeostasis
Lower blood glucose levels stimulates the pancreas to secrete glucagon.
Glucagon promotes breakdown of glycogen in the liver back into glucose which is released into the blood.

45. Nutritional Requirements
An animal must obtain organic carbon (from glucose) and organic nitrogen (from amino acids obtained during digestion of protein) in order to build organic molecules such as carbohydrates, lipids and proteins.

46. Nutritional Requirements
An animal’s diet must also supply essential nutrients in preassembled form.
An animal that is malnourished is missing one or more essential nutrients in its diet.

47. Nutritional Requirements
Herbivorous animals may suffer mineral deficiencies if they graze on plants in soil lacking key minerals.

48. Vitamins
Vitamins are organic molecules required in the diet in small amounts.
To date, 13 vitamins essential to humans have been identified.
Vitamins are grouped into two categories:
Fat-soluble
Water-soluble

49. Minerals
Minerals are simple inorganic nutrients that are usually required in small amounts.

50. Essential Fatty Acids
Animals can synthesize most of the fatty acids they need.
The essential fatty acids are certain unsaturated fatty acids.
Deficiencies in fatty acids are rare.

51. Essential Amino Acids
Animals require 20 amino acids and can synthesize about half of them from the other molecules they obtain from their diet.
The remaining amino acids, the essential amino acids, must be obtained from food in preassembled form.

52. Essential Amino Acids
A diet that provides insufficient amounts of one or more essential amino acids causes a form of malnutrition called protein deficiency.
Malnutrition is much more common than undernutrition in human populations.

53. Essential Amino Acids
Most plant proteins are incomplete in amino acid makeup.
Individuals who eat only plant proteins need to eat a variety to ensure that they get all the essential amino acids.