The basic unit of structure and
function in living organisms is the cell, cells are very minute, and they can
only be seen with the aid of a microscope. The
cell is defined as the structural and functional unit of a living organism. Living organisms are classified into two
(2) major groups based on the number of cells. These groups are: –
Unicellular Organisms: These are organisms which consist of
only one cell. Examples of unicellular organisms are Amoeba, Chlamydomonas,
Euglena and Paramecium. The single celled organisms can carry out all life
activities such as feeding, reproduction, excretion, growth, adaptation,
respiration, definite life span, sensitivity and movement.
Multicellular Organisms: These are organisms which consist of
two (2) or more cells. Examples are Volvox, Spirogyra, Plants, Fish, Bird and
Man. Multicellular Organisms have many cells and these cells also carry out all
life activities. Some cells will be involved in feeding, some reproduction, and
some growth and so on. There are 2 types of cells; these
are prokaryotic and eukaryotic cells. Prokaryotic
cells are simpler and smaller than eukaryotic cells. They do not have an
organized nucleus or the organelles found in eukaryotic cells. Examples
of prokaryotes are bacteria. Eukaryotic cells are more complex and bigger than
prokaryotic cells, they have an organized nucleus as well as several organelles
that are not found in prokaryotic cells. Examples of eukaryotes are plants,
animals, fungi and protists.
below shows the differences between a prokaryotic and eukaryotic cell.
Fig 2.1 The eukaryotic and prokaryotic cells
The Cell Theory
Robert Hooke is the father of cell. He
was the first human being to discover the honey-comb structure of the cell in
1665. He explained his observations of magnified thin slice of a cork of an oak
tree. He described that the cork is made up of thin component. He then named
the components cells.
Fig 2.2 Robert Hooke (1665).
The cell theory states that:
cell is the structural and functional unit of life.
living organisms are made of one (unicellular) or more cells (multicellular).
cells come from cells that have previously existed.
is no life apart from the life of cells.
Forms in Which Living Cells Exist
There are four (4) forms in which
living cells exists. These are:
independent or single free living organisms e.g. Amoeba, Paramecium etc.
as part of living organisms.
1. As independent or single free
living organisms: Independent and free living organisms are
organisms that possess only one cell and are able to live freely on their own.
Examples are Amoeba, Euglena, and Paramecium etc.
(i) Amoeba: Amoeba is a single cell organism (i.e.
unicellular). It has irregular shape and changes constantly. The protoplasm is
made of nucleus and cytoplasm. It is a cell of about 3000um across and it
houses the machinery with which it carries out all the functions of life. It feeds
itself and exchanges materials with its environment. It responds to stimuli in its
environment. It grows and reproduces. Amoeba moves by means of finger-like
projections of the body wall called pseudopodia. It also possesses food
and contractile vacuole.
Fig 2.3 Amoeba
(ii) Paramecium: Paramecium is a single-celled or unicellular
organism. As a unicellular organism, it has to carry out within one cell all
the functions performed by differentiated cells, tissues and organs in a
multicellular organisms. The macronucleus exercises control over metabolic
functions, including growth, while the micronucleus is necessary for sexual
cilia is responsible for locomotion (i.e. movement), the contractile vacuoles
for osmoregulation. The trichocysts are used for defense. Food particles swept
into the oral groove by ciliary action, and taken up into food vacuole formed
at the base of the gullet.
(iii) Euglena Viridis: Euglena Viridis
is a microscopic organism found in fresh waters. The spindle-shaped body is
broad in the middle. The presence of a gullet, flagellum, eye spot, contractile
vacuole, pellicle and myonemes often qualifies Euglena as an animal. Equally, the presence of
stellate chloroplast, pyrenoids and paramylum granules also qualifies it as a
plant. It is therefore capable of independent existence. It is an organism
sharing the characteristics of both plants and animals. It moves with the aid
Fig 2.5 Euglena
Animal Characteristics of Euglena
The characteristics of Euglena which make
it an animal include:
of flagellum used for movement.
of gullet for passage of food and as reservoir.
of contractile vacuole for osmoregulation.
of eye spot which enables it to respond to light.
of pellicle which makes it flexible.
has the ability to carry out holozoic mode of nutrition in the absence of
Plant Characteristics of Euglena
The characteristics of Euglena which make
it a plant include:
of chloroplast which enables it to carry out photosynthesis.
of pyrenoids where starch is stored.
of paramylum granules which is the form in which starch is stored.
Some organisms are made up of many similar cells which are clustered together,
these cells cannot be differentiated from each other. This aggregation or
constitution of independent cells is called a colony. Examples of organisms
which exist as colonies are Volvox, Pandorina and sponges.
3. As a filament: Some cells are
organized into filaments in which identical cells are joined end-to-end to form
an unbranched filament. Each cell functions as an independent living cell.
Filamentous organisms are Spirogyra, Zygnema, and Oscillateria etc.
as part of living organisms: Living things are made up of
cells. In multicellular organisms, a group of similar cells are arranged
together to form a tissue, a group of similar tissues are arranged together to
form an organ and group of organs are aggregated together to form a system.
Cells form the unit of living organisms.