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

Hello friends, today in this article we will learn about the cell, What is the cell theory? And knowing about it step by step well, then let’s start.

Definition of cell –

A cell is the smallest and complete explanation of the fundamental structure and function of all living organisms. It represents the unit of a structure function and heredity. it is also described as the unit of Protoplasm enclosed in a thin semi-permeable plasma membrane and has a nucleus within.

What is the cell theory?

The cell theory or Cell doctrine was formulated in 1838 by German botanist M.J. Schleiden For plants. it was extended to animals by German  Anatomist Theodor Schwann in 1839.

According to cell theory, “all living organisms ( Plants, animals, and microorganisms)   are sum total of activities and interaction of its cells”.

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 –

• Discoveries made at the beginning of the 19th century about the structure of plant and animal Tissue and cells by a number of zoologists and botanists contributed formulation of Cell Theory. the main contribution of cell theory is –
• Robert Brown 1831 discovered the nucleus.
• Dumortier (1831) studied cell division in algae, and Bon Mohl (1835) described cell division in animal cells.
• Purkinje and Von Mohl (1836 – 37) described cell Cytoplasm and established its importance in cell and Cellular activities.
• R. Virchow (1835)  expressed his famous amorphism  ‘Omnis cellulae a cellula’  which means all cells arise from pre-existing cells.  this established cell division Central phenomenon in the Reproduction of organisms.
•  Later on, after the development of Cell Theory Flemming (1880)  established that cells are in the continuity between the existing generation and the next generation by cell division.
• The discovery of reduction division at the time of gamete formation and the fertilization before the development of the embryo proved and supported cell theory and helped in the formation of the modern version of Cell Theory.

Essential Features of Cell Theory

The essential features of Cell Theory are-

• Cells are fundamental units of structure and function in all living organisms. it means a living organism is composed of cell or cells.
• Cells are units of heredity.
• New cells arise from pre-existing cells only.
• The activities of an organism are the outcome of the activities of its constituent cells.
•  Each cell has a life of its own in addition to its integrated rural in a multicellular organism.
• Thus a cell is the smallest unit of life. it is a unit of protoplasm surrounded by the plasma membrane. it consists of A nucleus (The controlling center)  and a cytoplasm or cytosol with the cell organelles ( the center of cellular activities).

Significance of Cell Theory

The modern concept of Cell Theory emphasizes The structural and functional relationship between the diverse living from bacteria to man.

All cells irrespective of their function and position have a nucleus embedded in the cytoplasm and are bounded by the cell membrane (unit in structure plan) and the same metabolic processes occur in all the cells primitive and specialized (unity of function).

This implies That all living beings have won monitored from the same primitive ancestral type that arose about 2 or  3 billion years ago.

Also, read – What is Taxonomy?

How many types of Cells?

There are two types.

1. Prokaryotic cells
2. Eukaryotic cells

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 having only a cell membrane. The membrane-bound organelles such as the endoplasmic reticulum, Golgi complex, mitochondria, chloroplast, and lysosomes are absent.

The double-formed nucleus is also absent. the hereditary material is a highly coiled circular chromosome lying naked in the cytoplasm. it is formed of DNA alone and is called nucleoid. bacteria blue green algae and pleuropneumococci Are prokaryotic cells.

Eukaryotic Cells –

These contain a true nucleus i.e., the hereditary material (DNA)  is associated with basic proteins and forms nucleoproteins,s and is separated from the cytoplasm by the nuclear envelope. the membrane-bound organelles are present.

From an evolutionary point of view, prokaryotes are primitive and eukaryotes have evolved from them.

