2.1.3 The Human Digestive System

Main Organs of the Digestive System:

1. Glands (such as the salivary glands and pancreas):

• These glands produce digestive juices that contain enzymes. These enzymes break down food into smaller molecules.

2. Stomach:

• The stomach produces hydrochloric acid which serves two main purposes:
• It kills bacteria that may be present in the food.
• It creates an acidic environment, which is the optimum pH for the enzyme pepsin (a type of protease) to work efficiently.

3. Small Intestine:

• This is where the absorption of soluble molecules (like glucose and amino acids) into the blood occurs. These nutrients are then transported around the body to be used by cells.

4. Liver:

• The liver produces bile, which is stored in the gallbladder. Bile helps in the digestion of lipids (fats) by breaking them down into smaller droplets, making it easier for enzymes to act on them.

5. Large Intestine:

• The large intestine absorbs water from the undigested food, forming solid waste (faeces). This waste is then expelled from the body through the rectum and anus.

Enzymes in Digestion:

• Enzymes are biological catalysts, meaning they speed up chemical reactions without being used up themselves.
• In the digestive system, enzymes are crucial because they break down large, complex molecules into smaller, simpler ones that can be absorbed by the body.
• Enzymes are proteins, and their function depends on their specific shape. Each enzyme has a uniquely shaped active site where a specific substrate molecule fits, much like a key fits into a lock.

The Lock and Key Hypothesis:

1. The substrate (the molecule the enzyme acts on) has a shape that is complementary to the enzyme's active site.
2. When the substrate binds to the enzyme's active site, an enzyme-substrate complex is formed.
3. The enzyme then catalyses the reaction, breaking down the substrate into products, which are then released from the enzyme.

Factors Affecting Enzyme Activity:

• Optimum Temperature:

  • Enzymes work best at a specific temperature, usually around 37°C (normal body temperature).
  • As temperature increases up to this point, the rate of reaction increases. However, if the temperature gets too high, the enzyme's structure can break down, changing the shape of its active site. When this happens, the enzyme is denatured and can no longer function.

• Optimum pH:

  • Most enzymes have an optimum pH around 7 (neutral), but enzymes in the stomach, like pepsin, work best in acidic conditions.
  • If the pH is too high or too low, it can disrupt the bonds that hold the enzyme's structure together, leading to a change in the shape of the active site. This also results in denaturation.

Types of Digestive Enzymes:

1. Carbohydrases:

• Convert carbohydrates into simple sugars.
• Example: Amylase breaks down starch into maltose.
• Amylase is produced in the salivary glands, pancreas, and small intestine.

2. Proteases:

• Convert proteins into amino acids.
• Example: Pepsin is a protease found in the stomach; other proteases are produced in the pancreas and small intestine.

3. Lipases:

• Convert lipids (fats) into fatty acids and glycerol.
• Lipases are produced in the pancreas and small intestine.

Absorption and Use of Nutrients:

• Soluble nutrients like glucose, amino acids, fatty acids, and glycerol are absorbed into the bloodstream through the walls of the small intestine.
• These nutrients are then transported to cells throughout the body where they are used to build new carbohydrates, lipids, and proteins, or to release energy through respiration.

Food Tests:

• To identify the presence of specific nutrients in food, various tests can be used:
  • Benedict's Test for sugars (turns brick red if positive).

•                                                              **Heat in a water bath***
  

• Iodine Test for starch (turns blue-black if positive).

  

• Biuret Test for protein (turns purple if positive).

  

• Emulsion Test for lipids (a cloudy layer forms when ethanol is added if lipids are present).

  

• Sudan III Test for lipids (a red layer forms on top if lipids are present).

  

Role of Bile:

• Bile is produced by the liver and stored in the gallbladder. It is released into the small intestine to perform two main functions:
  1. Neutralising Acid: Bile is alkaline and neutralises the hydrochloric acid that comes from the stomach, creating a more suitable pH for enzymes in the small intestine.
  2. Emulsification: Bile breaks down large fat droplets into smaller ones, increasing their surface area. This allows lipase enzymes to work more effectively, speeding up the digestion of lipids into glycerol and fatty acids. These processes ensure that the food you eat is broken down into smaller, absorbable molecules that your body can use for energy, growth, and repair.