1. Introduction to Biomolecules

2. Carbohydrates

Introduction

Carbohydrates are one of the four major groups of biological molecules, also known as biomolecules. They are found in almost every living organism and are the body's main source of energy. Whether you eat rice, bread, fruit, potatoes, or pasta, you are consuming carbohydrates that your body can break down to provide the energy needed for movement, growth, and everyday life.

Not all carbohydrates are the same. Some, such as glucose, provide energy almost immediately, while others, such as starch and glycogen, store energy for later use. Another carbohydrate, cellulose, performs an entirely different role by providing structural support in plants. Understanding the different types of carbohydrates helps explain how living organisms obtain, store, and use energy.

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What Are Carbohydrates?

Carbohydrates are organic molecules made primarily of:

• Carbon (C)
• Hydrogen (H)
• Oxygen (O)

Many carbohydrates have a hydrogen-to-oxygen ratio of approximately 2 : 1, similar to water.

Carbohydrates are produced by plants during photosynthesis and are one of the most abundant groups of molecules on Earth.

Their main functions include:

• Providing energy.
• Storing energy.
• Forming structural materials.

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Figure 1. Carbohydrates are organic molecules composed mainly of carbon, hydrogen, and oxygen.

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The Structure of Carbohydrates

Carbohydrates are built from small units called monosaccharides (simple sugars).

These simple sugars can join together to form larger carbohydrates.

The main types are:

• Monosaccharides – One sugar unit
• Disaccharides – Two sugar units joined together
• Polysaccharides – Many sugar units joined together

As carbohydrates become larger, they usually become less soluble and take longer to digest.

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Carbohydrates as an Energy Source

Carbohydrates are the primary source of energy for most living organisms.

Cells break down carbohydrates during cellular respiration to release energy.

This energy is used for:

• Growth
• Movement
• Cell repair
• Active transport
• Maintaining body temperature

The energy released is stored temporarily in molecules of ATP (adenosine triphosphate), which cells use to power their activities.

Because carbohydrates can be broken down relatively quickly, they provide a readily available source of energy.

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Figure 2. Cells break down carbohydrates during cellular respiration to release energy for life processes.

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Simple Carbohydrates

Simple carbohydrates contain one or two sugar molecules.

They are:

• Digested quickly.
• Absorbed rapidly.
• Provide fast energy.

Examples include:

Glucose

• Main energy source for cells.
• Produced during photosynthesis.
• Carried in the blood of animals.

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Fructose

• Found in fruits.
• Very sweet.

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Sucrose

• Common table sugar.
• Made of glucose and fructose.

Simple carbohydrates are especially useful when the body needs energy quickly.

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Complex Carbohydrates

Complex carbohydrates are made of many glucose molecules joined together.

They:

• Take longer to digest.
• Release energy more slowly.
• Often serve as energy storage or structural materials.

Important complex carbohydrates include:

• Starch
• Glycogen
• Cellulose

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Figure 3. Simple carbohydrates provide quick energy, while complex carbohydrates store energy or provide structural support.

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Glucose

Glucose is one of the most important carbohydrates.

Functions:

• Main fuel for cellular respiration.
• Produced during photosynthesis.
• Transported in the bloodstream.
• Used immediately or stored.

Plants use glucose to make:

• Starch
• Cellulose

Animals use glucose to make:

• Glycogen

Without glucose, cells could not produce enough energy to survive.

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Starch

Starch is the main energy storage carbohydrate in plants.

Plants convert excess glucose into starch for storage.

Starch is found in:

• Potatoes
• Rice
• Wheat
• Corn
• Seeds

When needed, plants break starch back into glucose to provide energy.

Humans and many animals also digest starch as an important food source.

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Glycogen

Glycogen is the energy storage carbohydrate in animals.

It is stored mainly in:

• Liver
• Skeletal muscles

When blood glucose levels fall, glycogen is broken down into glucose.

This provides energy between meals and during exercise.

