Cell Theory and Fundamental Cellular Structures

The Cell Theory

Cells were observed for the first time by Robert Hooke in 1665, in a slide of cork, where, by means of a microscope, he saw something similar to a hive. That's why he called these structures cells (cellula is a Latin word meaning "small room").

In 1838, two German scientists, botanist Matthias Schleiden and zoologist Theodor Schwann, stated the Cell Theory, which can be synthesized in the following tenets:

• All living things are made up of one or more cells.
• A cell is the smallest and simplest living thing.
• In a multicellular organism, cells keep their own activity, although a coordination among them occurs.
• All cells can only arise from pre-existing cells. (That idea was added later on by Rudolf Virchow. Originally, Schleiden and Schwann supported spontaneous generation.)

Functions and Structure of Cells

Cell Functions

All cells carry out the three basic vital (or life) functions: Nutrition, Interaction, and Reproduction.

Cell Structure

All cells share three main parts:

• Cell membrane: Acting as a boundary that controls which substances can enter or leave the cell.
• Cytoplasm: The space within the cell membrane.
• Genetic material: In charge of the cell activity.

Types of Cells

• Size: Cells are very small structures, only visible using a microscope. Their size varies from less than a micron to a few tens of microns.
• Shape: Cells have many different shapes, depending on their function. Plant cells are polygonal, whereas animal cells are more irregular.
• Structure: According to their structure, two types are differentiated:
  • Prokaryotic cells: Without a differentiated nucleus.
  • Eukaryotic cells: With a differentiated nucleus.

The Eukaryotic Cell

All living things, apart from bacteria, are made up of one or more eukaryotic cells. The cytoplasm in eukaryotic cells is made up of:

• Hyaloplasm (or ground substance): The clear fluid portion of cytoplasm, constituted by an aqueous solution. It's also called cytosol.