22.2 Structure of Prokaryotes: Bacteria and Archaea
3 min read•june 14, 2024
Prokaryotes, the simplest forms of life, have unique cellular structures that set them apart from eukaryotes. These tiny organisms pack a lot into their small size, with components like the , , and working together to keep them alive and thriving.
Reproduction in prokaryotes is fast and efficient, allowing for rapid population growth. They can also swap genetic material through processes like transformation and conjugation, giving them the ability to adapt quickly to new environments and challenges.
Prokaryotic Cell Structure and Components
Components of prokaryotic cells
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- Cell envelope
- Consists of the cell wall and plasma membrane
- Provides structural support, protection, and selective permeability
- Cell wall
- Rigid structure surrounding the plasma membrane provides protection and maintains cell shape
- Composed of in , a complex polymer of sugars and amino acids
- Helps protect against osmotic stress and mechanical damage
- Plasma membrane
- Selectively permeable barrier controls the passage of substances (nutrients, waste) in and out of the cell
- Contains embedded proteins for various functions such as transport, energy production (ATP synthase), and signal
- Composed of a phospholipid bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward
- Nucleoid
- Region within the cytoplasm where the genetic material (DNA) is located, not enclosed by a nuclear membrane like in eukaryotes
- DNA is circular and highly condensed, associated with DNA-binding proteins (histones in , HU proteins in bacteria)
- Contains essential genes for cell function and replication
- , small circular DNA molecules, may also be present in the cytoplasm
- Ribosomes
- Responsible for protein synthesis in prokaryotes
- Smaller than eukaryotic ribosomes (70S vs. 80S)
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- Storage structures for nutrients or metabolic products
- Examples include glycogen granules and gas vesicles
Cell wall comparisons in microorganisms
- bacteria (Streptococcus, Staphylococcus)
- Thick cell wall composed of many layers of peptidoglycan (up to 90% of cell wall)
- Teichoic acids are present, which contribute to cell wall rigidity and antigenicity
- Retains crystal violet dye in Gram staining procedure
- bacteria (Escherichia coli, Salmonella)
- Thin cell wall with a single layer of peptidoglycan (10-20% of cell wall)
- Outer membrane present, containing lipopolysaccharides (LPS) which act as endotoxins
- Periplasmic space between the outer membrane and the plasma membrane contains enzymes and transport proteins
- Does not retain crystal violet dye in Gram staining procedure
- Archaea (Methanococcus, Halobacterium)
- Cell wall composition varies among different species
- Some have (methanogenic archaea), similar to peptidoglycan but with different chemical linkages
- Others have S-layer proteins (a single layer of glycoproteins) or lack a cell wall entirely (Thermoplasma)
- Resistant to antibiotics that target peptidoglycan synthesis (penicillin, cephalosporins)
Prokaryotic cytoskeleton and membrane structures
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- Provides structural support and aids in cell division
- Includes proteins similar to eukaryotic actin, tubulin, and intermediate filaments
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- Invaginations of the plasma membrane
- May play a role in cell division and energy production
Prokaryotic Reproduction and Genetic Exchange
Reproduction and genetic exchange in prokaryotes
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- Asexual reproduction method in prokaryotes results in two genetically identical daughter cells
- DNA replication followed by equal division of the cell
- Occurs rapidly under favorable conditions, allowing for exponential growth (E. coli can divide every 20 minutes)
- Transformation
- Uptake of naked DNA from the environment by competent cells (Streptococcus pneumoniae, subtilis)
- Incorporated DNA can be expressed and passed on to daughter cells, leading to genetic variation
- Used in genetic engineering to introduce recombinant DNA into cells
- Transduction
- Transfer of genetic material from one bacterium to another via a bacteriophage (virus)
- Two types: generalized (any bacterial DNA packaged into phage) and specialized (specific DNA adjacent to phage insertion site)
- Important mechanism for and spread of antibiotic resistance genes
- Conjugation
- Direct transfer of genetic material between two bacterial cells through a conjugation pilus (F pilus)
- Requires cell-to-cell contact, often mediated by sex pheromones
- Plasmids (F ) or conjugative transposons are transferred from donor to recipient cell
- Allows for the spread of antibiotic resistance and virulence factors among bacterial populations
- Horizontal gene transfer
- Encompasses transformation, transduction, and conjugation
- Enables rapid adaptation and evolution in prokaryotes
Key Terms to Review (32)
Archaea: Archaea are a domain of single-celled microorganisms that are distinct from bacteria and eukaryotes, known for their ability to thrive in extreme environments. They possess unique biochemical and genetic characteristics that set them apart, emphasizing their significance in the broader context of prokaryotic cells and the evolutionary history of life on Earth.