Glycogen is sometimes called animal starch because it performs a similar storage function to starch in plants.

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Figure 4. Plants store energy as starch, while animals store energy as glycogen.

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Cellulose

Cellulose is a structural carbohydrate found in plant cell walls.

Unlike starch:

• It is not used for energy storage.
• It provides strength and support.

Functions:

• Supports stems and leaves.
• Protects plant cells.
• Helps plants remain upright.

Humans cannot digest cellulose because we lack the enzyme needed to break it down.

Instead, cellulose forms an important part of dietary fibre, which helps maintain healthy digestion.

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Comparing Important Carbohydrates

Although all four are carbohydrates, they perform very different functions.

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Figure 5. Different carbohydrates have specialised roles in living organisms.

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Why Carbohydrates Are Important

Carbohydrates are essential because they:

• Supply energy.
• Store energy.
• Build plant structures.
• Support growth.
• Fuel muscles and the brain.
• Form the base of many food chains.

Plants produce carbohydrates through photosynthesis, and animals obtain them by eating plants or other animals.

As a result, carbohydrates are one of the most important sources of energy in ecosystems.

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Worked Example

Question

Complete the table.

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Real-World Connection

Foods rich in carbohydrates provide the energy needed for daily activities. Athletes often eat carbohydrate-rich meals such as pasta, rice, or potatoes before endurance events to increase glycogen stores in their muscles. During exercise, this glycogen is broken down into glucose, providing a steady supply of energy for working muscles.

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Did You Know?

The average adult stores about 400–500 grams of glycogen in the liver and muscles. During prolonged exercise, these glycogen stores gradually become depleted. This is why marathon runners and cyclists often consume carbohydrate-rich foods or sports drinks during long events to maintain their energy levels.

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Key Terms

Carbohydrate – An organic molecule made mainly of carbon, hydrogen, and oxygen that provides energy, stores energy, or forms structural materials.

Cellular respiration – The process by which cells break down glucose to release energy.

Cellulose – A structural carbohydrate that forms plant cell walls.

Complex carbohydrate – A carbohydrate made of many sugar molecules joined together.

Glucose – A simple sugar that serves as the main source of energy for cells.

Glycogen – The main energy storage carbohydrate in animals.

Monosaccharide – A simple sugar consisting of a single sugar unit.

Polysaccharide – A large carbohydrate made of many sugar units linked together.

Simple carbohydrate – A carbohydrate made of one or two sugar molecules that is digested quickly.

Starch – The main energy storage carbohydrate in plants.

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Key Takeaways

• Carbohydrates are organic molecules composed mainly of carbon, hydrogen, and oxygen.
• They are the primary source of energy for most living organisms.
• Simple carbohydrates provide rapid energy, while complex carbohydrates store energy or provide structural support.
• Glucose is the main fuel for cellular respiration.
• Starch stores energy in plants, glycogen stores energy in animals, and cellulose forms strong plant cell walls.
• Carbohydrates are essential for energy production, growth, and the survival of both plants and animals.

3. Lipids

Introduction

Lipids are one of the four major groups of biological molecules found in living organisms. Although they are often called fats, the lipid group also includes oils, waxes, and phospholipids. Lipids perform many essential functions, including storing energy, insulating the body, protecting organs, forming waterproof coatings, and building the membranes that surround every living cell.

Unlike carbohydrates, which provide energy that can be used quickly, lipids are mainly used for long-term energy storage. Because lipids contain much more energy per gram than carbohydrates, they are an efficient way for organisms to store energy for future use.

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What Are Lipids?

Lipids are a group of organic molecules made mainly of:

• Carbon (C)
• Hydrogen (H)
• Oxygen (O)

Compared with carbohydrates, lipids contain:

• Much more hydrogen.
• Less oxygen.

Lipids are generally:

• Insoluble in water.
• Soluble in many organic solvents.

The major types of lipids include:

• Fats
• Oils
• Waxes
• Phospholipids

Although these molecules have different structures, they all belong to the lipid family.

