1. The Digestive System

Introduction

Every day, your body needs energy and nutrients to grow, repair tissues, and carry out thousands of chemical reactions. However, the food you eat cannot be used by your cells immediately. It must first be broken down into smaller molecules that can be absorbed into the bloodstream. This is the job of the digestive system.

The digestive system is a series of organs that work together to digest food, absorb nutrients, and remove undigested waste. Along the way, several accessory organs—including the liver, pancreas, and gallbladder—produce substances that help make digestion more efficient. Without the digestive system, your body could not obtain the nutrients needed to survive.

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What Is the Digestive System?

The digestive system is an organ system that:

• Breaks food into smaller molecules.
• Absorbs nutrients into the bloodstream.
• Eliminates undigested waste.

Digestion involves two main processes:

• Mechanical digestion – Physically breaking food into smaller pieces.
• Chemical digestion – Using enzymes and digestive juices to break large molecules into smaller ones.

Together, these processes make nutrients small enough to be absorbed by the body.

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Figure 1. The digestive system breaks down food, absorbs nutrients, and removes waste.

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Major Organs of the Digestive System

The digestive tract consists of:

• Mouth
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Rectum
• Anus

Three important accessory organs assist digestion:

• Liver
• Pancreas
• Gallbladder

Each organ has a specialised role.

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The Mouth

Digestion begins in the mouth.

The mouth performs both mechanical and chemical digestion.

Functions

• Teeth chew food into smaller pieces.
• The tongue mixes food with saliva.
• Salivary glands produce saliva.
• Saliva contains the enzyme amylase, which begins the digestion of starch.

Chewing increases the surface area of food, making chemical digestion more efficient.

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The Esophagus

The esophagus is a muscular tube connecting the mouth to the stomach.

Food is moved by peristalsis—wave-like contractions of smooth muscles.

Peristalsis pushes food toward the stomach even when a person is lying down.

The esophagus does not digest food but transports it safely to the stomach.

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Figure 2. Food travels from the mouth through the esophagus to the stomach by peristalsis.

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The Stomach

The stomach is a muscular organ that stores and digests food.

Functions

• Churns food to mix it with digestive juices.
• Produces hydrochloric acid (HCl).
• Produces enzymes such as pepsin, which digests proteins.
• Kills many harmful microorganisms.

The stomach turns food into a thick liquid called chyme, which is gradually released into the small intestine.

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The Small Intestine

The small intestine is the main site of digestion and nutrient absorption.

It is about 6 metres long in adults.

Functions

• Completes chemical digestion.
• Absorbs nutrients into the bloodstream.
• Contains tiny finger-like projections called villi, which greatly increase the surface area for absorption.

Nutrients such as glucose, amino acids, fatty acids, vitamins, and minerals pass through the villi into the blood.

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Figure 3. Villi provide a large surface area for absorbing nutrients into the bloodstream.

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The Large Intestine

The large intestine absorbs:

• Water
• Mineral salts

from undigested food.

Helpful bacteria living in the large intestine:

• Produce some vitamins.
• Help break down certain materials.

The remaining waste becomes faeces and is stored in the rectum before leaving the body through the anus.

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The Pathway of Food

Food follows this route through the digestive system:

Mouth → Esophagus → Stomach → Small Intestine → Large Intestine → Rectum → Anus

Each organ performs a specialised function before food moves to the next stage.

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Figure 4. Food passes through each organ in a specific order during digestion.

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Accessory Organs

Although food does not pass through them, these organs are essential for digestion.

Liver

The liver is the largest internal organ.

Functions include:

• Producing bile.
• Processing absorbed nutrients.
• Storing glycogen.
• Detoxifying harmful substances.

Bile helps break large fat droplets into smaller ones, making them easier for enzymes to digest.

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Gallbladder

The gallbladder stores bile produced by the liver.

When fatty food enters the small intestine, the gallbladder releases bile into the digestive tract.

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Pancreas

The pancreas produces:

• Digestive enzymes.
• Bicarbonate solution.

The enzymes digest carbohydrates, proteins, and fats.

Bicarbonate neutralises the acidic chyme entering from the stomach, protecting the lining of the small intestine.

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Figure 5. The liver, pancreas, and gallbladder are accessory organs that assist digestion.

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How the Digestive System Maintains Life

The digestive system supplies the body with:

• Glucose for energy.
• Amino acids for growth and repair.
• Fatty acids for energy storage and cell membranes.
• Vitamins and minerals for healthy body functions.
• Water for cellular processes.

