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Cell Biology: Cell Sizes, Types, and Theories


All life on earth is made of a fundamental structure called a Cell. They can be of two types of organisms: Unicellular and Multicellular. Unicellular are made of a single cell (Uni – one, in latin) cell and Multicellular organisms are made of many cells (Multi – many). Cell size is limited because the relative area of the surface should be proportional to the volume of the cell. The Cell Theory propounds that all organisms are made of cells, the cell is the basic unit of life and all cells descend from older cells. There are basically Prokaryotic and Eukaryotic & Plant and Animal Cells.

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Cell Size, Cell Theory and Types of Cells

Living organisms are made of cells which are the fundamental structural and functional unit of life. All the cells descend from pre-existing cells. Cells are of two types – Prokaryotic and Eukaryotic cells. Prokaryotic cells are simpler and smaller than Eukaryotic cells. Prokaryotic cells have ribosomes enclosed in a plasma membrane, they possess genetic material like the DNA or RNA but it is not enclosed in the nuclear membrane, hence they are called Pro – before, eukaryotes – possessing the nucleus.

Prokaryotes are mostly Unicellular organisms – organisms made of one cell, unless they live in a colony. They are older than the recent multicellular organisms, e.g., Archaea and Bacteria. The evolution of Multicellular organisms occurred giving rise to larger, more complex and different types of cells and myriads of organisms, called the Eukaryotic cells (Eu means True Karyon = nucleus).

These organisms possess a more evolved cell with the nuclear material enclosed in the double membraned nuclear envelope enclosing the DNA which is the brain or the CPU of the cell, besides this the cell contains a more complex complement of organelles. Plants, Animals, Fungi and Protista belong to this class and can be single or multicellular (port,2007).

The Cell Size Is Limited: The cell size is governed by the ratio of surface area to the internal volume of the cell – as the cell gets larger the volume becomes larger than the surface area, this affects the per square unit absorption of oxygen, water and nutrients. As deeper the cell the lesser the essentials will reach to its depth and slower will be the detoxification of the cells (port,2007).

The shape and size of the cell transcend the Prokaryotic and Eukaryotic realms, the shape varies according to function and the size according to age of the cell, the nutrition received and the genetics of the cell. The function of the cell to causes a difference in the cell size – e.g. the neuron can be as long as a 3 metres, with its dendrites and axon spanning the entire length of the elephant or run down the neck of a giraffe! To the tiniest cell known as mycoplasm which measures 0.0001 mm in diameter.

Shapes are different too some are oblong – neuron, some oval like the parenchyma cells of potatoes, some flattened discs like the RBCs in humans, some cells are enclosed in shells like the radiolarians and diatoms, others have fluid plasma membranes like the amoeba (Campbell, Reece, & Simon, 2006). The animal cells do not have the external cell wall the plant cells do. Did you know the largest unicellular cell was the Ostrich’s egg, yes, eggs are cells! Acetabularia are the largest unicellular marine algae (Farabee, 2007).

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As there are varying organisms of varying sizes they are measured by a metric system explained as following –

“1 meter (m)= 100 centimeter (cm)= 1,000 milimeter(mm) = 1,000,000 micrometer (µm) = 1,000,000,000 nanometer (nm), hence 1 centimeter (cm) = 1/100 meter = 10 mm, 1 millimeter (mm) = 1/1000 meter = 1/10 cm, 1 micrometer (µm) = 1/1,000,000 meter = 1/10,000 cm, 1 nanometer (nm) = 1/1,000,000,000 meter = 1/10,000,000 cm.” (Farabee, 2007).

Viruses are the smallest known organism at the borderline of living and non-living. They are even smaller than the Eukaryotic Chloroplasts, then come to the bacteria and the other plant and animal cells.


Campbell, N. A., Reece, J. B., & Simon, E. J. (2006). Essential biology. New York: Benjamin Cummings.

Farabee, M. (2007). ceIIs: Cellular organization. Web.

Port, T. (2007). Prokaryotic and eukaryotic cells. Web.

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"Cell Biology: Cell Sizes, Types, and Theories." StudyCorgi, 27 Oct. 2021,

1. StudyCorgi. "Cell Biology: Cell Sizes, Types, and Theories." October 27, 2021.


StudyCorgi. "Cell Biology: Cell Sizes, Types, and Theories." October 27, 2021.


StudyCorgi. 2021. "Cell Biology: Cell Sizes, Types, and Theories." October 27, 2021.


StudyCorgi. (2021) 'Cell Biology: Cell Sizes, Types, and Theories'. 27 October.

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