Cell Definition Theory Types and Structure

“The cell can be defined as the structural and functional unit of life. It is the smallest unit that can carry out all activities of life. Cells are building blocks of complex multicellular organisms”.


“The cell is the smallest part of living organism. It is the small piece of cytoplasm contains a nucleus and covered by an outer protective layer. i.e. cell membrane”.

                The cell was discovered by Robert Hooke. In the cell, nucleus was studied by Robert Brown.

Cell Theory

It is a fundamental theory about cell. This theory was proposed by three well known scientists, Schleiden (1838), Schwann (1839) and Virchow.

The important features of this theory are as follows:

  1. The body of living organisms is composed of one or many cells.
  2. Cells are the structural and functional unit of life. It is a denovo structuree. it is raised from a non-cellular structure.
  3. Virchow Point: A new cell is always produced from a pre-existing alive cell. It indicates that cell is not a denovo structure.

Structure of Generalized Cells

Structure of a cell can be studied under light or electron microscope. Different parts of cells can be isolated by a technique called Cell Fraction Technique. Therefore, the structures and functions of these isolated parts can be studied in detail.structure of cell

Technique to Isolate Different Components of Cells

“Isolation of cellular components to determine their chemical composition is known as cell Fraction”.


“The technique to isolate different components of a cell is called Fraction Technique”.

The different parts of cells are isolated separately step by step, and then these parts are studied in well-accurate manner in detail. In this process an apparatus in used. The working of this apparatus is called centrifugation.

Steps of Fraction:

  1. Many similar cells are kept in cold environment in a homogenizer present in test tube.
  2. Spinning action is applied i.e. shaking of material rapidly, the process is called centrifugation.
  3. Larger particles like nuclei are separated and settled down in the test tube. These are separated to study. Smaller particles remain in the fluid, which are transferred to other test tube.
  4. The fluid with particles is centrifuged at high speed. Different fractions are obtained in different steps. These are separated and used to study.

Parts of a Cell

A cell consists of following three parts:

  1. Plasma membrane, also a cell wall in plant cells.
  2. Cytoplasm, containing cell organelles.
  3. Nucleus, with nuclear and chromatin material.

Types of Cells

There are two types of cells:

A).          Prokaryotic Cells:

It is a primitive type of cell in which cell lake definite nucleus. The nuclear membrane is absent. The nuclear material directly submerged in the cytoplasm and it is not separated from cytoplasm by membrane. The organisms which contain prokaryotic cells are called Prokaryotes. Examples are bacteria and blue-green algae.


These cells are found in simple living organisms like blue-green algae, and bacteria. These cells do not contain a complete and true nucleus. Nuclear membrane and nucleolus are absent so instead of a true nucleus, the genetic material is found in those organelles, which are covered by membrane such as Mitochondria, Golgi bodies, Lysosomes and Chloroplasts. These cells contain mesosome. Their nuclear material does not produce true chromosomes.

B).          Eukaryotic Cells:

These cells contain a definite nucleus. Nuclear membrane is present and chromatin material is bounded by a membrane. The organisms which contain eukaryotic cells are called Eukaryotes. Examples are all plants and animals.


These cells are present in algae, fungi, protozoa, plants and animals. These cells have true nucleus. They also contain other organelles like Mitochondria, Golgi bodies, Lysosomes and Chloroplasts. Their nucleus produces true chromosomes. These cells do not contain mesosome.

Structure of Eukaryotic Cells:

A typical eukaryotic cell consists of following parts:

  1. Plasma membrane (cell membrane) is present.
  2. Cell Wall is found in plant cells, bacterial cells, fungal cells, but absent in animal cells.
  3. Nucleus is present.
  4. Cytoplasm and Cytoplasmic Organelles are present.

Size of Cells

The eukaryotic cells vary in size. The biggest cell is egg of Ostrich. Most of the cells are microscopic and cannot be seen with naked eye. Their size is measured in micrometer (μm). One μm is 0.000,001 meter (1×10-6 of a meter).

Difference between Plant and Animal Cells

plant cell

animal cell

Plant Cells

Animal Cells

·         They contain cell wall. ·         They do not contain cell wall.
·         Centrioles are absent in them. ·         They contain centrioles.
·         They contain a large central vacuole. ·         They do not contain this vacuole.
·         Plastids are present in plant cells. ·         Plastids are absent in them.


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