Characteristics of a Prokaryotic cell –

 The main characteristics  features of  prokaryotic cells are:

1.  These are very small in size about the size of Mitochondria.
2.  These are without a Distinct nucleus i.e., The nucleolus and nuclear envelope are absent.
3.  Hereditary material form a single chromosome. the prokaryotic chromosome is a single circular chromosome formed of the double standard molecule of the DNA.
4. Basic proteins histone is absent.
5. The covering is the plasma membrane. it is different from the Eukaryotic plasma membrane in the following features:
   •  it does not have Sterols.
   •  The ratio of protein to phospholipid is High (2:1) in bacteria while It is low (1:1) in eukaryotes.
6. A cell wall or capsule may be present outside the plasma membrane but it is noncellulosic. It is formed of carbohydrates and amino acids.
7. Membrane-bound organelles, such as Endoplasmic reticulum Golgi complex lysosome mitochondria and chloroplast are absent.
8.  Plasma membrane in some cases is folded inward for mesosomes and chromatophores.
9. The inner surface of the plasma membrane contains enzymes of respiratory metabolism photosynthesis and lipid metabolism.
10. Prokaryotes ribosomes are 70S type.
11. Their cytoplasm does not exhibit Streaming movement.
12. Prokaryotic cells may contain flagella and Pili.
13.  They store polymerized fatty acids like beta-hydroxybutyrate, Glycogen, and phosphate granules.

Bacterial cell (Prokaryotic cell)

Bacteria cells are typical and most common prokaryotic cells.

Structure of bacterial cell

Size and shape –

The bacteria are non Cellular microscopic prokaryotic cells. the average 1.25 microns in diameter. The smallest bacterium is dialister pneumosintes about 0.15 to 0. 3 microns In length and the largest bacterium is spirillum volutans 13 to 15 microns in length.

These may be spherical in form (cocci) or rod-shaped (bacilli), comma-shaped (spirillum), or coiled and screw-like (spirochaetes).

Structure –

 A bacterial cell consists of the following structures:

 Capsule –  In a large number of bacteria is slimy capsule is present around the bacterial cell outside the cell wall. it is composed of polysaccharides and serves as an additional protective covering.

Cell wall – Bacterial cells are covered by a strong rigid cell wall. it is about 49 Tu 140 microns in thickness and lies just outside the plasma membrane. It is formed of carbohydrate Polypeptide (peptidoglycans) teichoic acid (Polyphosphate polymer) some lipids phosphorus inorganic salts and a derivative of glucose called muramatic acid.  It also contains an amino acid diaminopimelic acid.

Plasma membrane –  bacterial plasma membrane is similar to the plasma membrane of Eukaryotic cells. but it contains oxidative or respiratory chain enzymes. the bacteria plasma membrane is modified to form a varied structure. 

Mesosome –   tubercle bacillus and bacillus subtilis Plasma membranes are invaginated into whorls of convoluted membranes called mesosome of chondroid. These contain enzymes of the electron transport system. Mesosomes help in respiration secretion and synthesis of material for the cell walls; Receive DNA during conjugation and are sites of DNA replication enzymes. these also help in The distribution of chromosomes ( DNA) to daughter bacterial cells.

Desmosomes – In Thiovulvum majus the plasma membrane sinks deep into cytoplasm forming multi-layered structure a called desmosomes.

Cytoplasm –  The bacterial cytoplasm contains granules of fats glycogen and proteins and volutin granules, etc. Certain photosynthetic bacteria have chromoplast having bacteriochlorophyll or some other photosynthetic pigment (carotenoids).

The pigment and the enzymes are found associated with the internal membranes which are arranged either as lamellae or tubules or vesicles. These form the so-called chromatophores.

The cytoplasm contains 70S  ribosomes. these are also called bacterial ribosomes. these occur either freely in the cytoplasm or form polyribosomes.  Each bacterial ribosome consists of two subunits 30S and 50S.

Nuclear material or nucleoid genophore –

It consists of a double helical DNA molecule that forms a circular chromosome. it lies free in the cytoplasm and is not enclosed in a nuclear membrane. the clear area of cytoplasm along with the bacterial chromosome is called nucleoid or genopore. 

It is folded into several supercoiled loops.

 Plasmids –

 E.coli  and many bacteria highway a number of small double-stranded circular DNA molecules other than the bacterial chromosome. These are called plasmids.

These have individuality And replicate independently. sometimes plasmid DNA joins bacterial chromosomes and becomes their integral part. such a structure is called episome.

