Cell As A Living Unit: Definition, Classification, Cell Theory And Forms

Topic: The Cell
Table Of Contents

  • Definition Of A Cell
  • Classification of Living Organisms Based On The Number of Cells
  • History of the Cell
  • The Cell Theory
  • Forms In Which Living Cells Exist

At the end of this article, readers should be able to:

  • Explain the forms in which living cells exist and give an example in each case.
  • Briefly describe the history of cell.
  • Explain briefly the cell theory.
  • Name scientists who contributed to the development of the cell theory

Cell As A Living Unit


Definition of a Cell

The cell is defined as the structural and functional unit of a livng organism. In other words, the cell is the simplest, the smallest and basic unit of life. All living things (plants and animals) are made of cells.
The cell is regarded as the basic unit of all living things because it can carry out all life activities such as feeding, reproduction, excretion, growth, adaptation, respiration, definite life span, sensitivity and movement. All these activities possessed by a cell are the characteristics of living plants and animals.

Classification of Living Organisms Based on the Number of Cells

Living organisms (both plants and animals) are classified into two major groups based on the number of cells. These groups are:

1. Unicellular or Acellular organisms:

These are organisms which consist of only one cell. Examples of unicellular organisms are Amoeba, Chlamydomonas, Euglena, Paramecium etc.

2. Multicellular organism:

These are organisms which consist of two or more cells. In other words, these organisms have many cells. Examples of multicellular organisms are Volvox, Hydra, Spirogyra, flowering plants, fish, bird and man.

History of the Cell

Many scientists contributed to the history of the cell. Among the scientists are:
1. Robert Hooke, an English scientist could be seen as father of cells. He was the first human being to discover the honey-comb structure of the cell in 1665. In his book, Micrographia, he described his observations of a magnified thin slice of a cork of an oak tree. He established that the cork is made up of thin components or rooms. He then named the components cells.
2. Felix Dujardin, a french biologist in 1835 discovered that the cell was made up of living substance. He however named the livng substance protoplasm.
3. Matthias Schleiden, a German botanist in 1838 revealed that the bodies of plants are made of cells which were described as units of life.
4. Theodor Schwann, another German zologist in 1839 also discovered that the bodies of all animals are composed of cells. The discoveries of Schleiden and Schwann led to postulation of the cell theory in 1839.
5. Rudolf Von Virchow, a German biologist in 1855 concluded in his research that all cells come from previously existing cells.

Cell As A Living Unit
The Cell Theory

The cell theory states that:
1. The cell is the structural and functional unit of life.
2. All living organisms are made of cells.
3. All cells come from previously existing cells.
4. There is no life apart from the life of cells.
5. All living things are either single cells (unicellular) or group of cells (multicellular).
Forms In Which Living Cells Exist
There are four forms in which living cells exists. These are:

1. As Independent or Single and Free living Organisms

Independent and free living organisms are organisms which possess only one cell and are capable of living freely on their own. Eache organism, even though it has only one cell can carry out all the life processes such as feeding, movement, reproduction, sensitivity, excretion, growth, respiration etc. Examples of independent or free-living organisms are Amoeba, Euglena, Paramecium, Chlamydomonas etc.
i. Amoeba: Amoeba has irregular shape and changes constantly. The protoplasm is made of nucleus and cytoplasm. Embedded in the cytoplasm are food vocuole and contractile vacuole. Amoeba moves with aid of pseudopodia.
ii. Paramecium: Paramecium is often described as having a slippery shape. The cytoplasm is composed of ectoplasm and endoplasm. The nucleus consists of two micronucleus and meganucleus. The cytoplasm also houses the food vacuole, contactile vacuole and cytostome. Paramecium moves with the aid of cilia.
iii. Euglena Viridis: Euglena viridis is a protist and a typical example of an organism sharping the characteristics of plants and animals. The organisms posses flagellum, gullet, contractile vacuole, eye spot, pellicle, myonemes etc which make it an animal and chloroplasts, pyrenoids and paramylum granules which also make the organism a plant.
iv. Chlamydomonas: Chlamydomonas is a simple microscopic plant. It is unicellular plant, having flagella for movement, eye spot, chloroplasts, food vacuole and contractile vacuole.

2. As a Colony

Some organisms are made of many similar cells which are joined or massed together but cannot be differentiated from each other. In other words, these cells form a loosely arranged association of two or more cells but the cells cannot be differentiated from each other. This aggregation of independent cells or protists is called a colony. Examples of organisms which exist as colonies are Volvox, Pandorina and Sponges.

3. As a Filament

Certain cells are organised into filaments in which identical cells are joined end to end to form unbranched filaments. Each cell functions as an independent living cell. Such organisms are multicellular and therefore exist as filament. Popular examples of filamentous organisms are the Spirogyra, Zygnema, Oscillateria and Oedogonium.

4. Cells as Part of a Living Organism

In multicellular organisms, a group of numerous, similar cells arranged together and performing a specific function is called a tissue. A group of similar tissues forming a layer in an organism which performs a specific function is called an organ. A group of organs which work together to perform specific function is called system. From the explanation above, it is deducted that cells lead to tissues, tissues lead to organs while organs lead to system. From the above, it can also be seen that the cells form the unit of living organisms.

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