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]The cell is the smallest structural and physiological unity of living organisms, and it is able to growth,  differentiation, reaction, movement and reproduction.

Said in simplest words, the cell can be seen as a little but intricate chemistry laboratory, in which a lot of reactions take place in order to build structural macromolecules, reactive organic molecules and energy life. Some of the most important biochemical reactions are the DNA reproduction ( the custodian-molecule of genetic information for a  living organism), protein synthesis and the production of chemistry energy life from organic substrates.

One cell can live alone and compose a single organism ( unicellular), or live in more cell aggregates by constituting a multicellular organism.  In this latter case, we find differentiated cell in tissues. Every tissues is made up different cells, in shape and dimension, which perform detailed jobs.

Each cells own a common structure: a thin double phospholipid layer membrane, containing a high amounts of proteins (plasma membrane) which divide the cell from outside place and surround the cytoplasm, a solution between water and other complex macromolecules in which metabolic processes go on. The solution is gelatinous.

The cytoplasm fill up the cell, in which there are immersed various organelles, whose number depend on the cellular complexity.

For the first time, in 1665, H. Hooke saw a piece of cork at microscopy, and noticed that cork is composed of little lockups, which were being called by Hooke “cells”. Actually, Hooke observed dead cell, in which just the cell wall ( presents only in plant cells) was remained. More later, microscopy underwent to a development, and scientists discovered that every plants were composed of cells. Just in XIX century was ascertained that also animals were cell masses and, in this way, the organisms cellular theory were published, according to every living organisms are composed of cells.

Towards the middle of XX century, with the electron microscopy, scientists succeeded to watch subcellular structure and dividing the cell into two complexity levels: prokaryotic cell ( the simplest) and that one eukaryotic ( the most complex ).

Prokaryotic cell structure. Prokaryotic cell is represented by bacteria, unicellular organisms.

Prokaryotic cell is the most primitive cell ( mainly the cell of Archaebacteria), and it’s featured by the lack of specific organelles and, above all, there isn’t even a real nucleus. In fact, the genetic material ( DNA) is found in direct contact with cytoplasm, joined with a particular place in cell membrane called mesosome. Bacterial DNA has got a spiral shape. Cyclic portions of DNA may be found in the cytoplasm, called plasmids, and keep not important information for the cell life, but they can be shared between other bacteria or enter in the nucleotide. In this way, we are present to a genetic change which strengthen the cell with new genes, which can contribute to new enzyme production against antibiotics ( drug resistance phenomena). The passage of genetic material is permitted by the pilus, thinner than flagella, which form themselves from membrane cell.

The only organelles owned by prokaryotic cells are the ribosomes. Ribosomes are the particles made up ribosomal RNA and are the place in which occurs the translation of genetic code to proteins.

Prokaryote’s membrane cell outwardly covered  by a cell wall which gives the shape to the cell and guard it against osmotic pressure. Cell wall has a glycoprotein nature, typically constituted by peptidoglycans. ]

 A further defense structure is the capsule which is needed to mechanic protection against the phagocytosis.

Cell may have appendix outside the membranes which are used to move in the environment. These appendixes are called flagella.  The rapid movements of flagella are caused by  protein microtubules, which utilize ATP energy molecule to slide over one another.

Another locomotors apparatus are cilium, shorter than flagella, which surround the cell’s body.

Prokaryotic cells are large 1 micron, smaller than eukaryotic one, and their facility to survive in inhospitable environments, fast growth and reproduction had gave to bacteria the necessary features to became a ubiquitous microorganisms.

Eukaryotic cell structure. Immediately, the enormous difference between the two cell types is in the organization. Eukaryotic cell has a lot of organelles in the cytoplasm, each one with a precise job to do. Every organelle is protect by a membrane and is characterized by a own shape and dimension.

The particularity of eukaryotic cell, more evolved than prokaryotic cell, is the presence of the nucleus well visible, bordered by a double membranes. As for prokaryotic creatures, also eukaryotic cell has a membrane made up a double layer of phospholipids. The difference is the superior amounts of sterols in eukaryotic cells, which give rigidity to cell. The ordered structure and very complex allows to membrane many works, such as, absorbing selectively nutrients from outside, ejecting waste materials and supplying an anchorage point for the communication with other cell. Glycoproteins molecule are found in the extern layer of the membrane by composing the  glycocalyx, necessary to recognition between identical cells in a tissue. In the end, membrane is also composed by tiny quantities of cholesterol, involved in the cellular homeostasis.]

