🧠 Biology IGCSE (0610): The Digestive System Study Notes

Hello future biologists! This chapter is all about how your body breaks down that huge pizza or healthy salad into tiny molecules that your cells can actually use. We call this process digestion. It’s one of the most vital things your body does, and understanding it means understanding how you get the energy to study (and relax!). Let’s dive in!

7.2 The Five Stages of Nutrition

The entire process of getting food and converting it into energy for the body happens in five key stages.

1. Ingestion

This is simply the act of taking food (or drink) into the body, usually through the mouth.

2. Digestion

This is the main event! It is the breakdown of large, insoluble food molecules into small, soluble molecules. Digestion occurs in two ways:

  • Physical Digestion (e.g., chewing).
  • Chemical Digestion (using enzymes).

3. Absorption

Once the food is broken down into small, soluble nutrient molecules (like glucose and amino acids), they need to enter the blood. Absorption is the movement of these small molecules from the intestines into the blood (or lymph).

4. Assimilation

After the nutrients are absorbed, they are transported in the blood to the body’s cells. Assimilation is the uptake and use of these nutrients by the cells for energy, growth, and repair.

Analogy: Think of the body as a city. Digestion is breaking down raw materials into building blocks. Absorption is loading those blocks onto trucks (the blood). Assimilation is when the construction crews (cells) actually use the blocks to build new structures or power their tools.

5. Egestion

Not everything is digestible (like fibre). Egestion is the removal of undigested food from the body as faeces (poo) via the anus.

Quick Review Tip: A handy way to remember these stages in order is: I. D. A. A. E. (Ingestion, Digestion, Absorption, Assimilation, Egestion)

7.2 The Main Organs of the Digestive System

The digestive system is made up of the alimentary canal (the long tube food passes through) and several associated organs (which produce substances needed for digestion).

The Alimentary Canal (The Food's Pathway)

This tube runs from the mouth to the anus:

  • Mouth: Ingestion starts here; physical digestion (chewing) and early chemical digestion (salivary amylase).
  • Oesophagus: A muscular tube that pushes food down to the stomach via peristalsis (wave-like muscle contractions).
  • Stomach: A muscular sac that churns food (physical digestion) and contains gastric juices for chemical digestion.
  • Small Intestine: The main site for digestion and absorption. It has two parts:
    • Duodenum: Where most chemical digestion occurs, receiving juices from the pancreas and gall bladder.
    • Ileum: Where the digested nutrients are absorbed into the blood.
  • Large Intestine: Primarily for absorbing water. It has three parts:
    • Colon: Main site of water absorption.
    • Rectum: Stores faeces temporarily.
    • Anus: Exit point for egestion.
Associated Organs (The Helpers)

These organs produce substances (like enzymes and bile) that are crucial for digestion but food does not pass through them.

  • Salivary Glands: Produce saliva containing amylase.
  • Pancreas: Produces powerful digestive enzymes (amylase, protease, lipase) which are released into the duodenum.
  • Liver: Produces bile.
  • Gall Bladder: Stores bile before releasing it into the duodenum.

Key Takeaway: The alimentary canal is a continuous tube where food is broken down, absorbed, and waste is collected. Associated organs release necessary digestive chemicals.

7.3 Physical Digestion: The Mechanical Breakdown

Physical digestion is the mechanical process of breaking food into smaller pieces without changing the molecules chemically.

Why Do We Need Physical Digestion?

The smaller pieces of food have a much larger surface area compared to the original large chunk. This is critical because enzymes can only act on the surface of the food.

More surface area = Faster enzyme action = Faster chemical digestion.

Teeth (Core Content)

Your teeth are specialised for different types of physical breakdown. Teeth are embedded in the bone and gums.

Structure of a Human Tooth:

  • Enamel: Hardest substance in the body; protects the crown.
  • Dentine: Bone-like material under the enamel.
  • Pulp: Contains soft tissue, including nerves (why you feel pain!) and blood vessels (supplying nutrients).
  • Cement: Secures the tooth to the jaw bone.

Types and Functions:

  • Incisors: Chisel-shaped, used for cutting and slicing food (like scissors). (4 in total).
  • Canines: Pointed, used for tearing meat (like sharp spikes). (4 in total).
  • Premolars: Have a flat surface with ridges (cusps), used for chewing and grinding. (8 in total).
  • Molars: Larger and broader than premolars, used for heavy grinding and crushing. (12 in total, including wisdom teeth).
Physical Digestion in the Stomach

The stomach wall is highly muscular and churns the food, mixing it thoroughly with gastric juices. This mechanical mixing further breaks down the food.

The Role of Bile (Supplement Content)

Bile, produced by the liver and stored in the gall bladder, plays a physical role in fat breakdown:

  • It emulsifies large fat globules.
  • Emulsification means breaking large fat droplets into tiny droplets.
  • This dramatically increases the fat’s surface area, making it easier for the lipase enzyme to act.

