All living organisms are grouped into two types, namely prokaryotes and eukaryotes, as these are the two basic types of cells. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms. Examples of prokaryotes are archaebacteria, eubacteria, and blue-green algae. Eukaryotes include animals, plants, fungi and protists.
Prokaryotes
Prokaryotes or prokaryotic organisms are single-celled and are the simplest form of life.
They lack nucleus. Due to absence of a nucleus, their DNA floats freely in the cell and generally remains clumped together.
Prokaryotic cells also lack extensive functional compartmentalization found in eukaryotic cells, but they do possess internal membranes, a nucleoid region, and ribosomes.
Prokaryotes are divided mainly of two types: Archaebacteria and bacteria.
Archaebacteria
Archaebacteria are the oldest organisms living on the Earth. They are unicellular prokaryotes and belong to the kingdom ‘Archaea’. Most archaebacteria appear to be bacteria when observed under the microscope. However, they are quite different from bacteria and eukaryotic organisms.
There are three main types of archaebacteria: methanogens, halophiles, and thermophiles. Each group has some distinct feature about their chemical composition, metabolism, structure, and nucleic acid sequences which sets them apart.
Methanogens
Methanogens inhabit oxygen-free environments like bogs, sewage, and intestines. They are obligate anaerobes, i.e. they cannot survive in the presence of oxygen. They generally get energy by converting hydrogen and carbon dioxide into methane to generate ATP.
Methanogens are particularly prevalent in the intestines of grazing animals such as cows because they help digest the cellulose in grass and other plants. They are also used in sewage treatment plants and water purification plants.
Extreme Halophiles
These unicellular organisms flourish in salty environments like salt lakes and near hydrothermal vents. They use aerobic respiration, or they can convert light into energy using a pigment called bacteriorhodopsin to get energy. This pigment absorbs everything but purple light, giving the cells a purplish color.
Extreme Thermophiles
Extreme thermophiles thrive in very hot and acidic conditions. They can be found in areas with temperatures up to 230 degrees Fahrenheit, or with a pH as low as 2.
Thermophiles produce ATP by taking electrons from hydrogen sulfide. They are mostly chemoautotrophs.
Bacteria
Bacteria are microscopic single-celled organisms that thrive in diverse environments. They can live within soil, in the ocean and inside the human gut.
Bacteria are single-celled microbes. The cell structure is simple as there is no nucleus or membrane bound organelles. Some bacteria have an extra circle of genetic material called a plasmid. The plasmid often contains genes that give the bacterium some advantage over other bacteria. For example, it may contain a gene that makes the bacterium resistant to a certain antibiotic.
Bacteria can be divided based on the structure of their cell wall and how it responds to Gram staining technique, as either gram-positive or gram-negative bacteria.
How can such tiny single-celled organisms survive long term?
Prokaryotes divide rapidly. The doubling time across the group varies massively; some divide in minutes (E. coli – 20mins and C. difficile – 7mins under suitable conditions) others in hours (S. aureus – in an hour) and some over days (T. pallidum – in about 35 hours).
As natural selection works on the generational time scale, if more generations pass, more time is required to select the genes for or against the process of evolution. Even the longest of these doubling times is still hugely faster than the reproduction rates of eukaryotes.
Almost all prokaryotes have a cell wall. A prokaryotic cell wall usually consists of peptidoglycan. Many prokaryotes have slender, rigid flagella for propulsion and some prokaryotes have pili that help in their attachment and genetic exchange. Bacteria have very short generation times thus mutation and genetic recombination play very important role in producing and maintaining their genetic diversity.
Table showing differences in archaebacteria and bacteria
| Features | Archaebacteria | Bacteria |
| Habitat | Extreme and harsh condition/environments like hot springs, salt lakes, gut of ruminants and humans | Widespread |
| Cell wall | Pseudopeptidoglycan | Peptidoglycan and lipopolysacchrides |
| Growth and reproduction | Reproduce asexually by the process of binary fission, budding and fragmentation | Similar to the archaebacteria but also form spores |
| Shape | Rods, cocci, spirals, plates, or coiled | Same as archaebacteria |
| Flagella | Present | Present |
| RNA | Three types of RNA | Single RNA |
| Metabolic activity | Methanogenesis | Aerobic/anaerobic respiration, fermentation, photosynthesis |
Eukaryotes
Eukaryotes are organisms made up of cells that possess a membrane-bound nucleus and organelles. Genetic material in eukaryotes is contained within a nucleus within the cell and DNA is organized into chromosomes. Eukaryotes may be multicellular or single-celled organisms.
