There are three large clusters of related organisms, called domains from which living things have evolved. These domains are archaea, bacteria and eukaryota. Archaea and bacteria are prokaryotes. They are small simple cells surrounded by a membrane and a cell wall. They have a circular strand of DNA that contains their genes.

Almost all of the everyday things that we see and notice everyday, belong to the third domain, which includes plants and animals. This domain is eukaryota. The cells in eukaryota are much more complex then prokaryotes. The DNA is linear and found within a nucleus. Eukaryotic cells also have mitochondria, which are their own personal power plants. Mitochondria not only produce energy but also hold the secret of the understanding of the eukaryotic cells evolution.

The eukaryotic cells ushered in a whole new era for life on earth by evolving into multicellular organisms. How did this evolution take place? Endosymbiosis seems to be the answer.

There is evidence, that millions of years ago, two separate prokaryotic cells were joined together in a symbiotic union. The mitochondrion seems to have once been free-living bacteria that was “eaten” or taken over by another cell engulfing it. It ended up staying as a permanent houseguest. The mitochondrion produced energy that the host cell needed and the host cell offered a protected nutrient rich environment for the mitochondrion. In this way, both profited from this relationship. This is an example of Endosymbiosis.

Endosymbiosis occurs when one cell takes up the residence inside of another cell and eventually evolving into a single cell.

There are plenty of evidences to support endosymbiosis. Although it was not until the 1980s when biologist Lynn Margulis’ book, “Symbiosis in Cell Evolution”, came out with the theory that was given any real consideration. After all, no one was around over a billion years ago to observe the start of endosymbiosis. Why should we believe that a mitochondrion used to live free in its own right?

The most important piece of evidence is the similarities between mitochondria and prokaryotes. They both have their own cell membranes. Although mitochondrion has a smaller DNA genome, it is circular like that in bacteria. Also, mitochondria pass its DNA to its offspring separately from the host cell’s genome in the nucleus. The way mitochondria multiply is by breaking itself in half. Every new mitochondrion has to be produced from parent mitochondria. If the mitochondrion is removed from the host cell then the host cannot build new mitochondria. Bacteria also reproduce by breaking itself in half.

Mitochondria look a lot like tiny little bacteria living inside of eukaryotic cells. Endosymbiosis seems to be the best explanation of eukaryotic cells. The scientific community has decided to support Lynn Margulis’s theory.

Endosymbiosis could also apply to other cellular organelles, such as chloroplasts. Chloroplasts are tiny little organelles living within plant cells that convert the energy from sunlight into sugars. Evidence suggests that these chloroplast organelles were also free-living bacteria once.

All eukaryotes have mitochondria suggesting that the event that triggered the endosymbiotic relationship happened many billions of years ago.

The origin of eukaryotic cells comprising of mitochondria and plastids or chloroplasts is the basis for the Endosymbiotic theory. This theory states that these organelles were once separate prokaryotic organisms. These separate organisms were taken into the eukaryotic cell as endosymbionts. Chloroplasts developed from cyanobacteria and mitochondria developed from proteobacteria.

This theory was initially put forth in 1905 by russian botanist Konstantin Mereschkowski. Andreas Schimper, another botanist had observed that the division of chloroplasts in some green plants resembled that of some free-living cyanobacteria in 1883. And in the 1920s, Ivan Wallin extended the idea of an endosymbiotic origin to mitochondria. All these theories were dismissed. It wasn’t until 1981 that Lynn Margulis popularized the theory in her book, “Symbiosis in Cell Evolution”.

The way the things are connected, categorized and named or the taxonomy and phylogeny boils down to questions of who beget who and who was the original begetter. Sometimes, this is a simple question in which gradually changing conditions create gradually changing adaptions; sometimes it is much more complex. Like dramatic changes in conditions create dramatic and significant evolutionary change. Some time a fortuitous event can make a sudden splash in the evolutionary pond. With the origin of eukaryotes, there appears to have been a big splash. The splash is endosymbiosis. This is a condition where one organism lives inside of another.

