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Topic 2: Cells
2.1
2.1.1
2.1.2
Cell Theory
Outline the cell theory.
Discuss the evidence for the cell theory.
Read these pages first:
Pages 7-8 Orange book
Pages 9-10 in the green e-book
Homework 1: Discuss Cell Theory (9 marks)
Due Date: 27th August, 2010
It has taken several hundred years of research to formulate modern cell theory. Many scientists have
contributed to developing the three main principles of this theory. These are:
 All organisms are composed of one or more cells.
 Cells are the smallest units of life.
 All cells come from pre-existing cells.
This theory has amassed tremendous credibility, largely through use of the microscope – an important tool.
Robert Hooke first described cells in 1665 while observing cork with a microscope he built himself. A few
years later, Antonie van Leeuwenhoek observed the first living cells and referred to them as ‘animalcules’,
meaning little animals. In 1838, botanist Mathias Schleiden stated that plants are made of ‘independent,
separate beings’ called cells. One year later, the zoologist Theodor Schwann made a similar statement
about animals.
The second principle continues to gain support today, as we have not been able to find any living entity that
is not made of at least one cell.
Some very famous scientists, such as Louis Pasteur in the 1860s, have performed experiments to support
the last principle. After sterilizing chicken broth by boiling. Pasteur showed that living organisms would
not ‘spontaneously’ reappear. Only after exposure to pre-existing cells was life able to re-establish itself
in the sterilized chicken broth.
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TOK
Theories are the accumulation of evidence that allows a hypothesis to become a theory; whether a theory
should be abandoned when there is evidence that it does not offer a full explanation; and what evidence is
needed for a theory to be adopted or rejected.
Question (5 marks)
1. Viruses are not considered to be living things. Discuss this statement in reference to ‘Cell Theory’.
(5)
The idea that all living things are composed of cells developed over many years and is strongly linked to the
invention and refinement of the microscope. Early microscopes in the 1600’s (such as Leeuwenhoek’s below)
opened up a whole new field of biology: the study of cell biology ad microorganisms. The cell theory is a
fundamental idea of biology.
Cell Theory
The idea that cells are fundamental units of life is part of cell theory. The basic principle of the theory
(as developed by early biologists) are:
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All living things are composed of cells and cell products
New cells are formed only by the division of preexisting cells
The cell contains inherited information (genes) that are used as instructions for growth, functioning
and development
The cell is the functioning unit of life; the chemical reactions of life take place within the cells.
Questions (4 marks)
2. Briefly describe the impact the invention of microscopes has had on biology.
(2)
3. Before the development of ‘cell theory’, it was commonly believed that living organisms could arise by
spontaneous generation. Explain what this term means and why it has been discredited as a theory. (2)