Bacillus: Bacillus refers to a genus of rod-shaped bacteria that can be found in various environments, including soil, water, and the human body. These bacteria are characterized by their distinct shape and can be either beneficial or pathogenic, playing a vital role in ecological systems as well as human health.
Bacteria: Bacteria are single-celled prokaryotic microorganisms that are ubiquitous in nature, found in various environments, from soil and water to the human body. They play crucial roles in ecosystems, including decomposition, nutrient cycling, and even human health. Bacteria are distinct from eukaryotic cells due to their simpler structure, lack of a nucleus, and different methods of reproduction, notably asexual reproduction.
Binary fission: Binary fission is a form of asexual reproduction in which a single organism divides into two identical daughter cells. This process is primarily observed in prokaryotic cells and is crucial for their growth and reproduction, enabling rapid population increases. It also plays a significant role in the study of cell division, contributing to our understanding of how organisms reproduce and evolve.
Capsule: A capsule is a protective outer layer surrounding certain prokaryotic cells, primarily bacteria, which helps to prevent desiccation and provides a barrier against environmental stressors. This structure enhances the cell's ability to adhere to surfaces and evade the immune response in host organisms, playing a crucial role in the survival and pathogenicity of some bacteria.
Cell envelope: The cell envelope refers to the outermost layers of a prokaryotic cell, which includes the plasma membrane, the cell wall, and, in some cases, an outer membrane. This structure plays a vital role in maintaining the integrity of the cell, protecting it from environmental stress, and regulating the passage of substances in and out of the cell.
Coccus: A coccus is a type of spherical-shaped bacterium that can exist as a single cell or in clusters. These microorganisms are classified as prokaryotic cells, lacking a nucleus and membrane-bound organelles, which distinguishes them from eukaryotic cells. Cocci can be found in various arrangements such as pairs (diplococci), chains (streptococci), or clusters (staphylococci), reflecting their mode of division and growth patterns.
Endospore: An endospore is a highly resistant, dormant structure formed by certain bacteria as a survival mechanism under unfavorable conditions. These spores can withstand extreme temperatures, desiccation, radiation, and chemical exposure, allowing bacteria to persist in harsh environments until conditions become favorable for growth. This unique adaptation highlights the resilience and survival strategies of prokaryotic cells, particularly within the domains of Bacteria and Archaea.
Fimbriae: Fimbriae are short, hair-like structures found on the surface of prokaryotic cells, primarily bacteria. They play a crucial role in the adherence of these cells to surfaces, including host tissues, which is essential for colonization and infection. Fimbriae are distinct from flagella, as they do not provide motility but rather enhance the ability of bacteria to anchor themselves in various environments.
Firmicutes: Firmicutes is a major phylum of bacteria characterized by a thick cell wall and low G+C content in their DNA. These bacteria play significant roles in various environments, including human gut microbiota, and are involved in important biological processes such as fermentation and the degradation of complex carbohydrates.
Flagellum: A flagellum is a long, whip-like structure that protrudes from the cell body of certain prokaryotic cells, enabling them to move through liquid environments. This organelle is essential for motility, allowing bacteria and archaea to navigate toward nutrients or away from harmful substances. Flagella can vary in number and arrangement, impacting the movement style of the organism.
FtsZ ring: The FtsZ ring is a cytoskeletal structure found in prokaryotic cells, primarily involved in cell division. It forms a contractile ring at the future site of division, serving as a scaffold for the assembly of other proteins necessary for cytokinesis. This structure is crucial for ensuring that bacterial cells divide correctly and maintain their shape, linking cell division to the overall structure of prokaryotes like bacteria and archaea.
Gram negative: Gram-negative bacteria are a group of bacteria that do not retain the crystal violet stain used in Gram staining. They have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides.
Gram positive: Gram-positive bacteria are a group of bacteria that have a thick peptidoglycan layer in their cell walls, which retains the crystal violet stain used in the Gram staining procedure. They appear purple under a microscope after Gram staining.
Gram-negative: Gram-negative refers to a classification of bacteria based on their cell wall structure, which does not retain the crystal violet stain used in the Gram staining procedure. This characteristic is due to a thinner peptidoglycan layer and the presence of an outer membrane containing lipopolysaccharides, making these bacteria more resistant to certain antibiotics and detergents. Understanding gram-negative bacteria is crucial for studying their roles in disease, environmental interactions, and antibiotic resistance.
Gram-positive: Gram-positive refers to a classification of bacteria that retain the crystal violet stain used in the Gram staining procedure, appearing purple under a microscope. This characteristic is due to their thick peptidoglycan cell wall, which is a major component of their structure and plays a crucial role in their overall physiology and response to antibiotics.
Horizontal gene transfer: Horizontal gene transfer is the process by which an organism incorporates genetic material from another organism without being its offspring, allowing for the exchange of genes across different species. This phenomenon is particularly significant in prokaryotic cells, where it plays a crucial role in genetic diversity and evolution, impacting phylogenetic relationships, viral evolution, and the overall structure and diversity of prokaryotic life.
