Principles of organisation: Organs are aggregations of tissues performing specific functions.

The idea

An organ is a structure built from several different tissues that cooperate so the organ can carry out one or more specific jobs in the body. Each tissue is itself a group of similar cells with a shared role, so the hierarchy runs cells → tissues → organs. In the stomach, for example, muscular tissue contracts to churn food, glandular tissue secretes digestive enzymes and acid, and epithelial tissue lines the inside and outside surfaces. No single tissue could digest food alone; the organ's function emerges from the tissues working together. When you are asked to explain this idea, name an organ, identify at least two tissues inside it, and state the job each tissue does and how that contributes to the organ's overall function.

What earns the marks

• States that an organ is made of more than one tissue type, not just many identical cells.
• Names a named organ, such as the stomach, heart or leaf, and links it to a specific function.
• Identifies at least two tissues in that organ, for example muscular tissue and glandular tissue in the stomach.
• Explains how each named tissue contributes to the organ's overall function, for example muscle contraction churns food while glands release enzymes.
• Uses the hierarchy cells → tissues → organs correctly, showing that tissues are groups of similar cells.
• Recognises that the tissues in an organ are different from one another and work together rather than independently.

Common mistakes and how to put them right

• Saying an organ is a group of identical cells.

  Put it right: An organ contains several different tissues, each made of similar cells with a shared role.

• Confusing an organ with an organ system.

  Put it right: An organ is a single structure such as the stomach, while an organ system is a group of organs such as the digestive system.

• Describing only what the organ does without naming its tissues.

  Put it right: Name at least two tissues and link each to part of the organ's function.

Exam tips

• Use the phrase 'different tissues working together' and then justify it with two named tissues from one organ.
• Choose one organ you know well, such as the stomach, and rehearse its tissues and their jobs so you can answer quickly.
• If a question says 'explain', make the link explicit: tissue → its job → how that helps the organ's function.