Living Systems and Levels of Organisation
A human body, a gum tree and even a small insect are not just random collections of parts. Science explains them as living systems made of organised components that work together. This lesson builds the foundation from cells to tissues, organs and organ systems.
Download this lesson's worksheet
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Q1: If a body system is made of organs, what are organs made of, and what are those made of?
Many students know words like heart or lungs, but are less clear about how all the levels fit together. Write your best chain from smallest to largest before reading.
Q2: If you cut your finger, what levels of organisation are affected?
Think about what you see, what bleeds, and what helps it heal. This prepares you for the real-world anchor later in the lesson.
Know
- living things are organised into levels
- cells are the basic unit of living things
- cells, tissues, organs and organ systems are not interchangeable words
Understand
- each level builds on the one before it
- living systems work because components interact, not because parts exist separately
- plants and animals both show organisation, even if their structures differ
Do
- put levels of organisation in the correct order
- match examples to the correct level
- explain why organised levels are useful in living things
Living things are organised systems, not random collections of parts
A living system works because its parts are arranged in a useful way and interact with one another.
Science uses the idea of organisation to explain how living things are built. A cell does not do the same job as an organ, and an organ does not do the same job as a whole system. Each level has its own role, and together the levels allow the organism to survive.
The main order is cells, tissues, organs, then organ systems
Each level is made from the previous level, so the order is not arbitrary — it shows how living things are built.
A useful way to think about this is that each level is made from the previous level. Cells form tissues, tissues form organs, and organs work together in systems. This does not mean every cell is identical or every organ system looks the same. It means living things have organised structure.
Examples help show the difference between the levels
Seeing real examples makes the abstract levels concrete and memorable.
Cell and tissue
- a muscle cell is one cell
- muscle tissue is many similar muscle cells working together
Organ
- the heart is an organ
- it contains different tissues working together
Organ system
- the circulatory system includes the heart, blood and blood vessels
- its parts work together as a system
Misconceptions to Fix
Wrong: "Organs and organ systems are the same thing."
Right: An organ is one structure made of tissues. An organ system is a group of organs working together. The heart is an organ; the circulatory system is the organ system.
Wrong: "All living things have the same systems."
Right: Plants do not have circulatory systems like animals do, but they still show organisation. They have cells, tissues and organs such as roots, stems and leaves.
Right: They have different structures and roles. A cell is the basic unit, a tissue is similar cells working together, and an organ is different tissues working together.
Right: Plants also have organised systems. Their root system and shoot system are made of organs, tissues and cells that work together.
Diagram 2: Plant vs Animal Organisation
Side-by-side comparison showing a leaf cross-section (plant cell → photosynthetic tissue → leaf organ) and a muscle cross-section (animal cell → muscle tissue → heart organ), highlighting that both use the same levels.
Living Systems
Living things are organised systems made of interacting parts that work together.
Levels of Organisation
The main order is cells, tissues, organs, then organ systems.
Key Idea
Each level builds on the one before it, so the levels are related but not interchangeable.
Examples
A muscle cell is one cell, muscle tissue is many similar cells, the heart is an organ, and the circulatory system is an organ system.
Activities
Activity 1: Put It in Order
Put these in order from smallest level to largest level: organ system, cell, organ, tissue. Then explain why that order makes sense.
Activity 2: Fix the Weak Answer
A student says: "The heart is an organ system because it pumps blood." This answer is weak. Use the Claim-Evidence-Reasoning frame below to fix it.
Claim-Evidence-Reasoning Frame
Claim: State what the heart actually is.
Evidence: Use facts from the lesson about organs and organ systems.
Reasoning: Explain how the evidence supports your claim.