All animals are eukaryotes. Other eukaryotes include plants, fungi, and protists. A typical eukaryotic cell contains the following organelles:
- Plasma membrane
- Nucleolus
- Nucleus
- Chromosomes
- Ribosome
- Vesicle
- Endoplasmic reticulum
- Golgi apparatus
- Cytoskeleton
- Cytoplasm
- Lysosome
- Mitochondria
- Centriole
Table showing comparison between Prokaryotic and Eukaryotic cell
| Characteristic and Cell Components | Prokaryotic cell | Eukaryotic cell |
| Size of the cell | 0.2-2.0mm in diameter | 0.10 -100mm in diameter |
| Cell Type | Usually unicellular (some cyanobacteria may be multicellular) | Multicellular |
| Plasma Membrane | Present | Present |
| Nucleus | Absent | True nucleus consisting of nuclear membrane and nucleoli |
| Flagella | Present, consisting of two protein building blocks (microtubules) | Present and is complex; consisting of multiple microtubules |
| Membrane bound vacuoles | Absent | Present |
| Ribosomes | Smaller size (70s) | Larger than prokaryotic ribosomes |
| Mitochondria | Absent | Present |
| Lysosomes and peroxisomes | Absent | Present |
| Microtubules | Absent or rare | Present |
| Endoplasmic reticulum | Absent | Present |
| Glycocalyx | Present as a capsule or slime layer | Present in some cells that lack cell wall |
| Genetic Recombination | Partial, and undirectional transfer of DNA | Meiosis and fusion of gametes |
| Gas vacuoles | May be present | Absent |
| Extrachromosomal plasmid | Present. Non-essential prokaryotic genes are encoded on extra chromosomal plasmid | Absent |
| Nitrogen fixation | Present in a few | Absent |
| Respiration | Mainly anaerobic | Aerobic with few exceptions which become facultative anaerobes due to modifications |
| Photosynthetic enzymes | Bound to plasma membrane as composite chromatophores | Enzymes packed in plastids bound by membrane |
| Chromosome | Usually single circular without histones | Multiple linear with histones |
| Cell division | Binary fission or budding | Mitosis or meiosis |
| Examples | Bacteria and Archaebacteria | Animals and Plants |
Can you guess why are chloroplasts found in plant cells and not in animal cells?
Plant cells use sunlight for energy. Chloroplasts are structures at effectively use sunlight to get energy. Animal cells do not use this mechanism for energy and therefore do not need chloroplast.
Structurally, plant and animal cells are similar as they both are eukaryotic. They contain membrane-bound organelles such as, nucleus, mitochondria, endoplasmic reticulum, golgi apparatus, lysosomes, and peroxisomes. Both also contain similar membranes, cytosol, and cytoskeletal elements. The functions of these organelles are extremely similar between two classes of cells. However, the differences that exist between plant and animals are very significant.
Table showing comparison between Plant and Animal Cells
| Characteristics | Plant cell | Animal cell |
| Size | 10-100 micrometers in length | 10-30 micrometers in length |
| Shape | Usually rectangular or cubical | Usually round or irregular shapes |
| Energy storage | Store energy in the form of glycogen | Store energy as starch |
| Growth | Mainly increase cell size by becoming larger | Increase in size by increasing in cell numbers |
| Cell wall | Cell wall composed of cellulose as well as a cell membrane | Cell wall absent but they have cell membrane |
| Centrioles | Absent | Present |
| Cilia | Usually absent | Present |
| Glyoxysomes | Present | Not found in animal cells |
| Lysosomes | Rarely present | Always present |
| Vacuole | Large central vacuole that can occupy up to 90% of the cell’s volume | Have many small vacuole |
| Structure |  |
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