1.4 Prokaryotic and eukaryotic cells
5 min read•august 16, 2024
Cells come in two main flavors: prokaryotic and eukaryotic. Prokaryotes, like , are simpler and lack a . Eukaryotes, including plants and animals, have a nucleus and other fancy parts. These differences shape how they work and evolve.
Prokaryotes and eukaryotes organize their genes differently. Prokaryotes have a single DNA loop, while eukaryotes pack DNA into chromosomes inside a nucleus. This affects how they make proteins and adapt to their environment.
Prokaryotic vs Eukaryotic Cells
Structural Differences
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- lack membrane-bound nucleus and organelles while possess these structures
- Prokaryotes measure 0.1-5 μm compared to eukaryotes at 10-100 μm with simpler internal organization
- Prokaryotes include bacteria and whereas eukaryotes encompass protists, fungi, plants, and animals
- Eukaryotic cells contain a (, , ) less developed or absent in prokaryotes
- Prokaryotes typically reproduce through
- Involves replication of single circular chromosome
- Cell elongates and splits into two identical daughter cells
- Eukaryotes undergo and meiosis for cell division and reproduction
- Mitosis produces two genetically identical daughter cells
- Meiosis produces four genetically diverse haploid cells (gametes)
Genetic Material Organization
- Prokaryotes contain single molecule in nucleoid region
- Eukaryotes possess organized into multiple chromosomes within membrane-bound nucleus
- Eukaryotic DNA associates with histone proteins forming chromatin structure
- Allows for compaction and regulation of gene expression
- Chromatin can condense further into chromosomes during cell division
- Prokaryotic DNA typically lacks histone association
- Prokaryotes often contain plasmids (small circular DNA molecules) separate from main chromosome
- Plasmids can carry genes for antibiotic resistance or virulence factors
- Can be transferred between bacteria through conjugation
- Eukaryotes generally lack plasmids with some exceptions in certain protists (mitochondrial DNA)
Genetic Material Organization
Gene Structure and Expression
- Prokaryotic genes often organized in operons transcribed as single mRNA
- Example: lac operon in E. coli controls lactose metabolism
- Allows for coordinated regulation of related genes
- Eukaryotic genes typically individually transcribed and contain introns and exons
- Introns removed during RNA splicing
- Exons can be alternatively spliced to produce different protein isoforms
- Prokaryotic transcription and translation coupled in cytoplasm
- Ribosomes can begin translating mRNA while it's still being transcribed
- Eukaryotic transcription occurs in nucleus, translation in cytoplasm
- Spatial separation allows for additional regulation and processing
Post-transcriptional Modifications
- Eukaryotic genes undergo extensive post-transcriptional processing
- Splicing removes introns and joins exons
- 5' capping adds modified guanine nucleotide to protect mRNA
- 3' polyadenylation adds poly-A tail for stability and export
- Prokaryotic mRNA generally lacks post-transcriptional modifications
- Some exceptions exist (tRNA processing, rRNA modifications)
- Eukaryotic mRNA processing enables additional layers of gene regulation
- Alternative splicing can produce multiple protein isoforms from a single gene
- RNA editing can alter the nucleotide sequence of mRNA
Unique Features of Prokaryotes
Cell Envelope and External Structures
- Prokaryotic cells possess peptidoglycan cell wall for structural support and protection
- have thick peptidoglycan layer
- have thin peptidoglycan layer with outer membrane
- External structures aid in various functions
- enable motility (example: H. pylori uses flagella to move through stomach mucus)
- facilitate adhesion and genetic exchange (example: F pilus in E. coli for conjugation)
- provide protection and enhance virulence (example: Streptococcus pneumoniae capsule resists phagocytosis)
Specialized Internal Features
- involve specialized membrane invaginations
- Participate in DNA replication and cell division
- Aid in energy production and protein secretion
- regulate buoyancy in aquatic prokaryotes
- Example: Cyanobacteria use gas vesicles to position themselves in water column for optimal light exposure
- enable magnetotaxis in certain bacteria
- Example: Magnetospirillum magnetotacticum orients itself along Earth's magnetic field
- Prokaryotic ribosomes (70S) found free in cytoplasm
- Smaller than eukaryotic ribosomes (80S)
- Composed of 30S and 50S subunits
Metabolic Diversity and Adaptations
- Prokaryotes exhibit remarkable metabolic diversity
- in cyanobacteria and purple bacteria
