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Definition of a Cell

The cell is defined as the structural and functional unit of a living organism. Also, 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 possesses all the characteristics of living things such as feeding, reproduction, excretion, growth, adaptation, respiration, definite life span, sensitivity and movement.


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:

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

b) Multicellular organisms: These are organisms that 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, birds and man.


History of the Cell

Many scientists contributed to the history of the cell. Among the scientists are:

  • Robert Hooke, an English scientist could be seen as the father of cells. He was the first human being to discover the honeycomb structure of the cell in 1665.
  • Felix Dujardin, a French biologist in 1835 discovered that the cell was made up of a living substance that he called protoplasm.
  • Matthias Schleiden, a German botanist in 1838 revealed that the bodies of plants are made of cells which were described as units of life.
  • 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
  • Rudolf Von Virchow, a German biologist in 1855 concluded in his research that all cells come from previously existing cells.


The Cell Theory

cell theory is a scientific theory first formulated in the mid-nineteenth century, that living organisms are made up of cells

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:

  1. Compound microscope
  2. Light microscope
  3. Electron microscope
  4. 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 Who 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 tum 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:

(a) Coarse adjustment knob: This is used in bringing the object into proper focus.

(b) Fine adjustment knob: This is used to ensure a cleaner view by sharpening the focused object

(10) The objective lens: This lens which is usually placed slight above the object is used for magnification



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 Chlamy-domonas.


  1. 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.
  2. 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 mega nucleus. The cytoplasm also houses the food vacuole, contractile vacuole and cyto-stome. 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
  1. 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 words, these cells form a loosely arranged association of two or more cells but the cells cannot be differentiated from each from each other. This aggregation of independent cells or protists is called a colony.

Examples of organisms which exist as colonies are Volvox, Pandorina, Eudorina 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 Chladophoral, Ulothrix, Oedogonium, etc.


Differences Between Colonial Organism and Filamentous Organism

Colonial Organism Filamentous Organism
There is absence of intercellular


There is presence of

Intercellular wall


The identical cells form a mass The identical cells form

end-to-end arrangement

in linear form

Cells are connected by cytoplasmic materials, i.e., physio logically dependent All cells are physio-

logically independent

Examples of colonial organisms are Volvox, Pandorina Examples of filamentous

organisms are organisms are Spirogyra Zygnema and Oscilateria.



(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 are called a system.

cells lead to tissues; tissues lead to organs while organs lead to system.

Examples are cheek cell, onion cell, epidermal cell, parenchyma cell.


Structures of Plant and Animal Cells and Functions of Their Components

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 of organelles are outlined in the below.

Cell components

or Organelles

Description of Structure Functions of cell components
i) Nucleus The nucleus has aspherical body which

is covered by a double membrane which

contains hereditary materials. chromosome

and genes often centrally located in the

cell, embedded in cytoplasm.

(i) It controls all life activities of the cell.

(ii) It stores hereditary information as it

contains DNA inside chromosomes

which take part in cell division.


(ii) Chromosome These are located in the nucleus and

contain deoxyribonucleic acid or DNA

It contains the DNA which stores genetic


(iii) Mitochondria These are oval or rod-shaped. They are

bounded by a double membrane. The

inner membrane is folded and the interior

is filled with matrix. The matrix contains


It is described as the power-house of the

cell. They are sites of respiration or where

energy is released from simple sugar.

(iv) Vacuole It occupies a large central portion of plant

cell. It is lined with a membrane and filled

will cell sap. The cell sap acts as a “store

house” for many substances.

It contains cells sap which act as an

Osmo-regulator by helping to remove

excess water in cells.


(iv) Nucleolus They are dense structures within the


They produce the ribosome for protein


(vi) Endoplasmic They are membrane-like structures that

reticulum form channels within the cytoplasm

Aid the transport of materials

within the cytoplasm.

(vii) Golgi bodies These are series of disc-shaped sacs. They function in synthesis, packaging

and distribution of materials.

(vii) Chloroplasts These are large green organelles in plant

cells. They contain chlorophyll.

They contain chlorophyll which aid

photosynthesis in green plants

(ix) Lysosomes These are thin-wall bodies and they

contain enzymes

They are sites for respiratory enzymes.
(x) Ribosomes These are small round bodies attached to

endoplasmic reticulum.

They are responsible for protein


(xi) Cell wall It is a tough, fairly rigid structure that

is freely permeable in plant cells.

(i) It provides protection, shape and

mechanical support for the cell.

(ii) It also allows free passage of

nutrients in and out of the cell.

(xii) Cell membrane This is a flexible membrane made up

of mainly proteins and lipids. It is

selectively permeable.

(i) It plays a great role in selective

absorption of materials.

(ii) It also protects the cell.

(xiii) Centrioles These are two small granules near

nucleus of animal cells

from which flagella or cilia arise.

They are important in cell division.

They may also serve as basal body

(xiv) Starch granules These are oval or round structures

mostly found in plant cells.

They store starch for the cell.





Both plants and animal’s cells have in common the following organelles:

(i) Golgi bodies

(ii) Nucleus

(iii) Mitochondria

(iv) Cytoplasm

(v) Chromosomes

(vi) Endoplasmic reticulum

(vii) Nucleolus

(viii) Ribosome

(ix) Lysosomes

(x) Cell membrane

(xi) Reticulum



  Plant Cell Animal Cell
i Has cellulose cell wall Lacks cellulose cell wall


ii Has one large central vacuole Has numerous

small vacuoles

iii Has chloroplasts Lacks chloroplast
iv Contains starch granules in its cytoplasm Contains glycogen granules in its


v Usually larger in size Usually, smaller

in size

vi Does not have lysosome Has lysosome


vii Does not have centrioles/


Has centrioles/



viii Has a regular/ definite cell shape


Has an irregular/

indefinite cell shape

ix Stores lipid as oil Stores lipid as fat


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