Without digestion and absorption, cells would not receive the nutrients needed to survive.

The digestive system also works closely with the:

• Circulatory system, which transports absorbed nutrients.
• Muscular system, which moves food through the digestive tract.
• Nervous system, which helps regulate digestion.
• Endocrine system, which releases hormones that control digestive processes.

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Worked Example

Question

Complete the pathway of food through the digestive system.

Mouth → ________ → Stomach → ________ → Large intestine → Rectum → Anus

Solution

Mouth → Esophagus → Stomach → Small intestine → Large intestine → Rectum → Anus

Food is digested and nutrients are mainly absorbed in the small intestine.

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Real-World Connection

After eating a meal, carbohydrates are broken down into glucose, which is absorbed through the villi of the small intestine into the bloodstream. The circulatory system carries this glucose to body cells, where it is used in cellular respiration to release energy. This energy powers everything from thinking and walking to growing and repairing tissues.

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Did You Know?

The small intestine is about 6 metres long, but it fits comfortably inside your abdomen because it is tightly coiled. Its lining contains millions of villi and microscopic microvilli, creating an enormous surface area—about the size of a tennis court—for absorbing nutrients efficiently.

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Key Terms

Amylase – An enzyme in saliva that begins the digestion of starch.

Bile – A digestive fluid produced by the liver that helps break large fat droplets into smaller ones.

Chyme – The semi-liquid mixture of partially digested food and digestive juices that leaves the stomach.

Digestion – The mechanical and chemical breakdown of food into smaller molecules.

Esophagus – The muscular tube that carries food from the mouth to the stomach.

Gallbladder – An organ that stores and releases bile.

Peristalsis – Wave-like muscle contractions that move food through the digestive tract.

Small intestine – The main site of digestion and nutrient absorption.

Villi – Tiny finger-like projections that increase the surface area of the small intestine for absorption.

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Key Takeaways

• The digestive system breaks down food, absorbs nutrients, and removes undigested waste.
• Food follows the pathway: mouth → esophagus → stomach → small intestine → large intestine → rectum → anus.
• The mouth, stomach, and small intestine perform both mechanical and chemical digestion.
• Most nutrient absorption occurs in the small intestine, while the large intestine mainly absorbs water and salts.
• The liver, pancreas, and gallbladder are accessory organs that produce or store substances needed for digestion.
• The digestive system works with other organ systems to provide the nutrients and energy needed to maintain life.

2. Mechanical and Chemical Digestion

Mechanical and Chemical Digestion

Digestion is the process of breaking food into smaller substances that the body can absorb and use.

There are two main types of digestion:

• Mechanical digestion – physically breaking food into smaller pieces.
• Chemical digestion – breaking large food molecules into smaller molecules using enzymes.

Both processes work together as food moves through the digestive system. Mechanical digestion includes chewing and stomach mixing, while chemical digestion uses enzymes to break molecules into absorbable units.

 

Why Is Digestion Necessary?

Many food molecules are too large to pass through the wall of the small intestine.

Before they can be absorbed, they must be broken into much smaller molecules. For example:

• starch must be broken into simple sugars
• proteins must be broken into amino acids
• lipids must be broken into smaller lipid products such as fatty acids and monoglycerides

This gives the overall sequence:

Large food molecules

↓

Digestion

↓

Small molecules

↓

Absorption

↓

Transport to body cells

Chemical digestion therefore prepares nutrients for absorption.

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Mechanical Digestion

Mechanical digestion is the physical breakdown of food into smaller pieces without changing the chemical identity of the molecules.

The main examples are:

• chewing in the mouth
• churning in the stomach

Mechanical digestion is important because it increases the surface area of food.

A greater surface area allows digestive enzymes to work more effectively.

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Chewing

Digestion begins in the mouth.

The teeth cut, tear, crush, and grind food into smaller pieces.

Different teeth have different roles:

• incisors cut
• canines tear
• premolars crush
• molars grind

The tongue helps move food around and mixes it with saliva.

Chewing does not chemically change starch, proteins, or fats. Instead, it creates smaller pieces that enzymes can attack more easily.

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Saliva and the Mouth

While food is being mechanically broken down, it is also mixed with saliva.

Saliva:

• moistens food
• makes swallowing easier
• contains the enzyme amylase

Amylase begins the chemical digestion of starch in the mouth.

This means that both mechanical and chemical digestion begin in the mouth.

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Stomach Churning

After swallowing, food moves through the oesophagus into the stomach.