- Chemosynthesis in sulfur-oxidizing bacteria (example: Thiobacillus)
- Anaerobic respiration using alternative electron acceptors (nitrate, sulfate)
- allows survival in extreme conditions
- Example: Bacillus anthracis spores can survive harsh environments for years
- Prokaryotes adapt to various ecological niches
- thrive in hot springs (example: Thermus aquaticus)
- live in high-salt environments (example: Halobacterium)
Complexity of Eukaryotic Cells
Membrane-bound Organelles
- Nucleus contains genetic material and regulates gene expression
- Nuclear pores control transport between nucleus and cytoplasm
- Nuclear lamina provides structural support
- generate ATP through oxidative phosphorylation
- Contain own DNA and ribosomes (endosymbiotic origin)
- Cristae increase surface area for ATP production
- synthesizes and modifies proteins and lipids
- Rough ER studded with ribosomes for protein synthesis
- Smooth ER involved in lipid synthesis and detoxification
- modifies, sorts, and packages proteins for secretion or transport
- Consists of stacked cisternae (flattened membrane sacs)
- Forms vesicles for protein transport
- contain hydrolytic enzymes for cellular digestion
- Break down cellular debris, damaged organelles, and engulfed particles
- Maintain cellular homeostasis through autophagy
Cytoskeleton and Intracellular Transport
- Complex cytoskeleton provides structural support and enables cell movement
- Microfilaments (actin) involved in cell shape and muscle contraction
- Intermediate filaments provide mechanical strength
- Microtubules facilitate intracellular transport and form mitotic spindle
- Sophisticated mechanisms for intracellular trafficking
- Vesicle-mediated transport moves proteins between organelles
- Motor proteins (kinesin, dynein) transport cargo along microtubules
- Signal sequences target proteins to specific organelles (example: nuclear localization signal)
Compartmentalization and Regulation
- Spatial and temporal regulation of cellular processes
- Compartmentalization allows for specialized environments (example: low pH in lysosomes)
- Separation of transcription and translation enables additional regulation
- Endomembrane system interconnects organelles
- Includes nuclear envelope, ER, Golgi, lysosomes, and vesicles
- Facilitates protein synthesis, modification, and transport
- Semi-autonomous organelles support
- Mitochondria and chloroplasts contain own DNA and ribosomes
- Evolved from ancient prokaryotic endosymbionts
Key Terms to Review (41)
70s ribosomes: 70s ribosomes are a type of ribosome found primarily in prokaryotic cells, consisting of a large 50s subunit and a small 30s subunit. These ribosomes are essential for protein synthesis, translating mRNA into polypeptides, and are distinguished from eukaryotic ribosomes, which are larger and designated as 80s. The 's' in 70s and 80s refers to the Svedberg unit, a measure of sedimentation rate that correlates with size and shape.
80s ribosomes: 80s ribosomes are a type of ribosome found in eukaryotic cells, which are crucial for protein synthesis. They are composed of two subunits: a larger 60s subunit and a smaller 40s subunit, and they play a central role in translating messenger RNA (mRNA) into proteins. This structure distinguishes them from prokaryotic ribosomes, which are smaller (70s), emphasizing the differences in cellular machinery between eukaryotes and prokaryotes.
Animal cells: Animal cells are eukaryotic cells that are characterized by the absence of a cell wall and the presence of membrane-bound organelles. These cells are essential building blocks of animal tissues and organs, allowing for complex structures and functions that support life processes. The unique features of animal cells distinguish them from prokaryotic cells and plant cells, facilitating their roles in multicellular organisms.
Archaea: Archaea are a group of single-celled microorganisms that are distinct from bacteria and eukaryotes, characterized by unique biochemical and genetic properties. They often thrive in extreme environments, such as hot springs and salt lakes, and possess unique features like ether-linked lipids in their cell membranes and distinct ribosomal RNA sequences. These properties set them apart from other prokaryotic organisms and highlight their evolutionary significance.
Bacteria: Bacteria are single-celled prokaryotic organisms that lack a nucleus and membrane-bound organelles, making them distinct from eukaryotic cells. They can be found in various environments, playing crucial roles in processes like decomposition, nutrient cycling, and even human health. Bacteria have diverse metabolic pathways and can reproduce rapidly, which makes them important both as beneficial microorganisms and as agents of disease.
