Digestive System

1. Digestive System Thursday November 24th, 2011

2. Why do we need to digest? • Heterotrophic! Obtain nutrients by consuming other organisms. • Food that we consume is in the form of large, complex molecules. • We need nutrients to be small enough to enter our cells. (pass through cell membrane) • Every major organ system requires nutrients to function.

3. The 4 Components of the Digestive System #1: Ingestion The taking in of nutrients #2: Digestion  The breakdown of complex organic molecules into small molecules. (By enzymes) #3: Absorption  The transport of digested nutrients to the tissues of the body. #4: Egestion  The removal of waste food materials from the body.

4. Digestive Enzymes • Catalyst: A compound that speeds up the rate of a chemical reaction without being used up in the reaction. • Enzymes are biological catalysts, they speed up chemical reactions in the body. • Increases temp helps speed up chemical reactions, but we can’t do this inside our bodies. • Enzymes are required to break down nutrient molecules during digestion. • Without enzymes, digestion would take too long and we would die before the nutrients were ever absorbed.

6. Important macromolecules • Macromolecule – very large molecules consumed in our diets. • Molecules must be broken down into smaller monomers in order to be absorbed by our cells. • Proteins  amino acids • Carbohydrates  simple sugars • Fats  fatty acids, glycerol

7. Digestion in Simple Organisms

8. Ingestion in Earthworms and Birds • Digestive systems that have a separate opening for a mouth and an anus are called alimentary canals. • Mouth Pharynx Esophagus Crop  Gizzard • Pharynx: Muscular area past the mouth that helps push food down the esophagus. • Esophagus  Carries food from the pharynx to the stomach. • Crop Stores undigested food. • Gizzard  Muscular chamber, usually containing small stones that helps to physically break down food.

10. Ingestion in Humans • Mouth • Physical and chemical digestion • Physical = breakdown of food into smaller pieces by teeth.

11. Incisors  Cutting • Canine  Tearing • Premolars  Grinding • Molars  Crushing

12. Chemical breakdown in mouth • Saliva  Mix of water and enzymes produced by the salivary glands in the mouth. • Contains the enzyme Amylase, which breaksdown complex carbohydrates into simple sugars. • Saliva also helps lubricate the food to make it easier to swallow.

13. The Esophagus • Connects the pharynx to the stomach. • Very muscular • Waves on muscle contractions send food down to the stomach. • Called peristalsis • Swallowed food = bolus

14. The Stomach • Function: Muscular sac that stores food and performs chemical digestion. • Can hold approx 1.5L • The inner wall of the stomach contains many folds called Rugae.

15. Stomach continued • Rugae allow the stomach to expand as more food enters. • The stomach contains many gastric cells that secrete gastric juices. • Gastric juice  Hydrochloric acid (pH 2-3), mucus, and enzymes. • Mucus is secreted to protect the stomach from digesting itself. • Hydrochloric acid breaks down the food into smaller pieces. • Enzyme Pepsin: Digests proteins.

16. Cardiac Sphincter  Circular muscle that controls the movement of food into the stomach. • Pyloric Sphincter  Controls the movement of food from the stomach into the small intestine.

17. Pepsin • Different enzymes have different conditions under which they function the most efficiently. • Pepsin digests proteins in the stomach, so it has an optimum pH of 2-3. • Trypsin is an enzyme that functions in the small intestine. It has an optimal pH of 7-8.

19. The Small Intestine • Site for the greatest amount of digestion. • Narrow diameter (2-2.5cm) but very long (7-8m) • 3 parts: duodenum, jejunum, and ileum. • Most digestion occurs in the duodenum.

20. Partially digested food from the stomach enters the duodenum. • This sets off a chemical signalling process. Acidity of food entering duodenum causes prosecretin to be converted into secretin. Secretin is absorbed into bloodstream and travels to pancreas. Secretin stimulates pancreas to release a bicarbonate solution (basic)

21. Bicarbonate solution moves into the small intestine, neutralizing the gastric juices from pH of 2.5 to 9. Now the partially digested food will not harm the small intestine. The change in pH deactivates the pepsin enzyme.

22. Absorption in the Small intestine • Inner wall of the small intestine is made up of finger-like projections called villi. • Villi increase the surface area of the small intestine, maximizes absorption of nutrients. • Villi contain capillary bed, allow digested nutrients to enter bloodstream. • Also contain a lacteal which is part of the lymphatic system (absorb fatty acids).

23. The Pancreas • Besides releases bicarbonate solution to neutralize the acid from the stomach, the pancreas also releases digestive enzymes into the duodenum. • Trypsin Continues to digest proteins. • Erepsin  Complete the digestion of proteins. • Amylase  Breaks down carbohydrates. • Lipase  Breaks down fats.

25. The Liver • Produced bile. • Bile is essential for the digestion of fats. • Bile is stored in the gall bladder before being released into the duodenum during digestion.

26. The Large Intestine • Main function is the absorption of water. • Approx 90% of water is absorbed back into bloodstream. • Billions of bacteria are present, help break down undigested materials. • Many of these bacteria produce important vitamins which are absorbed back into the bloodstream. (vitamin B, vitamin K)

27. Homework • Biozonepg 63,69-70,78-81 • Handout: The Human Digestive System • IB only: read pg 151-156. • 3U only: pg 238 # 1,3,4,10