The stomach has muscular walls that contract and mix the food.

This churning:

• breaks food into smaller pieces
• mixes food with stomach secretions
• increases contact between food and digestive enzymes

This is mechanical digestion.

At the same time, proteins begin undergoing chemical digestion in the stomach.

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Comparing Mechanical and Chemical Digestion

Mechanical Digestion	Chemical Digestion
Physically breaks food into smaller pieces.	Breaks large molecules into smaller molecules
Does not change molecular identity	Changes molecules chemically
Includes chewing	Uses enzymes
Includes stomach churning	Digests carbohydrates, proteins, and lipids
Increases surface area	Produces molecules suitable for absorption

A good way to remember the difference is:

Mechanical = size changes

Chemical = molecules change

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Chemical Digestion

Chemical digestion uses enzymes to break large biological molecules into smaller molecules.

Enzymes are biological catalysts.

A catalyst speeds up a chemical reaction without being permanently used up.

Different digestive enzymes act on different nutrients.

 

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How Do Digestive Enzymes Work?

An enzyme usually acts on a particular molecule called its substrate.

For example:

Amylase → starch

Protease → protein

Lipase → lipids

The enzyme helps break the substrate into smaller products.

A useful pattern is:

Enzyme + substrate → products

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Carbohydrate Digestion

Carbohydrates include substances such as starch and sugars.

The enzyme amylase helps digest starch.

A simplified reaction is:

Starch → smaller sugars

Amylase is found in saliva and is also produced by the pancreas. Carbohydrate digestion begins in the mouth and continues in the small intestine.

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Protein Digestion

Proteins are large molecules made from amino acids.

Proteins must be broken down before their components can be absorbed.

The enzymes that digest proteins are called proteases.

A simplified pathway is:

Proteins → smaller peptides → amino acids

One important stomach protease is pepsin. Other proteases act later in the small intestine.

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Lipid Digestion

Lipids include fats and oils.

They are digested by enzymes called lipases.

Lipases break lipids into smaller products that can eventually be absorbed.

At school level, this is often represented as:

Lipids → fatty acids + glycerol

Much of lipid digestion occurs in the small intestine.

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Main Digestive Enzymes

Enzyme	Nutrient Digested	Main Products
Amylase	Starch	Smaller sugars
Protease	Protein	Amino acids
Lipase	Lipids	Fatty acids and glycerol

A simple memory aid is:

Amylase → carbohydrates

Protease → proteins

Lipase → lipids

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The Role of Bile

Lipids do not mix well with water.

This creates a challenge because digestive enzymes work at surfaces.

Bile helps by breaking large fat droplets into many smaller droplets.

This process is called emulsification.

The effect is:

Large fat droplet

↓

Many smaller droplets

↓

Greater surface area

↓

Lipase can work more efficiently

Bile is not an enzyme. It assists digestion by emulsifying lipids and helping create suitable conditions in the small intestine.

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Where Does Digestion Occur?

Different parts of the digestive system perform different roles.

Mouth

Mechanical digestion:

• chewing

Chemical digestion:

• amylase begins starch digestion

Stomach

Mechanical digestion:

• muscular churning

Chemical digestion:

• proteases begin major protein digestion

Small Intestine

Chemical digestion continues here:

• carbohydrates are further digested
• proteins are further digested
• lipids are digested

The small intestine is also the main site of nutrient absorption.

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Digestion and Absorption Are Different

Digestion means breaking large molecules into smaller ones.

Absorption means moving digested nutrients through the wall of the digestive system into the body's transport systems.

For example:

Protein

↓

Digestion

↓

Amino acids

↓

Absorption

↓

Bloodstream

Without digestion, many food molecules would simply be too large to absorb efficiently.

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The Small Intestine and Villi

The inner surface of the small intestine contains many tiny projections called villi.

Villi increase the surface area available for absorption.

This allows digested nutrients to move into the body efficiently.

Suggested placement: Use a villi or digestive-process diagram here to connect digestion with absorption.

Once molecules are small enough:

• sugars can enter the blood
• amino acids can enter the blood
• lipid digestion products are absorbed through specialised pathways in the villi

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Putting It All Together

Imagine eating a sandwich.

Step 1 – Mouth

The teeth chew the food.

Mechanical digestion

Salivary amylase begins breaking down starch.

Chemical digestion

Step 2 – Stomach

Muscles churn the food.

Mechanical digestion

Proteases begin breaking down proteins.

