Cell theory

What is the cell theory?

In plain words: every living thing is built of cells (and of what cells make, such as bone matrix or a cell wall), and every cell comes from a cell that already existed. This describes how cells arise in present-day reproduction; it does not explain the origin of the first cells. The theory explains why the cell is the unit of life. The first tenet goes back to Schwann’s hypothesis (1839); the second is Virchow’s addition (1855).

Schleiden and Schwann

• In 1838, Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells, which form the tissues of the plant (page 87).
• At about the same time, Theodor Schwann (1839), a German zoologist, studied different types of animal cells and reported that cells had a thin outer layer, today known as the plasma membrane. Based on his studies of plant tissues, he also concluded that the presence of a cell wall distinguishes plant cells from the animal cells he studied (page 88). This historical plant–animal comparison does not mean only plants have walls: NCERT also describes fungal and bacterial walls.
• On this basis Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells. Schleiden and Schwann together formulated the cell theory (page 88).

NCERT spells his first name “Theodore”; the usual spelling is Theodor.

Virchow’s addition

This theory however, did not explain as to how new cells were formed.

NCERT Biology (Class 11), Section 8.2, page 88

In NCERT’s historical account, Rudolf Virchow (1855) explained that cells divide and that new cells are formed from pre-existing cells: omnis cellula e cellula, which NCERT prints as “Omnis cellula-e cellula” (page 88). He modified the hypothesis of Schleiden and Schwann and gave the cell theory its final shape.

Solved examples

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Quick revision summary

• Schleiden (1838, botanist): all plants are composed of different kinds of cells forming tissues.
• Schwann (1839, zoologist): animal cells have a thin outer layer (plasma membrane); a cell wall distinguishes plants from animals in his comparison; animals and plants are composed of cells and products of cells.
• Schleiden and Schwann together formulated the cell theory, which did not explain how new cells form.
• Virchow (1855): cells divide and new cells form from pre-existing cells (omnis cellula e cellula); he gave the theory its final shape.
• Today: all living organisms are composed of cells and products of cells; all cells arise from pre-existing cells.

Questions students ask

Who proposed the cell theory, and when?

The cell theory was proposed by Matthias Schleiden, a German botanist, and Theodor Schwann, a German zoologist: NCERT says they together formulated it, after Schleiden found in 1838 that all plants are composed of cells and Schwann studied animal cells in 1839. Rudolf Virchow (1855) did not formulate it; he modified it, adding that new cells are formed from pre-existing cells, and gave it its final shape. Robert Brown, who discovered the nucleus, and Leeuwenhoek, who first saw a live cell, had no part in proposing it.

What is the difference between the classical and the modern cell theory?

The classical cell theory of Schleiden and Schwann (1838–1839) held that the bodies of animals and plants are composed of cells and products of cells, but it did not explain how new cells were formed. The modern cell theory, which NCERT calls the cell theory “as understood today”, extends this to all living organisms and adds Virchow’s point (1855) that all cells arise from pre-existing cells, omnis cellula e cellula (NCERT prints “Omnis cellula-e cellula”). Beyond the NCERT text, the Latin means ‘every cell from a cell’, and some books add further tenets, such as that cells pass on hereditary information (DNA) when they divide.

What are the exceptions and limitations of the cell theory?

The main exception to the cell theory is viruses: NCERT calls them non-cellular organisms with an inert crystalline structure outside the living cell, not truly ‘living’ if living means having a cell structure (Chapter 2, page 19). The main limitation of the original theory of Schleiden and Schwann was that it did not explain how new cells were formed, which Virchow (1855) supplied. Beyond the NCERT Class 11 text, the theory also cannot explain how the first cell arose: NCERT Class 12 (Evolution) notes that proving life comes only from pre-existing life “did not answer how the first life form came on earth”.

Was Schwann a British or a German zoologist?

Theodor Schwann was a German zoologist: the current NCERT reprint (2026–27) calls him “a German Zoologist”, and Matthias Schleiden “a German botanist” (Section 8.2, pages 87–88). Older NCERT reprints, such as 2021–22, printed “a British Zoologist”, an error still found in some notes and question banks. Beyond the NCERT text, Schwann was born in Neuss, in today’s Germany, and did his studies of animal cells in Berlin; a question on his nationality expects German.

How does the cell theory reject spontaneous generation?

The cell theory rejects spontaneous generation through its second tenet, that all cells arise from pre-existing cells (Virchow, 1855), in present-day cell reproduction. It does not rule out the ancient chemical origin of the first life. Spontaneous generation was the old belief that life came out of decaying and rotting matter like straw and mud; NCERT Class 12 (Evolution) credits Louis Pasteur with showing, by careful experiments with pre-sterilised flasks, that life comes only from pre-existing life. Pasteur’s experiments tested contamination and growth in those conditions; neither account supplies a mechanism for the first cell’s origin.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. NCERT Exercise 2 (page 103): new cells generate from (a) bacterial fermentation, (b) regeneration of old cells, (c) pre-existing cells, (d) abiotic materials. Which is right?

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   (c): all cells arise from pre-existing cells, Virchow’s omnis cellula e cellula (NCERT page 88).

2. Q2. What did Schwann conclude about the cell wall?

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   In his plant–animal comparison, that a cell wall characterises plant cells (NCERT page 88). Fungi and many prokaryotes also have walls, so this is not a universal claim that walls occur only in plants.

3. Q3. What does “omnis cellula e cellula” mean?

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   All cells arise from pre-existing cells (NCERT page 88).

4. Q4. Who gave the cell theory its final shape?

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   Rudolf Virchow (NCERT page 88).