Tissues, Organs, Organ Systems
In 2013, scientists at Stanford University mapped every cell type in the human body and counted roughly 37 trillion cells — all organised into 11 organ systems that run without you thinking about them.
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Q1 · Your heart is made of millions of cells, but you'd never call a single cell a "heart". What's the difference between a heart cell and a heart?
Q2 · Name three different organs in your body and what each one does.
● Know
- The five levels of organisation: cells → tissues → organs → systems → organism
- Definitions of tissue, organ and organ system
- Examples of each level, especially the circulatory system
● Understand
- Why a group of cells working together can do more than one cell alone
- Why an organ usually contains more than one type of tissue
- How systems link up — your circulatory system depends on your respiratory system, etc.
● Can do
- Order any biological structure into the correct level
- Trace a full chain from cells to organism
- Match each system to its main job
- Tissue
- Organ
- Organ system
- Organism
- Hierarchy
- Group of organs working together (e.g. circulatory)
- Group of similar cells doing one job
- Levels arranged from smallest to largest
- Structure made of several tissues (e.g. heart)
- A whole individual living thing
Flex your hand into a fist and watch the tendons slide under your skin — what you are seeing is tissue, organ, and organ system all working in sync, built from trillions of individual cells none of which can see the bigger picture.
| Level | What it is | Example |
|---|---|---|
| 1. Cell | The smallest building block of life. | A single muscle cell. |
| 2. Tissue | A group of similar cells working together on one job. | Muscle tissue (many muscle cells). |
| 3. Organ | Several tissues working together to do one specific job. | Heart (made of muscle tissue, nerve tissue, blood vessels and more). |
| 4. Organ system | A group of organs working together to do one big job. | Circulatory system (heart + blood vessels + blood). |
| 5. Organism | The whole individual living thing — all systems combined. | You. |
Think of it like Lego: cells are the bricks, tissues are simple Lego shapes, organs are bigger structures, systems are whole vehicles, and the organism is the entire Lego city.
Many similar form a . Several tissues form an . Several organs form an organ . All systems together make an organism.
Let's trace one full chain through all five levels using your heart.
| Level | Example |
|---|---|
| Cell | A single cardiac muscle cell — one tiny twitchy fibre. |
| Tissue | Cardiac muscle tissue — millions of those cells joined together, all contracting at the same time. |
| Organ | The heart — cardiac muscle tissue PLUS nerve tissue (that triggers the beat) PLUS connective tissue (that holds it together) PLUS blood vessel tissue. |
| Organ system | The circulatory system — heart + arteries + veins + capillaries + blood. |
| Organism | The whole human — circulatory system plus respiratory, digestive, nervous, etc. |
Notice how each level needs the one below it. No cells, no tissues. No tissues, no organs. No systems, no organism. They build up like floors of a building.
Humans have about 11 major organ systems. You'll meet most of them properly in Year 8. Here are a few to keep in mind now.
| System | Main job | Key organs |
|---|---|---|
| Circulatory | Move blood (oxygen + nutrients) around the body | Heart, blood vessels, blood |
| Respiratory | Take in oxygen, get rid of CO₂ | Lungs, trachea, diaphragm |
| Digestive | Break down food and absorb nutrients | Stomach, intestines, liver, mouth |
| Nervous | Carry signals, control thought and movement | Brain, spinal cord, neurons |
| Skeletal | Support the body and protect organs | Bones, joints, cartilage |
| Muscular | Produce movement | Skeletal muscles, smooth muscles |
Plants have systems too. The two main ones are the root system (anchors the plant, absorbs water) and the shoot system (stems and leaves — collects light, makes food).
- Circulatory
- Respiratory
- Digestive
- Nervous
- Skeletal
- Supports the body and protects organs
- Breaks down food and absorbs nutrients
- Moves blood around the body
- Carries signals and controls thought
- Takes in oxygen, gets rid of CO₂
Wrong: "A tissue is the same as an organ." A tissue is just one type of cell working together. An organ contains several different tissues stitched together.
Right: A tissue = many of the SAME type of cell. An organ = several DIFFERENT tissues combined.