Chemical digestion

Step 3 – Small Intestine

Enzymes continue digesting:

• carbohydrates
• proteins
• lipids

Bile helps emulsify fats.

Step 4 – Absorption

Small molecules pass through the wall of the small intestine and enter the body's transport systems.

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Worked Example

A student eats a meal containing bread, chicken, and butter.

Bread

Main nutrient:

Starch

Main enzyme:

Amylase

Product:

Smaller sugars

Chicken

Main nutrient:

Protein

Main enzyme:

Protease

Product:

Amino acids

Butter

Main nutrient:

Lipid

Main enzyme:

Lipase

Products:

Fatty acids and glycerol

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Common Misconceptions

Bile is not an enzyme.

It helps by breaking large fat droplets into smaller ones.

Chewing is not chemical digestion.

It physically breaks food into smaller pieces but does not change the nutrient molecules themselves.

Digestion is not the same as absorption.

Digestion prepares molecules for absorption.

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Did You Know?

The stomach does much more than simply store food.

Its muscular walls repeatedly mix the food, combining mechanical digestion with chemical digestion. The resulting semi-liquid mixture is called chyme, which gradually enters the small intestine.

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Key Terms

Digestion – The breakdown of food into smaller substances.

Mechanical digestion – Physical breakdown of food into smaller pieces.

Chemical digestion – Chemical breakdown of large molecules using enzymes.

Enzyme – A biological catalyst.

Amylase – An enzyme that digests starch.

Protease – An enzyme that digests proteins.

Lipase – An enzyme that digests lipids.

Bile – A digestive fluid that helps emulsify lipids.

Emulsification – Breaking large fat droplets into smaller droplets.

Absorption – Movement of digested nutrients through the intestinal wall into the body.

Villi – Small projections in the small intestine that increase surface area for absorption.

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Key Takeaways

• Digestion includes both mechanical and chemical digestion.
• Mechanical digestion includes chewing and stomach churning.
• Mechanical digestion increases the surface area of food.
• Chemical digestion uses enzymes.
• Amylase digests carbohydrates such as starch.
• Proteases digest proteins.
• Lipases digest lipids.
• Bile helps lipid digestion by emulsifying fat.
• Digestion produces molecules small enough to be absorbed.
• Most nutrient absorption occurs in the small intestine.
• The villi provide a large surface area for efficient absorption.

3. Absorption in the Small Intestine

Introduction

After food has been digested into small molecules, the body must absorb these nutrients so they can be used by cells. Digestion alone is not enough—unless nutrients enter the bloodstream or lymphatic system, they cannot provide energy, support growth, or repair tissues.

Most nutrient absorption takes place in the small intestine, a long, highly specialised organ designed to maximise the movement of nutrients into the body. Its inner surface is covered with millions of tiny structures called villi and even smaller projections called microvilli, creating an enormous surface area for efficient absorption.

What Is Absorption?

Absorption is the process by which digested nutrients move from the small intestine into the bloodstream or lymphatic system.

Only small molecules produced during digestion can be absorbed.

Examples include:

• Glucose
• Amino acids
• Fatty acids
• Glycerol
• Vitamins
• Minerals
• Water

Once absorbed, these nutrients are transported to cells throughout the body.

 

 

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Figure 1. Digested nutrients pass through the wall of the small intestine into the blood or lymph.

The Small Intestine

The small intestine is the main site of nutrient absorption.

It is about 6 metres long in adults and is specially adapted for efficient absorption.

Features include:

• Great length.
• Folded inner lining.
• Millions of villi.
• Microscopic microvilli.
• Rich blood supply.

These adaptations allow nutrients to be absorbed quickly and efficiently.

Villi

Villi are tiny finger-like projections that cover the inner lining of the small intestine.

Each villus contains:

• A network of blood capillaries.
• A lymph vessel called a lacteal.

Functions of villi:

• Greatly increase surface area.
• Absorb nutrients.
• Transfer nutrients into blood or lymph.

Without villi, nutrient absorption would be much less efficient.

 

 

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Figure 2. Each villus contains blood capillaries and a lacteal for absorbing nutrients.

Microvilli

Each epithelial cell covering a villus has thousands of tiny projections called microvilli.

Microvilli:

• Increase the surface area even further.
• Form the brush border.
• Help maximise nutrient absorption.

Together, villi and microvilli create an enormous absorbing surface.

Scientists estimate that the total surface area of the small intestine is roughly 30–40 square metres in an adult—about the size of half a badminton court (estimates vary depending on how it is measured).