Key Takeaway: Physical digestion (chewing, churning, emulsification) is crucial preparation. It increases the surface area so enzymes can work efficiently in the next stage.

7.4 Chemical Digestion: The Enzyme Action

Chemical digestion is the breakdown of large insoluble molecules (like starch and protein) into small soluble molecules (like glucose and amino acids) using digestive enzymes.

Why do they need to be small and soluble? Because only small, soluble molecules can diffuse across the gut wall into the blood during absorption.

The Three Key Digestive Enzymes (Core Content)

Enzymes are specific. We need different enzymes to digest the three main food groups:

1. Carbohydrases (Amylase and Maltase)

  • Function: Break down starch.
  • Process (Core): Amylase breaks down starch into simple reducing sugars.
  • Process (Supplement): Specifically, amylase breaks starch into maltose. Maltase (found on the small intestine lining) then breaks maltose into glucose.
  • Secretion/Action Site: Salivary glands (mouth) and Pancreas (small intestine).

2. Proteases (Pepsin and Trypsin)

  • Function: Break down proteins.
  • Process: Protease breaks down proteins into amino acids.
  • Secretion/Action Site (Core): Stomach and Pancreas (small intestine).
  • Process (Supplement): Pepsin works in the acidic stomach. Trypsin works in the alkaline small intestine.

3. Lipases

  • Function: Break down fats and oils.
  • Process: Lipase breaks down fats/oils into fatty acids and glycerol.
  • Secretion/Action Site: Pancreas (small intestine).

Did you know? Glucose, amino acids, fatty acids, and glycerol are the final small molecules ready for absorption!

Controlling the Environment: Acid and Alkali

Enzymes work best at their optimum pH. The digestive system has two very different pH zones:

1. Stomach Acid (Hydrochloric Acid, HCl) (Core Content)

  • Function 1: Kills harmful microorganisms (bacteria) present in the food.
  • Function 2: Provides the very acidic pH (around pH 2) necessary for the protease enzyme, pepsin, to work optimally.

2. Bile (Alkaline) (Supplement Content)

  • Bile is secreted into the duodenum.
  • The mixture of food leaving the stomach is highly acidic.
  • Bile is an alkaline mixture that neutralises this acidic mixture.
  • This neutralisation provides a suitable alkaline pH (around pH 8) for the enzymes in the small intestine (like trypsin and lipase) to work optimally.

Quick Review Box: Chemical Digestion
Starch $\xrightarrow{\text{Amylase}}$ Maltose $\xrightarrow{\text{Maltase}}$ Glucose (Small Intestine)
Protein $\xrightarrow{\text{Protease}}$ Amino Acids (Stomach/Small Intestine)
Fats $\xrightarrow{\text{Lipase}}$ Fatty Acids + Glycerol (Small Intestine)

7.5 Absorption: Moving Nutrients into the Blood

Once digestion is complete, the small, soluble nutrients must be moved into the bloodstream. This happens mainly in the small intestine (specifically the ileum).

Structure of the Small Intestine: Maximising Surface Area (Supplement Content)

The small intestine is incredibly long, but its structure is further enhanced to maximise the rate of absorption:

  • The inner wall is highly folded, covered in tiny, finger-like projections called villi (singular: villus).
  • Each villus surface is covered with even tinier projections called microvilli.

The significance of the villi and microvilli is that they vastly increase the internal surface area of the small intestine. This large area allows nutrients to be absorbed quickly and efficiently.

Structure of a Villus (Supplement Content)

Each villus is adapted for rapid absorption because it is:

  • Very thin walled (only one cell thick), providing a short diffusion distance.
  • Has a rich network of blood capillaries.
  • Contains a central lymph vessel called a lacteal.
Roles of Capillaries and Lacteals

The different nutrients take different routes out of the villus:

  • Capillaries: They absorb glucose and amino acids. These enter the bloodstream directly and are carried away to the liver.
  • Lacteals: They absorb fatty acids and glycerol. These molecules are recombined and packaged and enter the lymphatic system before eventually joining the bloodstream.

Analogy: Villi are like microscopic sponges covering the intestinal wall, designed to soak up every nutrient possible!

Water Absorption (Core Content)

While nutrients are absorbed mainly in the small intestine, water absorption occurs in two places:

  • Most water is absorbed from the mixture (called chyme) in the small intestine.
  • Some water is also absorbed from the remaining contents in the colon (large intestine). If too little water is absorbed here, the result is diarrhoea. If too much is absorbed, the result is constipation.

Key Takeaway: The small intestine is the nutrient absorption powerhouse, designed with villi and microvilli to maximize surface area. Glucose and amino acids go into the blood, while fats go into the lacteal.