The key of the success of this partnership is the mitochondrion and the chloroplast. These are the two powerful and mutually supportive organelles.

The mitochondrion is an oxygen consuming organelle. It efficiently extracts energy from carbon sources like glucose and produces water and carbon dioxide in the process.

The chloroplast, however consumes water and carbon dioxide. It extracts energy from light and turns it into the chemical energy of glucose and releases oxygen at the same time.

The Endosymbiotic theory states that these two organelles were prokaryotic cells at one time. These cells were once living inside a larger host cell. They may have once been parasites or eaten by a larger cell and escaped being digested.

However, it happened originally that these cells must have quickly become willing captives to a happy warden. The prisoner or chloroplast provides crucial nutrients, while the mitochondrion provides protection and a place to live.

There are several lines of evidence that supports this theory. To start with endosymbiosis happens elsewhere in biology. Mitochondria and chloroplasts are similar in structure, biochemistry and genetic make up to other prokaryotes. The fact is that these prokaryotes have their own genetic information and separate from each other argues favorably for the theory.

It has been argued that because all eukaryotes have some sort of mitochondria but only photosynthetic eukaryotes have chloroplasts. Endosymbiosis must have occurred twice. First a larger cell took in a heterotrophic prokaryote. Then over time, the prokaryote evolved with the host cell, where it eventually became similar to a mitochondrion. After that, a photosynthetic prokaryote was taken in by a mitochondrion-containing cell. This form of eukaryote origin is known as serial endosymbiosis.

A cell contains protoplasm which is enclosed in a membrane called plasma membrane. The protoplasm has two parts cytoplasm and nucleus. Cells are mainly divided into the two main categories – the prokaryotic cell and the eukaryotic cell. The main difference between thise two cell types is that in prokaryotic cells the nuclear membrane is absent while in the eukaryotic cells, the membranous nuclear covering is there.

In the structure of prokaryotic cells, there are mainly three distinctive regions – bacterial flagella and pili,bacterial cell wall with cell envelope capsule and the cytoplasmic region with the cell genome mainly the nucleiod. The flagella and the pili actually forms in the surface of cell wall, these kind of structures helps in the locomotion and the communication between the cells , these are made up of proteins. The bacteria cell is covered by cell envelope which consists of both the cell wall and the plasma membrane. These contain the polysaccharides and sometimes also made up of protein and lipids. This kind of cell membrane provides the rigidity to the cells and used to give protection to the inner part of the cell from the surrounding environment. Though incase of mycoplasma and thermoplasma prokaryotes, the cell wall is not seen. The prokaryotes cells have peptidoglycon which acts as an extra barrier for protection. This kind of cell walls helps the prokaryotic cells from bursting when they are in contact with the hypotonic environment due to osmotic pressure. In bacterial prokaryotic cells, the nuclear material or the cell genome called as DNA and ribosome are concentrated in the cytoplasmic region .Though here formation of nucleus is not taking place but the DNA is concentrated to the nucleoid .

Eukaryotic cell is composed of a jelly like translucent protoplasm bound by the plasma membrane .The cytoplasm and the nuclueus can be easily identified. Cytoplasm has number of organelles, the structure and functions are also different .The plasma membrane of eukaryotic cell is typical protein-lipid–protein structure , the various contents and organelles are being separated from the surrounding environment by the plasma membrane. Here carbohydrate also has been found. The cell membrane structure actually follows the fluid mosaic model. In the nucleus, group of RNA located in the nucleolus which is responsible for the synthesis of ribosomes and proteins. The Chromatin material is composed of DNA and protein which contains the hereditary in formation to be passed on to the next generation of cells and thisis also necessary for cellular functions. The Eukaryotic DNA is more organized and stays in the chromosome which is associated with the protein histone.

