CHAPTER 66
Digestion and Absorption in the Gastrointestinal Tract
General Overview of Digestion
1. Main Food Groups and Their Breakdown:
◦ The body’s main nutrients are carbohydrates, fats, and proteins.
◦ These nutrients cannot be absorbed in their natural form. They need to be broken down (digested) into
smaller molecules for absorption.
◦ Digestion involves enzymes and a process called hydrolysis to break down large molecules.
2. What is Hydrolysis?
◦ Hydrolysis is the process where water molecules (H₂O) are used to break down bonds in large molecules.
◦ For example:
▪ Carbohydrates → Monosaccharides (e.g., glucose).
, ▪ Fats → Glycerol + Fatty Acids.
▪ Proteins → Amino Acids.
Carbohydrate Digestion
1. Major Sources of Carbohydrates in the Diet:
◦ Sucrose: Commonly known as table sugar (disaccharide).
◦ Lactose: Found in milk (disaccharide).
◦ Starches: Found in grains and potatoes (polysaccharides).
◦ Cellulose: Found in plants, but humans cannot digest it (it acts as fiber instead).
2. Where and How Carbohydrates Are Digested:
◦ In the Mouth:
▪ Saliva contains an enzyme called ptyalin (α-amylase), which begins breaking down starch into
maltose (a disaccharide).
▪ However, digestion in the mouth is brief, breaking down only 5% of starch.
◦ In the Stomach:
▪ Starch digestion continues for up to an hour until stomach acid deactivates ptyalin.
▪ Around 30–40% of starch is digested to maltose before mixing with stomach acid.
Digestion and Absorption in the Gastrointestinal Tract
General Overview of Digestion
1. Main Food Groups and Their Breakdown:
◦ The body’s main nutrients are carbohydrates, fats, and proteins.
◦ These nutrients cannot be absorbed in their natural form. They need to be broken down (digested) into
smaller molecules for absorption.
◦ Digestion involves enzymes and a process called hydrolysis to break down large molecules.
2. What is Hydrolysis?
◦ Hydrolysis is the process where water molecules (H₂O) are used to break down bonds in large molecules.
◦ For example:
▪ Carbohydrates → Monosaccharides (e.g., glucose).
, ▪ Fats → Glycerol + Fatty Acids.
▪ Proteins → Amino Acids.
Carbohydrate Digestion
1. Major Sources of Carbohydrates in the Diet:
◦ Sucrose: Commonly known as table sugar (disaccharide).
◦ Lactose: Found in milk (disaccharide).
◦ Starches: Found in grains and potatoes (polysaccharides).
◦ Cellulose: Found in plants, but humans cannot digest it (it acts as fiber instead).
2. Where and How Carbohydrates Are Digested:
◦ In the Mouth:
▪ Saliva contains an enzyme called ptyalin (α-amylase), which begins breaking down starch into
maltose (a disaccharide).
▪ However, digestion in the mouth is brief, breaking down only 5% of starch.
◦ In the Stomach:
▪ Starch digestion continues for up to an hour until stomach acid deactivates ptyalin.
▪ Around 30–40% of starch is digested to maltose before mixing with stomach acid.