E. coli has three types of Plasmids –

F factors –  These are also called sex Factors. The bacterial cell has F-factor called F⁺ cells or Donor cells. The cells without the F factor are called F^– cells or recipient cells. F factor initiates conjugation between F⁺ and F^– cells.

R factors –  These contain genes that provide resistance to bacteria cells against antibiotics.

Col factors – The presence of these factors make the bacteria secrete colicins. These are antibiotics. the cells without col factor are destroyed by colicins.

Flagella –  Flagella are fine thread-like cytoplasmic structures. these come out of the cell wall and cell coat. These help bacterial cells in swimming. The bacterial flagellum is much simpler than the eukaryotic Flagellum.

Without membrane covering and is formed of single five deal about 120 to 150 angstroms thick. it is formed of rounded subunits of flagellin. These units are arranged in a spiral fashion. 

Based on the number and arrangement of flagella bacteria are classified into the following types:

Atrichous – Without flagella.

Example – Pasteurella

Monotrichous – With one flagellum only

Examples – Thiobacillus and pseudomonas. 

Lophorichous – With many flagella but all on one end of the bacterial cell.

Examples – spirillum volutans

Amphitrichous – Flagella are many and present at both ends of bacterial cells.

Example – Nitrosomonas

Peritrichous – Flagella are present all over the body.

Flagella – Some bacteria possess whip-like flagella. A bacterial flagellum is formed of a single fibril about 100 to 200 angstroms thick.  It arises from a basal granule and is composed of a protein flagellum.

Pili or fimbriae – Some bacteria possess Fine hair-like outgrowth from their surface. these are composed of the helically arranged subunits of protein-pilin and are called pili. These help in attachment.

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 cytosome of the cell is bounded by an invisible exceptionally thin lipoprotein covering called plasma membrane. It is formed of a double layer of liquid molecules sandwiched between two layers of protein.

It is perforated by minute pores that are concerned with the material entering for leaving the cell and allowed the passes of molecules of specific size to and fro. the plasma membrane act as selectively permeable.

Functions –

(I) Plasma membrane provide mechanical support and definite external form to the cell.

(ii) It farms a part  aap its living machinery because everything that enters of leaves the cell must pass through it.

(iii) It exerts  elective influence on the substance  entering or leaving the cell.

Surrounding the plasma membrane there may be a tough non living and supporting membrane or cell wall. it is lacking in animal cells but is thick and formed of cellulose in plants.

Nucleus – 

Structure –  The nucleus occurs as a central body in Eukaryotic cells. commonly it is oval spherical are discoidal but may be lobed or long and Ribbon like. It may be a single body formed of several separate pieces are in the form of chromatin granules in the cell substance.

The nucleus is surrounded by a distinct nuclear envelope  enclosing the nuclear or the nucleoplasm.  In prokaryotic cells the nuclear deoxyribonucleic  acid is not separated from the cytoplasm by nuclear membrane.

The nuclear envelope in similar to plasma membrane Instructure and function accept that it is formed of two to memory separated by perinuclear space.

The nucleoplasm is a clear and transparent Homogeneous liquid of variable consistency. it contains the nuclear reticulum and nucleolus.

Nuclear reticulum –  The nuclear reticulum is a fine network of chromatin three beaded with coarser chromatin granules. During cell division the chromatin condenses into threads or Rods known as chromosomes. 

These are important hereditary vehicles. the chemical constituents  of chromatin  are nucleoprotein which resolved into 4 major molecules a low molecular weight protein in histone, A complex deoxyribonucleic acid  (DNA)  and  ribonucleic acid (RNA).

Function – 

1. The nucleus control the metabolic activities of the cell.
2. It takes an active part during cell division.
3.  It transmit hereditary information from generation to generation.

Cytosol or cytoplasm –

 The interior of the cell is filled with a colour translucent liquid of variable consistency. it is known as cytoplasm. it consists of submicroscopic  fibrils,  endomembrane system membrane bound organelles and non membrane organelles structureless ground substance or cytoplasmic matrix. The various component of cytoplasm are –

Cytoplasmic fibril and cytoskeleton –

The cytoskeleton of cell is formed of thin, thick ok and intermediate filaments and micro tubeless.  These form in network. The cytoskeleton filaments remain interconnected by a network of fine thread like microtrabecular Lattice. This lattice is also interconnects Many membranous organelles.

