Principles of organisation: Organs are organised into organ systems, which work together to form organisms.

The idea

Organs do not work in isolation: several organs combine into an organ system, and the organ systems together make up the whole organism. The digestive system illustrates this well. Its organs include the stomach, small intestine, large intestine, liver and pancreas. Each organ has its own tissues and its own job, but they act in sequence so that food is digested and nutrients are absorbed. The digestive system then works alongside other systems, such as the circulatory system that transports the absorbed nutrients and the respiratory system that supplies oxygen. The full hierarchy is cells → tissues → organs → organ systems → organism. When explaining this, name a system, list two or more of its organs, and state how those organs depend on one another and on other systems.

What earns the marks

• States that an organ system is a group of organs working together to carry out a major function.
• Names a named organ system, such as the digestive, circulatory or respiratory system, and its overall role.
• Lists at least two organs belonging to that system, for example stomach and small intestine in the digestive system.
• Explains how the organs within the system cooperate, for example the stomach digests food and the small intestine absorbs the products.
• Describes how different organ systems interact, for example the circulatory system transporting substances produced by the digestive system.
• Places the hierarchy correctly as cells → tissues → organs → organ systems → organism.

Common mistakes and how to put them right

• Treating an organ system as a single large organ.

  Put it right: An organ system is a group of separate organs, each with its own tissues, working together.

• Listing organs without linking them to the system's function.

  Put it right: State the shared job of the system and how each named organ contributes to it.

• Forgetting that systems interact.

  Put it right: Give an example such as the digestive and circulatory systems working together to deliver nutrients.

Exam tips

• Learn one organ system in detail, including at least four organs and the role of each, so you can answer with confidence.
• When asked about the whole organism, finish by stating that all the organ systems together form the organism.
• Use connectives such as 'so that' and 'which allows' to show how one organ or system enables the next step.