Binary fission: Binary fission is a type of asexual reproduction in which a single organism divides into two identical daughter cells, each inheriting the genetic material of the parent cell. This process is a primary method of reproduction for prokaryotic organisms, such as bacteria, and it highlights the differences in reproductive strategies between prokaryotic and eukaryotic cells. Understanding binary fission is essential to comprehending how cells replicate and maintain their populations.
Capsules: Capsules are protective outer structures found in some prokaryotic cells, primarily bacteria, that serve to enhance their survival and pathogenicity. These gel-like layers can help bacteria evade the immune response, prevent desiccation, and facilitate adherence to surfaces or host tissues. Capsules are made up of polysaccharides or polypeptides and can vary in thickness and composition, playing a crucial role in bacterial virulence.
Cell theory: Cell theory is a fundamental concept in biology that states all living organisms are composed of one or more cells, the cell is the basic unit of life, and all cells arise from pre-existing cells. This theory emphasizes that cells are the building blocks of life and underscores the relationship between structure and function in living organisms, as well as the continuity of life through cellular division.
Cellular respiration: Cellular respiration is the metabolic process through which cells convert nutrients into energy, primarily in the form of adenosine triphosphate (ATP), by breaking down glucose and other organic molecules. This process takes place in different cellular compartments and involves various organelles that play specific roles, highlighting the complexity of energy production in both simple and complex organisms.
Chloroplast: Chloroplasts are organelles found in plant cells and some algae that are responsible for photosynthesis, the process by which light energy is converted into chemical energy in the form of glucose. These double-membraned structures contain chlorophyll, the green pigment that captures light energy, along with various enzymes and other molecules essential for the photosynthetic process. Chloroplasts are key players in converting solar energy into a usable form for plants, thus playing a crucial role in the ecosystem's energy flow.
Circular DNA: Circular DNA is a type of DNA molecule that has a closed-loop structure, which is commonly found in prokaryotes and some eukaryotic organelles. This unique configuration allows for efficient replication and expression, playing a vital role in the genomic organization and function of these organisms. Unlike linear DNA found in the chromosomes of eukaryotic cells, circular DNA is typically smaller and can replicate independently, making it crucial for processes like gene regulation and plasmid function.
Cytoskeleton: The cytoskeleton is a dynamic network of protein filaments and tubules that provides structural support, shape, and organization to eukaryotic cells. It plays a crucial role in cell motility, intracellular transport, and the maintenance of cell shape. In both prokaryotic and eukaryotic cells, the cytoskeleton is essential for various cellular processes, contributing to the overall function and integrity of cells.
Endoplasmic reticulum (ER): The endoplasmic reticulum (ER) is an extensive network of membranous tubules and sacs that plays a crucial role in the synthesis, folding, modification, and transport of proteins and lipids within eukaryotic cells. It is divided into two distinct regions: the rough ER, which is studded with ribosomes and primarily involved in protein synthesis, and the smooth ER, which is associated with lipid synthesis and detoxification processes. The ER is essential for maintaining cellular function and homeostasis.
Endospore formation: Endospore formation is a survival mechanism used by certain bacteria to produce a highly resistant structure called an endospore, allowing them to endure extreme environmental conditions. This process is crucial for the survival of prokaryotic cells, enabling them to persist in harsh conditions such as high temperatures, desiccation, and exposure to harmful chemicals, highlighting a key adaptation that differentiates these organisms from eukaryotic cells.
Endosymbiotic theory: The endosymbiotic theory proposes that certain organelles in eukaryotic cells, specifically mitochondria and chloroplasts, originated from free-living prokaryotic organisms that were engulfed by ancestral eukaryotic cells. This process led to a mutually beneficial relationship, where the engulfed prokaryotes provided essential functions like energy production and photosynthesis, while the host cell offered protection and nutrients. The theory explains key features of eukaryotic cells, highlighting their evolutionary transition from simpler prokaryotic ancestors.
Eukaryotic cells: Eukaryotic cells are complex, membrane-bound cells that contain a nucleus and organelles, which perform specialized functions. They are typically larger than prokaryotic cells and include a wide range of organisms, from unicellular protists to multicellular plants and animals. The presence of a defined nucleus allows eukaryotic cells to store genetic material more efficiently and regulate gene expression in ways that prokaryotic cells cannot.
