# Cell Structure and Cell Organelles

## Introduction to Cell Biology

The cell is the basic unit that makes up all known life forms. It is the smallest self-sustaining and self-reproducing unit of life. Cells were first discovered by Robert Hooke (1635-1703) in Cork. However, it was Theodor Schwann and Matthias Schleiden who first postulated in 1838 that all organisms consist of cells. Nearly all known cells have significant components in common:

• Genetic information storage , which serves as the blueprint for all of the other cell components;
• , which largely determine the structure of the cell and, as enzymes, enable its metabolism; and
• , which separate the cell from its environment, act as a selective filter for contact with the outer world, and divide more complex cells into separate reaction spaces, known as .
Fig.1
Structure of a prokaryotic cell
Mariana Ruiz Villarreal (this work is public domain)

Prokaryotic cells consist of a single compartment that is separated from its surroundings by a membrane or structural cell wall. Genetic information is stored in the form of circular DNA or sometimes small DNA rings (plasmids). In addition, these cells contain ribosomes, represented here in blue, that are capable of protein biosynthesis. Small pili or larger flagelli provide locomotion.

Cells are classified as either or , depending on their complexity. It is now generally assumed that the more complex eukaryotes arose from the endosymbiosis of various prokaryotes (endosymbiotic theory).

The cells of all organisms also demonstrate common basic capabilities and properties, which mostly also represent the general characteristics of life:

• Identical reproduction (propagation) of the organism through cell division. This is closely related to another property: heredity.
• A , which consists of the absorption of raw materials, their conversion into new components or useable energy, and the disposal of products that are unusable or dangerous to the organism (waste). Many cells are also mobile which is another indicator of life.
• Another ability common to all cells is protein biosynthesis by means of the transcription of DNA to RNA and translation of the latter into proteins.
• Even primitive cells react to the environment and the conditions within the organism. They thus react to stimuli.
• In the end, death is also a property of living systems because no individual life form is immortal.

Multicellular organisms contain many differentiated cell types. The human body is thus made up of about 220 different types of cells and the tissues they form. The size of eukaryotic cells varies from 10 µm to 100 µm. The total number of cells in a human exceeds $1013$.

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