In this comprehensive exploration of prokaryotes, we will scrutinise their distinct features, such as the nucleoid, plasmids, cell wall, and ribosomes. We will also investigate the differences and similarities between the two main domains of prokaryotes: bacteria and archaea.
Key Features of Prokaryotes
Nucleoid
The nucleoid is a central feature of prokaryotic cells, which unlike eukaryotes, do not possess a true nucleus enclosed by a nuclear membrane. Instead, they have a nucleoid - a region within the cell where the DNA is located. This DNA is not segregated from the rest of the cell's contents, but instead floats freely within the cytoplasm. Prokaryotic DNA is circular and exists as a single, large, coiled chromosome.
The nucleoid's positioning within the cell isn't random. Instead, the location is thought to result from the cell's need to distribute its genetic material evenly during cell division, ensuring genetic material is transferred accurately to offspring cells.
Plasmids
Another unique feature of prokaryotic cells is the presence of plasmids. These small, circular DNA molecules exist separately from the main DNA in the nucleoid and can replicate independently. Plasmids often carry genes that may offer an evolutionary advantage to the organism, such as antibiotic resistance genes or genes necessary for metabolising uncommon substrates.
Plasmids can be transferred between prokaryotic cells in a process known as horizontal gene transfer. This process allows for the rapid spread of beneficial traits through a population, highlighting the adaptability of prokaryotic organisms.
Cell Wall
The cell wall is a vital structure in prokaryotes. It provides physical protection and rigidity to the cell and prevents the cell from bursting under different osmotic pressures. The composition of the cell wall varies between bacteria and archaea.
Bacteria possess a peptidoglycan cell wall, a complex lattice structure that encloses the entire bacterium and provides structural strength. This peptidoglycan layer is vital for maintaining cell shape and is unique to bacteria, making it a popular target for antibiotics.
Archaea, on the other hand, do not contain peptidoglycan in their cell walls. Instead, archaeal cell walls can contain a variety of other polysaccharides, proteins or pseudopeptidoglycan, providing a unique chemical signature that separates archaea from bacteria.
Ribosomes
Ribosomes are the sites of protein synthesis in all living cells, translating the genetic code from messenger RNA into amino acid sequences to create proteins. Prokaryotic ribosomes are 70S (Svedberg units), smaller than the 80S ribosomes found in eukaryotes.
Prokaryotic ribosomes, consisting of a large (50S) and a small (30S) subunit, can be found scattered throughout the cytoplasm or attached to the plasma membrane. This distribution allows for simultaneous transcription and translation, a feature not seen in eukaryotes due to the separation of the nucleus and cytoplasm.
Bacteria and Archaea
Bacteria
Bacteria represent the most abundant group of prokaryotes and are among the earliest forms of life on Earth. They are metabolically diverse, capable of exploiting a wide range of organic and inorganic substances to obtain energy. Bacteria can thrive in a variety of environments, including extreme conditions such as hot springs and the deep sea, to more familiar settings like soil, water, and the human body.
Bacterial cells are typically a few micrometres in length and come in several shapes, including spheres (cocci), rods (bacilli), and spirals (spirilla). Certain bacteria also have additional structures like pili (used for attachment), a capsule (for protection), and flagella (for movement).
Archaea
Archaea are single-celled prokaryotic organisms similar to bacteria in form and function, but their genetic makeup and biochemical characteristics distinguish them significantly. Some of the genes and metabolic pathways in archaea more closely resemble those in eukaryotes, leading some scientists to propose archaea as a stepping stone in the evolution of eukaryotic life.
Archaea are known for their ability to survive in extreme environments, such as high salt concentrations (halophiles), high temperatures (thermophiles), and anaerobic conditions like swamps and animal guts (methanogens). Archaea's unique properties and survival mechanisms have led to intense scientific research, investigating their potential applications in biotechnology and their implications for life in extreme environments.
FAQ
The high surface area to volume ratio in prokaryotes is crucial for their survival. Because they are small, they can rapidly exchange materials with their environment, allowing for quick nutrient uptake and waste removal. This also means that they can respond rapidly to changes in environmental conditions. Furthermore, the high surface area to volume ratio supports efficient cell division and growth. This aspect is a significant factor behind the success of prokaryotes in diverse and changing environments.
The bacterial capsule is a layer that lies outside the cell wall in some bacteria. Composed of polysaccharides, the capsule serves several functions. It provides protection against desiccation and harsh environmental conditions. It can also protect bacteria from being engulfed by host immune cells, enhancing the organism's pathogenicity. Additionally, the capsule helps in attachment to surfaces and in the formation of biofilms, which can further protect bacteria from environmental stress and antibiotics.
Prokaryotes reproduce through a process known as binary fission, which is a form of asexual reproduction. In this process, the DNA is replicated, and the cell divides into two daughter cells, each having a copy of the original DNA. Additionally, prokaryotes can transfer genetic material through horizontal gene transfer, which involves the transfer of genes from one organism to another that is not its offspring. This process can occur through transformation, conjugation, or transduction, providing a means of increasing genetic diversity in prokaryotic populations.
Prokaryotes have several methods of movement, most commonly through the use of flagella, which are long, whip-like appendages that rotate to propel the cell forward. In addition to flagella, some prokaryotes utilise pili for a form of movement called twitching. Some prokaryotes also exhibit a form of movement known as gliding, the mechanism of which is still not fully understood. Not all prokaryotes are motile, and those that can switch between modes of movement in response to environmental cues.
Prokaryotes play essential roles in ecosystems. They are critical in nutrient cycling, aiding in processes such as nitrogen fixation, decomposition, and carbon cycling. Furthermore, certain prokaryotes form symbiotic relationships with other organisms, such as bacteria in the human gut, that aid in digestion. Prokaryotes are also involved in primary production in aquatic ecosystems, including the generation of a substantial amount of the world's oxygen. Their roles are diverse and crucial to the functioning of ecosystems worldwide.
Practice Questions
Plasmids play a pivotal role in the survival and evolution of prokaryotic organisms. These small, circular DNA molecules carry genes that can offer a competitive advantage to the organism, such as antibiotic resistance genes. This enables the bacteria to survive in environments with the antibiotic present, promoting the proliferation of resistant strains. Moreover, plasmids can be transferred between prokaryotic cells via horizontal gene transfer, facilitating the rapid spread of advantageous traits through a population. This mechanism enhances the adaptability of prokaryotic organisms, aiding their survival in fluctuating environments and contributing to their evolution.
The composition of the cell wall in bacteria and archaea significantly differs, influencing their survival in different environments. Bacterial cell walls contain a unique compound, peptidoglycan, which gives structural strength and protects the bacteria from osmotic lysis. This composition makes bacteria susceptible to certain antibiotics that target peptidoglycan synthesis. Archaea, on the other hand, lack peptidoglycan in their cell walls. Instead, they contain other polysaccharides, proteins, or pseudopeptidoglycan, enabling them to survive in extreme environments where bacteria can't thrive. The cell wall's distinct composition in archaea also makes them resistant to antibiotics that target peptidoglycan, demonstrating how cell wall differences can impact survival.