The golgi apparatus which is associated with the production of secretions are stored in typical membranous stack, they process and diapatch the protein through the cells in and out process. The nuclear membrane is of typical membrane structure which separates the nucleus from the cytoplasm. The Endoplasmic reticulum are found in folds of membrane which forms large network of branching canals and sheet like structure , they form the surface for the chemical reactions and mechanical strength. On the surface of the Endoplasmic reticulum, the ribosome resides like small particles. The vacoules are the membrane sac which is in large size in the animal cell, they takes the maximum portion of the cell volume. They are used to store water and cell products and often called as the container of materials. The lysosomes are found in cells which contains the enzymatic ingredients to engulf and digest, sometimes it engulfs the other organelles that is why its called as suicidal bag, they are capable of digesting different carbohydrate, protein and polysaccharides. During the endocytosis process, lysosome takes part in digestion of the intracellular parts.

The mitochondria is found in the cytoplasm which involves in energy synthesis and works as the site of cellular respiration , it is called as power house of the cell. The nucleus is the largest structure which is just in the centre of the cell; this nucleus contains the chromosome which is the genetic material of the cell. Many eukaryotic cells like the protozoa is having the flagella and cilia for the locomotion. Chloroplast is also present in the cell as the double membrane container of chlorophyll which is the site of photosynthesis.

Every living thing on the planet are made of cells, including people, animals, plants, mushrooms, bacterias etc… Every cell is similar to other cells, and have similar organelles, and every cell function on the similar way. However there are some differences between them.
There are two big groups of cell types: prokaryotic cells and eukaryotic cells. Prokaryotic cells are much simpler then eukaryotic ones.
Although all cells have DNA as genetic material, they make proteins on same or similar ways, and have same metabolism processes on basic level, there are many variations of cells.
First if we look at cell size, it can be so different, from invisible bacteria cells, to ostrich egg which can be 15 cm long and weight about 1.4kg, or nerve cells in especially long animals, such as the Giant Squid and Colossal Squid, which may have nerve cells as long as 12 m.
Cells can differ in their function too, some can produce food only, another can be for reproduction…

Based on structure, hereditary material, and chemistry we can split all cells into three families:
1.Eucarya – this are cells of humans, animals, plants, fungi and protists
2.Bacteria – Single-celled microorganisms They can be human friends but also can be worst enemies, like those that cause serious diseases.
3.Archaea – Single-celled microorganisms. They can be found in all types of environment, including ones with very extreme living conditions – like on extremely hot or cold places.

But more often all cells are split to prokaryotes and eukaryotes. Therefore, Eucarya belongs to eukaryotes and Bacterias and Archaeas belongs to prokaryotes.

The eukaryote word consists of two words. “Eu” which mean true, and “karyote” which means seed, so eukaryote can be translated as true-seed. The reason for this name is because eukaryotes’ cells have nucleus which are enveloped by membrane, and nucleus contain cell DNA . Also eukaryotes’ cells have organelles which are enclosed by membrane, and every organelle perform some function which help cell to survive. Because this eukaryote cells are much bigger then prokaryotic cells.

On the other side, word prokaryote is coming from words “pro” which mean prior or before, and “karyote” which means seed, so we can translate them as before-seed. The reason for this name is because prokaryote doesn’t have nucleus, and their genetic material (DNA) is contained in cytoplasm. Their organelles are not enclosed by membrane, and for this reason, prokaryotes are few times smaller then eukaryotes.

It is believed that prokaryote cells are formed first from molecule, because they are much simpler and smaller then eukaryote, but this is not confirmed.

The Prokaryotic and the Eukaryotic cells are having the generic material, the DNA in common. Apart from this common DNA , both the Prokaryotic and the Eukaryotic cells have RNA also. This ribosome are the only few protein factories which is present in the cells and they are non membrane bound organelle. Though mainly in the Prokaryotic cells, the Ribosomes are found to be floating freely, but in eukaryotic, the Ribosome actually adheres to the endoplasmic reticulum. The ribosome actually makes up the protein. Both the Eukaryotic and the Prokaryotic cells are having cell membranes , both of them consists these bilayer of Lipid made plasma membrane which consists the boundary both inside and outside the cell.

