3.3 Prokaryotic Cell Structure
3 min read•august 7, 2024
, like , have unique structures that set them apart from eukaryotes. Their genetic material floats freely in the cytoplasm, and they have special features like and a tough . These structures help them survive and thrive in various environments.
Bacteria also have cool appendages like and . Pili help them stick to surfaces and share DNA, while flagella let them swim around. These features, along with their simple internal organization, make prokaryotes incredibly adaptable and successful organisms.
Genetic Material
Nucleoid Structure and Function
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- consists of a single circular molecule of DNA not enclosed within a nuclear membrane
- Houses the bacterial chromosome
- Lacks histone proteins (eukaryotic chromosomes)
- Condenses the DNA molecule to fit inside the cell
- Controls cellular processes through gene expression and regulation
Plasmids: Extrachromosomal DNA
- Plasmids are small, circular pieces of DNA separate from the main bacterial chromosome
- Contain genes that provide additional functions or traits to the cell (antibiotic resistance)
- Can be transferred between bacteria through processes like conjugation
- Replicate independently of the bacterial chromosome
- Utilized in genetic engineering and biotechnology to introduce foreign genes into bacteria
Ribosomes: Protein Synthesis Machinery
- are small, spherical organelles composed of rRNA and proteins
- Serve as the site of protein synthesis in the cell
- Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S)
- Consist of two subunits: the small (30S) and large (50S) subunits
- Translate mRNA into polypeptide chains using tRNA molecules and amino acids
- Can be found free in the cytoplasm or attached to the cell membrane
Cell Envelope Structures
Cell Wall Composition and Function
- Cell wall is a rigid structure surrounding the
- Provides shape, support, and protection against osmotic stress
- Composed of , a mesh-like polymer of sugar and amino acid chains
- bacteria have a thick cell wall with many layers of peptidoglycan
- bacteria have a thin cell wall with few layers of peptidoglycan and an outer membrane
- Maintains cell integrity and prevents cell lysis in hypotonic environments
- Acts as a barrier against harmful substances and enzymes
Capsule: Protective Coating
- is a thick, slimy layer of polysaccharides or proteins that surrounds the cell wall
- Protects the cell from phagocytosis by host immune cells
- Enhances bacterial adherence to surfaces and other cells
- Contributes to , a community of bacteria encased in an extracellular matrix
- Some capsules are virulence factors in pathogenic bacteria (Streptococcus pneumoniae)
Cell Surface Appendages
Pili: Attachment and DNA Transfer
- Pili are short, hair-like protein structures that extend from the cell surface
- Facilitate bacterial attachment to host cells or surfaces
- Sex pili (F pili) are specialized structures that allow for DNA transfer during conjugation
- Donor cell extends a sex pilus to establish contact with a recipient cell
- DNA is transferred from the donor to the recipient through the hollow sex pilus
- Type IV pili are involved in twitching motility, a form of surface movement
- Some pili act as receptors for bacteriophages (viruses that infect bacteria)
Flagella: Bacterial Locomotion
- Flagella are long, whip-like protein structures that protrude from the cell surface
- Provide mobility and allow bacteria to swim towards attractants or away from repellents
- Composed of three main parts: the filament, hook, and basal body
- Filament is the long, helical portion that extends from the cell surface
- Hook connects the filament to the basal body and acts as a flexible joint
- Basal body is embedded in the cell membrane and cell wall, anchoring the flagellum
- Rotate to generate propulsive force, enabling bacterial movement through liquid environments
- Flagellar arrangement varies among bacterial species (monotrichous, lophotrichous, amphitrichous, peritrichous)
Key Terms to Review (23)
Archaea: Archaea are a group of single-celled microorganisms that are distinct from bacteria and eukaryotes, characterized by their unique genetic and biochemical properties. They often thrive in extreme environments, such as hot springs and salt lakes, but can also be found in more common habitats. Archaea play important roles in various ecological processes and have unique features in their cell structure, setting them apart from other prokaryotic organisms.
