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.

---

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.

---

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

---

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.

---

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.

---

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

---

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

---

Structural Proteins

Structural proteins provide strength and support.

Examples include:

Collagen

Found in:

• Skin
• Bones
• Tendons
• Cartilage

Function:

Provides strength and flexibility.

---

Keratin

Found in:

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

Function:

Provides toughness and protection.

---

Actin and Myosin

Found in muscle cells.

Function:

Allow muscles to contract and produce movement.

---

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

---

Functional Proteins

Many proteins perform specialised jobs inside cells.

Examples include:

Haemoglobin

Found in red blood cells.

Function:

Transports oxygen around the body.

---

Antibodies

Produced by the immune system.

Function:

Recognise and destroy pathogens.

---

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.

---

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.

---

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

---

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.

---

Comparing Protein Functions

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

---

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

---

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.

---

Worked Example

Question

Complete the table.

---

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.

---

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.

---

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.

---

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.