The cell is defined as the structural and functional unit of a living 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 things.


Classification of Living Organisms Based on the Number of Cells

Living organisms are classified into two major groups based on the number of cells. These groups are:

Unicellular or Acellular organisms: These are organisms which consist of only one cell. Examples of unicellular organisms are Amoeba, Chlamydomonas, Euglena and Paramecium.

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 the father of cells. He was the first human being to discover the honey-comb structure of the cell in1665. 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 living 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 zoologist in 1839 also discovered that the bodies of all animals are composed of cells. The discoveries of Schleiden and Schwann led to the 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.


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).


The Microscope

Definition: The microscope is an instrument used in the laboratory to observe tiny structures of living organisms which cannot be seen or observed by the naked eyes. Organisms which can only be seen with the aid of microscope are called microscopic organisms.

When very small objects and tiny living things are observed through the microscope, they become magnified or enlarged and the detail structures can be seen properly. The study and use of the microscope will enable us to observe and identify tiny living things and the structure they are made of, especially the cell.



Types of Microscope

The various types of microscope include:

  • Compound microscope.
  • Light microscope.
  • Electron microscope.
  • Hand lens.


The hand lens is the simplest and the most commonly used magnifier in the laboratories for magnifying tiny living things and other objects.


Parts of a microscope

The microscope is made up of many parts which include:

  1. The plane mirror: The plane mirror helps to direct light rays to the object for proper lightening so that the object can be seen properly.
  2. The base: This part represents the metallic base which enables the microscope to rest properly on the table so as to prevent it from falling.
  3. The stage: This part represents where the object to be examined is placed.
  4. Clips: These are tiny structures which help to hold the object for proper viewing.
  5. Handle or arm: This part is used to carry the microscope.
  6. Condenser: The condenser consists of a powerful lens which condenses the light rays coming from the plane mirror and directs them to the object under observation.
  7. The rotatory nasal piece: This part is where the objective lenses of varying magnifications are fitted. It can be rotated in order to turn on the objective lens with a better magnification.
  8. The eye piece lenses: This part represents where the observer places his eyes when viewing the object through the microscope.
  9. Adjustment knobs: These are made up of two components. These are:
  10. Coarse adjustment knob: This is used in bringing the object into proper focus.
  11. Fine adjustment knob: This is used to ensure a cleaner view by sharpening the focused object.
  12. The objective lens: This lens which is usually placed slight above the object is used for magnification.


How to use the microscope

  • Bring out the microscope gently from where it is stored with the aid of its handle.
  • Clean the microscope gently with soft linen or cotton wool. Parts to be cleaned include the eye piece, objective lenses, condenser and other parts of the microscope.
  • Adjust the plane mirror in the direction of light in order to catch and direct the rays of light into the microscope.
  • Where necessary, open the lid of the condenser.
  • Place the slide of the object to be viewed on the stage and use the clip to hold it properly.
  • Proper adjustment is made on the objective lens so as to rest on the slide carefully to avoid cracking of the slide or shaking of the object to rotating the nasal piece, starting with the low power lens, then increase to high power lens.
  • Adjust the coarse knob to bring the object into focus.
  • When the object is brought into sharp focus, the fine adjustment knob is then used to make the object sharper for a clearer view.
  • The object or specimen is then examined carefully and all observations recorded.
  • The object or specimen is then translated into diagram using the Biology practical notebook.

The Biology teacher is expected to guide and demonstrate the use of microscope to students in the laboratory.



There are four forms in which living cells exist. 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. Each 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 and Chlamydomonas.

Amoeba Structure: Amoeba has irregular shape and changes constantly. The protoplasm is made of nucleus and cytoplasm. Embedded in the cytoplasm are food vacuole and contractile vacuole. Amoeba moves with the aid of pseudopodia.


Paramecium Structure: Paramecium is often described as having a slipper shape. The cytoplasm is composed of ectoplasm and endoplasm. The nucleus consists of two micronucleus and meganucleus. The cytoplasm also houses the food vacuole, contractile vacuole and cytostome. Paramecium moves with the aid of cilia.


Euglena Viridis Structure: Euglena viridis is a protist and a typical example of an organism sharing the characteristics of plants and animals. The organisms possess 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 protist. Euglena moves with the aid of flagellum.


Chlamydomonas Structure: Chlamydomonas is a simple microscopic plant. It is a 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 they 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.


4) 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.


Differences Between Colonial Organism and Filamentous Organism

Colonial Organism

  • There is absence of intercellular wall.
  • The identical cells form a mass.
  • Cells are connected by cytoplasmic materials, i.e ., physiologically dependent.
  • Examples of colonial organisms are Volvox, Pandorina.


Filamentous Organism

  • There is presence of intercellular wall.
  • The identical cells form end-to-end arrangement in linear form.
  • All cells are physiologically independent.
  • Examples of filamentous organisms are Spirogyra, Zygnema and Oscilateria.


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 are called a system. From the explanation above, it is deduced 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.

See also  HEART



Structure of the cell: The structure of plant cell and animal cell can fully be understood through the use of microscope. The cell is composed of protoplasm which can be divided into two main parts: the cytoplasm and nucleus. Each cell (plant or animal) is bounded by a thin membrane. The cytoplasm is a fluid material that consists of cytoplasmic organelles such as lysosome, golgi bodies, endoplasmic reticulum, mitochondria, vacuoles etc. The nucleus is bounded by a nuclear membrane and it consists of chromosomes (chromatin granules) and nucleolus.

The animal cell in addition has centrosomes. The plant cells in addition also has starch granules, cellulose cell wall and some plastids, e.g. chloroplasts. The structure and functions of the components of the cells or organelles are outlined in the table below.



Similarities: Both plants and animals cells have in common the following organelles:

  1. Nucleus.
  2. Golgi bodies.
  3. Mitochondria.
  4. Cytoplasm.
  5. Chromosomes.
  6. Endoplasmic reticulum.
  7. Nucleolus.
  8. Ribosome.
  9. Lysosomes.
  10. Cell membrane.


Plant Cell

  1. Plant cell has chloroplasts.
  2. It is usually rectangular and definite in shape.
  3. It has rigid cell wall
  4. It has no flexible cell membrane.
  5. It has large vacuoles.
  6. It stores lipid as oil.
  7. It has nucleus at the edge of cytoplasm.
  8. There is absence of centrosomes and centriole.


Animal Cell

  1. Animal cell has no chloroplasts.
  2. Animal cell is usually spherical or has no definite shape.
  3. It has no cell wall.
  4. It has flexible cell membrane.
  5. Animal cell has small vacuoles.
  6. It stores lipid as fat.
  7. It has nucleus at the centre of cytoplasm.
  8. Centrosomes and centriole are present in animal cell.


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