Welcome to Digestion and Absorption!

Hi there! This chapter is all about the amazing journey food takes once it enters your body. We will learn how that delicious meal is broken down into tiny pieces so your body can actually use the nutrients for energy, growth, and repair.
Don't worry if the names of organs or enzymes seem long—we will break down the process step-by-step, just like your body breaks down food! This is Topic 3 of your syllabus, and it’s crucial for understanding nutrition.

Section 1: What is Digestion?

The Goal: From Big Molecules to Small Molecules

Digestion is the process of breaking down large, complex food molecules (like proteins, starches, and fats) into small, soluble molecules (like amino acids, glucose, and fatty acids) that can be absorbed into the bloodstream.
Think of it like dismantling a large Lego structure so the individual blocks can be shipped out and used to build something new elsewhere.

Two Types of Digestion
  • Mechanical Digestion: Physical breakdown of food into smaller pieces. This increases the surface area for enzymes to work on. (Example: chewing in the mouth, churning in the stomach).
  • Chemical Digestion: The breakdown of molecules using special chemicals called enzymes. This is where the complex molecules are split apart.

Section 2: Enzymes – The Catalysts of Digestion

Enzymes are special types of proteins that act as catalysts. This means they speed up chemical reactions in the body without being used up themselves.
Analogy: If digestion is like building a car, enzymes are the specialised tools that perform specific tasks quickly, like tightening a bolt or cutting a wire.

Key Enzymes You Need to Know (Grouped by Nutrient)

Different enzymes are specific to the macronutrient they break down.

1. Enzymes that digest Carbohydrates: (They break down complex sugars and starches into simple sugars like glucose)

  • Amylase: Breaks down starch (a polysaccharide) into simpler sugars (like maltose). Found in the saliva and pancreatic juice.
  • Maltase: Breaks down maltose into glucose.
  • Lactase: Breaks down lactose (the sugar in milk) into glucose and galactose.
  • Invertase (Sucrase): Breaks down sucrose (table sugar) into glucose and fructose.

2. Enzymes that digest Proteins: (They break proteins down into amino acids)

  • Pepsin: Starts protein breakdown in the stomach.
  • Trypsinogen: This is an inactive enzyme released by the pancreas. It is converted to active Trypsin in the duodenum, where it continues to break down proteins.
  • Erepsin: Completes protein digestion, breaking peptides into final amino acids (found in the ileum).
  • Rennin: A specific enzyme (important in infants) that curdles milk protein (casein) in the stomach, making it easier for pepsin to act upon.

3. Enzymes that digest Fats:

  • Lipase: Breaks down fats (lipids) into fatty acids and glycerol. (Released by the pancreas).

Quick Memory Tip: Think about the end result of the nutrient. Enzymes often have "-ase" at the end, and the name usually relates to what they break down (e.g., Lactase breaks down Lactose).


Section 3: Digestion – The Journey Through the System

We will follow a bite of food through the four main digestive sites listed in your syllabus: the mouth, stomach, duodenum, and ileum.

Stage 1: The Mouth (Start of Mechanical and Chemical Digestion)

  • Mechanical: Chewing (mastication) breaks food into smaller pieces, mixing it with saliva.
  • Chemical: Saliva contains the enzyme Amylase, which starts the breakdown of starch into simpler sugars.
  • Key Takeaway: Starch digestion begins here!

Stage 2: The Stomach (Protein Digestion Starts)

The stomach is a muscular bag that churns food (mechanical digestion) and mixes it with strong digestive juices (chemical digestion).

  • Hydrochloric Acid (HCl): Makes the stomach contents very acidic (pH 1.5–3.5). This acid kills bacteria and provides the optimum pH environment for the enzyme Pepsin to work.
  • Pepsin: The key enzyme here. It begins the breakdown of protein into polypeptides.
  • Rennin: Curdles milk (especially important for babies) to keep it in the stomach long enough for pepsin to act on it.

Stage 3: The Duodenum (The Mixing Pot)

The duodenum is the first, short section of the small intestine. It receives digestive juices from two major accessory organs: the pancreas and the liver/gall bladder.

The acidic food (called chyme) coming from the stomach is neutralised here by alkaline fluids.

A. Juices from the Pancreas (Pancreatic Juice)

The pancreas sends powerful enzymes into the duodenum to break down all three macronutrients:

  • Pancreatic Amylase: Continues starch digestion.
  • Lipase: Breaks down fats into fatty acids and glycerol.
  • Trypsinogen: Activated into Trypsin to continue protein digestion.
B. The Role of Bile

Bile is produced by the liver and stored in the gall bladder. It enters the duodenum, but it is NOT an enzyme.

  • Function: Bile’s main role is emulsifying fats.
  • Emulsification: This process breaks large droplets of fat into tiny, smaller droplets. This greatly increases the surface area for the enzyme Lipase to work on, making fat digestion much faster and more efficient.

Stage 4: The Ileum (Final Digestion and Absorption)

The ileum is the longest part of the small intestine. This is where digestion is completed and where almost all nutrients are absorbed.

  • Final Enzymes: The walls of the ileum release enzymes that finish the job: Erepsin (completes protein digestion) and the disaccharidases (Maltase, Lactase, Invertase) that break down the last remaining simple sugars into monosaccharides (like glucose).
  • End Products Ready for Absorption:
    Proteins -> Amino Acids
    Carbohydrates -> Monosaccharides (Glucose)
    Fats -> Fatty Acids and Glycerol

Section 4: Absorption of Nutrients

Once the food is fully digested into its smallest forms, it must be absorbed into the bloodstream so it can travel to where it’s needed (like muscles and organs). This process happens mainly in the ileum.

The Structure of the Ileum: Maximising Surface Area

To absorb nutrients efficiently, the ileum must have a massive surface area. It achieves this using two special structures:

1. Villi (Plural) / Villus (Singular)

The inner lining of the ileum is covered in millions of tiny, finger-like projections called villi.

  • Structure: Each villus is extremely thin-walled (only one cell thick) and rich in blood capillaries and a central lymphatic vessel called a lacteal.
  • Function: The thin walls and dense blood supply allow for quick diffusion of nutrients directly into the blood.
2. Microvilli (The Brush Border)

Each cell on the surface of a villus has even tinier projections called microvilli.

  • The combination of folds, villi, and microvilli gives the small intestine a surface area equivalent to a tennis court—about 250 square meters!
  • More Surface Area = Faster and More Efficient Absorption.

Did You Know? The structure of the ileum, with its folds and projections, is similar to how a highly absorbent towel is woven—lots of texture and loops to soak up as much liquid (nutrients) as possible.

How Nutrients are Absorbed

  • Amino Acids, Glucose, Vitamins, and Minerals: These are absorbed directly through the villi cell walls into the blood capillaries and are then carried via the hepatic portal vein to the liver.
  • Fatty Acids and Glycerol: These molecules reform into tiny fat droplets and are absorbed into the lacteal (the lymphatic vessel) in the centre of the villus. They enter the bloodstream later, bypassing the liver initially.

Quick Review: Digestion and Absorption


Key Term: Enzymes are catalysts.
Mouth: Starch broken down by Amylase.
Stomach: Protein broken down by Pepsin (in acidic conditions).
Duodenum: Receives Bile (to emulsify fats) and pancreatic enzymes (Amylase, Lipase, Trypsinogen).
Ileum: Final digestion by Erepsin, Maltase, Lactase, Invertase. Absorption occurs using villi and microvilli to maximise surface area.