Levels of organisation: from cells to whole organisms

Learning objectives

By the end of this lesson, you should be able to:

• explain what biological organisation means
• describe the sequence from cell to organism
• distinguish cells, tissues, organs and organ systems
• give accurate examples from flowering plants and mammals
• explain why specialised cells are suited to their functions

What biological organisation means

A living organism is not simply a collection of unrelated parts. Its structures are arranged in levels. Small structures work together to form larger structures, and each level contributes to the functioning of the whole organism. This arrangement is called levels of biological organisation.

In a typical multicellular organism, the sequence is:

cell → tissue → organ → organ system → organism

The cell is the basic structural and functional unit of life. A tissue is a group of similar cells, usually together with their products, performing a particular function. Different tissues combine to form an organ. Several organs work together in an organ system, and all the organ systems together make an organism.

The sequence is a hierarchy: each level is built from the level below it. Do not reverse the order in an examination answer.

The cell: the basic unit

Cells are specialised for different jobs. A red blood cell transports oxygen; a neurone carries nerve impulses; a root hair cell absorbs water and mineral ions; and a palisade mesophyll cell carries out much of the photosynthesis in a leaf.

Specialisation means that a cell has structural features suited to its function. A root hair cell has a long projection that increases its surface area for absorption. A red blood cell has a biconcave shape that gives a large surface area for gas exchange and, in mammals, loses its nucleus when mature to leave more room for haemoglobin.

Cells in a multicellular organism do not usually work independently. They depend on other cells for supplies, waste removal, protection and coordination.

Tissues: groups of cells working together

In animals, examples include:

• Epithelial tissue: covers body surfaces and lines organs and cavities. It may protect, absorb or secrete.
• Muscular tissue: contracts to produce movement. Skeletal muscle moves bones; smooth muscle moves materials through organs; cardiac muscle contracts rhythmically in the heart.
• Nervous tissue: contains neurones and supporting cells and carries information through electrical impulses and chemical signals.
• Connective tissue: supports, binds or transports. Examples include blood, bone, cartilage, tendons and ligaments.

In plants, important tissues include:

• Epidermal tissue: forms the protective outer covering of leaves, stems and roots.
• Xylem: transports water and dissolved mineral ions and also provides mechanical support.
• Phloem: transports dissolved organic substances, especially sugars, from sources to places where they are used or stored.
• Meristematic tissue: contains actively dividing cells responsible for growth.

A tissue is more than a group of cells in the same place. The cells are organised to perform a shared role.

Organs: different tissues cooperating

An organ contains two or more tissue types working together to perform a function.

A leaf is a plant organ. Its epidermis protects it, the palisade mesophyll contains many chloroplasts for photosynthesis, the spongy mesophyll provides air spaces for gas exchange, and the veins contain xylem and phloem. The leaf works because these tissues cooperate.

The human heart is an animal organ. Cardiac muscle produces contractions, connective tissue supports the structure, nervous tissue helps regulate the heartbeat, and blood vessels supply the heart muscle itself.

Other examples include the kidney, stomach, brain, root, stem and flower. A root is an organ because it contains several tissues that work together in anchorage and absorption.

Organ systems and the organism

An organ system is a group of organs that cooperate in a major body function. In humans, the digestive system includes the mouth, oesophagus, stomach, small intestine, large intestine, liver and pancreas. These organs work together to take in food, digest it, absorb nutrients and eliminate undigested material.

The circulatory system includes the heart, blood and blood vessels. It transports oxygen, nutrients, hormones and waste products around the body. The respiratory system brings oxygen into the body and removes carbon dioxide. The systems interact: the respiratory system supplies oxygen to the blood, while the circulatory system transports it to cells.

In a flowering plant, roots, stems and leaves form a connected shoot-and-root system. Roots absorb water and mineral ions; stems support leaves and connect transport tissues; leaves manufacture sugars by photosynthesis. Reproductive organs such as flowers contribute to producing seeds.

An organism is an individual living thing. Its survival depends on coordination between its organs and systems. For example, during physical exercise, muscles need more oxygen and glucose. Breathing rate and heart rate increase to help meet this demand.

Unicellular organisms are an important exception

Not every organism has tissues, organs or organ systems. A unicellular organism, such as Amoeba, consists of one cell that carries out all essential life processes. That single cell obtains nutrients, respires, removes waste, responds to its environment and reproduces.

Therefore, the cell-to-organism hierarchy describes the organisation of multicellular organisms. It should not be used to claim that every living organism has organs or organ systems.

WASSCE examination focus

For a question asking you to arrange the levels of organisation, write the sequence exactly: cell, tissue, organ, organ system, organism. If asked to define a tissue or an organ, include the idea that its parts cooperate to perform a function. Give a named example and explain the contribution of at least two components.

A common error is to describe a leaf as a tissue. A leaf is an organ because it contains several tissues, including epidermal, mesophyll and vascular tissues.

Quick check

1. Arrange cell, organ, tissue and organ system from the smallest to the largest level.
2. Why is a leaf classified as an organ rather than a tissue?
3. Name two plant tissues and state one function of each.
4. Does Amoeba have an organ system? Explain your answer.

Answers: 1. Cell → tissue → organ → organ system. 2. It contains several different tissues that cooperate in photosynthesis, gas exchange and transport. 3. Xylem transports water and mineral ions; phloem transports dissolved sugars. 4. No. Amoeba is unicellular and its single cell performs all essential life processes.