Bacilli: Bacilli are a class of rod-shaped bacteria that play a vital role in various ecological and biological processes. They are characterized by their cylindrical shape, which can influence their movement, reproduction, and interactions with other organisms. Bacilli can be found in numerous environments, including soil, water, and the human gut, where they can have both beneficial and harmful effects.
Bacteria: Bacteria are single-celled microorganisms that lack a nucleus and other membrane-bound organelles, classifying them as prokaryotes. These tiny organisms play a crucial role in various ecosystems, including human health, where they can be both beneficial and harmful. Understanding their structure, classification, and interaction with the immune system helps to explain their importance in science and medicine.
Binary fission: Binary fission is a type of asexual reproduction commonly found in prokaryotic organisms, particularly bacteria, where a single cell divides into two identical daughter cells. This process is fundamental for the growth and reproduction of these organisms, enabling them to quickly increase their population size. The mechanism involves DNA replication, cell elongation, and the formation of a septum that separates the two new cells.
Biofilm formation: Biofilm formation is the process by which microorganisms adhere to surfaces and each other, creating structured communities that are encased in a self-produced extracellular matrix. This phenomenon is crucial for the survival and proliferation of prokaryotic cells, enabling them to resist environmental stressors and enhance nutrient absorption. Biofilms can form on both biotic and abiotic surfaces, leading to various implications for health, industry, and ecosystems.
Capsule: A capsule is a thick, protective layer that surrounds the cell wall of certain prokaryotic cells, primarily bacteria. This structure serves various functions, such as providing protection from desiccation and shielding the cell from the host immune system. Capsules also play a crucial role in adherence to surfaces and contribute to the overall virulence of some pathogenic bacteria.
Cell Wall: The cell wall is a rigid outer layer found in certain types of cells, primarily in plants, fungi, and prokaryotes. It provides structural support, protection, and helps maintain the shape of the cell. In prokaryotic cells, the cell wall is essential for survival, as it protects against environmental stress and determines cell shape. The composition of cell walls varies among organisms, influencing their properties and functions.
Chemoautotrophy: Chemoautotrophy is a process by which certain organisms, primarily some prokaryotes, obtain energy through the oxidation of inorganic compounds and utilize that energy to convert carbon dioxide into organic compounds. This unique mode of nutrition allows these organisms to thrive in environments devoid of sunlight, making them crucial in various ecosystems, particularly in extreme habitats such as deep-sea vents and sulfur springs.
Cocci: Cocci are a type of spherical bacteria that are classified based on their shape and arrangement. These microorganisms can exist singly, in pairs, chains, or clusters, impacting their identification and behavior in various environments. Their distinctive round shape is crucial in differentiating them from other bacterial forms and plays a significant role in how they interact with their surroundings and host organisms.
Flagella: Flagella are long, whip-like structures that protrude from the surface of certain cells, enabling movement through liquid environments. These structures are crucial for the motility of various organisms, particularly prokaryotic cells like bacteria, and they play a significant role in their ability to navigate their surroundings, find nutrients, and evade predators.
Gram-negative: Gram-negative refers to a classification of bacteria that do not retain the crystal violet stain used in the Gram staining procedure, resulting in a pink or red appearance under a microscope. This characteristic is primarily due to their cell wall structure, which is composed of a thin layer of peptidoglycan surrounded by an outer membrane that contains lipopolysaccharides. Gram-negative bacteria are often more resistant to antibiotics and can be more virulent compared to gram-positive bacteria due to this unique structural composition.
Gram-positive: Gram-positive refers to a category of bacteria that retain the crystal violet stain used in the Gram staining procedure, resulting in a blue or purple appearance under a microscope. This characteristic is primarily due to a thick peptidoglycan layer in their cell wall, which is crucial for their structural integrity and resistance to certain antibiotics. Understanding gram-positive bacteria is essential as it connects to various aspects of bacterial structure, growth, reproduction, and their interactions with the environment.
