Living organisms are built from simple parts that are organised into increasingly complex levels.
The levels in order from simplest to most complex are:
CELL → TISSUE → ORGAN → ORGAN SYSTEM → ORGANISM
This hierarchy is the foundation of biology — understanding it helps you understand how the body works as a whole, and why problems at one level (e.g. a faulty cell) can affect the whole organism.
Cells — The Basic Unit of Life
A cell is the smallest unit capable of carrying out all the processes of life.
Every living organism is made of at least one cell.
Different cell types are specialised for different jobs — a muscle cell is built to contract, a red blood cell is built to carry oxygen, a root hair cell is built to absorb water.
All cells in one organism contain the same DNA, but different genes are switched on in different cell types.
Tissues
A tissue is a group of similar cells working together to carry out a particular function.
Key examples of animal tissues:
Muscle tissue — cells that can contract and relax to produce movement.
Epithelial tissue — thin cells that line surfaces (e.g. gut lining, airway lining, skin surface).
Glandular tissue — cells that produce and release substances such as enzymes, hormones or mucus.
Nervous tissue — neurones that carry electrical impulses.
Key examples of plant tissues:
Mesophyll tissue — cells in leaves packed with chloroplasts for photosynthesis.
Xylem tissue — hollow dead cells that transport water upwards.
Phloem tissue — living cells that transport dissolved sugars.
Organs
An organ is a structure made of several DIFFERENT types of tissue working together to perform a specific function.
Key examples:
The STOMACH — contains muscle tissue (churns food), epithelial tissue (lines the stomach wall), glandular tissue (secretes HCl and pepsin enzymes).
The HEART — contains cardiac muscle tissue (pumps blood), valves (prevent backflow), coronary blood vessels (supply oxygen to heart muscle).
The KEY DIFFERENCE between a tissue and an organ: a tissue is made of ONE type of cell. An organ contains MULTIPLE different tissue types.
Organ Systems and Organisms
An organ system is a group of organs that work together to perform a major function.
Examples:
Digestive system — mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas. Function: breaks down food and absorbs nutrients.
Circulatory system — heart, blood vessels, blood. Function: transports oxygen, nutrients and waste products.
Respiratory system — lungs, trachea, bronchi, diaphragm. Function: gas exchange (O₂ in, CO₂ out).
Nervous system — brain, spinal cord, nerves. Function: detects stimuli and coordinates responses.
An ORGANISM is the complete living individual — all organ systems working together to sustain life.
⚠️ Common Mistake
Students confuse tissue and organ. Remember: a tissue is ONE type of cell doing ONE job. An organ contains MULTIPLE tissue types. The stomach is an ORGAN (it has muscle tissue, glandular tissue and epithelial tissue). Muscle is a TISSUE.
📌 Key Note
Cell → Tissue (same cells, one function) → Organ (multiple tissues) → Organ System (multiple organs) → Organism. Each level is more complex than the one before.
🎯 Matching Activity — Match the Level of Organisation
Match each example to its correct level of organisation. — drag the symbols on the right to match the component names on the left.
Cell
Drop here
Tissue
Drop here
Organ
Drop here
Organ system
Drop here
Organism
Drop here
A complete human being — all systems functioning together
The stomach — contains muscle, glandular and epithelial tissue
A single red blood cell carrying oxygen in the blood
Muscle — a group of muscle cells that contract together
The digestive system — stomach, intestines, liver and pancreas
🎯 Test Yourself
Question 1 of 4
1. What is the correct order of organisation from simplest to most complex?
2. What is a tissue?
3. The stomach contains muscle tissue, glandular tissue and epithelial tissue. What level of organisation is the stomach?
4. Which of these is an example of an organ system?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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