Wrong: "Blood isn't a tissue — it's just a liquid." Blood is actually a connective tissue. It's made of specific cells (red blood cells, white blood cells, platelets) working together in a liquid called plasma.
Right: Blood IS a tissue — it has specialised cells doing a coordinated job (carrying oxygen, fighting infection, clotting).
Wrong: "Plants don't have organs — they only have leaves and roots." Leaves and roots ARE organs! Each is made of several different tissues and does a specific job.
Right: Plant organs include leaves, roots, stems, flowers and fruits. Each is a true organ made of multiple tissues.
The cleverest part of the hierarchy isn't just that systems exist — it's that they all depend on each other.
For example, your circulatory system relies on:
- the respiratory system to load the blood with oxygen at the lungs;
- the digestive system to add glucose and nutrients to the blood;
- the nervous system to control how fast the heart beats;
- the skeletal system to make new red blood cells (bone marrow).
Knock out any one of these and the others struggle. That's why doctors think about systems as a team, not as separate organs. The same idea applies to plants — the root system can't feed the leaves without the stem in between.
A bacterium is a single-celled organism — it has no tissues, no organs, and no organ systems. Yet it can still take in food, release energy, reproduce, and respond to its environment. Predict: how can a single cell do all those jobs that a human body needs entire organ systems for? Write 1–2 sentences, then reveal.
How close was your prediction?
Nice — you linked the size of the cell to the need for systems.
That's fine — the idea that "size forces specialisation" is a deep one in biology.
At the start of this lesson you were asked how any one cell knows what 37 trillion others are doing — and who is in charge.
Now that you understand how cells, tissues, organs and organ systems are organised, write your full answer. How does the nervous system and the organ-system level of organisation help answer that question?
Q1. List the five levels of organisation in order from smallest to largest, and define each level in your own words. (3 marks)
Q2. Trace a full chain from a single cell to an organism using the human heart as your example. Name something specific at each of the five levels. (4 marks)
Q3. A student says: "The circulatory system works on its own — it doesn't need any of the other systems." Explain why this statement is wrong by referring to at least TWO other organ systems and what they provide. (4 marks)
Answers
▾MCQ 1
B — The hierarchy goes: cell (smallest) → tissue → organ → organ system → organism (largest).
MCQ 2
D — A tissue is a group of similar cells doing one job. A single cell is not a tissue (option A); systems and organs are higher levels (B, C).
MCQ 3
A — A leaf is an organ — it's made of several different tissues (palisade, spongy, epidermis, vein) doing one big job (photosynthesis). Muscle tissue is a tissue, the circulatory system is a system, a red blood cell is a cell.
MCQ 4
C — The circulatory system is heart + blood vessels (+ the blood). Stomach/liver/intestines = digestive. Lungs/trachea = respiratory. Brain/spinal cord = nervous.
MCQ 5
B — A bacterium is small enough that gases and food can leak in and out of a single cell easily. Big organisms are too thick for that — their inner cells are too far from the outside — so they need specialised cells, tissues, organs and systems.
Short Answer 1
Model answer: (1) Cell — the smallest building block of life. (2) Tissue — a group of similar cells working together on one job. (3) Organ — a structure made of several tissues working together to do one specific job. (4) Organ system — a group of organs working together to do one big job. (5) Organism — the whole individual living thing, made of all its systems combined.
Short Answer 2
Model answer: Cell — a single cardiac muscle cell. Tissue — cardiac muscle tissue (millions of cardiac muscle cells together). Organ — the heart (cardiac muscle tissue + nerve tissue + connective tissue + blood vessels). Organ system — the circulatory system (heart + arteries + veins + capillaries + blood). Organism — a human being.
Short Answer 3
Model answer: The statement is wrong because organ systems depend on each other. The circulatory system needs the respiratory system to load the blood with oxygen at the lungs — without it, the blood being pumped around would have nothing useful in it. It also needs the digestive system to add glucose and nutrients to the blood at the small intestine, so they can be delivered to every cell. Other systems matter too: the nervous system controls how fast the heart beats, and the skeletal system (bone marrow) makes new red blood cells. If any of these systems stop working, the circulatory system stops being useful.