Why Is a Large Surface Area Important?

A larger surface area allows:

• More nutrients to be absorbed at the same time.
• Faster absorption.
• Greater efficiency.

The small intestine achieves this through:

• Its long length.
• Circular folds.
• Millions of villi.
• Billions of microvilli.

These features allow the body to absorb enough nutrients to meet its needs.

 

 

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Figure 3. Villi and microvilli greatly increase the surface area available for absorption.

Where Different Nutrients Go

Different nutrients enter different transport systems.

Carbohydrates

Digested into:

• Glucose and other simple sugars.

These pass into the blood capillaries inside each villus.

Proteins

Digested into:

• Amino acids.

These also enter the blood capillaries.

Lipids (Fats)

Digested into:

• Fatty acids.
• Glycerol.

These enter the lacteals, which are part of the lymphatic system.

Eventually, they enter the bloodstream.

Transporting Nutrients

Once absorbed:

Blood Capillaries

Carry:

• Glucose
• Amino acids
• Vitamins
• Minerals
• Water

The blood transports these nutrients first to the liver through the hepatic portal vein, where many are processed before being distributed to the rest of the body.

Lacteals

Carry:

• Fatty acids.
• Glycerol (mainly as part of larger fat particles called chylomicrons).

The lymphatic system eventually returns these fats to the bloodstream.

The circulatory system then delivers nutrients to cells for:

• Energy.
• Growth.
• Repair.
• Storage.

 

 

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Figure 4. Different nutrients enter either blood capillaries or the lacteal inside each villus.

Adaptations of the Small Intestine

The small intestine is highly adapted for absorption.

These adaptations make the small intestine one of the most efficient absorbing organs in the body.

Why Absorption Is Essential

Without absorption:

• Cells would not receive glucose for respiration.
• Growth would stop.
• Tissue repair would not occur.
• Organs could not function properly.

Absorption allows the nutrients released during digestion to become available to every cell in the body.

 

 

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Figure 5. After absorption, nutrients are transported throughout the body to support cellular functions.

Worked Example

Question

Complete the table.

Solution

Glucose and amino acids enter the bloodstream directly, while most fats first enter the lymphatic system.

Real-World Connection

Athletes often eat carbohydrate-rich foods before endurance events. During digestion, starch is broken down into glucose, which is absorbed through the villi into the bloodstream. The circulatory system transports this glucose to muscle cells, where it is used in cellular respiration to produce the energy needed for running, cycling, or swimming.

Did You Know?

The tiny microvilli on the cells lining the small intestine are so numerous that they form what scientists call the brush border. This surface not only increases the area available for absorption but also contains enzymes that complete the final stages of digestion before nutrients enter the body.

Key Terms

Absorption – The movement of digested nutrients from the small intestine into the blood or lymph.

Blood capillary – A tiny blood vessel where nutrients such as glucose and amino acids enter the bloodstream.

Brush border – The layer of microvilli covering the epithelial cells of the small intestine.

Lacteal – A lymph vessel inside each villus that absorbs most fats.

Lymphatic system – The body system that transports lymph and plays important roles in immunity and fat transport.

Microvilli – Microscopic projections on epithelial cells that greatly increase surface area for absorption.

Small intestine – The main site of digestion and nutrient absorption.

Villus (plural: villi) – A finger-like projection that increases the surface area of the small intestine.

Key Takeaways

• Absorption is the movement of digested nutrients from the small intestine into the blood or lymphatic system.
• The small intestine is specially adapted for absorption with its long length, folds, villi, and microvilli.
• A large surface area allows nutrients to be absorbed more quickly and efficiently.
• Glucose and amino acids enter the blood capillaries, while most fatty acids and glycerol enter the lacteals.
• The circulatory and lymphatic systems transport absorbed nutrients throughout the body.
• Efficient absorption ensures that every cell receives the nutrients needed for energy, growth, repair, and normal body function.

4. Nutrients and Their Functions

Introduction

Every cell in the human body needs nutrients to survive. Nutrients provide the energy needed for movement, help build and repair body tissues, support growth, and regulate thousands of chemical reactions that keep the body functioning properly. Without a balanced supply of nutrients, the body cannot grow, stay healthy, or respond effectively to disease and injury.

There are six major classes of nutrients required by the human body: carbohydrates, proteins, lipids (fats), vitamins, minerals, and water. Each nutrient has a specific role, and together they provide everything the body needs to maintain life.

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What Are Nutrients?

