Prokaryotic Cell Structure and Function: Study Guide

Section 1: Prokaryotic Cell Basics

Overview of Prokaryotic Cells

Prokaryotic cells are fundamental units of life that lack a membrane-bound nucleus and organelles. They are structurally simpler than eukaryotic cells and include bacteria and archaea.

• Definition: Prokaryotes are unicellular organisms without a true nucleus or membrane-bound organelles.

• Cellularity: Prokaryotes are generally unicellular, but some can form colonies or simple multicellular structures.

• Organelles: Prokaryotes do not contain membrane-bound organelles such as mitochondria or endoplasmic reticulum.

• Shapes and Arrangements: Common prokaryotic shapes include cocci (spherical), bacilli (rod-shaped), and spirilla (spiral). Arrangements can be single, chains, clusters, etc.

• Reproduction: Most prokaryotes reproduce asexually by binary fission, a process where the cell divides into two identical daughter cells.

• Photosynthetic Prokaryotes: Some bacteria, such as cyanobacteria, are photosynthetic and can produce their own food using sunlight.

Example: Escherichia coli is a rod-shaped bacterium commonly found in the intestines of humans and animals.

Section 2: Extracellular Structures

Cell Walls and Surface Structures

Prokaryotic cells possess various extracellular structures that provide protection, shape, and aid in interactions with their environment.

• Cell Wall: Most bacteria have a cell wall composed of peptidoglycan, which provides structural support and shape.

• Gram Stain: Bacteria are classified as Gram-positive or Gram-negative based on their cell wall structure and response to Gram staining.

• Outer Membrane: Gram-negative bacteria have an additional outer membrane that contains lipopolysaccharides.

• Glycocalyx: Many bacteria produce a glycocalyx, a sticky layer outside the cell wall that can be a capsule (well-organized) or slime layer (loose).

• Flagella and Pili: Flagella are used for motility, while pili (fimbriae) are involved in attachment and conjugation.

• Biofilms: Communities of microorganisms attached to a surface and embedded in a self-produced matrix.

Example: The capsule of Streptococcus pneumoniae helps it evade the host immune system.

Gram-Positive vs. Gram-Negative Bacteria

• Gram-Positive: Thick peptidoglycan layer, no outer membrane, stains purple.

• Gram-Negative: Thin peptidoglycan layer, outer membrane present, stains pink/red.

Table: Comparison of Gram-Positive and Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present
Teichoic Acids	Present	Absent
Lipopolysaccharide (LPS)	Absent	Present
Gram Stain Color	Purple	Pink/Red

Transport Across Cell Membranes

• Passive Transport: Movement of substances across the membrane without energy input (e.g., diffusion, facilitated diffusion, osmosis).

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Passive movement of molecules via transport proteins.

Equation:

$J = -D \frac{dC}{dx}$ Where: J = flux (rate of movement) D = diffusion coefficient dC/dx = concentration gradient

Section 3: Intracellular Structures

Nucleoid and Ribosomes

Prokaryotic cells contain essential internal structures for genetic information storage and protein synthesis.

• Nucleoid: The region in a prokaryotic cell where the circular DNA chromosome is located. Not surrounded by a membrane.

• Plasmids: Small, circular, double-stranded DNA molecules that replicate independently of the chromosome.

• Ribosomes: Structures responsible for protein synthesis. Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits).

• Sedimentation Rate: The Svedberg (S) unit measures how fast particles sediment in a centrifuge; prokaryotic ribosomes are smaller than eukaryotic (80S) ribosomes.

Equation:

$S = \frac{m(1 - \rho / \rho_0)}{f}$ Where: S = Svedberg unit m = mass of particle ρ = density of particle ρ₀ = density of medium f = frictional coefficient

Endospores

• Definition: Endospores are highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium) to survive harsh conditions.

• Function: Protects genetic material during extreme heat, desiccation, chemicals, and radiation.

• Vegetative Cells: Regular, metabolically active bacterial cells (as opposed to dormant endospores).

Example: Bacillus anthracis forms endospores that can survive in soil for decades.

Endosymbiotic Theory

• Definition: The endosymbiotic theory proposes that eukaryotic organelles such as mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Mitochondria and chloroplasts have their own DNA, ribosomes, and reproduce independently within the cell.

Example: Chloroplasts in plants are believed to have evolved from cyanobacteria.

Additional info: Some explanations and context were expanded for clarity and completeness based on standard microbiology textbooks.