Endomembrane system –

The endomembrane system of cytoplasm includes Endoplasmic reticulum Golgi  Complex and nuclear envelope. The membranous organelles are mitochondria chloroplast lysosome and peroxisome.

Endoplasmic reticulum – 

It consists of interconnected channels of physical and tumulus in the cytoplasm matrix. it is Present in almost every cell except the mammalian erythrocytes and the prokaryotic cells.

The tubeless are bounded by unit membranes and may even form interconnected channels. Some of them are connected with the nuclear membrane with plasma membrane will a few may be connected with both forming a direct  passage from outside  to the nuclear material.

The channels may be branched or unbranched. attached to some membranes are endoplasmic reticulum are ribosomes.

Depending upon the presence  or  absence of ribosome the endoplasmic reticulum has been differentiate into the Rough surface  endoplasmic reticulum (RER) and smooth surface endoplasmic reticulum (SER).

Function –

1. It provide mechanical support.
2. It facilities exchange of materials between the cell and its surroundings.
3. It assists in intracellular transport.
4. Ribosomes attaches to Endoplasmic  reticulum membrane Participate in protein synthesis.
5. SER is associated with the synthesis and storage of cholesterol and steroid hormones.

Golgi complex –

 The Golgi bodies are thick laminated plate like structure. Which body consists of flattened  sacs  or  cisternae  arranged parallel to each other. these are rich in fatty materials.

 Function –

1.  Golgi complex is associated with the cell secretion.
2.  This form lysosome and  peroxisome.
3. Then help in processing and packaging a product that passes through the endoplasmic reticulum.

Membrane bound organelles – 

Lysosome –

These are spherical vesicles,  filled  with hydrolytic  enzyme. these are bounded by a single membrane.

Function –

Due to the presence of hydrolytic enzymes lysosome help in the digestion of ingulfed  bacteria and  Cellular debris.

These are also called suicide  bags because if needed the lysosome burst out releasing the enzymes which cause autolysis of cell. 

These help in destroying diseased and exhausted cells. help in intracellular digestion of  ingested particle (intracellular digestion).

Peroxisomes –  

These are smaller than lysosome are enclosed by a single membrane and contain enzymes peroxidases that peroxides.

Mitochondria – 

The mitochondria are virtually either scattered single or in lumps in every types of living cell where they are In constant it  motion.  These range in size from 2 Micron to 5 Micron and in safe from spheres  to Rods or granules.

These are surrounded by a double lipoprotein membrane. the inner membrane in Thrown into Tu numerous convoluted  folds. These are  called cristae.  The central space is filled with  matrix. it contains oxidative enzymes.

Function – 

Inside mitochondria tricarboxylic cycle or Kreb’s cycle of respiration is completed and energy is released in the form of ATP by the oxidation of carbohydrate proteins and fats. Hence mitochondria are are commonly known as power house of cell.

Plastids –

The plastids  are of common occurrence in plant cells but limited in animal cells. These are of various forms and colour those green are known as chloroplast.

Function – 

1. These are centre of chemical activity. the chloroplast synthesis carbohydrates during photosynthesis. (chloroplast).
2. Store synthesized food.
3. Contains pigment (chromoplast).

Vacuoles –

These are hollow space in cytoplasm. these are line with a definite membrane and filled with water and other soluble substance . These are more frequent in plant cell.

The principal function maintain a proper internal pressure in the cell although the food vacuole and contractile vacuoles of protozoa aid in digestion and excretion respectively.

Nonmembranous organelles –

Ribosomes – 

These are tiny  granules.  these are present either free in the cytoplasm or Attached to the  membrane  of Endoplasmic reticulum. these are formed of ribonucleic acid and protein. These help in synthesis of protein from amino acid.

 Secretory granules are zymogen granules – 

 These represent preliminary stages in the production of enzymes and secretion.

Ergastic substances –

The various lifeless materials are found interspersed Inside the cytoplasm as granules spheres droplet.These are usually reserved for food. 

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