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Figure 1. Lipids include fats, oils, waxes, and phospholipids, each with specialised biological functions.

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The Structure of Lipids

Most fats and oils are made from:

• One glycerol molecule.
• Three fatty acid molecules.

Together they form a molecule called a triglyceride.

Scientists often represent triglycerides as:

• Glycerol "backbone"
• Three fatty acid "tails"

This structure allows lipids to store large amounts of chemical energy.

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Figure 2. Most fats and oils are triglycerides made from one glycerol molecule and three fatty acids.

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Lipids as Long-Term Energy Storage

One of the main functions of lipids is long-term energy storage.

Animals store excess energy as fat.

Plants store oils, especially in:

• Seeds
• Nuts
• Some fruits

When food is scarce, stored lipids are broken down to release energy.

Lipids store more than twice as much energy per gram as carbohydrates, making them an efficient energy reserve.

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Comparing Lipids and Carbohydrates

Both lipids and carbohydrates provide energy, but they are used differently.

Cells usually use carbohydrates first because they provide energy more rapidly.

Lipids become especially important during prolonged exercise or periods without food.

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Figure 3. Carbohydrates provide quick energy, while lipids are the body's long-term energy store.

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Fats

Fats are lipids that are usually solid at room temperature.

Examples include:

• Butter
• Animal fat
• Lard

Functions:

• Long-term energy storage.
• Thermal insulation.
• Protection of internal organs.
• Energy reserve during fasting.

Many mammals use body fat to help maintain their body temperature.

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Oils

Oils are lipids that are usually liquid at room temperature.

Examples include:

• Olive oil
• Sunflower oil
• Canola oil
• Fish oil

Plants commonly store energy as oils inside their seeds.

When seeds germinate, these oils provide energy for the young plant before it begins photosynthesis.

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Waxes

Waxes are protective lipids.

Functions include:

• Preventing water loss.
• Protecting surfaces.
• Providing waterproof coatings.

Examples:

• Plant leaf cuticle.
• Earwax.
• Beeswax.
• Waterproof coatings on fruits.

The waxy cuticle helps reduce water loss from leaves.

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Figure 4. Different types of lipids have specialised roles in living organisms.

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Phospholipids

Phospholipids are specialised lipids that form the cell membrane.

Unlike triglycerides, phospholipids have:

• One glycerol molecule.
• Two fatty acid tails.
• One phosphate-containing head.

The phosphate head is:

• Hydrophilic ("water-loving")

The fatty acid tails are:

• Hydrophobic ("water-repelling")

Because of these properties, phospholipids naturally arrange themselves into a phospholipid bilayer, forming the basic structure of all cell membranes.

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Figure 5. Phospholipids form the bilayer that surrounds every living cell.

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Lipids and Cell Membranes

Every living cell is surrounded by a cell membrane.

The membrane:

• Controls what enters the cell.
• Controls what leaves the cell.
• Protects the cell.
• Maintains internal conditions.

The phospholipid bilayer creates a flexible barrier that allows cells to function properly.

Proteins are also embedded within the membrane, helping transport substances across it.

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Other Functions of Lipids

In addition to storing energy, lipids:

• Insulate the body.
• Cushion organs.
• Form cell membranes.
• Waterproof leaves, feathers, and skin.
• Help absorb vitamins A, D, E, and K.
• Serve as building blocks for some hormones.

Lipids therefore perform many important functions beyond energy storage.

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Worked Example

Question

Complete the table.

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Real-World Connection

Many animals rely on stored fat to survive periods when food is scarce. Bears, for example, build up large fat reserves before hibernation. During the winter, they break down these stored lipids to provide energy while they remain inactive for several months. Similarly, migrating birds store fat before long flights because lipids provide a lightweight, energy-rich fuel for extended journeys.

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Did You Know?

Lipids provide about 9 kilocalories (kcal) of energy per gram, while carbohydrates provide only about 4 kilocalories per gram. This means lipids store more than twice as much energy in the same mass, making them the most efficient long-term energy storage molecules in the body.

