Prokaryotic Cells

Cell Categorization

• Cells are categorized into two major groups: prokaryotic and eukaryotic.

Prokaryotic Cells

• These cells lack a distinct nucleus and membrane-bound organelles.
• They are simpler in structure compared to eukaryotic cells.
• Bacteria and archaea are examples of prokaryotic cells.
• The term "prokaryote" means "before a nucleus" in Greek.
• Genetic material is typically organized as a single circular chromosome in the nucleoid region.
• Prokaryotic cells are usually smaller and simpler than eukaryotic cells.
• They consist of a single compartment, the cytoplasm, surrounded by a plasma membrane.
• Internal membrane-bound organelles are absent; ribosomes are present for protein synthesis only.
• Prokaryotic cells are typically unicellular, but some species can form simple colonies.
• Exhibit diverse metabolic capabilities and differ in needing oxygen for survival.
• Prokaryotic cells include bacteria and Archaea, which are two of the three domains of life
• Are among the earliest forms of life and inhabit diverse environments, from extreme conditions to the human body.

Types of Prokaryotes: Bacteria

• Diverse and found in virtually every habitat, including soil, water, air, and living organisms.
• Possess cell walls containing peptidoglycan.
• Exhibit varied shapes, including spheres(cocci), rods (bacilli), and spirals.
• They encompass various species such as Escherichia coli, Streptococcus pneumoniae, and Staphylococcus aureus.

Types of Prokaryotes: Archaea

• Often thrive in extreme environments like hot springs, acidic or alkaline conditions, deep-sea vents, and salt flats, but also inhabit moderate habitats.
• Have unique membrane lipids and cell walls, differing from bacteria (e.g., pseudopeptidoglycan).
• Present diverse metabolic pathways, being either autotrophic or heterotrophic.
• Methanogens (methane producers) and halophiles (high-salt-thriving organisms) are examples.

Prokaryotic Morphology: Size

• Generally smaller than eukaryotic cells, ranging from about 0.2 - 2 micrometers in diameter.
• Some such as Mycoplasma, are among the smallest cells, while others, like Epulopiscium fishelsoni, can be relatively large.

Prokaryotic Morphology: Cellular Arrangement

• May occur singly or in groups based on their reproduction mode and environmental conditions.
• Some bacteria form pairs (diplo), chains (strepto), clusters (staphylo), or packets (sarcinae) based on division patterns.

Prokaryotic Morphology: Plasma Membrane

• Is a vital structure surrounding the cell, composed of a lipid bilayer with proteins.
• Functions as a selective barrier, controlling substance entry and exit, and communicates with the environment.

Prokaryotic Morphology: Cell Wall

• Provides structural support, protecting against osmotic changes and mechanical stress.
• Bacterial cell walls contain peptidoglycan (a complex polymer of sugars and amino acids).
• Archaeal cell walls may lack peptidoglycan and contain pseudopeptidoglycan, S-layer proteins, or glycoproteins.

Prokaryotic Morphology: Surface Structures

• Additional structures may be present on their surface
• Capsule: Gelatinous layer providing extra protection and adheres to surfaces.
• Pili (or Fimbriae): Hair-like appendages assists adherence to surfaces while facilitating DNA transfer (conjugation).
• Flagella: Whip-like structures extending from the cell surface that enable movement by propellers.

Prokaryotic Morphology: Cytoplasm

• Gel-like substance filling the interior of the cell.
• Site of metabolic reactions, including protein synthesis and nutrient metabolism.

Prokaryotic Morphology: Nucleoid Region

• Lack a true nucleus; instead, genetic material concentrates in the nucleoid.
• Contains a single, circular chromosome of DNA, along with proteins that organize and maintain the DNA.

Prokaryotic Morphology: Ribosomes

• Cellular structures responsible for protein synthesis
• Free-floating in the cytoplasm or attached to the cell membrane.
• Consist of ribosomal RNA (rRNA) and protein molecules, functioning by reading mRNA transcripts to synthesize proteins.

Prokaryotic Morphology: Plasmid

• Small, circular DNA molecules found in some prokaryotes.
• Carry extra genes conveying advantages like antibiotic resistance or metabolizing compounds.
• Transferable between cells through processes like conjugation, facilitating the spread of genetic material.

Shapes of Prokaryotes

• Coccus (plural: cocci): Spherical or round-shaped e.g. Streptococcus pneumoniae and Staphylococcus aureus.
• Bacillus (plural: bacilli): Rod-shaped, can be short, long, or slightly curved e.g. Escherichia coli and Bacillus anthracis.
• Spirillum (plural: spirilla): Spiral-shaped with a rigid helical shape and may have one or more twists e.g. Spirillum volutans.
• Vibrio: Curved or comma-shaped, resembling a curved rod e.g. Vibrio cholerae, the bacterium responsible for cholera.
• Filamentous: Long, thin, and filament-like with branching structures e.g. Actinomycetes, important decomposers in soil.

