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All living things are highly organised. This organisation occurs in levels. The simplest structures are found at the lowest levels and they interact to build up more complex structures at the next level and so on.


Levels of Organisation of Life

There are four levels of organisation of life in organisms. These are the cells, tissues, organs and systems. The simplest of all these levels is the cell.


Cells (First Level)

The cell is defined as the smallest unit of living organism. It is the first level of organisation of life. All plants and animals are made up of cells. Some of these plants and animals are made of only one cell hence they are called unicellular organisms while others are made up of many cells and are therefore called multicellular organisms. Examples of unicellular organisms are Amoeba, Euglena, Paramecium, Plasmodium, Trypanosome, Chlamydomonas. These organisms have only one cell each and are capable of carrying out all life processes such as movement, respiration and reproduction.

Examples of cells in higher plants are phloem cells, xylem vessels, while examples of cells in higher animals are rod and cone cells in the eyes, ova or eggs, spermatozoa, nerve cells, red blood cells, white blood cells and epidermal cells.


Tissues (Second Level)

A tissue is a group of similar cells forming a layer in an organism which performs a particular function. In other words, a tissue consists of two or more different types of cells aggregating together to perform a specific function. Examples of tissues in higher plants are mesophyll layer in leaves, epidermal tissue, sclerenchyma tissue, xylem tissue, parenchyma tissues in stem. Examples of tissues in higher animals include muscle, bone, cartilages and blood (a liquid tissue). Examples of organisms which exist at the tissue level of organisation of life are Hydra, Algae, sponges and fungi.


Organs (Third Level)

An organ is a group of similar tissues forming a layer in an organism which performs a specific function. Examples of organs in plants are leaves, flowers, roots, stems and seeds. Examples of organs in animals are skin, eyes, ears, stomach, brain, heart, liver and kidney. These organs are known to perform specific functions in the body.


Systems (Fourth Level)

A system is a group of similar organs which work together to perform specific functions. Examples of systems in plant are the shoot system and root system. Examples of systems in animals are digestive, reproductive, respiratory, skeletal, nervous, excretory and circulatory systems. These systems work together to make up an organism. For an organism to perform well, all the cells, tissues, organs and systems must also function normally.



There is an increase in complexity from unicellular organisms to multicellular organisms. Even though unicellular organisms can perform all life processes, they still lack vital tissues, organs or systems that is efficient and capable of ensuring their survival. As a result of this, higher organisms have advantages and some disadvantages in complexity over the unicellular organisms.


Advantages of Complexity in Higher Organisms

  1. It leads to cellular differentiation: As a result of complexity, group of similar cells are differentiated to form tissues that carry out similar functions.
  2. It leads to internal structural specialisation: Differentiation leads to internal structural specialisation in which the group of similar cells or tissues become specialised to carry out one or more functions.
  3. Mutual interdependence between component cells: There is always mutual interdependence between component cells. In other words, there is division of labour among the component cells.
  4. Complexity leads to efficiency: Complexity makes higher organism to be efficient in carrying out life processes.
  5. Complexity leads to increase in size: Complexity leads to increase in size of organisms because there are spaces between cells for growth.
  6. It increases adaptation to environment: Complexity makes higher organisms to become more resistant to adverse conditions within the environment.


Disadvantages of Complexity in Higher Organisms

  1. Inability of individual cells to exist on their own: In complexity, cells lose their independence and become increasingly dependent on one another’s activities.
  2. Difficulties in acquisition of oxygen and food materials: Increased size creates difficulties in acquisition of respiratory gas (oxygen), food materials and loss of waste products.
  3. It leads to slower rate of diffusion: Complexity leads to slower rate of diffusion of oxygen or respiratory gas to individual cells.
  4. Slower rate of expulsion of waste products: Complexity leads to slower rate of expulsion of waste products from cells.
  5. Difficulties in reproduction: Complexity also leads to difficulties in reproduction in higher organisms.
  6. Decrease in ability to regenerate: Ability to regenerate decreases with complexity of organisms.
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