Horizontal gene transfer (HGT): Horizontal gene transfer (HGT) is the movement of genetic material between organisms other than through vertical transmission (from parent to offspring). It plays a significant role in the evolution of many species, including bacteria and archaea.
Inclusion Bodies: Inclusion bodies are distinct cytoplasmic structures found within prokaryotic cells, primarily in bacteria and archaea, that serve as storage granules or aggregates of proteins. These structures can contain a variety of substances, including nutrients, waste products, and proteins, and play a crucial role in the metabolic processes and survival strategies of prokaryotic organisms under various environmental conditions.
Mesosomes: Mesosomes are invaginations or infoldings of the plasma membrane found in some prokaryotic cells, particularly in bacteria. These structures are thought to increase the surface area for cellular processes and play a role in various functions such as cell division, DNA replication, and energy production. Though their exact function remains debated, they are often associated with the organization of cellular components in prokaryotes.
Methicillin-resistant Staphylococcus aureus (MRSA): Methicillin-resistant Staphylococcus aureus (MRSA) is a type of bacteria that is resistant to several widely used antibiotics, making it difficult to treat. It is a common cause of hospital-acquired infections and can lead to severe health complications.
Nucleoid: The nucleoid is a region within prokaryotic cells where the cell's circular DNA is located, playing a crucial role in cellular processes such as replication and gene expression. Unlike eukaryotic cells, which have a defined nucleus, prokaryotic cells have their genetic material concentrated in this non-membrane-bound area, allowing for efficient regulation of DNA functions in processes like cell division and adaptation.
Peptidoglycan: Peptidoglycan is a polymer that forms a mesh-like structure in the cell wall of most bacteria, providing strength and rigidity to the cell. It consists of sugars and amino acids, allowing it to maintain the shape of the bacterial cell and protect it from external stressors. This unique structure is crucial for distinguishing between different types of prokaryotic cells and plays a vital role in their survival and replication.
Pili: Pili are hair-like structures found on the surface of many prokaryotic cells, particularly bacteria, that play critical roles in attachment and mobility. These structures can help bacteria adhere to surfaces, other cells, and even facilitate the exchange of genetic material through a process called conjugation. Understanding pili is essential for grasping how prokaryotic cells interact with their environment and each other.
Plasmid: A plasmid is a small, circular piece of DNA that exists independently of chromosomal DNA in prokaryotic cells, primarily found in bacteria and some archaea. Plasmids carry genes that can provide various advantages to the host cell, such as antibiotic resistance or the ability to metabolize unusual substances. Their independent replication allows for easy transfer between cells, promoting genetic diversity and adaptation.
Plasmids: Plasmids are small, circular DNA molecules found in prokaryotic cells that replicate independently of chromosomal DNA. They often carry genes that confer advantageous traits such as antibiotic resistance.
Prokaryotic cytoskeleton: The prokaryotic cytoskeleton is a network of protein filaments and tubules found in prokaryotic cells, particularly in bacteria and archaea, that provides structural support, shape, and organization. This cytoskeletal system plays a critical role in various cellular processes such as cell division, maintaining cell shape, and facilitating intracellular transport, highlighting its importance in prokaryotic cell structure and function.
Proteobacteria: Proteobacteria is a major phylum of bacteria that includes a wide variety of gram-negative microorganisms, known for their diverse metabolic capabilities and ecological roles. This group encompasses a significant number of pathogenic species as well as important nitrogen-fixing bacteria, making them essential in various environments and biological processes.
Pseudopeptidoglycan: Pseudopeptidoglycan is a polymer that serves as a structural component of the cell wall in certain archaea, resembling the peptidoglycan found in bacterial cell walls but differing in its chemical composition. It provides rigidity and shape to archaeal cells while playing a crucial role in distinguishing archaea from bacteria, particularly in terms of cell wall structure and composition.
Ribosomes: Ribosomes are molecular machines within cells responsible for synthesizing proteins by translating messenger RNA (mRNA) sequences into polypeptide chains. These essential structures link amino acids together in the order specified by the mRNA, making them crucial for the synthesis of biological macromolecules like proteins, which play various roles in cellular functions and structures.
Spirillum: Spirillum is a genus of bacteria characterized by its spiral or helical shape. These prokaryotic cells are typically found in aquatic environments and are known for their motility, which is often facilitated by flagella. Spirilla are distinct from other bacterial shapes, such as cocci (spherical) and bacilli (rod-shaped), and they play a role in various ecological processes.
Transduction: The process by which DNA is transferred from one bacterium to another by a virus (bacteriophage). It plays a crucial role in bacterial genetic variation and evolution.