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:
- The cell is the structural and functional unit of life.
- All living organisms are made of cells.
- All cells come from previously existing cells
- There is no life apart from the life of cells.
- All living things are either single cells (unicellular) or group of cells (multicellular).
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 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
FORMS IN WHICH LIVING CELLS EXIST
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.
- 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 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
- 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
|The identical cells form a mass||The identical cells form
in linear form
|Cells are connected by cytoplasmic materials, i.e., physio logically dependent||All cells are physio-
|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.
|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
|They are sites for respiratory enzymes.|
|(x) Ribosomes||These are small round bodies attached to
|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
|(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.|
SIMILARITIES AND DIFFERENCES BETWEEN PLANT CELL AND ANIMAL CELL.
Both plants and animal’s cells have in common the following organelles:
(i) Golgi bodies
(vi) Endoplasmic reticulum
(x) Cell membrane
|Plant Cell||Animal Cell|
|i||Has cellulose cell wall||Lacks cellulose cell wall
|ii||Has one large central vacuole||Has numerous
|iii||Has chloroplasts||Lacks chloroplast|
|iv||Contains starch granules in its cytoplasm||Contains glycogen granules in its
|v||Usually larger in size||Usually, smaller
|vi||Does not have lysosome||Has lysosome
|vii||Does not have centrioles/
|viii||Has a regular/ definite cell shape
|Has an irregular/
indefinite cell shape
|ix||Stores lipid as oil||Stores lipid as fat|