The cell theory is one of the fundamental concepts of biology. It states that all living things are composed of cells, which are the basic units of life.

The theory has three generalizations that have been accepted by the scientific community for centuries. Let’s explore these generalizations in more detail.

Generalization 1: All living things are composed of cells

This generalization is the foundation of the cell theory. It means that every living organism, whether it’s a plant, animal, or bacteria, is made up of at least one cell.

Cells are microscopic structures that perform various functions within an organism. They can be compared to building blocks because they come together to form tissues, which then form organs and eventually entire organisms.

Generalization 2: Cells are the basic units of life

All living organisms are made up of cells, and these cells are considered as the basic units of life because they perform all the necessary functions to keep an organism alive. Cells can reproduce themselves through cell division and carry out metabolic processes such as respiration and digestion.

Subgeneralization 1: Cells can be unicellular or multicellular

While all living things are composed of cells, not all organisms have the same number of cells. Some organisms like bacteria have only one cell and are called unicellular organisms. Others like humans have trillions of cells organized into tissues and organs, making them multicellular.

Subgeneralization 2: Cells have a common structure

Despite differences in size and function, all cells share a common structure. They have a plasma membrane that encloses their contents and separates them from their environment. They also contain genetic material in the form of DNA, which carries instructions for cellular processes.

Generalization 3: Cells arise from pre-existing cells

This generalization is also known as the principle of biogenesis and states that all cells come from pre-existing cells. This means that new cells are produced through cell division, where a parent cell divides into two or more daughter cells. This process is essential for growth, repair, and reproduction.

Subgeneralization: Exceptions to the principle of biogenesis

While the principle of biogenesis holds true for the majority of living organisms, there are some exceptions. Viruses, for example, are not considered living organisms because they cannot reproduce on their own and require a host cell to replicate.


The cell theory is a fundamental concept in biology that has three generalizations. All living things are composed of cells, which are the basic units of life and arise from pre-existing cells. These generalizations have been accepted by the scientific community for centuries and have helped us understand the complexity of life on earth.