Horizontal gene transfer: Horizontal gene transfer is the process by which organisms exchange genetic material directly, rather than through traditional inheritance from parent to offspring. This phenomenon is particularly significant in prokaryotic cells, as it enables bacteria to acquire new traits rapidly, such as antibiotic resistance or metabolic capabilities. The ability to exchange genes allows prokaryotes to adapt quickly to changing environments and contributes to genetic diversity within microbial populations.
Lack of nucleus: The lack of nucleus refers to the absence of a membrane-bound nucleus in certain cells, particularly prokaryotic cells. This key characteristic distinguishes prokaryotes from eukaryotes, where a defined nucleus houses genetic material. In prokaryotes, the genetic material is located in a region called the nucleoid, which is not enclosed by a membrane, allowing for different cellular processes and organization compared to eukaryotic cells.
Nucleoid: The nucleoid is a distinct region within prokaryotic cells where the genetic material, primarily DNA, is located. Unlike eukaryotic cells that have a defined nucleus, the nucleoid is not surrounded by a membrane and is instead characterized by a more open structure, often containing a single circular DNA molecule that holds the cell's genetic information. This unique organization allows for essential processes such as replication and transcription to occur in close proximity, influencing gene expression and cellular function in prokaryotes.
Peptidoglycan: Peptidoglycan is a polymer that forms a crucial part of the bacterial cell wall, providing structural support and shape. This molecule consists of a glycan chain made of alternating N-acetylglucosamine and N-acetylmuramic acid, cross-linked by peptide chains. The presence of peptidoglycan differentiates bacteria from eukaryotic cells, as it is absent in the cell walls of plants and animals, making it an important target for antibiotics that inhibit its synthesis.
Photoheterotrophy: Photoheterotrophy is a metabolic process where organisms use light as their energy source while obtaining carbon from organic compounds instead of carbon dioxide. This unique mode of nutrition is primarily found in certain prokaryotes, allowing them to thrive in various environments by utilizing both light and organic molecules for growth.
Pili: Pili are thin, hair-like structures found on the surface of many prokaryotic cells, especially bacteria. These appendages play essential roles in various functions, including attachment to surfaces, facilitating movement, and aiding in the exchange of genetic material during conjugation. The presence of pili enhances a bacterium's ability to adhere to host tissues and contributes to its pathogenicity.
Plasma Membrane: The plasma membrane is a biological barrier that surrounds and protects the contents of a cell, composed mainly of a phospholipid bilayer with embedded proteins. This structure is crucial for maintaining homeostasis by controlling the movement of substances in and out of the cell, thereby playing a key role in the overall functionality of prokaryotic cells and bacteria.
Plasmids: Plasmids are small, circular pieces of DNA found in prokaryotic cells that exist independently of chromosomal DNA. They often carry genes that provide additional traits to the bacterium, such as antibiotic resistance, and play a crucial role in genetic variation and adaptability. Due to their ability to replicate independently, plasmids are widely used in biotechnology and genetic engineering to transfer specific genes into organisms.
Prokaryotic Cells: Prokaryotic cells are simple, unicellular organisms that lack a nucleus and membrane-bound organelles. These cells are characterized by their smaller size and a more primitive structure compared to eukaryotic cells. Prokaryotic cells are essential in understanding the diversity of life, as they include bacteria and archaea, which play crucial roles in ecosystems, biotechnology, and human health.
Ribosomes: Ribosomes are complex molecular machines found within all living cells that serve as the site of protein synthesis. They translate messenger RNA (mRNA) sequences into amino acid chains, forming proteins that perform vital functions in the cell. Ribosomes can be free-floating in the cytoplasm or attached to the endoplasmic reticulum, playing a critical role in the overall function and health of both eukaryotic and prokaryotic cells.
Smaller size: In biological terms, smaller size refers to the generally reduced dimensions of prokaryotic cells compared to eukaryotic cells. This smaller size is significant because it allows prokaryotes to have a higher surface area-to-volume ratio, which enhances their efficiency in nutrient uptake and waste elimination. Additionally, the compact nature of prokaryotic cells contributes to their rapid reproduction and adaptability in various environments.