Eukaryotic Cells

• They possess a true nucleus enclosed within a nuclear membrane in addition to membrane-bound organelles.
• Examples of membrane-bound organelles include mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes.
• Organisms belonging to the domain Eukarya, including animals, plants, fungi, and protists, contain eukaryotic cells.
• Appeared approximately one billion years ago.
• "Eukaryotes" means Eu = true karyot = nucleus, where plants and animals have a real nucleus and a nuclear membrane.
• Generally more advanced than prokaryotes.
• Nuclear membrane surrounds genetic material/DNA.
• Chromosomes are contained within a membranous nuclear envelope.
• The region between the nucleus and the plasma membrane is the cytoplasm.
• The cytoplasm has membrane-bound organelles of specialized form and function.
• Eukaryotic cells are generally bigger compared to prokaryotic cells.
• Eukaryotic cells are typically 10-100 microns in diameter.

Prokaryotic vs Eukaryotic Cells

• Nucleus: Absent vs. Present.
• Genetic Material: Circular DNA (nucleoid region) vs. Linear DNA (enclosed in nucleus).
• Membrane-bound Organelles: Generally absent vs. Present (e.g., mitochondria, endoplasmic reticulum).
• Size: Generally smaller (0.1-5 µm) vs. Generally larger (10-100 µm).
• Cell Wall Composition: Peptidoglycan (bacteria), pseudopeptidoglycan or other substances (archaea) vs. Varies such as cellulose in plants and chitin in fungi).
• Ribosomes: Smaller (70S) vs. Larger (80S in cytoplasm, 70S in organelles).
• Reproduction: Binary fission (asexual) vs. Including mitosis and meiosis (sexual).
• Organelles: Few and without membrane-bounds vs. Numerous and membrane-bound.
• Complexity: Simpler in structure and organization vs. More complex in structure and organization.
• Examples: Bacteria, Archaea vs. Animals, plants, fungi, protists.

Similarities Between Prokaryotes and Eukaryotes

• Both cell types possess cell membranes, which serve as the outer covering of the cell.
• Both cell types contain ribosomes.
• Both cell types have DNA.
• Both cell types have cytoplasm (liquid environment).

Endosymbiosis Theory

• Proposed by Lynn Margulis, suggests that certain organelles in eukaryotic cells, like mitochondria and chloroplasts, originated from symbiotic relationships between primitive prokaryotic cells.
• Ancestral prokaryotes were engulfed by early eukaryotic cells through endocytosis, established a mutually beneficial relationship, and eventually evolved into organelles.
• Endosymbiotic theory: Margulis suggests that mitochondria and chloroplasts were once free-living prokaryotic organisms, possibly related to modern-day bacteria.
• These ancestral prokaryotes were engulfed by larger host cells through endocytosis(one cell engulfs another).
• Instead of being digested, the engulfed prokaryotes sustained a symbiotic relationship to create energy or photosynthesis.
• Over time, the engulfed prokaryotes evolved into organelles, losing original features and becoming dependent on the host cell for survival.
• Eventually led to the formation of eukaryotic cells with membrane-bound organelles.

Origin of the Nucleus and Plasma Membrane

• Is a hypothesis for the evolutionary development of eukaryotic cells.
• Suggests that the nucleus and internal membrane systems, like the endoplasmic reticulum and the Golgi apparatus, originated from infoldings of the plasma membrane of ancestral prokaryotic cells.
• Early stages of eukaryotic cell revolution involved infoldings of the plasma membrane of primitive prokaryotic cells, which increased surface area.
• Internal membranes separated cellular processes and allowed complexity and specialization.
• Compartments enclosed the genetic material, forming the nucleus.
• Endoplasmic reticulum and the Golgi apparatus came about due to further infoldings and allowed protein synthesis and regulation.
• Eukaryotic cells grew to accomplish many functions with efficiency given by specialization by internal membranes.

Evidence Supporting the Endosymbiotic Theory

• Mitochondria and chloroplasts have their own DNA that is similar to prokaryotic DNA.
• Mitochondria and chloroplasts divide like prokaryotes, independently of the cell.
• Mitochondria and chloroplasts have double membranes resembling bacteria.
• Mitochondria and chloroplasts have simple ribosomes of the same size and structure as prokaryotes' ribosomes.
• Fossil records show ancient cells with structures resembling mitochondria and chloroplasts.