Digestive System Pt 1 (Bio 11 Unit 3)

The Small Intestine and Pancreas

• Most digestion takes place in the small intestine, named for its narrow diameter.
• In humans, it's up to 7 m long but only 2.5 cm in diameter.
• The large intestine, by comparison, is only 1.5 m in length, but 7.6 cm in diameter.
• In mammals, the length of the small intestine is related to diet.
• Meats are relatively easy to digest, while plant materials are more difficult to digest.
• Carnivores (wolves, lions) have short small intestines.
• Herbivores (rabbits) have long small intestines.
• Omnivores (raccoons, pigs, bears, humans) have small intestines of intermediate length.
• The majority of digestion occurs in the duodenum (first 25 to 30 cm of the small intestine).
• The second and third components of the small intestine are called the jejunum and ileum.
• The three segments are differentiated by cell shape.
• Partially digested foods reach the small intestine already soaked in HCl and pepsin.

Protection of the Small Intestine

• When acids enter the small intestine, prosecretin is converted into secretin.
• Secretin is absorbed into the bloodstream and carried to the pancreas.
• Secretin signals the release of a solution containing bicarbonate ions (HCO_3^-).
• Bicarbonate ions are carried to the small intestine, where they neutralize the HCl in gastric fluid.
• This raises the pH from about 2.5 to 9.0.
• The now basic pH inactivates pepsin.
• Thus, the small intestine is protected from stomach acids by the release of secretin.

Pancreatic Secretions

• The pancreatic secretions also contain enzymes that promote the breakdown of proteins, carbohydrates, and lipids.
• Trypsinogen (a protein-digesting enzyme) is released from the pancreas.
• Enterokinase (an enzyme) converts trypsinogen into trypsin, which acts on partially digested proteins.
• Trypsin breaks down long-chain polypeptides into shorter-chain peptides.
• Erepsins (enzymes) are released from the pancreas and small intestine.
• They complete protein digestion by breaking the bonds between short-chain peptides, releasing individual amino acids.
• The pancreas also releases amylase enzymes, which continue the digestion of carbohydrates begun in the mouth by salivary amylase.
• The intermediate-size chains are broken down into disaccharides.
• The small intestine releases disaccharide enzymes (disaccharidases), which complete the digestion of carbohydrates.
• Amylase is the enzyme that breaks down amylose, and disaccharidases break down disaccharides into monosaccharides.
• Lipases are enzymes released from the pancreas that break down lipids (fats).
• Pancreatic lipase (the most common) breaks down fats into fatty acids and glycerol.
• Phospholipase acts on phospholipids.

Lactose Intolerance

• Many people are unable to digest lactose (milk sugar) because their bodies do not produce sufficient quantities of the enzyme lactase.
• Normally, the disaccharide lactose is broken down into two monosaccharides, which are then absorbed into the blood.
• Lactose-intolerant people are unable to break down lactose in the small intestine, so when it moves to the large intestine, water is drawn in by osmosis, causing diarrhea.

Summary Points

• Most digestion occurs in the duodenum.
• When acids enter the small intestine, prosecretin is converted to secretin.
• This causes the pancreas to signal the release of bicarbonate ions, which help neutralize HCl and inactivate pepsin.
• Pancreas secretions (such as trypsinogen and erepsins) play a large role in protein digestion.
• The pancreas also secretes amylase enzymes, which continue the digestion of carbohydrates that was started in the mouth.
• The pancreas also releases lipases, which are lipid-digesting enzymes.
• The small intestine secretes disaccharidases, which complete the digestion of carbohydrates.

Starch Digestion Experiment

Introduction

• Very little starch is broken down in the mouth.
• The low pH of the digestive fluids in the stomach halts digestion of carbohydrates such as starch until the carbohydrates leave the stomach and enter the small intestine.

Questions

• What is the best pH for starch digestion?
• What is the best temperature for starch digestion?

Hypothesis/Prediction

• Form a hypothesis about the ideal pH level for starch digestion.
• Predict whether a very cold temperature or very warm conditions would promote the most complete breakdown of starch.

Design

• A cornstarch suspension will be mixed with an enzyme solution at different pH levels and at different temperatures to see which acidity level and which temperature result in the most complete breakdown of starch.
• The efficiency can be measured by how much sugar is produced.
• Benedict’s reagent is used to indicate the presence of maltose, a disaccharide.

Materials

• apron
• goggles
• 10 test tubes
• test-tube rack
• 1% cornstarch suspension
• 5% amylase solutions at pH 2.0, 7.0, and 12.0
• Benedict’s reagent
• hot plate
• ice cubes
• thermometers
• two 250-mL beakers
• utility stand
• ring clamp
• tap water
• 25-mL graduated cylinder
• labelling materials
• eyedropper
• timer or watch
• rubber stoppers for test tubes
• glass stirring rod

Procedure: Part 1 - The Effect of pH on Starch Digestion

1. Copy Table 2 in your notebook and complete it as you perform each step in the activity.

2. Put on your apron and goggles.
3. Label 4 test tubes from 1 to 4. Set up a water bath as shown in Figure 4.
4. Place 15 mL of the 1% cornstarch suspension into each of the 4 test tubes.
5. Add 5 drops of the pH 2.0 amylase solution to test tube 2. Add 5 drops of the pH 7.0 amylase solution to test tube 3. Add 5 drops of the pH 12.0 amylase solution to test tube 4. Put a rubber stopper in each test tube and shake.
6. Let the test tubes sit for 20 min. Record your observations about each test tube. A colour change from blue to yellow to orange indicates maltose.
7. Add 5 mL of Benedict’s reagent to each of the 4 test tubes and place them in the hot water bath at 100°C. If you use the same cylinder as in Step 4, make sure to rinse and dry it first. Record your observations after 5 min. Do not let the test tubes sit in the hot water bath for more than 5 min.

Procedure: Part 2 - The Effect of Temperature on Starch Digestion

8. Copy Table 3 in your notebook and add 5 more rows, numbered up to 6. Complete the table as you perform each step in the activity.

9. Label 6 test tubes from 1 to 6.

10. Place 15 mL of cornstarch suspension in each test tube.

11. Add 5 drops of amylase solution at pH 7.0 to test tubes 1, 3, and 5.

12. Place test tubes 1 and 2 in the hot water bath and heat until the cornstarch suspension reaches 50°C. Do not heat the contents of the test tubes above 50°C.

13. Place test tubes 3 and 4 in a beaker of ice water. Let the cornstarch suspensions chill to a temperature between 0°C and 5°C. Stirring the water with a stirring rod may speed the cooling process.

14. Keep test tubes 5 and 6 at room temperature. Record the temperature of the cornstarch suspension. Record your observations about each test tube.

15. Let all test tubes stand for 20 min. Maintain temperature conditions for the test tubes.

16. Add 5 mL of Benedict’s reagent to each test tube and place them in a hot water bath at 100°C for 5 min. Record your observations in the table.