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Key Terms

Cell membrane – A thin barrier surrounding the cell that controls the movement of substances into and out of the cell.

Fat – A lipid that is usually solid at room temperature and stores energy in animals.

Fatty acid – A long hydrocarbon chain that forms part of most lipid molecules.

Glycerol – A three-carbon molecule that forms the backbone of triglycerides and phospholipids.

Hydrophilic – Attracted to water.

Hydrophobic – Repelled by water.

Lipid – An organic molecule used for energy storage, insulation, waterproofing, and cell membrane structure.

Oil – A lipid that is usually liquid at room temperature.

Phospholipid – A specialised lipid that forms the main structure of cell membranes.

Triglyceride – A lipid made of one glycerol molecule joined to three fatty acids.

Wax – A protective lipid that reduces water loss and provides waterproof coatings.

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Key Takeaways

• Lipids are organic molecules made mainly of carbon, hydrogen, and oxygen.
• Most fats and oils are triglycerides, consisting of one glycerol molecule and three fatty acids.
• Lipids are the body's main form of long-term energy storage and contain more than twice as much energy per gram as carbohydrates.
• Fats, oils, waxes, and phospholipids each have specialised functions in living organisms.
• Phospholipids form the cell membrane, which controls the movement of substances into and out of cells.
• In addition to storing energy, lipids provide insulation, protection, waterproofing, and structural support for cells.

4. Proteins

Introduction

Proteins are one of the four major groups of biological molecules and are among the most versatile substances found in living organisms. Every cell contains thousands of different proteins, each carrying out a specific job. Proteins build muscles, strengthen bones and skin, transport substances through the body, defend against disease, and speed up the chemical reactions that keep cells alive.

Unlike carbohydrates and lipids, which mainly provide energy, proteins are primarily responsible for the structure and function of cells. The enormous variety of proteins is possible because they are built from different combinations of smaller molecules called amino acids. The order of these amino acids determines the shape of each protein, and its shape determines its function.

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What Are Proteins?

Proteins are large organic molecules made mainly of:

• Carbon (C)
• Hydrogen (H)
• Oxygen (O)
• Nitrogen (N)

Some proteins also contain sulfur.

Proteins are built from smaller molecules called amino acids.

Their major functions include:

• Growth
• Tissue repair
• Structural support
• Transport
• Defence
• Cell communication
• Enzyme activity

Almost every process in living cells depends on proteins.

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Figure 1. Proteins are large molecules made by linking together many amino acids.

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Amino Acids: The Building Blocks of Proteins

Amino acids are the basic building blocks of proteins.

Living organisms use 20 different amino acids to build thousands of different proteins.

Amino acids join together through peptide bonds to form long chains called polypeptides.

One protein molecule may contain:

• Dozens
• Hundreds
• Or even thousands of amino acids

The exact sequence of amino acids determines the properties of the protein.

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Protein Structure

Proteins are not simply long chains.

After being produced, the amino acid chain folds into a specific three-dimensional shape.

This folding is essential because:

Shape determines function.

If the shape changes, the protein may no longer work properly.

For example:

• An enzyme's active site must have the correct shape to bind its substrate.
• Haemoglobin must have the correct shape to carry oxygen.

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Figure 2. A protein's function depends on its unique three-dimensional shape.

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Proteins in Growth and Repair

One of the best-known functions of proteins is building and repairing tissues.

Proteins help:

• Build muscles.
• Repair damaged cells.
• Produce new tissues.
• Heal wounds.
• Support growth during childhood.

Because body cells are constantly replaced, organisms require a regular supply of amino acids from food.

Good sources of protein include:

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

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Structural Proteins

Structural proteins provide strength and support.

Examples include:

Collagen

Found in:

• Skin
• Bones
• Tendons
• Cartilage

Function:

Provides strength and flexibility.

