Cell Theory and Prokaryotic Cell Structure (2B) - MCAT Biological and Biochemical Foundations of Living Systems

Peptidoglycan (murein). Peptidoglycan provides rigidity and shape, unique to bacterial cell walls and absent in eukaryotes.

All organisms are cellular; cells are basic unit; cells arise from cells. These tenets, established by Schleiden, Schwann, and Virchow, form the foundation of understanding cellular organization and reproduction in biology.

Peptidoglycan. Lysozyme hydrolyzes $eta$-1,4 linkages in peptidoglycan, weakening bacterial cell walls.

Pink (Gram-negative). Gram-negative bacteria have these structures, leading to decolorization and pink staining with safranin.

Prokaryotic cell. These features indicate a prokaryote, lacking eukaryotic nuclear membranes and larger ribosomes.

The tenet that cells arise only from preexisting cells. This claim supports spontaneous generation, contradicting the biogenesis principle of cell theory.

Asexual division producing two genetically identical daughter cells. Binary fission ensures rapid reproduction without genetic variation, typical of prokaryotic growth.

Conjugation pilus; transfers DNA between bacteria. The sex pilus forms a bridge for horizontal gene transfer during conjugation, enabling genetic diversity.

Cells contain hereditary information (DNA) passed to daughter cells. This addition incorporates the role of DNA in inheritance, reflecting advances in genetics since the original cell theory.

Energy flow (metabolism) occurs within cells. This emphasizes that cells are sites of biochemical activity, aligning with modern understanding of cellular energetics.

All cells have fundamentally similar chemical composition. This reflects the universal biochemical similarities among cells, supporting evolutionary relatedness.

It is rejected; new cells arise only from preexisting cells. Cell theory's third tenet directly opposes spontaneous generation by asserting biogenesis.

No membrane-bound nucleus or membrane-bound organelles. Prokaryotes lack compartmentalization provided by membranes, distinguishing them from eukaryotes with true nuclei and organelles.

Non-membrane-bound region containing the bacterial chromosome. The nucleoid organizes the prokaryotic genome without a nuclear envelope, allowing direct cytoplasmic access.

Small circular extrachromosomal DNA that replicates independently. Plasmids provide bacteria with accessory genes, often for advantages like antibiotic resistance, separate from the main chromosome.

Prokaryotic $70S$; eukaryotic cytosolic $80S$. Ribosome sizes differ due to subunit compositions, with prokaryotic ones being smaller and targeted by certain antibiotics.

$50S$ large subunit and $30S$ small subunit. The $70S$ ribosome assembles from these subunits to facilitate protein synthesis in prokaryotes.

Lipopolysaccharide (LPS), especially lipid A. LPS in the outer membrane triggers intense immune responses, contributing to pathogenicity in Gram-negative infections.

Gram-negative bacteria. The outer membrane prevents crystal violet retention, allowing counterstain with safranin for pink appearance.

Short surface appendages used mainly for attachment. Fimbriae promote adherence to host tissues or surfaces, crucial for colonization and biofilm formation.

Motility via rotation driven by a proton motive force. Flagella enable chemotaxis and movement, powered by ion gradients across the membrane.

External polysaccharide layer; protects and aids adhesion/biofilms. The capsule enhances virulence by evading host immunity and facilitating surface attachment.

Gram-positive bacteria. The thick layer retains crystal violet dye during Gram staining, indicating structural differences from Gram-negative bacteria.

Plasma membrane, cytosol, DNA, and ribosomes. These universal components enable basic cellular functions like enclosure, metabolism, heredity, and protein synthesis across all cell types.