The fundamental units of living organisms are cells, which may be part of…

The idea

Cells are the basic building blocks of every living organism. A single cell can carry out all life processes, but in multicellular organisms cells become specialised for particular jobs. Similar specialised cells group together as a tissue, such as muscle tissue or xylem tissue. Different tissues combine to form an organ, for example the stomach or a leaf. Organs work together as an organ system, such as the digestive system or the transport system in plants. This hierarchy allows large organisms to perform living processes efficiently because each level is adapted to its role.

For example: A root hair cell has a long extension that increases surface area for water uptake, and the many root hair cells form root tissue that absorbs water for the whole plant.

What earns the marks

• Cells are the fundamental units of living organisms; all living processes occur in or are controlled by cells.
• Specialised cells have structural adaptations that suit a particular function, such as a sperm cell having a tail for swimming.
• A tissue is a group of similar cells working together to carry out a shared function, for example muscle tissue or xylem tissue.
• An organ is a group of different tissues working together to perform a specific function, such as the heart or a leaf.
• An organ system is a group of organs working together to carry out a major life process, such as the digestive system.
• The levels of organisation form a hierarchy: cells, tissues, organs, organ systems and the whole organism.

Common mistakes and how to put them right

• Saying that a tissue is a group of organs.

  Put it right by stating that a tissue is a group of similar cells, while an organ contains several different tissues.

• Confusing an organ with an organ system.

  Put it right by noting that an organ system is a group of organs, for example the stomach is an organ in the digestive system.

• Describing a cell as adapted without linking the adaptation to a function.

  Put it right by naming the feature and explaining how it helps the cell carry out its job, such as many mitochondria releasing energy for muscle contraction.

Exam tips

• Use the correct sequence when describing organisation: cell, tissue, organ, organ system, organism.
• When asked about adaptation, name the structural feature and then explain the advantage it gives, for example a large surface area increases the rate of absorption.
• Give named examples from a familiar organism, such as root hair cells, xylem tissue, leaves and the transport system in a plant, to make your answer precise.