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Prokaryotic and eukaryotic cells compared

Prokaryotic and Eukaryotic Cells

In this essay I will describe the structure of a prokaryotic cell and contrast it with the structure of an eukaryotic one.
The prokaryotic cell is quite simply built, and therefore I would like not only to mention its parts - called organelles - but also to discuss some of their general functions. I would start with the major property: size. Prokaryotes are very small - 0.3 µm (Mycoplasma) up to 10 µm (Spirochete). Bigger, but still not visible for the human eye, are most eukaryotes: 5 µm (animal cell) up to 100 µm (euglena). And I say most, because there are some exceptions, like the eukaryotic egg (yolk) cell, which complained with any other cell looks absolutely huge. Both eukaryotic and prokaryotic cells are covered with permeable cell wall that protects the cell and maintains its shape; animal cells, which are eukaryotes, lack it, and that is why they are spherically shaped. Under the cell wall there is a semipermeable plasma membrane (also called cell membrane) with bilayer phospholipids, used for protection, communication between cells and regulation of passing materials. The gel-like cytoplasm is the interior of both eukaryotes and prokaryotes, but there is a difference in organelles placed in it. Let’s start with the most common organelle, the nucleus. Actually, by prokaryotes this is called nucleoid, because it has only one, circularly - shaped chromosome of DNA or RNA and is also not protected by a membrane. The one that is protected, that is the eukaryotes’ “true” nucleus - a more complex a structure, covered with membrane (-nuclear envelope with pores). Inside there are several linear chromosomes (DNA) and a small organelle, the nucleolus. Ribosomes, sometimes creating groups called polysomes or polyribosomes, contain RNA and also proteins (which they produce, as well). They are present in both eukaryotes and prokaryotes, exactly like flagella, “instruments” used for cell movement.
Eukaryotic cell does possess more organelles that the simple prokaryotic cell does. For example, flagella are not the only movement organelles. There is a whole cytoskeleton – microfilaments, microtubules and intermediate filaments together in one structure. Mitochondria with their two membranes (inner and outer) provide cellular chemical energy for further cellular activities (respiration, etc.) and generate high – energy compounds, such as ATP.

Lysosomes contain digestive enzymes and help recycle worn – out cell parts. Some materials produced in the cell are modified and packaged in the Golgi complex. Finally, there is another very interesting structure that prokaryotes lack - the endoplasmic reticulum - “network within the cell”. There are two main types of ER: smooth ER (SER, detoxifies poisonous chemicals and also manufactures lipids) and rough ER (RER, it is loaded with ribosomes and involved in protein synthesis). As a whole, ER modifies and transports pre – made materials. It is the continue of the nuclear envelope and has cavities and ribosomes used for proteosynthesis.
It may also depend on the type of the eukaryotic cell whether it will or will not have some particular organelles. Chloroplasts, a special type of plastids, have two lipid bilayer membranes and tylakoids that contain green – colored pigments which harvest and trap sun’s energy in chemical form (ATP). They produce and store food and that is what we call photosynthesis. Chloroplasts and other plastids, like chromoplasts and leucoplasts, are only present in plant cells. The same does the vacuole, the big watery thing inside the plant cell that contains solution of various storage products. It provides cell’s nutrition (endocytosis – phagocytosis, pinocytosis) and excretion (exocytosis). On the other hand, animal cells have some “special” organelles, as well: cilia for outer and centrioles, with their nine microtubule triplets, for inner cell movement – they both are parts of the cytoskeleton. Although - evolutionally speaking - the prokaryotic cell is older, it is less developed than the eukaryotic, and that is why they are so different. Compared with eukaryotes’ membrane - bounded nucleus, DNA in several linear chromosomes and many specialized organelles, the prokaryotes with their no – membrane nucleus, circular strand of DNA and few organelles look very simple, but, and that the main difference, prokaryotic cells exist only like single celled, absolutely independent organisms (some eukaryotes do, as well, but in most cases they are specialized and work like subunits or multicellular organisms). .

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