The basic chemical structure of the Prokaryotic and Eukaryotic cells are very same, each of the cells are made up of carbohydrate, fat , proteins, vitamins , nucleic acid and minerals. Both the prokaryotic and the Eukaryotic cells regulate perfectly specially the wastes and the nutrients that actually leave and enter the cell. Prokaryotic and eukaryotic cells both of them have the supportive cytoskeleton within the cell. This fact been recently seen in the Prokaryotic cell. In both Prokaryotic and Eukaryotic the basic of life process mainly the Photosynthesis and reproduction been seen. Both the types of cells require supply of appropriate energy for the survival. Each Prokaryotic and Eukaryotic cells consists of thin extensions of the cell membrane which is supported by the elements of the cytoskeleton, in the Eukaryotic cells which includes flagella and cilia, in case of Prokaryotic cells flagella, pili and fimbriae which helps in the motility or the movement of materials in the cell. Both the prokaryotic and Eukaryotic cells are having the fluidized matrix called as cytoplasm which actually fills the cell. Both the Eukaryotic and the prokaryotic cell have chemical noses to help them and keep them update what type of reactions occurs around the surrounding environment. The basic metabolism process is common in both the Eukaryotic and Prokaryotic cells. There are few prokaryotic and Eukaryotic cells which have glycocalyx , these are the sticky sugar based structure which helps the cell to attach to one another and stick to the wall which gives some protection to the cell also, though both the cells glycocalyx made up of different material. In Eukaryotic cells, where there is cell membrane, the glycocalyx is absent. Both the Prokaryotic and the Eukaryotic are amazingly diverse in form.

Major prokaryotic cells and few of the eukaryotic have the strong cell wall structure which actually helps the cell to burst out in the hypotonic environments. The Prokaryotic actually have cell envelope which is consisting of cell wall and plasma membrane just similar like Eukaryotic , in function wise both are same just there is little amount of difference in the set up. The Eukaryotic can move by the help of flagella or cilia just like the prokaryotic cells , just the flagella are more complex in the Eukaryotic than the prokaryotic cells. In both the Eukaryotic cell and Prokaryotic cell , the electron transport system is located ,but in the Prokaryotic it is located in the plasma membrane and in the Eukaryotic cell, it is situated in the inner wall of the mitochondria. Photosynthesis takes place in both the Eukaryotic cells and Prokaryotic cells but takes place in different places in both the cells, in Eukaryotic it takes place in the chloroplast and in the Prokaryotic cells it takes place in the folding or extensions of the cytoplasmic membrane. In both type of cells, the cell division takes place, Eukaryotic cells divides by meiosis and mitosis but in case of Prokaryotic cells, the cell division takes place by the help of binary fission. In both the Prokaryotic and the Eukaryotic cells , the cytoplasmic membrane is a fluid phospholipid layer, just the sterols are absent in Prokaryotic cells which is seen in the Eukaryotic cells. This kind of sterols like molecule though can be seen in many bacteria which is called as Hapanoids. In case of the Prokaryotic cells, the reproduction is always asexual but in case of Eukaryotic cells, the similar asexual reproduction is seen, though there is bisexual reproduction also.

In prokaryotic cell, the genetic material is not being enclosed by a particular membrane to form a nucleus but it lies freely in the cytoplasm part.  Here in the prokaryotic type, cell lacks all membrane bound organelles.  But in the eukaryotic cell type, the cell has a membrane bound nucleus.  It has a number of membrane bound organelles.  In the prokaryotic cell, the DNA is naked but in case of eukaryotic cell, the DNA is combined with the proteins and the eukarotic cells can obtain numerous linear chromosomes, mainly in human body it is of 46 chromosomes.

