Introduction:

Cell is the basic unit of living thing. It simply put, the smallest independently functioning unit in the structure of an organism, usually consisting of one or more nuclei surrounded by cytoplasm and enclosed by a membrane. Cells are highly organized to form a simple or complex organism. In order words the nature and form of the organisms is structurally and chemically determined by cells. The two types of cells that form living things are the prokaryotic and Eukaryotic cells.

Levels of cell Organization in Living Things:

Cell organization-Physiology - science which explains how organisms function and survive in constantly changing environments

Chemical Level: contains all chemical substances essential for life ( for instance, small portion - a heme group - of hemoglobin molecule, or amino acid); together form next higher level

Cellular Level: Cells are essential structural and functional units of organism's body. Cellular arrangement is of 2 kinds; Prokaryotic cells and Eukaryotic cells, and there are several different kinds of cells (like nerve, muscle, blood, and so on) found in body of living organisms.

Tissue Level: The tissue is group of cells which perform specific function and basic kinds of tissues in human body comprise nervous, epithelial, muscle, and connective tissues

Organ Level: The organ comprises of 2 or more tissues which carry out particular function (like stomach, heart, liver, and so on)

Types of cells:

Cells in our world come in two essential kinds, prokaryotic and eukaryotic. "Karyose" comes from the Greek word that means kernel, as in kernel of grain. "Pro" denotes "before," and "eu" denotes "true," or "good." So "Prokaryotic" denotes "before the nucleus (cells, such as bacteria, contain no nucleus), and "eukaryotic" denotes "having the true nucleus. Prokaryotic cells have no nuclei, whereas eukaryotic cells do contain true nuclei.

Evolution of cells:

Origin of cells has to do with origin of life that started with history of life on Earth. There are many theories about origin of small molecules which could lead to life in the early Earth. Another is that they were developed at deep-sea vents. They were synthesized by lightning in the reducing atmosphere; though it is not clear if Earth had such an atmosphere. There are fundamentally no experimental data stating what first self-replicating forms were. RNA is normally thought to be the earliest self-replicating molecule. Cells emerged at least 4.0-4.3 billion years ago. The significant characteristic of cells is cell membrane, made up of bilayer of lipids. Early cell membranes were possibly more simple and permeable than modern ones, with only single fatty acid chain per lipid. Lipids are known to impulsively form bilayered vesicles in water, and could have preceded RNA.

Evolutionary relationship of eukaryotic and prokaryotic cells:

Prokaryotic and eukaryotic cell are all related with larger size and greater complexity. This leads to the significant observation. Though, they are clearly more alike than different, and they are evidently evolutionarily associated to each other. Eukaryotic cell is evidently developed from prokaryotic cell.

Within eukaryotic cells there is fascinating organelle known as mitochondrion. And in plant cells, additional family of organelles knew as plastids. Mitochondria and chloroplasts roughly certainly contain similar evolutionary origin. Both are pretty clearly descendants of independent prokaryotic cells that have taken up permanent residence within other cells through the renowned and very common phenomenon known as endosymbiosis. Similarities in appearance between the structures are pretty clear. Mitochondrion is double-membrane organelle, with the smooth outer membrane and the inner membrane that protrudes in interior of mitochondrion in folds known as cristae. This membrane is similar in appearance to prokaryotic plasma membrane with its mesosomes.

Both mesosomes and cristae are utilized for same function: they play significant role in aerobic cellular respiration where much higher energy yield is generated than anaerobic respiration. Biologists think that mitochondrion evolved from once-independent aerobic prokaryotic cell that entered into endosymbiotic relationship with the larger, anaerobic cell. DNA has a number of genes that are essential to make materials required for aerobic cellular respiration. Certainly, mitochondria are totally dependent on cells in which they exist in.

Eukaryotic cells:

Eukaryotic cell appears to have evolved from symbiotic community of prokaryotic cells. DNA-bearing organelles such as mitochondria and chloroplasts are approximately certainly what remain of ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, where rest of cell appears to be derived from ancestral Archaean prokaryote cell -theory termed endosymbiotic theory. Sex, as stereotyped choreography of meiosis and syngamy which persists in almost all extant eukaryotes, may have played the role in transition from prokaryotes to eukaryotes. Eukaryotic cells are approx 15 times wider than typical prokaryote and can be as much as 1000 times greater in volume. Major difference between prokaryotes and eukaryotes is that eukaryotic cells has membrane-bound compartments in which particular metabolic activities occurs. Most significant among these is cell nucleus, membrane-delineated compartment which houses eukaryotic cell's DNA. This nucleus provides eukaryote its name that means true nucleus. Other differences comprise:

• Plasma membrane looks likes that of prokaryotes in function, with minor differences in setup. Cell walls may or may not be there.
• Eukaryotic DNA is organized in one or more linear molecules, known as chromosomes, that are related with histone proteins. Few eukaryotic organelles like mitochondria also have some DNA.
• Many eukaryotic cells are ciliated with primary cilia. Primary cilia play significant roles in chemosensation, mechanosensation, and thermosensation.
• Eukaryotes can move by using motile cilia or flagella. Flagella are more complex than those of prokaryotes.

Characteristics of Prokaryotic Cells:

• Prokaryotes, that comprise all bacteria and archaea (archaebacteria), are simplest cellular organisms.
• Prokaryotic cells are basically different in internal organization from eukaryotic cells. Notably, prokaryotic cells lack the nucleus and membranous organelles.

Characteristics of Eukaryotic Cells:

• Eukaryotic cells have membrane-bound nucleus and several membrane-enclosed organelles (like, lysosomes, mitochondria, Golgi apparatus) not found in prokaryotes.
• Prokaryotic cells are basically different in internal organization from eukaryotic cells. Particularly, prokaryotic cells lack the nucleus and membranous organelles.
• Nucleus is bounded by nuclear envelope, double membrane with several nuclear pores by which material enters and leaves. Fungi, animals, plants, and protists are all eukaryotes. Eukaryotic cells are more complex than prokaryotic cells and are found in the great several different forms.

Similarities of prokaryotic and Eukaryotic cells:

i. They carry out most of the same types of functions, and in some ways.

ii. Both are surrounded by plasma membranes, filled with cytoplasm, and

iii. They cytoplasm of both cells types are loaded with small structures known as ribosomes.

iv. Both have DNA that carries archived instructions for operating cell.

Differences of prokaryotic and Eukaryotic cells:

i) Eukaryotic cells are much larger and much more complex than prokaryotic cells.

ii) Eukaryotic cells contain true nucleus, bound by double membrane. Prokaryotic cells have no nucleus.

iii) Eukaryotic DNA is linear; prokaryotic DNA is circular (it has no ends).

iv) Eukaryotic DNA is complexed with proteins known as histones, and is organized in chromosomes; prokaryotic DNA is naked, meaning that it contains no histones related with it, and it is not formed in chromosomes.

v) The eukaryotic cell has a number of chromosomes; a prokaryotic cell has only one circular DNA molecule and varied assortment of much smaller circlets of DNA known as plasmids.

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