Microorganisms are ubiquitous. They can be found in air, water, soil, inside and on the surfaces of plants, animals and humans. They live in a variety of habitats and under a wide variety of environmental conditions. Microorganisms vary principally in their morphology and, depending on the degree of structural complexity are broadly classified as prokaryotes and eukaryotes.
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The word “prokaryotes” also spelled “procaryotes” is coined from two Greek words pro, before, and karyon, nut or kernel. It is used to describe unicellular (single-celled) organisms that lack true nucleus and membrane-bound cell organelles. This means that the genetic material in prokaryotes is not bound within a nucleus.
Prokaryotes are divided into two domains, bacteria and archaea. Bacteria used to be considered as the only category of prokaryotic cells, but in 1990 a second group, the archaea, were recognized as having equal status to bacteria.
Archaea tend to live in harsh environmental conditions (such as high temperatures, extremes pH or salinity etc.) and often possess unusual modes of metabolism. All other organisms including humans have the eukaryotic structure with relatively more complex architecture.
Prokaryotic cell (Source: www.courses.lumenlearning.com)
Prokaryotic cell structures and functions |
|
| Capsules and slime layers | Resistance to phagocytosis, adherence to surfaces |
| Cell wall | Gives bacteria shape and protection from lysis in dilute solutions |
| Endospore | Survival under harsh environmental conditions |
| Fimbriae and pili | Attachment to surfaces, bacterial mating |
| Flagella | Provides the power of motility or self-propulsion |
| Gas vacuole | Buoyancy for floating in aquatic environments. |
| Inclusion bodies | Storage of carbon, phosphate, and other substances |
| Nucleoid | Localization of genetic material (DNA) |
| Periplasmic space | Contains hydrolytic enzymes and binding proteins for nutrient processing and uptake’ |
| Plasma membrane | Selectively permeable barrier, mechanical boundary of cell, nutrient and waste transport, location of many metabolic processes (respiration, photosynthesis), detection of environmental cues for chemotaxis |
| Ribosomes | Protein synthesis |
Read Also: Microorganisms of Pharmaceutical Interest
Eukaryotes (eucaryotes) are organisms made up of cells that possess a membrane-bound nucleus. Just like prokaryotes, the word “eukaryotes” is derived from two Greek words eu, true, and karyon, nut or kernel.
A typical eukaryotic cell is surrounded by a plasma membrane and contains many different structures and organelles with a variety of functions. The major groups of microorganisms (fungi, protozoa and algae), as well as parasitic worms and mites, and all plants and animals up to and including humans all belong to this group.
Viruses do not have a cellular structure and so some scientists do not even regard them as living but merely mixtures of complex chemicals; nevertheless, they are indisputably agents of infection and for that reason are usually considered as part of the microbial world.
Eukaryotic cell (Source: www.biologydictionary.net)
Eukaryotic cell structures and functions |
|
| Cell wall and pellicle | Strengthen and give shape to the cell |
| Chloroplasts | Photosynthesis—trapping light energy and formation of carbohydrate from CO2 and water |
| Cilia and flagella | Cell movement |
| Cytoplasmic matrix | Environment for other organelles, location of many metabolic processes |
| Endoplasmic reticulum | Transport of materials, protein and lipid synthesis |
| Golgi apparatus | Packaging and secretion of materials for various purposes, lysosome formation |
| Lysosomes | Intracellular digestion |
| Microfilaments, intermediate filaments, and microtubules | Cell structure and movements, form the cytoskeleton |
| Mitochondria | Energy production through use of the tricarboxylic acid cycle, electron transport, oxidative phosphorylation, and other pathways |
| Nucleolus | Ribosomal RNA synthesis, ribosome construction |
| Nucleus | Repository for genetic information, control centre for cell |
| Plasma membrane | Mechanical cell boundary, selectively permeable barrier with transport systems, mediates cell-cell interactions and adhesion to surfaces, secretion |
| Ribosomes | Protein synthesis |
| Vacuole | Temporary storage and transport, digestion (food vacuoles), water balance (contractile vacuole) |
The various differences between prokaryotic and eukaryotic cells are listed in the table below.
| Characteristic | Prokaryote | Eukaryote |
| Size | Typically 1 – 5 µm | Normally greater than 10 µm |
| Cell nucleus | Do not possess a true nucleus | Have a nucleus surrounded by a nuclear membrane |
| Location of chromosomes | In the cytoplasm, usually attached to the cell membrane | Within a true nucleus separated from the cytoplasm by a nuclear membrane |
| Nuclear division and reproduction | Mitosis and meiosis are absent so reproduction is asexual |
Exhibit both mitosis and meiosis, so reproduction may be sexual or asexual or both depending on species |
| Nucleolus | Absent | Present |
| Genetic variation | Resulting largely from mutations | Resulting both from mutations and the creation of new gene combinations during sexual reproduction |
| Mitochondria, chloroplasts and ribosomes | Mitochondria and chloroplasts absent; ribosome size is 70s |
Mitochondria and chloroplasts may be present; ribosomes larger: 80s |
| Chemical composition | Do not possess sterols in the cell membrane but do usually have peptidoglycan in the cell walls |
Do possess sterols in the cell membrane but no peptidoglycan in the walls |
| Flagella | Structurally simple | Structurally complex |
| Pili | Present | Absent |
| Storage compounds | Poly -β- hydroxybutyrate often present | Poly -β- hydroxybutyrate absent |
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