Chromosomes are single and circular in case of prokaryotic cells but in case of eukaryotic cells, the chromosome is multiple.  Nucleolus is absent in prokaryotic cells but it is present in eukaryotic cells.  In prokaryotic cells, the cell division is done by the amitosis process, but in case of erokaryotic cells, the cell division is done by the mitosis or meiosis process.
The cell wall is non cellulosic in prokaryotic cells but cellulosic in the eukaryotic cell, mainly in plants.  The endomembranes are absent in the prokaryotic cells but it is present in the eukaryotic cells.
The ribosome is of 70s (50s+30s) in case of prokaryotic cells but it is of 80s(60s+40s) in eukaryotic cells. The organelles is not surrounded by an envelope in the prokaryotic cells but in eukaryotic, the organelles i.e nucleus, mitrochondria, chloroplasts are bound by envelope. The cytoplsamic inclusions like golgi apparatus, endoplasmic reticulum, vacuoles, lysosomes are absent in the prokaryotic cells where as they are present in the eukaryotic cell. The locomotion is of single fibril and flagellum in case of prokaryotic cells but the cilia and flagella locomotion is seen in the eukaryotic cells. The exocytosis and the endocytosis is absent in the prokaryotic cells but those process are seen in the eukaryotic cells.
The Photosynthesis process takes places on the membranes which shows no stacking as there is no chloroplasts in theprokaryotic cells  but in the eukaryotic cells the photosynthesis process is taken place in the chloroplast  which are stacked.
The respiration process takes place in the cytoplasmic membranes in the prokaryotic cells  but the process does in the mitochondria for aerobic respiration. Mainly the prokaryotic cells are much smaller when it is compared with the eukaryotic cells . Prokaryotic cells mainly measured within the 10 micrometer range but in case of eukaryotic cells, the size is much more, above the 10 micrometer to 1 millimeter. Eukaryotic cells are much advanced may have risen from the prokaryotic ones. The centromes and the centrosomes are present in case of some eukaryotic cells mainly in animal and some fungal cells , sometimes its been seen in plants , fungi and some species of algae but in prokaryotic cells these organelles are not present.
The mictotubules is not present in the prokaryotic cells but its seen in the eukaryotic cells. The cell division mode in the prokaryotic cells is quick, takes about 20 -25 minutes but in the eukaryotic cells , the process is rather slow, takes  about an hour. The cell division mode is by the formation of septa in case of prokaryotic cells but in the eukaryotic cells its done by mitosis or cytokinesis. The recombination technique in case of prokaryotic cells by conjugation but in the eukaryotic, the conjugation is done by the meiosis and cytokinesis.  The use of actomysin for the movement is seen in the eukarotic cells but its not seen in the prokaryotes.

The DNA amount is less in the prokaryotic cells but in the eukaryotic cells the amount of DNA is more , specially in haploidity.  Golgi apparatus actually is seen in the eukaryotic cells which secretes substances. The prokaryotic cells are actually simple than the eukaryotic cells   at various levels.  Eukaryotic cells have higher degree of organization when it is compared with the prokaryotic cells.  Eukaryotic cells are thought to be multicellular co-ordinate groups mainly in the organs and tissues but recently in some prokaryotic cells in colonial mass its seen.  The prokaryotes are mainly of two groups, the eubacteria and the archeabacteria  but the eukaryotic cells are seen in metazoa and protozoa also. The glycocalyx is present as a capsule or in the form of slim layer in the prokaryotic cells and in case of eukaryotic cells , it is present in those cells which lacks a cell wall.

Cell is the fundamental structural and functional unit of all living organisms. In living organisms, the metabolic reactions essential for life can occur in a delicately balanced environment that is different from any found in the non-living object. Cells are the life supporting chambers which have such a special environment. In this controlled environment of a cell, all life activities can occur in a desired manner. Prokaryotic means which is devoid of nucleus.