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Keratin

Found in:

• Hair
• Nails
• Outer layer of skin
• Animal claws and feathers

Function:

Provides toughness and protection.

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Actin and Myosin

Found in muscle cells.

Function:

Allow muscles to contract and produce movement.

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Figure 3. Structural proteins provide support, protection, and movement throughout the body.

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Functional Proteins

Many proteins perform specialised jobs inside cells.

Examples include:

Haemoglobin

Found in red blood cells.

Function:

Transports oxygen around the body.

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Antibodies

Produced by the immune system.

Function:

Recognise and destroy pathogens.

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Hormone Proteins

Some hormones are proteins.

Example:

Insulin

Function:

Helps regulate blood glucose levels.

These proteins allow different body systems to communicate and function together.

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Enzymes: Biological Catalysts

Enzymes are specialised proteins that act as biological catalysts.

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

Enzymes allow reactions to occur:

• Quickly.
• At normal body temperatures.

Without enzymes, many life processes would occur far too slowly to support life.

Examples include:

• Digestive enzymes breaking down food.
• Enzymes involved in cellular respiration.
• Enzymes used during DNA replication.

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Figure 4. Enzymes speed up chemical reactions by binding specific substrates at their active sites.

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Protein Structure Determines Function

Every protein has a unique shape.

The shape depends on:

• Amino acid sequence.
• Folding of the protein.

If the protein changes shape, its function may change or stop completely.

For example:

High temperatures or extreme pH can cause proteins to denature.

Denaturation means:

• The protein loses its three-dimensional shape.
• It can no longer function properly.

Many enzymes stop working after they are denatured.

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Comparing Protein Functions

Although all are proteins, each performs a unique role because of its different structure.

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Figure 5. Different proteins have different functions because each has a unique structure.

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Why Proteins Are Essential

Proteins are necessary for:

• Growth.
• Tissue repair.
• Muscle contraction.
• Immune defence.
• Transport of substances.
• Cell communication.
• Chemical reactions.

Without proteins, cells could not survive or function properly.

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Worked Example

Question

Complete the table.

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Real-World Connection

Athletes often increase their protein intake after exercise because muscles require amino acids to repair microscopic damage caused during training. Similarly, growing children need adequate protein to support the development of muscles, bones, skin, and other tissues. In hospitals, patients recovering from surgery or injury are often encouraged to consume enough protein to promote healing and tissue repair.

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Did You Know?

The human body contains tens of thousands of different proteins, each with a unique amino acid sequence and shape. Even a small change in the sequence of amino acids can dramatically alter a protein's function. For example, a single amino acid change in the protein haemoglobin causes sickle cell disease, demonstrating how closely protein structure is linked to function.

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Key Terms

Amino acid – The small molecule that serves as the building block of proteins.

Antibody – A protein produced by the immune system that recognises and helps destroy pathogens.

Biological catalyst – A substance, such as an enzyme, that speeds up chemical reactions without being consumed.

Denaturation – The loss of a protein's normal three-dimensional shape, causing it to lose its function.

Enzyme – A protein that speeds up chemical reactions in living organisms.

Haemoglobin – A protein in red blood cells that transports oxygen.

Peptide bond – The chemical bond that joins amino acids together.

Polypeptide – A long chain of amino acids linked by peptide bonds.

Protein – A large biological molecule made from amino acids that performs structural and functional roles in living organisms.

Structural protein – A protein that provides strength and support to cells and tissues.

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Key Takeaways

• Proteins are large biological molecules made mainly of carbon, hydrogen, oxygen, and nitrogen.
• Proteins are built from amino acids joined together by peptide bonds.
• The sequence of amino acids determines a protein's three-dimensional shape, and shape determines function.
• Proteins are essential for growth, tissue repair, transport, communication, defence, and many other cellular functions.
• Enzymes are specialised proteins that act as biological catalysts, speeding up chemical reactions.
• Different proteins perform different functions because each has a unique structure specifically adapted to its role.

5. Nucleic Acids (DNA and RNA)