What is the cell theory | History, Eukaryotic, Prokaryotic, Features

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What is the cell theory?

For the first time, the German botanist MJ Schleiden formulated the theory of the cell for plants in 1838. It was extended to animals in 1839 by the German physiologist Theodor Schwann.

According to cell theory – All living organisms (plants, animals, and micro-organisms) perform the activities of their cells. and are the sum total of the interactions.

Cell –

Cells are the basic units of structure and function in all living organisms. This means that all living organisms are made up of one cell or many cells.

History of cell theory –

In the early nineteenth century, discoveries made by many zoologists and botanists about the structure of plant and animal tissues and cells contributed to the formation of the cell theory.

Scientist Robert Brown discovered the nucleus in 1831.

Scientist Dumortier studied cell division in algae in 1831 and scientist Von Mohl described cell division in animal cells in 1835.

Scientists Purkinje and Von Mohl described the cytoplasm in 1836–37, and they established the importance of cell and cellular activities.

R. Virchow in 1835 expressed his famous amorphism ‘Omniscellulae a cellula’ which means. That all cells arise from already existing cells. They established cell division as a central event in the reproduction of organisms.

Later, following the development of cell theory, Fleming established that cells ensure continuity between the current generation and the next generation by cell division.

The discovery of reduction division at the time of gamete formation and fertilization prior to embryo development improved and supported the cell theory and helped in the creation of the mode version of the cell theory.

What is the cell theory
What is the cell theory?

Essential Features of Cell Theory

  1. The cell is the functional and structural unit of living organisms, which is surrounded by a membrane on all sides and is capable of becoming a structure like itself by reproduction,
  2. cells are the units of inheritance.
  3. New cells arise only from the already existing cells.
  4. The activities of an organism are the result of the activities of its constituent cells.
  5. Apart from its integral role in multicellular organisms, each cell has its own life.
  6. Hence cell is the smallest unit of life. It is a unit of living matter which is surrounded on all sides by the plasma membrane. It consists of a nucleus, which is also called the control center, and cytoplasm or cytosol with organelles, which is the center of cellular activity.
Significance of Cell Theory

The modern concept of cell theory emphasizes the structural and functional relationships between various living forms, from bacteria to humans.

Regardless of their function and position, a nucleus is embedded or embedded in the cytoplasm, surrounded by the cell membrane (the unit in the structural framework), and similar metabolic processes occur in all cells original or distinctive (unity of function). This means, that all living beings have originated from the same ancient ancestral or ancestral form. Which originated about two or three billion years ago.

Also read – What is Taxonomy?


Definition of cell –

The cell is the smallest and complete expression or unit of the basic structure and functions of all living organisms. It represents the unit of structure, function, and heredity. it is a thin semipermeable plasma membrane It is also described as a unit of protoplasm and has a nucleus within it.

Type of Cell –

There are two types.

Prokaryotic cell –

Prokaryotic is made up of two words from the Greek language, Pro + karyon. In which pro means primitive and karyon means nucleus. 

These are relatively simple cells. In which there is only a cell membrane. Membrane-bound organelles such as endoplasmic reticulum, Golgi complex, mitochondria, chloroplasts, and lysosomes are absent. A well-organized nucleus is also absent.

A chromosome is a hereditary material that is highly coiled and circular. Which is in the naked state in the cytoplasm. It is simply deoxyribonucleic acid or DNA Bacteria, blue-green algae and pleuropneumococci are prokaryotic cells.

Eukaryotic Cells –

They have a true nucleus, that is, the hereditary material (DNA) consisting of basic proteins and nucleoproteins, and separated from the cytoplasm by the nuclear envelope. In these membranes or membrane-bound organs are present.

From an evolutionary point of view, prokaryotic cells are ancient, and eukaryotic cells have evolved from these prokaryotic cells.

Characteristics of a Prokaryotic cell
  • The main characteristics of a prokaryotic cell are as follows.
  • It is of very small size. Approximately the size of mitochondria.
  • They are without a specific nucleus i.e. nucleolus and nuclear envelope are absent.
  • In these, the hereditary material is in the form of a single chromosome. A prokaryotic chromosome is a single circular chromosome made up of double-stranded DNA molecules.
  • They do not contain the basic protein histones.

