What is a Cell? Definition, Structure, Cell Theory, Types

Cell is basic fundamental unit of life or it is the structural functional unit of life or it is the smallest unit of life.

Brief History on Cell Theory

Let’s first discover something about cell theory. Let us go through its history a bit.

Sir Rudolf Peter said, “The living cell is the most important invention in nature”.

Cells are so minute that their structure can be studied only with the help of a microscope.

Robert Hooke in 1665 studied a thin section of cork with the help of a microscope and showed that it is made up of many small compartments like that of a beehive. He introduced the word “CELL” for those compartments. 

Anton van Leeuwenhoek (1632-1723) studied many organisms such as bacteria, protozoa etc. 

Robert Brown in 1831 discovered the nucleus which is a spherical body in a cell.

Schleiden in 1838 found nucleoli in the nucleus. 

Dujardin in 1835 described a cellular membrane in a cell which he called sarcode.

Later on, Hugo van Mohl in 1846 discovered a similar cellular content in plants and named it protoplasm.

In 1931, M. Knoll Ruska invented the electron microscope and much could be known about the cell. With this microscope, which has now been improved much, we can obtain magnifications more than 10,00,000 times. 

These were the scientists who contributed to the cell theory.

Who were cell theory scientists? What they proposed on cell theory?

German biologists Matthias Schleiden (1838) and Theodor Schwann (1839) proposed a famous cell theory according to which “the cells are organisms and animals as well as plants are aggregates of these organisms, arranged in accordance with definite laws”.

Certain changes have been made in the original cell theory on the basis of the knowledge gained in the present century. 

The modern cell theory is illustrated briefly by the following four points: 

  1. Each living body is made up of a single cell or many cells and aggregations arising from them.
  2. The cells are units of vital activities and metabolism of the living body.
  3. Cells are also hereditary units because they possess hereditary material.  
  4. New cells can be formed only from pre-existing cells.

Definition of cell

Different scientists have given different definitions of the cell.

These are given below.

  1. Cells are structural and functional units of animals and plants.
  2. They are the smallest living units that are capable of maintaining their independent existence and self-reproduction. 
  3. It is a small mass of living material that is limited by cell membrane regulating all exchanges of matter and energy.

All the above definitions give insufficient information and hence they are not accepted. On the basis of  modern research, the cell is defined thus: 

Cell is the smallest but complete expression of fundamental organization and function of organisms, it is limited by a semipermeable cell membrane and has the power of self-reproducing or regeneration in medium free of other living beings.

a blue green algae cell
A Blue Green Algae Cell

The shape of Cell

Cells may be of different shapes such as spherical, rectangular, cylindrical, tubular, triangular, polygonal or irregular. 

The shape of the cell is affected by two factors.

  1. External factors such as pressure and surface tension.
  2. Internal factors are various specialized activities taking place within the cell due to which extremely variable shapes occur.
various types of cells
Various Types of Cells

Size of cell

Mycoplasma galliseptium is the smallest cell known so far. Its diameter is 0.0001 mm. Some cells are large and visible with the naked eyes, such as frog’s eggs or bird eggs.

In the human body, the size of a cell ranges from 3-4 microns to 3-4 feet (from leucocytes to nerve cells respectively).

The size of cells does not increase after they have reached maximum size. The reason for this is the relationship between the cytoplasm and nucleus of a cell, This relationship is called karyoplasma relation.

The nucleus controls the various life activities such as nutrition, respiration etc. so long as this relation is existing, but due to continuous growth, the amount of cytoplasm increases so much that the nucleus is unable to control its activities and then karyoplasma relation breaks, and the cell divides into daughter cell alongside nucleus. This process is called cell division. It restores karyoplasma relation once again. 

Number of cells

There is a great variation in the number of cells present in the body of living beings. Some have only one cell such as the members of phylum Protozoa (Amoeba, Plasmodium, Euglena, Paramecium etc.) while others are multicellular, for example, Metazoa in which a number of cells are according to the size of the body.

