Recommended resources
The freely available resources this lesson will direct you to.
- Biology - Cells /Tissues/Organs/Organ Systems - Specialization (Middle School Science)www.youtube.com
- Cells, Tissues, Organs, Organ systems | Level of organisation in organisms | Easy science videowww.youtube.com
- How plants and animals are organised - Living organismswww.bbc.co.uk
- Learn: Tissues, organs, and organ systems (article) | Khan Academywww.khanacademy.org
Hello! Last lesson, you saw how mitotic division produces new cells for growth and repair. But a larger collection of cells is not yet an organ. In this final lesson of the cells and microscopy module, you’ll see how specialised cells work together at successively larger levels: cells, tissues, organs and organ systems. You’ll use that order to interpret examples in both animals and plants. Allow about 30–35 minutes, including the short video and readings.
A hierarchy of working parts
A hierarchy is an arrangement in which each level is built from parts at a smaller level. In a multicellular organism, the order from smallest to largest is:
- Cell — a living unit that may be specialised for a particular job.
- Tissue — a group of similar cells working together to perform a function.
- Organ — a structure made of different tissues working together.
- Organ system — a group of organs working together to perform a broader function.
- Organism — the complete living thing.
The important change at each level is not just size. It is what works together: cells within a tissue, different tissues within an organ, and organs within a system.
Watch Cognito USA’s Biology – Cells / Tissues / Organs / Organ Systems – Specialization for a visual introduction. It starts below the cell level, with organelles; keep your attention on the four levels named in this lesson.
Biology - Cells /Tissues/Organs/Organ Systems - Specialization (Middle School Science)
Cognito USA’s video builds the hierarchy using examples of specialised cells, tissues, organs and systems.
Watch the hierarchy. Notice the distinction made between a tissue (similar cells working together) and an organ (different tissues working together). Follow the stomach example through to the digestive system.
The diagram below gives you a second way to read the same idea. Follow the smooth muscle example from the cellular level down towards the complete organism.
An organelle is part of a cell, not an organ. Likewise, the water molecule in the diagram is not a tiny tissue. These are useful smaller-scale details, but the order you need to arrange for this lesson begins with the cell.
Why a tissue is not an organ
Consider the smooth muscle cells in the bladder wall. Each is a cell; many working together form smooth muscle tissue, which can contract. The bladder is an organ because it contains different tissues working together to store and release urine. The smooth muscle tissue is part of the bladder—it is not another name for the whole bladder.
The same distinction applies to the heart. Cardiac muscle tissue helps it pump, but the heart is more than muscle tissue alone. Read Khan Academy’s explanation to see how several tissues contribute to an organ, and how that organ fits into a system.
Learn: Tissues, organs, and organ systems (article) | Khan Academy
Read Khan Academy’s Tissues, organs, and organ systems to sharpen the boundary between an organ and an organ system.
In “Tissues make up organs,” start at the definition of an organ and read the heart example through the paragraph explaining what its tissues achieve together. Then, in “Organs make up organ systems,” read the circulatory example, ending with what the circulating blood carries. Focus on what is being grouped at each level; you do not need to memorise every named tissue.
The heart is an organ; the circulatory system includes the heart and blood vessels working together to move blood around the body. Calling the heart a system would skip a level. Calling cardiac muscle tissue an organ would also skip the distinction that an organ contains different tissues.
The diagram’s bladder offers another complete path through the hierarchy: smooth muscle cell; smooth muscle tissue; bladder; urinary system; organism. Each item names a different scale of organisation, even though all belong to the same living body.
The hierarchy also works in plants
Plants have specialised cells, tissues, organs and organ systems too. A root hair cell, for example, is a specialised cell on the surface of a root. It belongs to the root’s outer tissue. A root is an organ: it contains several tissues that work together, including tissues involved in absorption and transport. Multiple roots form a root system, part of the complete plant.
BBC Bitesize provides the plant tissue and organ examples behind this distinction.
How plants and animals are organised - Living organisms
Read the plant examples in BBC Bitesize’s How plants and animals are organised to see that the hierarchy is not limited to animals.
In “Tissues,” read the tissue definition and the “Plants” paragraph on dermal tissue. Then, in “Organs,” read the plant organs paragraph, from roots through the list of other organs. Focus on why a root is more than a group of identical root hair cells.
A useful safeguard is to ask “What kind of parts make up this structure?” A root contains different tissues, so it is an organ, not a tissue. A root system contains roots—organs working together—so it is an organ system. The whole plant is the organism, not one of its organs.
This hierarchy describes multicellular organisms. A single-celled organism is already a complete organism, even though it does not have tissues and organs.
Reading an unfamiliar example accurately
In an eAssessment diagram, you may meet a cell or organ you have not memorised. Use the relationships in the description rather than guessing from how important something looks. If the description says similar cells cooperate, it points to a tissue. If it says different tissues cooperate, it points to an organ. If it names several organs cooperating, it points to an organ system.
Be precise about the whole and its parts. For instance, “The bladder is part of the urinary system” correctly places an organ inside an organ system. “The bladder is urinary tissue” does not: it confuses the entire organ with one of the tissues it contains.
Key takeaways
For a multicellular organism, the order is cell, tissue, organ, organ system, organism. Similar cells working together form a tissue; different tissues working together form an organ; organs working together form an organ system. The order applies to plants as well as animals.
You have now completed the cells and biological organisation module. Next, you’ll move from how cells are organised to how substances move into and out of cells, beginning with diffusion.