Welcome to the Levels of Organisation!
Extension topic – this chapter is Extension Part content in the 2025 junior secondary science curriculum.
Your body is made of trillions of tiny cells. But a pile of cells is not a person! In this chapter you will learn how cells form tissues, how tissues build organs, and how organs team up into organ systems that keep you alive. You will meet four major organs – the lungs, stomach, brain and heart – and four systems: the breathing, digestive, nervous and transport systems. Let's climb the ladder of life!
1. The Ladder of Life: From Cells to You
In the chapter Cells - the basic units of living things, you learnt that cells divide, grow and become specialised. Now we go further up. Living things are organised in five levels, from smallest to largest:
1. Cell – the basic unit of life (e.g. a muscle cell).
2. Tissue – a group of similar cells working together (e.g. muscle tissue).
3. Organ – different tissues working together to do a job (e.g. the heart).
4. Organ system – different organs working together (e.g. the transport system).
5. Organism – a whole living thing made of many systems (e.g. you!).
\(\text{cell} \rightarrow \text{tissue} \rightarrow \text{organ} \rightarrow \text{organ system} \rightarrow \text{organism}\)
Analogy: Think of a housing block in Hong Kong. One brick is like a cell. A wall of many similar bricks is like a tissue. A room is like an organ – it needs different parts (walls, floor, windows, door) working together. A flat, with its kitchen, bathroom and bedrooms, is like an organ system. The whole building is like the organism.
Memory aid: "Cool Teens Often Study Online" = Cell, Tissue, Organ, System, Organism.
Key takeaway: Cells build tissues, tissues build organs, organs build organ systems, and the systems together make a whole organism.
2. Tissues: Teams of Specialised Cells
A tissue is a group of similar cells with a specialised structure that work together to carry out a particular function. The shape of the cells matches their job – this is "form and function". Here are three examples.
Nerve tissue – the body's messengers
Nerve tissue is made of nerve cells. They are long and thin, with fine branches at their ends, so they can carry electrical messages (nerve impulses) very quickly around the body. It is found in the brain, spinal cord and nerves. Some nerve cells running from the spinal cord to the toes are about 1 metre long!
Muscle tissue – the movers
Muscle tissue is made of long muscle cells that can contract (get shorter) and relax. When they contract together, they pull on body parts and cause movement. There are three kinds:
Skeletal muscle – attached to bones; moves your arms and legs when you choose to.
Heart muscle – only in the heart wall; contracts all your life without getting tired.
Smooth muscle – in the walls of the stomach, intestines and blood vessels; works without you thinking about it.
Bone tissue – the strong supporters
Bone tissue is made of living bone cells surrounded by a hard material rich in calcium compounds. This makes bone strong, so it can support the body, protect soft organs (the skull protects the brain; the ribs protect the heart and lungs) and work with muscles for movement. Because bone is alive and has a blood supply, a broken bone can slowly repair itself.
Did you know? An adult skeleton has 206 bones. A newborn baby has more, but some of them join together as the baby grows.
Other tissues
Blood is a liquid tissue that carries substances around the body. A lining tissue covers surfaces, like the inside of the stomach. Plants have tissues too, such as vascular tissue that carries water and food through the plant.
Key takeaway: A tissue is a group of similar specialised cells working together for one function – nerve tissue carries messages, muscle tissue causes movement, bone tissue supports and protects.
3. Organs: Different Tissues Working Together
An organ is a structure made of several different tissues that work together to carry out specific functions. The tissues cooperate, like players in a football team.
Example: what is the stomach made of?
Muscle tissue in the wall churns and mixes the food.
Gland tissue makes digestive juice containing acid and enzymes.
Lining tissue covers the inside surface.
Nerve tissue controls the muscles and glands.
Blood brings oxygen and food to all these cells.
Meet four major organs
Lungs – two spongy organs in the chest, protected by the ribs. They contain millions of tiny air sacs, where oxygen passes from the air into the blood and carbon dioxide passes from the blood into the air.
Stomach – a J-shaped muscular bag in the upper abdomen. It stores food for a few hours, churns it and continues to digest it with digestive juice (digestion has already begun in the mouth). Its acid also kills many germs in food.
Brain – protected by the skull and made mostly of nerve tissue. It is the body's control centre: it receives messages from the sense organs, makes decisions, sends out instructions, and lets us think, learn, remember and feel.
Heart – a hollow muscular pump about the size of your fist, in the chest between the lungs. Each beat pushes blood through blood vessels to every part of the body.
