Cell Theory

Cell Biology, Ecology, Zoology

Cell Theory Definition

Cell theory is a proposed and widely accepted view of how most life on Earth functions. Before the invention of advanced microscopes, microorganisms were unknown, and it was assumed that individuals were the basic units of life. However in the 1800s this view began to change, thanks to the microscope. Even with a microscope, it is not always possible to see the exact functioning of cell. Scientist formulated a general theory of how cells work that is very simple. All organisms are made of cells, and cells are the fundamental building blocks used to create larger organisms. The third, and probably most important part of the theory, is that cells can only arise from other cells.

Thus, organisms start as single cells, and grow through mitosis into multi-celled organisms. Mitosis is a form of cellular division that produces identical cells. These cells can then differentiate when given different signals to produce different types of tissues and organs. This is how large and complex organisms are made. Single-celled organisms divide as well, but when they divide, the cells separate and do not function together. This is asexual reproduction. Either way, organisms are mutated, changed, and reproduce either more or less successively. This is what drives evolution.

3 Parts of the Cell Theory

The 3 parts, or tenets, of cell theory are a little outdated in terms of modern biology, but still serve as a good general model. The three tenets of cell theory are discussed in detail below.

All organisms are made of cells

Cells are the smallest unit of life. Each cell is a membrane of semi-permeable phospholipids wrapped around cytosol or a solution of water and dissolved solutes. All cells rely on DNA to hold the information necessary to produce the molecules they use to obtain energy. Although the methods for obtaining energy vary widely, all organism obtain energy to grow and reproduce. This first tenet of the cell theory is mostly true, but the discovery of viruses lead to complications. Viruses, while they use DNA or RNA to reproduce, do not have cells or cellular membranes. Viruses typically use a host cell to replicate. In this case, the virus appears to be living, but does not create its own cell. Some scientist argue that viruses are not living, thus cell theory is not violated.

Cells are the most fundamental unit of life

Organisms can be single cells, which hold all of the components necessary for a metabolism, or they can be more complex. More complex organisms divide the various metabolic tasks into different groups of cells, called tissues. These tissues are arranged in compartments with membranes that separate them from other tissues. These groups of tissues are called organs. A group of organs functioning together is an organism, or an individual creature. Each cell is distinct from the cells next to it, and each functions independently, while contributing to the output of the organism as a whole. Again, modern cell theory is a bit more complicated because advances in science have revealed many different organelles within cells. These organelles are bound in membranes themselves, and serve different functions for eukaryotic cells. Some scientists argue that these are more fundamental units, but other scientists argue that like an organ outside of an organism, they could not function without the cell.

Cells come from other cells

As far as we know, no cell on Earth currently has arisen spontaneously. All cells are the result of cell division. When a cell is large enough, it replicates its DNA and important components. These components can then be divided into two daughter cells, which are copies of each other. Variations in the DNA in each cell can lead to changes in how they function, which can result in them dividing at different rates. The cell that reproduces more than the other cell will pass on more of its DNA. The purpose of every cell or organism is to reproduce the DNA in cells.

This third tenet of the cell theory has yet to be disproven. No scientist has ever created a functioning cell without replicating another cell, although some scientists are trying. If they were successful, it would give proof to how life could have evolved. It is thought that a self-replicating molecule mutated, developed the ability to produce a membrane, and thus the first cell was born. The cell was such a successful form of life that all life since has used the same basic template.

Related Biology Terms

  • Cellular Membrane – The phospholipid bi-layer that surrounds cells and is only permeable to certain substances.
  • Cell Division – The process by which cells replicate their DNA and divide their cell membrane in half.
  • Prokaryote – Organisms in which the DNA and metabolic enzymes exist freely within the cell, not bound by membranes.
  • Eukaryote – Organisms that separate their organelles with membranes, including a nucleus holding the DNA.


1. Some plants reproduce by spraying pollen into the air. This pollen reaches the female plant, and fertilizes and egg. This new zygote becomes another plant. Do plants fit into cell theory?
A. Yes
B. No
C. Only when not reproducing

Answer to Question #1

2. Why are viruses sometimes not considered living organisms?
A. They do not produce their own cell
B. They cannot reproduce
C. Their DNA doesn’t contain the same information

Answer to Question #2

3. A scientist in the lab is testing cells. The scientist has various chemicals that are applied to cells, and the reaction observed. The scientist puts some bleach on cells and watches the reaction. The bleach starts to destroy the cell membrane, and eats its way to the DNA, which is also destroyed. Will the cell reproduce?
A. Yes
B. No
C. Only of the hole is closed

Answer to Question #3

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