Prokaryotic organisms first evolved on the earth. Many differences exist between the prokaryotic and eukaryotic organisms. Hence, their cellular structures and compartmentalization also vary. The functions and the biochemical pathways also vary from each other. The eukaryotic cells possess different types of organelles. The prokaryotic cells do not have all of them. Most of the prokaryotic cells possess microscopic and sub-microscopic structures. These cells do not get visualized with the naked eyes. The only way to study them involves culturing them and visualizing them under a microscope. The eukaryotes, on the other hand, show a wide range of organisms such as plants, insects, animals, birds, and human beings. The eukaryotic organisms possess different types of bodies, different cells, different proteins, and cellular processes. Thus, both prokaryotic and eukaryotic cells involve topics of interest for the researchers. Let us discuss the different points.
Image: A rough sketch of a eukaryotic cell and a prokaryotic cell
Cell size and shapes: The size of a eukaryotic cell varies from 10-100um. The size of the prokaryotic cell varies from 1-10um. The shapes of the cells also vary. Eukaryotic cells possess different types of cells with varied shapes in a particular organism. For example, the shape of the sperm cells differs from the oocytes or the smooth muscle cells. Similarly, the neuronal cells possess different shape. The prokaryotic organisms have rod-shaped cells, cocci, spiral-shaped cells or cork-screw shaped cells, and many other shapes.
Nucleus: The core region in the cell, known as the nucleus, plays a crucial role in giving space to the genetic machinery and other cellular activities. Eukaryotic cells possess a well-defined nucleus. The prokaryotes do not have a well-defined nucleus. The eukaryotic cells possess more than one chromosome in the nucleus. The prokaryotic cells have only one or very few chromosomes. However, they show the presence of extrachromosomal material known as the plasmids. The eukaryotic cells possess a membrane-bound nucleus.
Chromosomes: The prokaryotic chromosomes mostly possess a single-stranded or a double-stranded DNA. They possess a circular or a linear structure. The bacterial or the archeal chromosomes are known as a nucleoid. Eukaryotes possess a specific number of chromosomes. For example, humans possess forty-six chromosomes. The packaging proteins in the prokaryotic chromosomes are known as HU proteins. The packaging proteins in the eukaryotic chromosomes are known as histone proteins.
Cell wall: Present in plants and fungi. The prokaryotic cell wall consists of complex structures. The bacterial cell walls possess peptidoglycan. It consists of polysaccharide chains cross-linked by unusual peptides. The plant cell wall consists of cellulose. The fungal cell walls contain chitin.
The permeability of the nuclear membrane: The eukaryotic nuclear membranes show selective permeability. The prokaryotic cells do not have permeability mechanisms. The cell membrane helps manage the movement of water, carbon dioxide, and oxygen. It exhibits a selective permeability towards the ions and the organic molecules.
The cellular type: The eukaryotic organisms possess many cells. Thus, they are known as multicellular organisms. The prokaryotic organisms have a single cell. They are known as unicellular organisms. The eukaryotic cells also possess different kinds of cells such as immune cells, blood cells, somatic cells, stem cells, gametic cells, and many other types of cells. Most of the cells possess a nucleus (exceptions include red blood cells). The chromosomal segregation of the somatic and the gametic cell also varies. The prokaryotic cells lack all the above points.
Mitochondria and Chloroplast: These organelles play a crucial role in producing energy in the form of ATP. They possess a separate DNA known as mitochondrial DNA or the mtDNA. The chloroplasts possess pigment chlorophyll. It plays a crucial role in photosynthesis. The eukaryotic cells possess a mitochondrion and chloroplasts (in plants and algae). The prokaryotic cells lack mitochondrion. They do not possess chloroplast. However, they show the presence of scattered chlorophyll. The origin of the mitochondria and chloroplast involved a free-living prokaryotic origin. These organisms, later on, invaded eukaryotic cells and got established there. The theory explaining the above concept is known as endosymbiont theory. Eukaryotic cells originated as anaerobic organisms. They lacked mitochondria and chloroplasts. After many years, a eukaryotic cell established a relationship with a purple non-sulfur bacteria. The purple non-sulfur bacteria involved a key process known as oxidative phosphorylation, which proved to be beneficial for the eukaryotic cell. Thus, it started getting atmospheric oxygen thereby depending on the prokaryotic cells. Thus, the mitochondrion came into existence. The chlorophyll producing plants ingested the oxygen-producing photosynthetic bacteria. Thus, the chloroplast came into existence.
Lysosomes and peroxisomes: The eukaryotic cells possess lysosomes and peroxisomes. The prokaryotes lack them. Lysosomes possess hydrolytic enzymes capable of breaking down many different biomolecules. They help in disposing of the unwanted things from the cell. The peroxisome is also known as a microbody. It plays an important role in catabolizing the long chain fatty acids.
Endoplasmic reticulum: Eukaryotes possess endoplasmic reticulum. The prokaryotes do not have an endoplasmic reticulum.
Ribosomes: Prokaryotes have 70S ribosome. Eukaryotes have 80S ribosomes. The eukaryotic ribosomes have a larger size as compared to the prokaryotic ribosomes. Ribosomes play a crucial role in translation (RNA to protein). The process of translation occurs slightly differently in the prokaryotic and eukaryotic cells. The initiator methionine gets modified to N-formyl-methionine in the case of the prokaryotic translation process. The eukaryotic translation process does not involve the modification of the methionine. Unlike prokaryotic sequences, the eukaryotic mRNAs do not possess Shine-Dalgarno sequences. Instead, they employ a short sequence known as the Kozak sequence.
Vesicles and the Golgi apparatus: The eukaryotic cells have the vesicles and Golgi apparatus. The prokaryotic cells lack Golgi complex and vesicles.
Genetic recombination: Meiosis and fusion of gametes occur in the eukaryotes. The eukaryotic DNA undergoes recombination. The prokaryotic DNA also undergoes recombination. However, it involves partial genetic recombination. It mostly chooses unidirectional DNA transfer or a vector mediated DNA transfer.
Microtubules: The eukaryotic cells possess microtubules. The prokaryotes do not have microtubules. However, rarely they occur in very few prokaryotic organisms. The rare bacterial microtubules possess a smaller diameter as compared to the eukaryotic microtubules.
References:
[1] How Eukaryotic and Prokaryotic Cells Differ, Raina G. Merchant and Lesli J. Favor
[2] Eukaryotic and Prokaryotic Cell Structures: Understanding Cells With and Without a Nucleus, By Leslie Favor, Ph.D.
[3] Website Diffen: Eukaryotic Cell vs. Prokaryotic Cell
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