cell the unit of life

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When you look around

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you see both living and non-living things. You

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must have wondered and asked yourself – ‘what is it that makes an

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organism living

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or what is it that an inanimate thing does not have which

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a living thing has’ ? The answer to this is the presence of the basic unit of

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life – the cell in all living organisms.

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All organisms are composed of cells. Some are composed of a single

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cell and are called unicellular organisms while others

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like us

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of many cells

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are called multicellular organisms.

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8.1 WHAT IS A CELL?

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Unicellular organisms are capable of (i) independent existence and

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(ii) performing the essential functions of life. Anything less than a complete

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structure of a cell does not ensure independent living. Hence

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cell is the

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fundamental structural and functional unit of all living organisms.

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Antonie Von Leeuwenhoek first saw and described a live cell. Robert

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Brown later discovered the nucleus. The invention of the microscope and

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its improvement leading to the electron microscope revealed all the

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structural details of the cell.

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8.2 CELL THEORY

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In 1838

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Matthias Schleiden

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of plants and observed that all plants are composed of different kinds of

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cells which form the tissues of the plant. At about the same time

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Theodore

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CELL: THE UNIT OF LIFE

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CHAPTER 8

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8.1 What is a Cell?

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8.2 Cell Theory

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8.3 An Overview of

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Cell

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8.4 Prokaryotic Cells

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8.5 Eukaryotic Cells

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Reprint 2025-26

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88 BIOLOGY

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Schwann (1839)

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a German Zoologist

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cells and reported that cells had a thin outer layer which is today known

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as the ‘plasma membrane’. He also concluded

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based on his studies on

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plant tissues

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that the presence of cell wall is a unique character of

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the plant cells. On the basis of this

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Schwann proposed the hypothesis

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that the bodies of animals and plants are composed of cells and

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products of cells.

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Schleiden and Schwann together formulated the cell theory. This

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theory however

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did not explain as to how new cells were formed. Rudolf

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Virchow (1855) first explained that cells divided and new cells are formed

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from pre-existing cells (Omnis cellula-e cellula). He modified the

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hypothesis of Schleiden and Schwann to give the cell theory a final

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shape. Cell theory as understood today is:

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(i) all living organisms are composed of cells and products of cells.

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(ii) all cells arise from pre-existing cells.

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8.3 AN OVERVIEW OF CELL

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You have earlier observed cells in an onion peel and/or human cheek

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cells under the microscope. Let us recollect their structure. The onion cell

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which is a typical plant cell

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has a distinct cell wall as its outer boundary

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and just within it is the cell membrane. The cells of the human cheek

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have an outer membrane as the delimiting structure of the cell. Inside

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each cell is a dense membrane bound structure called nucleus. This

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nucleus contains the chromosomes which in turn contain the genetic

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material

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DNA. Cells that have membrane bound nuclei are called

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eukaryotic whereas cells that lack a membrane bound nucleus are

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prokaryotic. In both prokaryotic and eukaryotic cells

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a semi-fluid matrix

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called cytoplasm occupies the volume of the cell. The cytoplasm is the

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main arena of cellular activities in both the plant and animal cells. Various

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chemical reactions occur in it to keep the cell in the ‘living state’.

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Besides the nucleus

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the eukaryotic cells h