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CLASS: SS1                                                                                                                         



  1. Cell as a living unit of an organism
  2. Forms in which cells exist
  3. The cell structure and functions
  4. Cell theory
  5. Differences and similarities between plants and animal cells


The cell is defined as the simplest, the smallest and basic unit of life. It is the structural and functional unit of a living organism. The cell can carry out all life activities such as feeding, respiration, movement etc.

Classification of Living Things Based on The Number of Cells

All living things are classified into two major groups depending on the number of cells they possess. These groups are:

  1. Unicellular or Acellular organism: Organisms which consist of only one cell e.g. Amoeba, Paramecium, Chlamydomonas and Euglena.
  2. Multicellular Organisms: Organisms which consist of two or more cells, e.g. Hydra, Flowering plants, Fish, Bird, Man etc.


Living cells exist basically in four forms;

  1. As single and free living organisms: These are organisms which possess only one cell and are able to live freely on their own. The single cell carries out all life processes such as feeding. Euglena, etc. Parts of the cell may be specialized to carry out a particular process, for example Paramecium.
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Paramecium is a fresh-water protozoan. It lives in ponds and puddles and feeds mainly on bacteria. It is often described as having a slipper shape and is about 0.25mm long. It has a stiff outer-covering called the pellicle which gives it a fixed shape. Its cytoplasm is divided into a clear outer layer called the ectoplasm and a granular inner layer called the endoplasm.

The endoplasm contains several food vacuoles, two contractile vacuoles and two nuclei. The larger one called the meganucleus and the smaller one the micronucleus. It moves (swims) in the water by means of cilia. The cilia are tiny projections of the ectoplasm through the pellicle.  The oral groove and gullet are specialized for feeding, while the contractile vacuole is specialized for water regulation.

  1. As a colony: A colony consists of many similar cells which are joined together. E.g. Volvox, Pandorina, etc. Diagram of volvox
  2. As a filament: This consists of many similar or identical cells joined end to end to form unbranched filaments. e.g. spirogyra, zygnema, oscillatoria and oedogonium. Each cell in a colony or filament behaves as an individual. Each carries out life processes independently of its neighbours.
  1. As a part of a living multicellular organism: In multicellular organism cells which perform the same function are grouped together to form a tissue, e.g. tissues of the ileum wall, the retina, and the blood (a tissues which perform a specific function form an organ and a group of organism working together to perform a specific task form a system.


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