The cell as we know is the basic unit of life. Microorganisms are cellular entities, and thus could be likened to a cell. They can either exist as unicellular organisms or organisms with multiple cells (i.e. as multicellular organisms). Microbial cells are ubiquitous in the natural environment, and are found in the soil, water, in the air, on and in our body, plants and animals and even in places where other life forms rarely exist (e.g. in hot springs). Biologists believe that microbial life existed since antiquity, and that microbial cells precede the emergence of multicellular or higher organisms on planet earth. A single cell is a unit, isolated or secluded from other cells by a membrane, and many cells also contain a cell wall outside the membrane. For example, bacteria are grouped into two main types based on their cell walls, viz: Gram positive bacteria and Gram negative bacteria.
A cell contains different chemicals and cellular structures including cytoplasm, ribosomes and cell wall to mention but a few. The membrane is semi-permeable and it forms a compartment which maintains the internal constituents of the cell and protects it from external forces while allowing important nutrients and wastes to enter and leave the cell with proper cell control. Cells communicate and exchange materials with their environments, and they are constantly undergoing changes, and they also show a high degree of metabolism. Cells can be considered as both chemical and coding devices because they can take up nutrient from their environment and chemically transform same for their upkeep or growth, and in turn can undergo reproduction. Thus, microorganisms are living cells because they all show the fundamental characteristics of living things except for viruses which exist as a non-living entity outside a living cell. There are basically two different types of microbial cells based on the orientation of the nucleus of each of these organisms and the revelations of microscopic examinations when living cells are scrutinized under an electron microscope.
The two types of microbial cells are prokaryotic cells and eukaryotic cells.
- PROKARYOTIC CELLS
Prokaryotic cells are microbial cells that have chromosomes that are not separated from the cytoplasm by a membrane. They are cells that lack a nucleus and other membrane-enclosed organelles like mitochondrion and chloroplast et cetera. Thus everything inside the cell of a prokaryote is openly accessible within the cell, and some of them are free-floating inside the cell because they are not membrane-bound. The phrase ‘prokaryote’ is from the Greek word pro which means before and kary or cary stands for nucleus or kernel. In general terms, prokaryote means ‘primitive cells’ in Greek; and the reason their ‘nucleus’ is said to be primitive is because their genetic material is not enclosed within a nuclear membrane (in this case: the nucleus). Their genetic material (i.e. DNA) is usually enclosed in a single circular molecule known as the nucleoid. The nucleoid of prokaryotes is not usually enclosed within a membrane (Figure 1). Based on this, both transcription and translation processes takes place and are combined in the cytoplasm as against the case in eukaryotic cells where their genetic material or DNA is enclosed within a nucleus.
The other parts of a prokaryotic cell may include flagella, cell wall, plasma membrane, fimbriae/pilus, endospores, capsules and starch granules. Microbial cells lacking a nucleus and other membrane-bound organelles such as mitochondria are generally referred to as prokaryotes. In addition to lacking membrane-bound organelles, prokaryotes also have smaller ribosomes (with sedimentation coefficient of 70S) than other non-prokaryotic cells. Basically, there are two types of prokaryotic cells and these are: Archaea and Bacteria. Prokaryotic cells are much simpler in form or structure than the eukaryotic cells, and they possess cell walls that contain peptidoglycan. They are exceptional in carrying out nitrogen-fixation, and most prokaryotes are chemolithoautotrophs or chemolithotrophs in nature (i.e. they are capable of oxidizing certain elements or inorganic chemicals such as hydrogen, sulphur and hydrogen sulphide to generate their own energy or food). Prokaryotes that are chemolithoautotrophs play significant roles in the ecosystem in that they help in recycling matter in nature and they are also employed in leaching activities.
- EUKARYOTIC CELLS
Eukaryotic cells are organisms or cells that have a membrane-bound nucleus. They are distinct group of organisms that have a unit membrane-enclosed nucleus and other organelles such as the mitochondria that are membrane-bound (Figure 2). The term eukaryote is derived from the Greek word eu which means ‘true’ and kary which means nucleus. In other words, eukaryotes are cells or organisms that have true nucleus. All eukaryotic cells have their genetic material or DNA enclosed in the nucleus (a membrane-bound organelle). They include cells from fungi, protozoa, algae, plants and animals. Eukaryotic cells have organelles with separate cellular and metabolic functions unlike the prokaryotes. Structurally, eukaryotes are more complex and organized than prokaryotic cells. All known living organisms that exclude bacteria and Archaea are known to be eukaryotes. Some of the organelles found in eukaryotic cells include chloroplast (found only in photosynthetic cells), mitochondria, cell wall (which contains chitin or cellulose when present), ribosomes (80S ribosomes) and cytoplasm amongst others.
There are other groups of microorganisms which are neither eukaryotes nor prokaryotes. These microbes which are not prokaryotic cells or eukaryotic cells are known as viruses. It is noteworthy that viruses, a group of microorganisms are neither eukaryotes nor prokaryotes. The reason for this is because viruses are not cells because they lack certain attributes of a cell (e.g. the ability to self-replicate). Viruses only exhibit the attributes of a cell when it infects a living cell, and they are known to infect all cells including other microorganisms. They are much smaller in size than the eukaryotic and prokaryotic cells. Outside a living cell, viruses are static and incapable in carrying out any form of metabolic activity. They only become active and replicate when they infect living cells whose biosynthetic machinery is used to drive their own. Thus, viruses are generally referred to as ‘obligate intracellular parasites’ because they can only replicate inside the cells of other living organisms that they infect.
Table 1. enumerates some of the basic differences between eukaryotic and prokaryotic cells.
Table 1. DIFFERENCES BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS
|1.||Nuclear membrane is absent.||Nuclear membrane is present.|
|2.||Chromosome structure is in direct contact with the cytoplasm.||Chromosome structure is not in direct contact with the cytoplasm but in a nuclear membrane bound organelle called nucleus.|
|3.||Cell division does not involve mitosis and meiosis.||Cell division involves mitosis and meiosis.|
|4.||Cell wall when present contains only peptidoglycan.||Cell wall when present contains chitin and cellulose but not peptidoglycan.|
|5.||Mitochondria are absent.||Mitochondria are generally present.|
|6.||Flagella when present have a simple structure.||Flagella when present have a complex structure.|
|7.||Chloroplasts are absent.||Chloroplasts are present but only in photosynthetic cells.|
|8.||Golgi bodies/apparatus and endoplasmic reticulum are absent.||Golgi bodies/apparatus and endoplasmic reticulum are present.|
|9.||Respiration is carried out through the cytoplasmic membrane.||Respiration is carried out through mitochondria.|
|10.||The chromosomes of prokaryotic cells are often haploid (i.e. contains single-strands of DNA).||The chromosomes of eukaryotic cells are diploid (i.e. it contains two or more strands of DNA).|
|11.||Carries out only asexual reproduction.||Carries out both sexual and asexual reproduction.|
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