Nutrients are substances in food that the body needs for:

• Energy.
• Growth.
• Repair.
• Maintaining healthy body functions.

The six major nutrients are:

• Carbohydrates
• Proteins
• Lipids (fats)
• Vitamins
• Minerals
• Water

Each nutrient performs different but equally important functions.

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Figure 1. The human body requires six major classes of nutrients to remain healthy.

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Macronutrients and Micronutrients

Nutrients can be divided into two groups.

Macronutrients

Macronutrients are nutrients that the body needs in large amounts.

They include:

• Carbohydrates
• Proteins
• Lipids
• Water

Macronutrients provide energy or are needed in large quantities for normal body functions.

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Micronutrients

Micronutrients are nutrients needed in small amounts.

They include:

• Vitamins
• Minerals

Although only tiny amounts are required, micronutrients are essential for health.

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Carbohydrates

Carbohydrates are the body's main source of energy.

During digestion:

• Starch and sugars are broken down into glucose.

Cells use glucose during cellular respiration to release energy.

Examples of carbohydrate-rich foods:

• Bread
• Rice
• Pasta
• Potatoes
• Fruit
• Cereals

Unused glucose may be stored as glycogen in the liver and muscles.

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Figure 2. Carbohydrates provide the body with its primary source of energy.

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Proteins

Proteins are essential for:

• Growth.
• Repair.
• Building body tissues.

During digestion:

• Proteins are broken down into amino acids.

Amino acids are used to make:

• Muscles.
• Skin.
• Hair.
• Enzymes.
• Hormones.
• Antibodies.

Protein-rich foods include:

• Meat
• Fish
• Eggs
• Beans
• Lentils
• Dairy products

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Lipids (Fats)

Lipids are an important source of long-term energy storage.

They also:

• Form cell membranes.
• Protect internal organs.
• Provide insulation.
• Help absorb vitamins A, D, E, and K.

Examples include:

• Oils
• Butter
• Nuts
• Seeds
• Avocados

Although fats are essential, excessive intake can contribute to health problems.

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Figure 3. Lipids store energy, protect organs, and help build cell membranes.

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Vitamins

Vitamins are organic substances required in small amounts.

They help regulate many body processes.

Examples include:

Vitamin	Main Function
Vitamin A	Healthy vision and skin
Vitamin C	Wound healing and immune function
Vitamin D	Calcium absorption and healthy bones
Vitamin K	Blood clotting
B Vitamins.	Energy release and nervous system function

Vitamin deficiencies can lead to disease.

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Minerals

Minerals are inorganic nutrients required in small amounts.

Examples include:

Mineral	Main Function
Calcium	Bones and teeth
Iron	Formation of haemoglobin in red blood cells
Potassium	Muscle and nerve function
Sodium	Fluid balance and nerve signalling
Magnesium	Muscle, nerve, and enzyme function

Minerals cannot be produced by the body and must come from food.

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Water

Water is the most abundant substance in the human body.

It performs many vital functions:

• Dissolves nutrients.
• Transports substances.
• Regulates body temperature.
• Removes wastes.
• Lubricates joints.
• Supports chemical reactions.

Adults should drink enough water each day to replace water lost through sweat, urine, and breathing.

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Figure 4. Water is essential for transporting substances, regulating temperature, and supporting life processes.

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Comparing Nutrient Functions

Nutrient	Main Function
Carbohydrates.	Main source of energy
Proteins	Growth and tissue repair
Lipids	Long-term energy storage, insulation, cell membranes
Vitamins	Regulate body processes
Minerals	Support body structures and functions
Water	Transport, temperature regulation, chemical reactions

A balanced diet provides all of these nutrients in appropriate amounts.

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How Nutrients Support Life

Together, nutrients allow the body to:

• Produce energy.
• Build new cells.
• Repair damaged tissues.
• Support growth.
• Fight infections.
• Maintain healthy bones and muscles.
• Regulate body functions.

Different nutrients often work together.

For example:

• Vitamin D helps the body absorb calcium.
• Proteins build enzymes that digest carbohydrates and fats.
• Water transports glucose and amino acids to cells.

---

 

 

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Figure 5. A balanced diet provides all the nutrients needed for growth, repair, and energy production.

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Worked Example

Question

Match each nutrient to its primary function.

