2.1.1 The Cell Theory

Key Principles of the Cell Theory:

1. All living organisms are composed of cells:

• Cells are the basic unit of structure and organisation in all living things, from unicellular to multicellular organisms.

2. The cell is the smallest unit of life:

• A cell is the simplest structure capable of performing all the functions necessary for life, including metabolism, growth, and reproduction.

3. All cells arise from pre-existing cells:

• Cells are formed by the division of existing cells through processes like mitosis or binary fission.

Additional Insights:

• Cells contain genetic material (DNA or RNA), which directs cellular activities and is passed on during cell division.
• Cellular activities rely on chemical reactions that occur within cells, often facilitated by enzymes.
• All cells are surrounded by a cell membrane, which regulates the movement of substances in and out of the cell.

Modern Extensions to the Cell Theory:

• Viruses are considered exceptions to the cell theory because they are not made of cells and cannot carry out life processes independently.
• The theory has been expanded with advances in molecular biology, showing the shared biochemistry of all cells (e.g., DNA as the genetic material and ATP as an energy source).