Genetic material is kept in the nucleus, in which is commonly viewed as chromatin, that is a mixture composed by DNA, RNA and specialized proteins. At the time of reproduction, chromatin condenses in bigger systems, called chromosomes, easily watched through microscopy.

Moreover in the nucleus, there are one or more nucleoli, which produce ribosomal RNA.

Nucleic membrane is in the cytoplasm. Cytoplasm contains encompasses all the organelles and give a material support to macromolecules in the cells, such as proteins. This fibers are joined together in a sort of web to form: microtubules ( present in flagella) which control the reproduction processes; microfilaments made up myosin and actin and allows cellular contraptions; intermediate filaments made up several protein subunits. This fibers compose the cytoskeleton.

Next to nuclear membrane, there is the endoplasmic reticulum, consisting in an elaborate and branched membrane system, which give form to flattened vesicles. Endoplasmic reticulum in majority occupies the cytoplasmic space. If these membrane are joined with ribosomes, endoplasmic reticulum is defined as rough, thus involved  in proteins synthesis. If there aren’t ribosomes in the surface, endoplasmic reticulum is called as smooth. Both the rough and smooth reticulum are involved to phospholipids synthesis and many modify reactions post-transcriptional of proteins, for example the partial hydrolysis, glycosylation and sulfation.  

Proteins which must be expelled from the cell are carried through the reticulum into Golgi apparatus.

Discovered by C. Golgi in 1898, Golgi apparatus is organized in membrane groups which form thanks. The thanks makes thin vesicles which move on to the cell membrane in order to reverse their content outside the cell.

The most important job of this apparatus consists in many modify post-transcriptional reactions started in endoplasmic reticulum, and in the correct positioning of cellular proteins  and structural macromolecules.

Lysosomes are spherical vesicles surrounded by a membrane and contain hydrolytic enzymes. These enzymes are used to cell to demolish phagocytized materials and unused protein portions of the cell. Similar organelles are the peroxisomes, containing enzymes which decay fatty acids and amino acids. The reaction bring to hydrogen peroxide production, toxic for the cell. Through the catalysis, this latter substance is split in oxygen and water.

The centrioles are small cylindrical organelles which are found in all eukaryotic cells. Centrioles are constituted by nine triplets of microtubules. From these organs, the mitotic spindle take place and is controlled by centrioles.

Seat of life energy are the mitochondria, one of the most numerous organelles in cell. They are covered by a double membrane. That one inside the structure is bend on itself and forms several cristae, which are immerged in a gelatinous substance called matrix. On the cristae there are breathing enzymes (cytochromes) which oxidize organic nutrients by molecular oxygen, producing water and carbon dioxide.

The high quantities of produced energy is transferred into phosphorus’ chemical bonds in ATP, the energy reserve of the cell.

The mitochondria are rich in ribosomes which have an own DNA and RNA, and they divide during cellular reproduction. This fact has led to the affirmation that mitochondrial are semiautonomous organelles. From there, the hypothesis that mitochondria formation was born from a symbiosis between a host cell and parasites organisms, a reciprocally advantaged relationship.

In the plant cell we also find the chloroplasts, seat of the photosynthesis. These organelles, capture the Sun light and convert it into chemistry energy, producing oxygen and carbohydrates. During the night, plant cells acquire energy through oxidation of carbohydrates with the mitochondria.]

Chloroplasts are very similar to mitochondria, but the inside part of membrane constitutes a special structure of disc-shaped folds called thylakoids, which contain chlorophyll and photosynthetic enzymes. Chlorophyll is a pigment which catches the light energy. This capture give to the substance the common green color of the leaves. Chloroplasts are part of a organelles class called plastids, which contains other pigment necessaries to photosynthesis processes. Each plastid has ribosomes, DNA and RNA. As for mitochondria, each plastid born from one plastid. For this motive, scientists think that plastid is born from a prokaryotic cell with photosynthetic activity in symbiosis with a eukaryotic cell.

Other organelles which we see in plant cells are the vacuoles, which grow with the cell. They consist in big vesicles with a membrane which keep waste material, nutrients and water.

In plant cell is present the cell wall. It is a rigid and porous coating, made up cellulose and other polysaccharides joined together by a cementing polymer. Cell wall protect the cell against the lysis due to osmotic pressure.  

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