Flagella: Flagella are long, whip-like structures that protrude from the cell surface and are primarily used for locomotion. These organelles enable cells, particularly prokaryotic and some eukaryotic organisms, to move through their environment by rotating or waving, providing a crucial means of motility. Flagella are essential for many bacteria in navigating toward nutrients or away from harmful substances, illustrating their importance in cellular behavior and survival.
Gas vesicles: Gas vesicles are small, buoyant structures found in some prokaryotic cells, composed of proteins that create a gas-filled chamber. These structures allow microorganisms to regulate their buoyancy in aquatic environments, enabling them to ascend or descend to optimal light or nutrient levels, which is essential for their survival and growth.
Golgi apparatus: The Golgi apparatus is a membrane-bound organelle found in eukaryotic cells, responsible for modifying, sorting, and packaging proteins and lipids for secretion or delivery to other organelles. This structure plays a critical role in the cell's secretory pathway, ensuring that proteins synthesized in the endoplasmic reticulum are properly processed before reaching their final destinations.
Gram-negative bacteria: Gram-negative bacteria are a class of bacteria that do not retain the crystal violet stain used in the Gram staining protocol, appearing pink or red after being counterstained with safranin. This classification is significant because gram-negative bacteria have a distinctive cell wall structure that includes an outer membrane containing lipopolysaccharides, which can contribute to their pathogenicity and resistance to certain antibiotics.
Gram-positive bacteria: Gram-positive bacteria are a group 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 layer in the cell wall, which helps them resist certain environmental stresses and antibiotics.
Halophiles: Halophiles are organisms that thrive in high-salinity environments, such as salt flats, salt mines, and seawater. These unique microorganisms, often classified within the domain Archaea, have evolved specialized adaptations that allow them to maintain cellular functions and survive in conditions that would be detrimental to most other life forms. Their ability to withstand extreme salinity sets them apart from many other prokaryotic and eukaryotic cells.
Intermediate filaments: Intermediate filaments are a type of cytoskeletal component found in eukaryotic cells that provide structural support and mechanical strength. They are more stable and less dynamic than microtubules and microfilaments, helping to maintain the shape of cells and anchor organelles in place. These filaments play a crucial role in various cellular processes, including cell division and the formation of tissue architecture.
Linear DNA: Linear DNA refers to a type of genetic material that is organized in a straight chain, as opposed to circular DNA found in many prokaryotes. This structure is characteristic of the genomes of eukaryotic organisms, where linear DNA is packaged into chromosomes and associated with proteins, allowing for efficient organization and regulation of gene expression. Understanding linear DNA is essential for grasping how genetic information is stored and transmitted in complex cellular environments.
Lysosomes: Lysosomes are membrane-bound organelles found in eukaryotic cells that contain digestive enzymes responsible for breaking down waste materials, cellular debris, and foreign pathogens. They play a crucial role in maintaining cellular homeostasis and recycling macromolecules, connecting their function to various cellular processes and structures.
Magnetosomes: Magnetosomes are specialized organelles found in certain prokaryotic cells, particularly magnetotactic bacteria, that contain magnetic iron minerals. These organelles enable the bacteria to orient themselves along the Earth's magnetic field lines, which assists them in navigating towards optimal environments for growth, such as areas with low oxygen concentrations. Magnetosomes illustrate the unique adaptations found in prokaryotic cells that enhance their survival in diverse environments.
Membrane-bound organelles: Membrane-bound organelles are specialized structures within eukaryotic cells that are enclosed by lipid membranes, allowing for compartmentalization of various cellular processes. These organelles enable efficient organization of biochemical reactions and play crucial roles in maintaining cellular function. Unlike prokaryotic cells, which lack such structures, eukaryotic cells benefit from having these distinct compartments that facilitate complex cellular activities.
Mesosomes: Mesosomes are folded invaginations of the plasma membrane found in prokaryotic cells, particularly in bacteria. They are believed to play a role in various cellular processes, such as DNA replication and cell division, and are often associated with the organization of enzymes and other proteins involved in cellular metabolism. Their unique structure highlights the differences between prokaryotic and eukaryotic cells, particularly in how cellular functions are compartmentalized.