Nutrient	Function
Carbohydrates.	Main source of energy
Proteins	Growth and repair
Lipids	Long-term energy storage
Calcium	Healthy bones and teeth
Vitamin C	Wound healing and immune function
Water	Transport of substances and temperature regulation

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Real-World Connection

A balanced breakfast containing whole-grain cereal, milk, fruit, and nuts provides several important nutrients. The carbohydrates supply glucose for energy, the protein supports growth and repair, the healthy fats provide long-term energy, the vitamins and minerals support body functions, and the water in food and drinks helps maintain hydration. Eating a variety of foods helps ensure the body receives all the nutrients it needs.

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Did You Know?

Water makes up about 60% of an adult's body mass. Blood, muscles, the brain, and even bones contain significant amounts of water. Because the body constantly loses water through breathing, sweating, and urination, it must be replaced regularly to maintain normal body functions.

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Key Terms

Carbohydrate – A nutrient that provides the body's main source of energy.

Lipid – A nutrient that stores energy, forms cell membranes, and provides insulation.

Macronutrient – A nutrient required by the body in relatively large amounts.

Micronutrient – A nutrient required by the body in relatively small amounts.

Mineral – An inorganic nutrient needed for healthy body functions.

Nutrient – A substance in food that supports growth, repair, energy production, and body functions.

Protein – A nutrient made of amino acids that is needed for growth and tissue repair.

Vitamin – An organic nutrient required in small amounts to regulate body processes.

Water – A nutrient that acts as a solvent, transport medium, temperature regulator, and reactant in many biological processes.

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Key Takeaways

• The human body requires six major nutrients: carbohydrates, proteins, lipids, vitamins, minerals, and water.
• Carbohydrates, proteins, lipids, and water are macronutrients, while vitamins and minerals are micronutrients.
• Carbohydrates provide the body's main source of energy.
• Proteins support growth, repair, and the production of enzymes and hormones.
• Lipids provide long-term energy storage, insulation, and form cell membranes.
• Vitamins, minerals, and water regulate body processes and help maintain health.
• A balanced diet supplies all the nutrients needed for energy production, growth, tissue repair, and normal body function.

 

 

 

5. Maintaining Digestive Health

Introduction

The digestive system works every day to break down food, absorb nutrients, and remove waste from the body. To function efficiently, it depends not only on healthy organs but also on good nutrition and healthy lifestyle habits. Eating a balanced diet, drinking enough water, exercising regularly, and maintaining a healthy population of beneficial bacteria all help keep the digestive system working properly.

When digestive health is poor, problems such as constipation, diarrhoea, indigestion, and nutrient deficiencies can occur. Understanding how to care for the digestive system helps promote overall health and well-being throughout life.

Why Is Digestive Health Important?

A healthy digestive system allows the body to:

• Break down food efficiently.
• Absorb nutrients effectively.
• Remove waste regularly.
• Support the immune system.
• Maintain healthy gut bacteria.

Healthy digestion ensures that every cell receives the nutrients needed for energy, growth, and repair.

 

 

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Figure 1. Good nutrition and healthy habits help keep the digestive system functioning efficiently.

Dietary Fibre

Dietary fibre is the part of plant foods that cannot be digested by human digestive enzymes.

Although fibre is not absorbed, it plays an important role in digestive health.

Functions of Fibre

• Adds bulk to food.
• Helps move food through the intestines.
• Promotes regular bowel movements.
• Reduces the risk of constipation.
• Supports beneficial gut bacteria.

Foods rich in fibre include:

• Whole grains
• Fruits
• Vegetables
• Beans
• Lentils
• Nuts
• Seeds

Eating enough fibre helps maintain a healthy digestive system.

Water and Digestion

Water is essential throughout the digestive process.

It helps to:

• Soften food during chewing.
• Form digestive juices.
• Dissolve nutrients.
• Help enzymes function effectively.
• Transport absorbed nutrients in the bloodstream.
• Prevent constipation by softening faeces.

Drinking enough water supports both digestion and overall health.

 

 

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Figure 2. Water supports digestion, nutrient transport, and healthy bowel function.

Lifestyle Choices That Promote Digestive Health

Several healthy habits improve digestive function.

Eat a Balanced Diet

A diet rich in:

• Fruits
• Vegetables
• Whole grains
• Lean proteins

provides fibre and essential nutrients.

Drink Enough Water

Adequate hydration supports digestion and nutrient absorption.

Exercise Regularly

Physical activity:

• Stimulates movement of the intestines.
• Helps prevent constipation.
• Supports overall digestive function.

Eat Regular Meals

Eating at regular times helps the digestive system work efficiently.

Practise Good Food Hygiene

Safe food preparation reduces the risk of food poisoning.

Healthy lifestyle choices protect both the digestive system and overall health.

