KS3 · Biology

Multicellular and unicellular organisms

Some living things are just one cell, and that cell keeps itself alive. You're made of a huge number of cells. So what's the real difference? Who does the work.

Sort it yourself

What do they share, and what sets them apart?

Some living things are made of just one cell. Others are made of many. For each statement, tick where it belongs: the unicellular organism, the multicellular organism, or both.

  • A Unicellular organism (one cell)
  • B Multicellular organism (many cells)
  1. It is made of cells
  2. It is a living organism in its own right
  3. Every life process it needs gets carried out
  4. It is made of just one cell
  5. One cell carries out every life process by itself
  6. Its single cell may have a structure such as a flagellum, cilia or an eyespot that suits its way of life
  7. It is made of many cells
  8. Its cells are specialised to carry out particular functions
  9. Its cells work together, organised into tissues, organs and organ systems

Built for the job

What is each structure for?

Pick a structure, then pick the job it helps the single cell do.

Still to sort

Moving (0)

Getting the cell from place to place

Where the line is: Two different structures belong here. A job can be done by more than one kind of structure.

Photosynthesis (0)

Making its own food

Detecting light (0)

Sensing where the light is

Where the line is: Detecting light is sensing it. That is a different job from using light to photosynthesise.

4 of 4 still to sort.

Some single-celled organisms have structures that suit their way of life. Match each structure to the job it does.

Levels of organisation

Build a multicellular organism, level by level

Put the levels in order, starting from the smallest building block and finishing with the whole living thing. Each level is built from the one before it.

1 · The smallest building block5 · The whole living thing

  1. Organ

  2. Cell

  3. Organism

  4. Tissue

  5. Organ system

Watch out: Easy mistake: swapping tissue and organ. Tissues come first, because organs are built from tissues.

Tissue or organ?

What is an organ actually made of?

You now know the order. The trap is what each level is made of, especially the difference between a tissue and an organ.

Which is closest to what you think an organ is made of?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Every living thing is made of cells. The big question is who does the work.

What you need to know

  • Every living organism is made of one or more cells. The cell is the basic unit of life.
  • A unicellular organism is made of one cell, and that cell carries out all of the organism's life processes.
  • A multicellular organism is made of many cells that are specialised to carry out particular functions and work together.
  • Some unicellular organisms have structural adaptations: a flagellum or cilia for moving, chloroplasts for photosynthesis, or a light-sensitive eyespot for detecting light.
  • The levels of organisation in a multicellular organism are cell, tissue, organ, organ system, organism.
  • Tissue: similar cells working together. Organ: different tissues working together. Organ system: organs working together.

The big picture

Every living organism is made of cells. A unicellular organism is a single cell that carries out every life process itself, and some have structures such as a flagellum, cilia, chloroplasts or an eyespot that suit their way of life. A multicellular organism is made of many specialised cells that work together, organised into tissues, organs, organ systems and the whole organism.

Key points

1Unicellular and multicellular organisms are both living organisms made of cells.
2The difference is who does the work: one cell doing everything, or many specialised cells sharing it.
3A structure such as a flagellum, cilia, chloroplasts or an eyespot suits a single cell's way of life.
4Each level of organisation is built from the level below it, from cell up to the whole organism.
5An organ is made of different tissues, not simply of lots of similar cells.

Worked example

Problem

A student is looking at a part of a multicellular organism. It is built from several kinds of tissue, all doing one job between them. The student says: "It's made of tissues, so it must be a tissue." Which level is it really?

⚠ Watch out

Mixing up tissue and organ. A tissue is a group of similar cells working together; an organ is a group of different tissues working together. An organ is not just a big pile of similar cells.

🧠

Memory hook

Count the teams. Similar cells team up into a tissue. Different tissues team up into an organ. Organs team up into an organ system. The systems together make the organism. For the order, try: Clever Tigers Organise Our Outings (cell, tissue, organ, organ system, organism).

✓

Check yourself

Cover the page. What do unicellular and multicellular organisms share, and what is the big difference? Then write the five levels in order, and say what a tissue and an organ are made of.

Flashcards

(11)
What is a unicellular organism?
An organism made of a single cell. That one cell carries out all the life processes the organism needs.
What is a multicellular organism?
An organism made of many cells. Its cells are specialised to carry out particular functions and work together.
What do all living organisms have in common, whether unicellular or multicellular?
They are all made of cells: one or more. The cell is the basic unit of living organisms.
What does it mean when a cell in a multicellular organism is specialised?
It is suited to carrying out a particular function, rather than doing every job itself.
Which structures help some single-celled organisms to move?
A flagellum or cilia.
What are chloroplasts for in a single-celled organism?
Photosynthesis: they let the cell make its own food.
What does a light-sensitive eyespot let a single-celled organism do?
Detect light.
List the levels of organisation in a multicellular organism, smallest first.
Cell → tissue → organ → organ system → organism.
What is a tissue?
A group of similar cells working together to carry out a function.
What is an organ?
A group of different tissues working together to carry out a function.
What is an organ system?
A group of organs working together to carry out a function.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 1 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.