2a Level of Organisation¶

Part of 2 Structure and Functions in Living Organisms.

Living things are organised in five levels: organelles, cells, tissues, organs and organ systems. Each level is built from the one below, and each can do a job that the level below cannot do alone, so the whole organism works only if every level works.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

The five levels of organisation, what each one is made of, and how to apply them to examples in both animals and plants.

The Organisational Hierarchy¶

Organelles are specialised structures inside a cell, each with its own job. The nucleus, mitochondria and ribosomes are examples, and 2b Cell Structure covers them in detail.

Cells are the basic structural and functional units of a living organism.

Tissues are groups of similar cells that work together to do the same job. Epithelial cells, for example, form epithelial tissue, and muscle cells form muscle tissue.

Organs are groups of different tissues working together to do a specific job. The lungs contain epithelial tissue, connective tissue and blood vessels, and together they exchange gases.

Organ systems are groups of organs with related functions that work together to carry out a major job for the body. The respiratory system includes the lungs and the airways that lead to them.

The word that separates a tissue from an organ is different: a tissue is made of similar cells, and an organ is made of different tissues.

Applying the Hierarchy¶

Any structure can be placed by asking what it is made of. The respiratory system contains the lungs (organs), which are made of epithelial tissue (tissue), which is made of epithelial cells (cells), which contain organelles such as mitochondria.

Plants follow the same pattern. A leaf is an organ, because different tissues work together inside it. Roots, stems and leaves form an organ system that transports substances around the plant.

Examples Across the Hierarchy¶

Level	Made of	Animal example	Plant example
Organelle	Part of a cell	Mitochondrion in a muscle cell	Chloroplast in a leaf cell
Cell	Organelles	Muscle cell	Palisade cell
Tissue	Similar cells	Muscle tissue	Xylem tissue
Organ	Different tissues	Heart	Leaf
Organ system	Different organs	Circulatory system	Plant transport system

Teacher insight

Plants also have organs and organ systems. A leaf counts as an organ because different tissues work together inside it, and plant organs such as roots, stems and leaves work together in transport.

Exam technique

When a question asks you to decide a level, name what the structure is made of and use the word different where it applies. "The heart is an organ because it is made of different tissues, such as muscle tissue and connective tissue, working together to pump blood" is a complete answer. "The heart is a big group of cells" is a description of a tissue.

Common Confusions¶

Plants have no organs: They do. A leaf is an organ, and roots, stems and leaves work together as an organ system.
Size decides the level: It does not. A very large block of identical cells is still only a tissue, because the level depends on what the structure is made of.
Organ and organ system: The heart is an organ. The circulatory system is the organ system that contains it, together with the blood vessels.

Check Yourself¶

Put these in order from smallest to largest: tissue, organ system, organelle, cell, organ.
State one difference between a tissue and an organ.
Name one plant organ and one plant tissue found inside it.
A student says: "Plants are too simple to have organs." Explain why this is wrong.
Muscle cells make up muscle tissue. Explain why the stomach, which contains muscle tissue, is described as an organ and not as a tissue.

Answers

Organelle, cell, tissue, organ, organ system.
A tissue is made of similar cells doing the same job, whereas an organ is made of different tissues working together.
Any plant organ with one of its tissues, such as a leaf containing xylem, or a root containing xylem or phloem.
Plants have organs. A leaf is made of different tissues working together, mainly for photosynthesis, and roots, stems and leaves work together as an organ system for transport.
The stomach contains several different tissues, such as muscle tissue and epithelial tissue, which work together to do a specific job. A structure made of different tissues is an organ.

Key Terms¶

Organelle: a specialised structure inside a cell that carries out a particular job.
Cell: the basic structural and functional unit of a living organism.
Tissue: a group of cells with a similar structure and function, working together to perform the same task.
Organ: a structure made of different tissues working together to perform specific functions.
Organ system: a group of organs that work together to carry out a major body function.