1.4.3 Digestion in the Small Intestine

Introduction

The small intestine is the longest part of the alimentary canal and the main site of chemical digestion. Here, starch, proteins, and fats are all enzymatically broken down into their simplest forms. The small intestine receives secretions from the liver, pancreas, and its own intestinal glands to complete the process of digestion. This topic explains what happens in the small intestine during digestion.

Definition

Digestion in the small intestine refers to the breakdown of starch, proteins, and fats into small soluble molecules by enzymes present in bile, pancreatic juice, and intestinal juice.

Main Idea

When food (chyme) enters the duodenum, it stimulates:

• The gall bladder to release bile.
• The pancreas to secrete pancreatic juice.
• The intestinal glands to secrete intestinal juice.

All these secretions are alkaline and neutralize the acidic chyme. The pancreatic and intestinal enzymes can work only in this alkaline medium.

Core Concept

Secretions in the Small Intestine

Detailed Explanation

1. Entry of Chyme into the Duodenum

When food enters the duodenum, the chyme stimulates:

(a) The gall bladder to release bile.

- Bile is released into the duodenum through the bile duct.

(b) The pancreas to secrete pancreatic juice.

- Pancreatic juice enters the duodenum through the pancreatic duct.

(c) The intestinal glands to secrete intestinal juice.

- Intestinal juice is secreted directly into the small intestine.

Simple explanation: When chyme enters the small intestine, it sends signals to the liver, gallbladder, and pancreas to release their digestive juices.

2. Neutralization of Chyme

All these secretions (bile, pancreatic juice, intestinal juice) are alkaline and neutralize the acidic chyme.

Why this is important:

• The chyme from the stomach is acidic (due to HCl).
• Pancreatic and intestinal enzymes can work only in an alkaline medium.
• If the chyme remains acidic, these enzymes will not function.

Simple explanation: The small intestine cannot work in acid. So, alkaline juices are added to neutralize the acid and create the right environment for enzymes to work.

3. Bile

What it is:

• Bile is a greenish-yellow liquid manufactured in the liver.
• Bile is stored in the gallbladder.
• Bile is released as soon as food enters the duodenum.

What it does:

• Bile emulsifies fats, causing them to break down into small droplets called emulsion.
• Emulsification provides relatively large surface areas of lipid for the action of lipase enzymes.

Simple explanation: Bile is like a detergent. It breaks large fat droplets into tiny droplets so that lipase can digest them faster.

Important point:

• Bile does not contain enzymes.
• Bile only emulsifies fats (physical breakdown).
• The actual digestion of fats is done by lipase.

4. Pancreatic Juice

The secretion of the pancreas is called pancreatic juice. It contains enzymes:

Pancreatic amylase:

• It is the starch-digesting enzyme.
• It converts polysaccharides to maltose and even to glucose.

Pancreatic lipase:

- It converts neutral fats (i.e., triglycerides) into glycerol and fatty acids.

Trypsinogen:

• It is secreted in inactive form.
• It is activated to trypsin by enterokinase (from intestinal juice).
• Trypsin converts proteins into polypeptides.

Simple explanation: Pancreatic juice is a complete digestive juice. It contains enzymes to digest starch, fats, and proteins.

5. Digestion in Jejunum and Ileum

The lining of the jejunum and ileum secretes several enzymes:

Erepsin:

- Converts peptides into amino acids.

Enterokinase:

- Activates pancreatic trypsinogen into its active form trypsin.

Lactase:

- Converts lactose to glucose.

Maltase:

- Converts maltose to glucose.

In the ileum, digestion process is complete.

Simple explanation: The jejunum and ileum produce enzymes that complete the digestion of peptides, lactose, and maltose. By the end of the small intestine, all food is fully digested.

Intermediate Level

Digestion in the small intestine involves three main secretions:

• Emulsifies fats.
• Does not contain enzymes.
• Increases surface area for lipase action.

• Pancreatic amylase: starch → maltose, glucose.
• Pancreatic lipase: fats → glycerol + fatty acids.
• Trypsinogen: proteins → polypeptides (after activation to trypsin).

• Erepsin: peptides → amino acids.
• Enterokinase: activates trypsinogen → trypsin.
• Lactase: lactose → glucose.
• Maltase: maltose → glucose.

• All secretions are alkaline.
• They neutralize the acidic chyme.
• Enzymes work best in alkaline medium.

• Starch → glucose
• Proteins → amino acids
• Fats → glycerol + fatty acids

Important Points

• - Starch, proteins, and fats are all enzymatically broken down in the small intestine.
• - When food enters the duodenum, it stimulates the gall bladder, pancreas, and intestinal glands.
• - Bile is manufactured in the liver and stored in the gallbladder.
• - Bile emulsifies fats, causing them to break down into small droplets.
• - Emulsification provides a large surface area for lipase action.
• - Pancreatic juice contains amylase, lipase, and trypsinogen.
• - Pancreatic amylase converts polysaccharides to maltose and glucose.
• - Pancreatic lipase converts triglycerides into glycerol and fatty acids.
• - Trypsinogen is activated to trypsin by enterokinase.
• - Trypsin converts proteins into polypeptides.
• - Intestinal juice contains erepsin, enterokinase, lactase, and maltase.
• - Erepsin converts peptides into amino acids.
• - Lactase converts lactose to glucose.
• - Maltase converts maltose to glucose.
• - All secretions are alkaline and neutralize the acidic chyme.
• - In the ileum, digestion is complete.

Remember

• - Small intestine = main site of digestion
• - Bile = from liver, stored in gallbladder, emulsifies fats
• - Pancreatic juice = amylase, lipase, trypsinogen
• - Intestinal juice = erepsin, enterokinase, lactase, maltase
• - Trypsinogen → Trypsin (by enterokinase)
• - Peptides → Amino acids (by erepsin)
• - Lactose → Glucose (by lactase)
• - Maltose → Glucose (by maltase)
• - All secretions are alkaline
• - Digestion is complete in the ileum

Common Mistakes

• 1. Thinking bile contains enzymes.
• - Bile does not contain enzymes. It only emulsifies fats.
• 2. Confusing emulsification with digestion.
• - Emulsification = physical breakdown of fats into small droplets.
• - Digestion = chemical breakdown of fats into glycerol and fatty acids by lipase.
• 3. Forgetting that trypsinogen is inactive.
• - Trypsinogen is the inactive form. It is activated to trypsin by enterokinase.
• 4. Mixing up the enzymes of pancreatic juice and intestinal juice.
• - Pancreatic juice = amylase, lipase, trypsinogen.
• - Intestinal juice = erepsin, enterokinase, lactase, maltase.
• 5. Thinking digestion is complete in the stomach.
• - Digestion is complete in the small intestine, specifically in the ileum.
• 6. Believing bile is released from the liver directly.
• - Bile is produced by the liver but stored and concentrated in the gallbladder. It is released from the gallbladder.
• 7. Confusing lactase and maltase.
• - Lactase = lactose → glucose.
• - Maltase = maltose → glucose.

Quick Revision

60-Second Revision

• Small intestine = main site of digestion
• Bile = from liver, stored in gallbladder, emulsifies fats (no enzymes)
• Pancreatic juice = amylase (starch → maltose), lipase (fats → glycerol + fatty acids), trypsinogen (proteins → polypeptides)
• Intestinal juice = erepsin (peptides → amino acids), enterokinase (activates trypsinogen), lactase (lactose → glucose), maltase (maltose → glucose)
• All secretions are alkaline = neutralize acidic chyme
• Digestion is complete in the ileum
• Warning: Bile emulsifies fats but does not digest them.