All living things found on the planet earth are divided into two major groups namely, prokaryotes and Eukaryotes based on the types of cells these organisms possess. Prokaryotic cells lack a well defined nucleus and have a simplified internal organization. Eukaryotic cells have a more complicated internal structure including a well defined, membrane- limited nucleus. Bacteria and Cyanobacteria are prokaryotes. Fungi, plants and animals are eukaryotes.
Prokaryotes
In general, Prokaryotes consist of a single closed compartment containing the cytosol and bounded by the plasma membrane. Although bacterial cells do not have a well defined nucleus, the genetic material, DNA, is condensed into the central region of the cell.
In all prokaryotic cells, most of or all the genetic information resides in a single circular DNA molecule, in the central region of the cell. This region is often referred to as incipient nucleus or nucleoid. In addition, most ribosomes, the cell’s protein synthesizing centres are found in the DNAfree region of the cell.
Some bacteria also have an invagination of the cell membrane called a mesosome, which is associated with synthesis of DNA and secretion of proteins. Thus we can not say that bacterial cells are completely devoid of internal organization. Bacterial cells possess a cell wall which lies adjacent to the external side of the plasma membrane. The cell wall is composed of layers of peptidoglycan, a complex of proteins and oligosaccharides. It protects the cell and maintain its shape. Some bacteria (eg E.coli) have a thin cell wall and an unusual outermembrane separated from the cell wall by the periplasmic space.
Such bacteria are not stained by Gram staining technique and thus are classified as Gram- negative bacteria. Other bacteria (eg. Bacillus polymyxa) that have a thicker cell wall without an outer membrane take the Gram stain and thus are classified as Gram positive bacteria.
Eukaryotes
Eukaryotes comprise all members of Plant Kingdom, Fungi and Animal Kingdoms, including the unicellular fungus Yeast, and protozoans. Eukaryotic cells, like prorkaryotic cells are surrounded by a plasma membrane. However, unlike prokaryotic cells, most eukaryotic cells contain internal membrane bound organelles.
Each type of organelle plays a unique role in the growth and metabolism of the cell, and each contains a set of enzymes that catalyze requisite chemical reactions. The largest organelle in a eukaryotic cell is generally the nucleus, which houses most of the cellular DNA. The DNA of eukaryotic cells is distributed among 1 to about 50 long linear structures called chromosomes. The number and size of the chromosomes are the same in all cells of an organism but vary among different species of organisms.
The total DNA ( the genetic information) in the chromosomes of an organism is referred to as its genome. In addition to the nucleus, several other organelles are present in nearly all eukaryotic cells, the mitochondria in which the cell’s energy metabolism is carried out, the rough and smooth endoplasmic reticula, a network of membranes in which proteins and lipids are synthesized and peroxysomes, in which fatty acids and amino acids are degraded.
Chloroplasts, the site of photosynthesis are found only in plants and some single celled organisms. Both plant cells and some single celled eukaryotes contain one or more vacuoles, large, fluid – filled organelles in which nutrients and waste compounds are stored and some degradative reactions occur. The cytosol of eukaryotic cells contains an array of fibrous proteins collectively called the cytoskeleton. Cytosol is the soluble part of the cytoplasm. It is located between the cell organelles. The plant cell has a rigid cell wall composed of cellulose and other polymers. The cell wall contributes to the strength and rigidity of plant cell.
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