Principles of Organisation¶

Part of Organisation.

Multicellular organisms are organised in levels. Cells form tissues, tissues form organs, organs form organ systems, and organ systems together make up the whole organism. Each level can do a job that the level below cannot do alone, which is why the order matters.

What You Need to Learn¶

Follow the organisation of a living body from individual cells to cooperating organ systems. Use examples to explain what each level contributes and why an organ needs several kinds of tissue.

Cells, Tissues, Organs and Organ Systems¶

Cells¶

The cell is the basic unit of a living organism. Cells are often called the building blocks of life. The exact claim is that everything an organism does, from contracting a muscle to digesting a meal, is done by cells or by something cells have made.

A single-celled organism such as yeast carries out every life process inside one cell. A human or an oak tree is made of many cells, and most of them are specialised: each type is adapted to do one job. A muscle cell contracts, a nerve cell carries electrical impulses, and a root hair cell absorbs water (see Cell Structure). One specialised cell cannot keep an organism alive, so specialised cells have to be organised to work together.

Tissues¶

A tissue is a group of similar cells that work together to do one job. Muscular tissue is made of muscle cells that contract together, which produces a force no single cell could. Glandular tissue is made of cells that make and release substances such as digestive juices. Epithelial tissue is made of cells that cover surfaces, both inside and outside the body. The two words that define a tissue are similar and one job.

Organs¶

An organ is a group of different tissues that work together to do a specific job. The stomach is the standard example. Its muscular tissue churns the food, its glandular tissue makes digestive juices, and its epithelial tissue covers the inside and outside surfaces. None of these tissues could do the whole job alone. The stomach is an organ because it combines them into one working unit.

Exam technique

An answer to "why is this an organ?" is strongest when it names at least two tissues and says what each one does. "It is made of different tissues" is the claim, and the named tissues are the evidence for it. For the stomach, that means muscular tissue churning the food and glandular tissue making digestive juices.

Organ systems¶

An organ system is a group of organs that work together to do a major job for the organism. The digestive system digests food and absorbs the products. The stomach churns food and starts digesting it, the pancreas releases digestive enzymes, the liver makes bile, and the small intestine finishes digestion and absorbs the products into the blood. Each organ does part of the job, and the system works only because they do it together.

From organ systems to the whole organism¶

Organ systems working together make up the organism, and each one depends on the others. The digestive system produces glucose and other soluble molecules. The circulatory system carries them to every cell. The respiratory system takes in oxygen, so that cells can use the glucose to release energy. If one system fails, cells elsewhere in the body suffer even though nothing is wrong with them.

The levels compared¶

Level	Made of	Does	Animal example	Plant example
Cell	sub-cellular structures	one specialised job	muscle cell	palisade mesophyll cell
Tissue	similar cells	one job	muscular tissue	palisade mesophyll tissue
Organ	different tissues	a specific function	stomach	leaf
Organ system	different organs	a major function	digestive system	roots, stem and leaves, which transport substances around the plant

Plants follow the same pattern. A leaf is an organ because its epidermal, palisade mesophyll, spongy mesophyll, xylem and phloem tissues work together. The detail is in Plant Tissues, Organs and Systems.

Common Confusions¶

Tissue vs organ: a tissue is made of similar cells doing one job, so a structure made of only one kind of cell is a tissue. An organ is made of different tissues, which is why the stomach counts as an organ and a sheet of muscle does not.
Organ vs organ system: the stomach is an organ. The digestive system is the organ system that contains it, along with the pancreas, liver and intestines. An organ system is not a large organ, because it is made of organs and not of tissues.
Size does not decide the level: a very large block of identical cells is still only a tissue. The level depends on what the structure is made of and not on how many cells it has.

Check Yourself¶

Put these in order from smallest to largest: organ, cell, organ system, tissue.
State one difference between a tissue and an organ.
A leaf contains epidermal tissue, palisade mesophyll tissue and xylem tissue. Explain why a leaf is an organ.
A researcher grows a large sheet of identical heart muscle cells in a dish. Is this a tissue or an organ? Explain your answer.
A student writes: "The stomach is part of the digestive organ." Explain what is wrong and rewrite the sentence correctly.
Explain why an organism needs its organ systems to work together. Use the digestive and circulatory systems in your answer.

Answers

Cell, tissue, organ, organ system.
A tissue is a group of similar cells doing one job, whereas an organ is made of several different tissues working together.
A leaf is made of different tissues (epidermal, palisade mesophyll and xylem) that work together to do a specific job, which is photosynthesis. No single tissue could do all of that alone.
It is a tissue. All the cells are identical and do one job, which is contracting. To be an organ it would have to contain different tissues, however many cells there were.
The digestive system is an organ system, not an organ, and the stomach is one of the organs in it. Correct sentence: "The stomach is an organ in the digestive system."
Each system does only one major job. The digestive system breaks food down into soluble molecules such as glucose, and the circulatory system carries them to the cells. Cells need those molecules, together with oxygen, to release energy, so if either system fails the cells cannot get what they need.

Key Terms¶

Cell: the basic building block of a living organism.
Tissue: a group of specialised cells working together to perform a function.
Organ: a structure made from different tissues working together.
Organ system: a group of organs that work together to carry out a major function.