The cell is surrounded by the plasma membrane. It differs from the eukaryotic plasma membrane. It has the following characteristics –

  • It does not contain sterols.
  • bacteria Protein to phospholipid ratio (2:1) is high. Whereas in eukaryotic (1:1) it is less.
  • A cell wall or a capsule may be present on the outside of the plasma membrane. But it is noncellulosic, it is made up of carbohydrates and amino acids.
  • Membrane-bound organelles such as endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, and chloroplasts are absent.
  • In some cases, the inner plasma membrane is folded. And it occurs in the form of mesosomes and chromatophores.
  • The inner surface of the plasma membrane contains enzymes of respiratory metabolism, photosynthesis, and lipid metabolism. 
  • The prokaryotic ribosome is of the 70S type.
  • Their cytoplasm does not exhibit swaying motion.
  • Prokaryotic cells consist of flagella and pili.
  • These store polymerized fatty acids such as beta-hydroxy-butyrate, glycogen, and phosphate granules.

Eukaryotic Cell

The cell structure –

A typical animal cell is made up of three parts.

  1. Plasma Membrane or Cell Membrane
  2. Nucleus
  3. Cytosome or Cytoplasm

First of all, we know about Plasma Membrane.

Cell Membrane or Plasma membrane –

Structure –

The cytoplasm of the cell is usually surrounded by a thin layer of lipoprotein that is not visible. called plasma membrane. It is made up of two layers with two layers of proteins sandwiched between lipid molecules. It has microscopic pores. Which is related to the substances entering and coming into the cell.

And these allow specially shaped molecules to move around. Meaning which type of molecules has to go inside the cell or not. That’s what decides. The plasma membrane chooses according to its functions whether it is capable of entering the cell or not.

Function –

  • The plasma membrane provides mechanical support and a definite external shape to the cell.
  • It forms a part of its living machinery. Because everything that enters or leaves the cell must pass through it.
  • It exerts a selective effect on substances entering or leaving the cell. That is, it first selects whether it is worth joining or exiting or not, allowing it to go or exit.

There may be a rigid non-living and supporting membrane around the plasma membrane. called cell wall or cell wall. It lacks animal cells but is thick and made up of cellulose in plants.


Structure –

the nucleus is found in the middle of the body in a eukaryotic cell. Generally, it is oval and spherical. But can also be segmented or long and lace-like. It may be a single body composed of several individual fragments or may be in the form of chromatin granules diffused in the cell substance.

Vesicular Nucleus A specialized nuclear envelope enclosed by nucleoplasm or nuclear juice. It is surrounded on all sides by it. Nuclear deoxyribonucleic acid (DNA) in prokaryotic cells is not separated from the cytoplasm by the nuclear membrane.

  1. The nuclear envelope is similar to the plasma membrane except in structure and function. It is made up of two membranes and is separated by the perinuclear space. Nucleoplasm is a clear and transparent homogeneous liquid of variable consistency. It includes the nuclear reticulum and nucleolus.
  2. Reticulum – Nuclear reticulum is a fine mesh of coarse chromatin grains with chromatin threads and beads. During cell division, chromatin gets condensed into threads or rods called chromosomes. They are important genetic carriers. Meaning that they work to pass on the qualities of the parents to the children. Chromatin and nucleoproteins are the chemical components in it. It consists of four main molecules. A low molecular weight protein is a histone and a complex DNA and RNA.  

Function –

  • The nucleus controls the metabolic activities of the cell or cell.
  • It takes an active part during cell division.
  • It transmits genetic information from generation to generation.
What is the cytoplasm? 
Cytosol or Cytoplasm –

The inner part or interior of the cell is filled with a colorless, transparent fluid with variable consistency. It is called cytoplasm. It consists of submicroscopic filaments, endomembranous system, membrane-bound organelles, nonmembrane organelles, structureless material, or cytoplasmic matrix. There are different components of the cytoplasm.

  1. Cytoplasmic Fibrils and Cytoskeleton
  2. Endomembrane System
  3. Membrane-bound Organelles
  4. Nonmembranous Organelles

Cytoplasmic Fibrils and Cytoskeleton

The cell skeleton of the cell is made up of thin, thick, and medium filaments and microscopic tubules. It is like a net. The cell skeleton filaments are interconnected by a network of the fine thread-like fine trabecular meshwork. This network is also interconnected with many membranous organs.