A typical cell is composed of following three elements:

Cell Membrane – It is a lipoprotein coat that serves as a selective barrier to the passage of substances into and out of the cell, and thereby keeps the cell distinct from the environment.

Cytoplasm – It is a watery solution containing controlled concentrations of inorganic and organic compounds.

Nuclear (Genetic) Material – It consists essentially of DNA. It serves to store and transmit information to direct the synthetic activities of the entire cell.

In addition to above three elements, the cells of protists, fungi, plants and animals have in their cytoplasm many membrane bound compartments called organelles which include nucleus, mitochondria, chloroplasts, lysosomes, vacuoles and endoplasmic reticulum.

Two Main Types of Cells –

There are two basic types of cells – prokaryotic cells and eukaryotic cells. This classification is based on three main features: (i) organization of biomembranes, (ii) variety of cytoplasmic organelles, (iii) complexity of nuclear material.

DIFFERENCE BETWEEN EUKARYOTIC AND PROKARYOTIC CELLS –

Prokaryotic cells are believed by the scientists as the very oldest and first form of life on the earth. They are the morphologically the most primitive cells. In the category of size of the cells, prokaryotic cells are ten times smaller than eukaryotic cells. Prokaryotic cell is a single membrane system whereas eukaryotic cell is a double membrane system. Cell membrane bears respiratory enzymes in prokaryotic cells but in eukaryotic the cell membrane lacks respiratory enzyme. In prokaryotic cells, cell membrane helps in distributing replication products of chromosomes between daughter cells but in the case of eukaryotic cells cell membrane has no role in distributing replication products of chromosomes between daughter cells. Cytoplasm lacks organelles such as ER, mitochondria, Golgi apparatus; centrosome etc, except ribosomes in prokaryotic cells but in eukaryotic cells cytoplasm contains such organelles. Ribosomes in prokaryotic cells are of type 70S and in eukaryotic cells it is of type 80S.Cell cycle is short in prokaryotic cells ,taking about 20-60 minutes to complete but it is very long and time taking process in eukaryotic cells ,taking about 12 -24 hours. In prokaryotic cells, there is no process comparable to meiosis, gamete formation or true fertilization. In eukaryotic cells meiosis, gamete formation and true fertilization occur in most cases in sexual reproduction. Prokaryotic cells contain cell wall which is made of the compound peptidoglycan which is composed of both amino acid and sugar but in eukaryotic cells cell walls are present, not composed of peptidoglycan.

The flagella in prokaryotic cells are diverse in both structure and composition from the flagella found in eukaryotic cells. Flagella are the cellular parts that present in the cell that used to help the cells move and make them motile. The flagella in eukaryotic cells are composed of several filaments and are more complex in nature than the flagella found in prokaryotic cells.

The process of synthesis of protein takes place in cytoplasm only in prokaryotic cells but in eukaryotic cells, protein synthesis occurs in the cytoplasm, mitochondria and plastids. Prokaryotic cells are haploid but eukaryotic cells are typically diploid, but in some cases haploid cells also occur. The process of transcription and translation, both occur in the cytoplasm only in the case of prokaryotic cell but in eukaryotic cell, transcription and translation occur in nucleus and cytoplasm respectively. Eukaryotic organisms can either be single-celled or multi-celled, but prokaryotic cells are only single celled. All bacteria and many single celled organisms are categorized under the prokaryotic cells and animals, plants, fungi and protists fall under the category of eukaryotic cells.

BESIDES DIFFERENCES THERE ARE SOME familiar THINGS IN PROKARYOTIC AND EUKARYOTIC-

• Both have DNA present.
• Both contain RNA.
• Both have ribosome.
• Both require a supply of energy.
• Both are composed of similar basic chemicals which include carbohydrates, proteins, nucleic acid, minerals, fats and vitamins.
• Both are covered by a cell membrane.
• The entering and leaving of the cellular liquid which contain nutrients and wastes control by both the cells.