Characteristics of Cell

  1. A cell provide structure and support to organisms.
  2. Its interior is organized into organelles.
  3. All living organism are made up of cells.
  4. It is the fundamental or basic unit of life. Also the smallest unit of life.
  5. Cells are tiny. Because of the tiny nature they are measured in micrometers.
  6. These are capable of independent existence. It gives rise to new cells.
  7. They have a capability to perform all the basic function of life. For example, absorption, digestion, modification, excretion are done.
  8. All cells arise from its pre-exiting ones.
  9. Nucleus hold genetic information.
  10. Nucleus and other cell organelles are present in the cytoplasm.

What are the types of cells?

On the basis of the presence or absence of mainly the nuclear membrane, the cells found in nature are of two types→ prokaryotic and eukaryotic. 

Prokaryotic and Eukaryotic cell
Labelled Diagram of (A) Prokaryotic and (B) Eukaryotic cell

1. Prokaryotic cell:

These cells are simple in structural organization and most primitive types. They include bacterial, blue-green algae etc. There is a complete absence of a nuclear membrane. 

The chromatin or the nuclear material, therefore, remains scattered in the cytoplasm instead of being organized.

Most of the cytoplasmic organelles are also absent in these cells, The functions such as respiration and photosynthesis are performed by the cell membrane and its infoldings. 

2. Eukaryotic cell:

These cells are found in higher animals, plants and microorganisms such as fungi, protozoa and most algae.

They are characterized by the presence of a distinct nuclear membrane that surrounds a definitely organized nucleus.

The cytoplasm of these cells contains various structurally distinct organelles such as mitochondria, Golgi apparatus, endoplasmic reticulum, lysosome, ribosomes, plastids etc. 

These organelles carry out activities like respiration, excretion, photosynthesis etc. The eukaryotic cell can be of many types, e.g, epithelial cell, muscle cells, connective cells.

Division of labor in the cell

Except for acellular animals, i.e., protozoans, the body of others consists of numerous cells where all these cells do not perform the same function.

The function of these cells is changed according to the need for excretion, reproduction etc.  This division of work among the cells in the body is known as the division of labor.

Difference between Animal and Plant Cell

Animal CellPlant Cell
The plasma membrane is present. The cell wall is absent.Outside the plasma membrane, a thick and non-living cell wall, composed of cellulose is present. 
The amount of cytoplasm is more but the number of vacuoles is less.The cytoplasm is less and generally forms a layer near the cell occupied by one or more vacuoles which are filled with liquid food and excretory substance.
Chloroplasts are absent.Chloroplast is present and contains green chlorophyll with the help of which they manufacture their food themselves by photosynthesis. 
Centrosomes, centrioles and Golgi complexes are present.Generally, these structures are absent.
At the time of cell division chromosomes, spindles and centrioles become clear and distinct. Chromosomes and spindles alone become distinct at the time of cell division and centrioles are not clear.
animal cell structure
Structure of an Animal Cell

Difference between Eukaryotic and Prokaryotic Cell

            Eukaryotic CellProkaryotic Cell
The nucleus is well-organized into the nuclear membrane, nucleolus and chromatin networks.The nucleus is not well-organized, simply represented by a nuclear membrane. 
Cell organelles like chloroplast, mitochondria, endoplasmic reticulum, Golgi bodies etc are surrounded by unit membranes.The plasma membrane is not the only unit membrane, and organelles like chloroplasts, mitochondria, endoplasmic reticulum, Golgi bodies etc are absent.
Usually have chromosomes (like bearers of hereditary characters) that have open fibrils of DNA.Hereditary material is present in the form of a closed-loop.
Histone proteins are present.Histone proteins are absent.
Usually divided by mitotic and meiotic division.Usually divided by amiotic division.
Ribosomes are usually present in the form of monosomes, on the surface of the endoplasmic reticulum.Ribosomes usually occur in polysomic conditions.
Ribosomes are of 80 S types.Ribosomes are of 70 S types.
m-RNA is monocistronic.m-RNA is polycistronic.
Sexual reproduction is common.Sexual reproduction is rare.
The flagellum is composed of 9+2 fibrils.Flagella have only one fibril.

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