BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.1. The Cell Concept

One of the fundamental concepts in biology is the idea that The Cell is the Structural and functional unit of living organisms. This concept, known as the Cell Theory, was formulated by two scientists: the Belgian botanist Matthias Schleiden in 1838 and the German zoologist Theodor Schwann in 1839. The Discovery of the cell was a result of the rapid development of Microscopy in the 19th century, which sparked immense interest among scientists in studying The Structure of living organisms. Later in the 19th century, and subsequently in the 20th century, many other important developments occurred in this field (listed in Table 5.1). An essential part of the cell theory, first stated in 1855, is the assertion that all new Cells arise only from pre-existing cells.

Class="center">Table 5.1. Some important Milestones in the history of cell biology

1590	Jansen invented the microscope, which achieved greater magnification by combining two lenses
1665	Robert Hooke, using an improved microscope, studied the structure of cork and first used the term cell to describe the structural units composing this tissue. He believed that cells were empty and that the living substance resided in the cell walls
1650-1700	Antony van Leeuwenhoek, using simple, well-ground lenses (x200), observed "animalcules" and various single-celled organisms, including Bacteria. Bacteria were first described in 1683
1700-1800	Many new descriptions and drawings of various Tissues, predominantly plant tissues, were published (though the microscope at this time was viewed largely as a toy)
1827	Dolland drastically improved lens quality. Following this, interest in microscopy grew rapidly and spread widely.
1831-1833¹	Robert Brown described The Nucleus as a characteristic spherical body found in plant cells
1838-1839¹	The botanist Schleiden and the zoologist Schwann synthesized the ideas of various scientists and formulated the "cell theory," which postulated that the basic unit of Structure and function in living organisms is the cell
1840¹	Purkinje proposed the name protoplasm for cell contents, having become convinced that it (rather than the cell walls) constituted the living matter. The term Cytoplasm was introduced later (cytoplasm + nucleus = protoplasm)
1855¹	Virchow showed that all cells arise from other cells through Cell Division
1866	Haeckel established that the nucleus is responsible for storing and transmitting hereditary traits
1866-1883	Cell division was studied in detail and Chromosomes were described
1880-1883	METABOLISM/14.html">Chloroplasts were discovered
1890	Mitochondria were discovered
1898	The Golgi apparatus was discovered
1887-1900	The microscope was improved, along with techniques for fixation, staining, and sectioning. Cytology² began to take on an experimental character. Cytogenetics³ emerged as a branch of cytology, focusing on The Role of the nucleus in the transmission of hereditary traits

1900	Mendel's Laws, forgotten since 1865, were rediscovered, giving a major impetus to The Development of cytogenetics. The Light Microscope nearly reached its theoretical limit of resolution, and the progress of cytology naturally slowed down
1930s	The Electron microscope was developed, providing much higher resolution
From 1946 to the present	The electron microscope became widely used in biology, making it possible to investigate Cell Structure in much greater detail. This fine structure became known as the ultrastructure
¹ A milestone event in the origin and Development of the concept of cellular structure in living organisms. ² Cytology is the branch of science that studies cell structure (primarily using a microscope). ³ Cytogenetics is the branch of science that combines cytology and genetics, largely by correlating breeding experiment results with chromosome behavior during cell division.

5.1.1. Why Cells?

A cell can be thought of as a bag filled with various chemicals that is alive and capable of self-Replication. The substances inside it differ in many ways from those in its external environment. Without a barrier separating it from the surroundings, these substances would freely diffuse away, and the differences between the internal contents and the external environment could not be maintained. This means life could not exist. The barrier surrounding the cell is an extremely thin membrane. Its role is akin to a national border: it regulates the flow of molecules moving in both directions—into and out of the cell. All living cells are surrounded by a membrane. It is called the Plasma Membrane to distinguish it from other membranes found inside the cell. The mechanisms regulating the exchange between a cell and its environment will be discussed in Section 5.9.8.

Last update: 06/08/2026

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