Endomembrane System –

The Endomembrane system of the cytoplasm includes the endoplasmic reticulum, Golgi complex, and nuclear envelope. Mitochondria, chloroplasts, lysosomes, and peroxisomes.

Endoplasmic reticulum –

It is interconnected by channels of vesicles and tubules in the cytoplasmic matrix. It is present in almost all cells except mammalian erythrocytes and prokaryotic cells. These tubules are surrounded by unit membranes and may even form interconnected channels.

Some of them are associated with the nuclear membrane, some are associated with the plasma membrane. While some may be associated with both. This creates a direct path outward for the nuclear material. These channels can be with branches and without branches. In this, some membranes of the endoplasmic reticulum are attached to the ribosomes.

Based on the presence or absence of ribosomes, the endoplasmic reticulum is divided into the Rough Endoplasmic Reticulum (RER) and smooth endoplasmic reticulum (SER).

Functions –

  • It provides mechanical support.
  • It facilitates movement between cells and their surrounding materials.  
  • It helps to move inside the cell.
  • Ribosomes attached to the endoplasmic reticulum participate in protein formation.
  • The smooth endoplasmic reticulum (SER) is involved in the synthesis and storage of cholesterol and steroid hormones.

Golgi Complex –

The Golgi body is a thick, flaky plate-like structure. Each body consists of flattened sacs or cisternae arranged parallel to each other. Fatty content is abundant in them.

Functions –

  • The Golgi complex is involved in cell secretion.
  • They form lysosomes and peroxisomes.
  • They help in the processing and packaging of products that pass through the endoplasmic reticulum.

Membrane-bound Organelles –

Lysosome –

These are filled with globular vesicles and hydrolytic enzymes. It is bound by a single membrane.

Functions –

  • Due to the presence of hydrolytic enzymes, lysosomes aid in the digestion of ingested bacteria and cellular debris.
  • These are also called suicide sacs because when needed, lysosomes burst and release enzymes. which cause the destruction of the cells themselves. It helps in destroying dead and weak cells.
  • They help in the intracellular digestion of the ingested particles.

Peroxisomes –

These are smaller than lysosomes and are surrounded by a membrane and contain peroxidase enzymes that break down peroxides.

Mitochondria –

In fact, mitochondria are found either scattered or in bulk in every type of living cell. where they are in constant motion. They range in size from 2µ to 5µ, and range in size from spherical to rods or particles. These are surrounded by a double lipoprotein membrane. Their inner membrane is laid down in several coiled layers. These are called Christie’s. Their central space is filled by a matrix. It contains oxidative enzymes.

The function of Mitochondria –

mitochondria – The tricarboxylic cycle of respiration or Kreb cycle is completed inside the mitochondria and energy is released in the form of adenosine triphosphate (ATP) by the oxidation of carbohydrates, proteins, and fats, hence mitochondria are called cells of the cell. powerhouse It is called

Plasmids –

Plasmids are commonly found in plant cells. But it is limited to animal cells. They have different forms and colors, in which the green color is known as chloroplast. Meaning the green color in them is due to the chlorophyll present in the chloroplast.

The function of Plasmids –

  1. These are centers of chemical activity, during photosynthesis chloroplast synthesizes carbohydrates.
  2. It stores synthetic food.
  3. They contain pigments (chloroplasts).

Vacuoles –

These are hollow spaces in the cytoplasm. These are lined with a fixed membrane and filled with water and other soluble substances. These are more frequent in plant cells, their main function is to maintain proper internal pressure in the cell. However, food vacuoles and contractile vacuoles of protozoa help in digestion and excretion respectively.

Nonmembranous Organelles –

  1. Ribosomes – These are in the form of small granules, they are present freely in the cytoplasm. or attached to the membrane of the endoplasmic reticulum. They are made up of Ribonucleic acid (RNA) and Protein. These help in the synthesis of proteins from amino acids.
  2. Secretory granules or zymogen granules – These represent the initial steps in the production of enzymes and secretions.
  3. Orgastic substance – Various non-living substances are found inside the cytoplasm in the form of granules, spheres, or droplets. These are usually reserved foods.

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