Cell Theory – Definition, Parts and Types

Biology begins with the cell Theory. So, if you are studying biology, you must thoroughly understand cell theory. Learn in detail about cell theory — definition, its history, importance, and all that you must know to understand three postulates completely.

What is Cell Theory? 

Cell theory is three statements describing cells and how they function. But before moving on to cell theory, it is important to revise what cells are. 

Cells are extremely thin, microscopic structures that cannot be seen with the naked eye. If you ask an expert what a cell is, they will tell you that it is the smallest unit of structure and function. However, this leaves you wondering what does this statement mean? 

Living beings share certain traits— they take nutrition, reproduce, utilize energy, grow, respond to stimuli, and adapt to the environment. A cell is one such smallest part of living beings capable of performing all of these tasks. 

A living organism can be just a single cell, and this single cell carries out all the necessary functions. However, most living things are multicellular, i. e., they are made up of many cells. 

So what is cell theory? 

In Science, the theory of cells is an idea supported by extensive research that describes cells and their operation. 

Parts of Cell Theory: The Three Principles

Cell theory consists of three main postulates:

1) All living things are made up of cells.

2) A cell is the smallest unit of structure and function in a living thing.

3) All cells arise from preexisting cells. 

Understanding the Cell Theory Postulates

So what do the statements of the cell theory mean? Here is an explanation of the three statements. 

All living things are made up of cells: The first statement of the cell theory is simple to understand. It implies that every living thing is made of cells. So, everything alive, including bacteria, viruses, plants, animals, and humans, is composed of cells. Cells consist of a group of specialized structures called organelles surrounded by a thin cytoplasmic membrane.

A cell is the smallest unit of structure and function in a living thing: The second postulate implies that cells are the building blocks of life. They not only makeup all the living things but also run the life processes. A human being consists of over 30 trillion cells. Everything from your hair, nails, skin, tissues, and more is composed of cells. So, every function the body performs is done at a cellular level that combines to bring forth the larger picture. 

All cells arise from preexisting cells: The third part of the cell theory implies that cells don’t just appear spontaneously, but the currently present cells divide to give rise to new cells. Cells replicate themselves continuously via cell division. However, it is to be kept in mind that all cells arise from preexisting cells, but not all cells give rise to new cells as some do not divide.

Cell Theory: Timeline

The journey of cell theory can be traced back to the discovery of cells by Robert Hooke, an English scientist who first viewed and described cells. Here is the complete timeline of the cell theory:

1665: Robert Hooke observed cells in a slice of cork under his microscope. 

1665: Francesco Redi disproved the theory of spontaneous generation. He conducted the experiment showing the growth of maggots on uncovered meat and not in an enclosed jar. His experiment later helped scientists in formulating the cell theory.

1670: The development of the compound microscope by Antonie van Leeuwenhoek allowed scientists to see cells clearly and understand their structure. 

1839: Matthias Schleiden and Theodor Schwann, the two German scientists, put forward the first two postulates of the cell theory. Schleiden found that all plants are composed of cells, while Schwann stated all animals consist of cells that perform several functions. Schleiden and Schwann are the developers of cell theory.

1855: Rudolf Virchow stated the third part of cell theory. After extensive research, he concluded that all cells arise from existing cells.

Types of Cells Theory

Cells can be broadly divided into two main types: 

• Prokaryotic cells: These are relatively small cells. They contain a plasma membrane surrounded by a characteristic cell wall. The cell wall might differ in composition depending on the particular organism. Prokaryotes lack a true nucleus, i.e., the nucleus is not surrounded by a nuclear membrane. Rather it lies freely in the cytoplasm. Their genetic material can be circular or linear DNA. 
• Eukaryotes: Eukaryotes are complex cells. They have a true nucleus and several specialized cell organelles. Animal cells do not have a cell wall surrounding their plasma membrane. They reproduce both sexually and asexually, unlike prokaryotes which reproduce only by binary fission.  

The following table summarises the key differences between prokaryotic and eukaryotic cells. 

	Prokaryotes	Eukaryotes
Type of Cell	Unicellular	Unicellular or multicellular
Cell size	The size ranges from 0.2 μm – 2.0 μm in diameter.	The size varies from 10 μm – 100 μm in diameter.
Cell wall	Mostly present and is chemically complex in nature	Present in plant cells and has a simple chemical composition as compared to prokaryotic cells.
Nucleus	The true nucleus is absent. They have a nucleoid region in the cell.	True nucleus present.
Ribosomes	They are smaller in size and are of the 70S type.	They are comparatively larger in size and are mostly of the 80S type.
DNA	Circular	Linear
Mitochondria	Absent	Present
Endoplasmic Reticulum	Absent	Present
Plasmids	Present	Very rare in eukaryotes
Lysosome and Centrosome	Lysosomes and centrosomes are absent	They are present in animal cells but not in plant cells.
Cell division	Binary fission	Mitosis and Meiosis
Flagella	It is smaller in size.	If present, it is larger in size.
Reproduction	Asexual	Both asexual and sexual

How is Cell Theory Important?

Cell theory might appear to be a combination of basic statements for a student today. Anyone who understands basic biology already knows what cells are. However, the cell theory’s importance can be understood by its obviousness. It is one of the fundamental principles.

Knowing that every living being is composed of cells facilitates understanding how they form, grow, and die. The information enables an understanding of how a new life is created, how cancer spreads, why diseases occur, and how they can be managed. The most advanced studies in the field of Biology are possible today because the three scientists provided the cell theory back then.  

Before cell theory, people had conflicting ideas regarding the occurrence of life. While some supported spontaneous generation, others did not. Disease infliction, spread, and communication were a mystery, and the conditions were hardly explainable. The cell theory brought about a change that completely transformed the outlook of biological concepts. 

Modern Interpretation of Cell Theory 

In addition to the three postulates, the following statements have become integral to cell theory interpretation. 

• All living things known today are made up of one or more cells. 
• Living cells arise from preexisting cells through the process of cell division.
• A cell is the basic unit of structure and function in all living organisms.
• The activity of an organism relies on the overall activity of independent cells.
• Metabolism occurs within cells.
• Cells consist of DNA found in the chromosome and RNA in the cell nucleus and cytoplasm.
• Cells of organisms of a similar species are the same in chemical composition. 

Conclusion 

Cell theory is the foundational biological theory with three main parts: 

1. All living things are made up of cells.
2. A cell is the smallest unit of structure and function in a living thing.
3. All cells arise from preexisting cells.

The pioneering efforts of Schleiden, Schwann, and Virchow have helped scientific studies by providing a strong ground for further research. Cell theory is crucial because it affects nearly every biological aspect, from understanding life and death to the occurrence of diseases and their management and more.

Frequently Asked Questions

Q1. How do cells arise from preexisting cells?

The currently existing cells divide to give rise to new cells. The process is called cell division. Cell division is of two main types: mitosis and meiosis. Mitosis is the cell division that gives rise to two diploid daughter cells. It occurs in somatic cells. Contrastingly, the cell division occurring in gonads and resulting in four haploid daughter cells is called meiosis.

Q2. Why is the cell called the basic unit of life?

The body or structure of all living organisms consists of cells/ cells. A cell performs all the metabolic and regulatory functions of every individual. Since the cell is behind every life process, it is called the basic unit of life. 

Q3. Which cells don’t divide after birth? 

The nerve cells do not divide after the birth of an organism because they do not contain the centrioles—the organelles responsible for cell division management.