Microfilaments: Microfilaments are thin, thread-like protein fibers that are part of the cytoskeleton in eukaryotic cells, primarily composed of actin. They play a crucial role in maintaining cell shape, enabling movement, and facilitating cell division. Microfilaments interact with other cytoskeletal components to support cellular structure and function.
Microtubules: Microtubules are dynamic, cylindrical structures made of tubulin protein subunits that play crucial roles in cell shape, transport, and division. They are part of the cytoskeleton in eukaryotic cells, providing structural support and facilitating various cellular processes such as intracellular transport, the separation of chromosomes during cell division, and the movement of cilia and flagella. Microtubules are not found in prokaryotic cells, highlighting a key difference between these two cell types.
Mitochondria: Mitochondria are double-membrane-bound organelles found in eukaryotic cells, often referred to as the 'powerhouses' of the cell due to their role in producing adenosine triphosphate (ATP) through cellular respiration. These organelles are essential for energy metabolism and play a significant part in various metabolic processes, such as the citric acid cycle and oxidative phosphorylation, making them crucial for cell function and survival.
Mitosis: Mitosis is the process of cell division that results in two genetically identical daughter cells from a single parent cell. This process is essential for growth, repair, and asexual reproduction in organisms. Understanding mitosis is crucial for grasping how eukaryotic cells duplicate their genetic material and distribute it evenly during division, while also recognizing its differences from the simpler binary fission seen in prokaryotic cells.
Nucleus: The nucleus is a membrane-bound organelle found in eukaryotic cells that contains the cell's genetic material, organized as DNA molecules. It acts as the control center for cellular activities, including gene expression and replication, thereby playing a vital role in cellular function and inheritance.
Phospholipid bilayer: The phospholipid bilayer is a fundamental component of cell membranes, consisting of two layers of phospholipids arranged tail-to-tail. This structure creates a semi-permeable barrier that separates the interior of the cell from the external environment, allowing for selective transport of substances and contributing to the fluidity and flexibility of the membrane.
Photosynthesis: Photosynthesis is the biological process through which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. This process not only provides energy for the organisms performing it but also produces oxygen as a byproduct, making it essential for life on Earth as it supports the energy needs of nearly all living organisms.
Pili: Pili are hair-like structures found on the surface of many bacteria, primarily prokaryotic cells, that play crucial roles in adhesion, motility, and genetic exchange. These appendages enable bacteria to attach to surfaces, which is vital for colonization and infection, especially in the context of infectious diseases. They can also facilitate the transfer of genetic material between bacterial cells, promoting genetic diversity and adaptability.
Plant cells: Plant cells are eukaryotic cells that are characterized by the presence of a rigid cell wall, chloroplasts for photosynthesis, and large central vacuoles for storage and maintaining turgor pressure. These features differentiate them from animal cells and are essential for the plant's growth, structure, and ability to perform photosynthesis.
Plasmid: A plasmid is a small, circular piece of DNA that exists independently of chromosomal DNA within a cell. It can replicate independently and is commonly found in prokaryotic organisms like bacteria, but can also be present in some eukaryotic cells. Plasmids often carry genes that provide advantageous traits, such as antibiotic resistance, and are crucial tools in recombinant DNA technology and cloning for gene manipulation and expression.
Prokaryotic Cells: Prokaryotic cells are single-celled organisms that lack a membrane-bound nucleus and other organelles. These cells are generally smaller and simpler than eukaryotic cells, featuring a distinct structure that includes a cell wall, plasma membrane, ribosomes, and genetic material typically organized in a single circular chromosome. Their simplicity allows them to thrive in a wide range of environments, making them essential to various ecological processes.
Selective permeability: Selective permeability is the property of cellular membranes that allows certain substances to pass through while restricting others. This characteristic is crucial for maintaining homeostasis within cells, as it regulates the movement of ions, nutrients, and waste products. It enables cells to control their internal environment and respond to external changes effectively.
Thermophiles: Thermophiles are microorganisms that thrive at high temperatures, typically between 45°C and 122°C (113°F and 252°F). These organisms are often found in extreme environments, such as hot springs, geothermal areas, and deep-sea hydrothermal vents. Their ability to survive and reproduce in such heat is due to specialized enzymes and cellular structures that prevent denaturation and maintain stability.