Beneficial Bacteria

The digestive system contains trillions of beneficial bacteria, especially in the large intestine.

Together, these microorganisms are called the gut microbiome.

Beneficial bacteria help to:

• Break down certain foods.
• Produce vitamins such as vitamin K and some B vitamins.
• Prevent harmful bacteria from growing.
• Support the immune system.

A healthy gut microbiome contributes to good digestion and overall health.

 

 

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Figure 3. Beneficial bacteria living in the large intestine help maintain digestive and overall health.

Common Digestive Disorders

Several common conditions affect the digestive system.

Constipation

Occurs when bowel movements become difficult or infrequent.

Risk can be reduced by:

• Eating more fibre.
• Drinking enough water.
• Exercising regularly.

Diarrhoea

Occurs when waste passes too quickly through the intestines.

Causes include:

• Infections.
• Food poisoning.
• Certain medications.

Drinking fluids is important to prevent dehydration.

Indigestion

Symptoms may include:

• Stomach discomfort.
• Bloating.
• Heartburn.

It may be reduced by:

• Eating smaller meals.
• Avoiding overeating.
• Limiting foods that trigger symptoms.

Food Poisoning

Usually caused by harmful microorganisms in contaminated food.

Good hygiene and proper food storage greatly reduce the risk.

Reducing the Risk of Digestive Disorders

Healthy habits include:

• Eating plenty of fibre.
• Drinking enough water.
• Exercising regularly.
• Washing hands before eating.
• Preparing food safely.
• Storing food correctly.
• Eating a balanced diet.

These habits help maintain a healthy digestive system and reduce the likelihood of illness.

 

 

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Figure 4. Healthy lifestyle choices can reduce the risk of many digestive disorders.

How the Digestive System Supports Overall Health

A healthy digestive system helps:

• Supply nutrients for growth.
• Provide energy for body cells.
• Support immune function.
• Maintain healthy body weight.
• Promote overall well-being.

The digestive system also works closely with:

• The circulatory system to transport nutrients.
• The immune system to defend against harmful microorganisms.
• The nervous system to regulate digestion.

Healthy digestion benefits the entire body.

 

 

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Figure 5. Good digestive health supports many other body systems and contributes to overall well-being.

Worked Example

Question

Which lifestyle choices help maintain digestive health?

Select all that apply.

• Eating enough dietary fibre
• Drinking plenty of water
• Regular physical activity
• Good food hygiene
• Eating a balanced diet

Solution

All of these choices promote healthy digestion by supporting nutrient absorption, bowel function, and protection against disease.

Real-World Connection

Many people experience constipation when they travel because changes in diet, reduced physical activity, or not drinking enough water can slow the movement of food through the intestines. Increasing fibre intake, staying hydrated, and remaining physically active often help restore normal bowel function. These simple habits are important for maintaining digestive health both at home and while travelling.

Did You Know?

Your digestive system is home to trillions of microorganisms, including bacteria, fungi, and other microbes. Collectively known as the gut microbiome, they help digest certain foods, produce vitamins, and interact with the immune system. Scientists continue to discover new ways in which these microorganisms influence not only digestion but also overall health.

Key Terms

Beneficial bacteria – Helpful microorganisms that live in the digestive system and support digestion and health.

Constipation – A condition in which bowel movements are difficult or infrequent.

Dietary fibre – The indigestible part of plant foods that helps maintain healthy bowel function.

Diarrhoea – Frequent, loose, or watery bowel movements.

Digestive health – The healthy functioning of the digestive system.

Food poisoning – Illness caused by consuming food contaminated with harmful microorganisms or toxins.

Gut microbiome – The community of microorganisms living in the digestive tract.

Indigestion – Discomfort in the upper digestive tract, often including bloating, heartburn, or stomach pain.

Key Takeaways

• Dietary fibre promotes healthy digestion by adding bulk to food, supporting regular bowel movements, and nourishing beneficial gut bacteria.
• Water is essential for digestion, nutrient transport, and preventing constipation.
• Healthy habits such as eating a balanced diet, exercising regularly, staying hydrated, and practising good food hygiene help maintain digestive health.
• The gut microbiome contains beneficial bacteria that support digestion, produce certain vitamins, and help protect against harmful microorganisms.
• Common digestive disorders include constipation, diarrhoea, indigestion, and food poisoning, many of which can be reduced through healthy lifestyle choices.
• A healthy digestive system supports nutrient absorption, energy production, immune function, and overall health.