Did you know? At about 70 beats a minute, your heart beats about 100,000 times a day: \(70 \times 60 \times 24 \approx 100\,000\).
Key takeaway: An organ is made of different tissues working together. The lungs exchange gases, the stomach stores and digests food, the brain controls the body and the heart pumps blood.
4. Organ Systems: Organs as a Team
An organ system is a group of different organs that work together to carry out a major life function. Here are four key human systems.
The breathing system
Main parts: nose (and mouth), windpipe (trachea), bronchi, bronchioles, air sacs, lungs, ribs, rib muscles and diaphragm.
Path of air: nose \(\rightarrow\) windpipe \(\rightarrow\) bronchi \(\rightarrow\) bronchioles \(\rightarrow\) air sacs in the lungs.
Role: takes oxygen into the body and removes carbon dioxide. Hairs and mucus in the nose trap dust and germs. The diaphragm (a sheet of muscle under the lungs) and the rib muscles change the size of the chest so that air moves in and out. So the organs involved in breathing are mainly the lungs, helped by the nose, windpipe, diaphragm and rib muscles.
The digestive system
Main parts: mouth (teeth and salivary glands), gullet (oesophagus), stomach, small intestine, large intestine, rectum and anus, helped by the liver and pancreas.
Role: breaks down food into small, soluble substances that are absorbed into the blood, mainly in the small intestine. Undigested waste leaves through the anus.
The nervous system
Main parts: brain, spinal cord and nerves, linked to sense organs such as the eyes, ears and skin.
Role: detects changes inside and outside the body, carries nerve impulses, and controls and coordinates the body's activities. The brain and spinal cord form the central nervous system; the backbone protects the spinal cord. Example: if you touch a hot wok, your hand pulls away in a split second!
The transport system
Main parts: heart, blood vessels and blood (also called the circulatory system).
Blood vessels: arteries carry blood away from the heart; veins carry blood back to the heart; capillaries are tiny thin-walled vessels where substances pass between blood and cells.
Blood: red blood cells carry oxygen; white blood cells fight germs; platelets help blood clot; the liquid part (plasma) carries digested food, carbon dioxide and other substances.
Role: delivers oxygen and digested food to every cell and carries away carbon dioxide and other wastes.
Key takeaway: The breathing system brings in oxygen, the digestive system supplies food, the transport system delivers both to every cell, and the nervous system controls everything.
5. Teamwork Keeps You Alive
No system works alone. Every cell needs a steady supply of oxygen and food to release energy, and its wastes must be removed. The systems share this work.
Case study: Sports Day 100 m race
Nervous system: your ears detect the starting signal and your brain sends messages to your leg muscles.
Muscles: contract quickly and need much more energy.
Breathing system: you breathe faster and deeper to take in more oxygen and remove more carbon dioxide.
Transport system: your heart beats faster to deliver oxygen and food to the muscles.
Digestive system: your breakfast (maybe a pineapple bun!) has been digested and absorbed, giving your blood the food that muscles use for energy.
If one system fails, the others suffer too. For example, if the lungs are damaged, the heart has to work harder to supply enough oxygen.
Did you know? Chinese White Dolphins in Hong Kong waters live in the sea but breathe with lungs, just like us. They must come to the surface to breathe air through the blowhole on top of their head.
Key takeaway: Organ systems depend on each other. Together they supply every cell with what it needs and keep the whole organism alive.
Common Mistakes to Avoid
Common mistake 1: "A tissue is any group of cells." Not quite! A tissue is a group of similar cells with a specialised structure working together for a particular function.
Common mistake 2: "An organ is made of one kind of tissue." Wrong – an organ has several different tissues. The heart has heart muscle, nerve tissue, lining tissue and blood.
Common mistake 3: "Breathing and respiration are the same." No! Breathing moves air into and out of the lungs. Respiration happens inside cells and uses oxygen to release energy from food. Breathing supplies the oxygen that respiration needs.
Quick Review
1. Cell \(\rightarrow\) tissue \(\rightarrow\) organ \(\rightarrow\) organ system \(\rightarrow\) organism.
2. Tissue = similar cells with a specialised structure and function (e.g. nerve, muscle, bone).
3. Organ = different tissues working together (e.g. lungs, stomach, brain, heart).
4. Organ system = different organs working together for a major life function.
5. Breathing system takes in oxygen and removes carbon dioxide; digestive system breaks food into small soluble substances; nervous system detects changes and controls the body; transport system carries oxygen and food to cells and removes wastes.
6. All systems depend on each other. Great job – you have climbed the whole ladder of life!