Functional Anatomy of Cells: Prokaryotic and Eukaryotic Structures

Prokaryotic Cell Structures & Functions

Shapes and Arrangements of Bacteria

Bacteria exhibit a variety of shapes and arrangements, which are important for identification and classification.

• Coccus: Spherical-shaped bacteria (e.g., Staphylococcus).

• Bacillus: Rod-shaped bacteria (e.g., Bacillus subtilis).

• Spirillum: Spiral-shaped bacteria.

• Arrangements: Bacteria may occur singly, in pairs (diplo-), chains (strepto-), clusters (staphylo-), or other groupings.

Example: Streptococcus pyogenes forms chains of cocci.

Glycocalyx: Capsule vs. Slime Layer

The glycocalyx is a gelatinous, sticky substance surrounding the outside of some bacterial cells.

• Capsule: Organized and firmly attached to the cell wall; protects against phagocytosis.

• Slime Layer: Unorganized and loosely attached; aids in adherence to surfaces.

• Virulence: Capsules enhance pathogenicity by preventing immune recognition and phagocytosis.

Example: The capsule of Streptococcus pneumoniae is essential for its virulence.

Flagella: Structure, Arrangement, and Function

Bacterial flagella are long, whip-like appendages used for motility.

• Three Parts: Filament, hook, and basal body.

• Arrangements: Monotrichous (single), lophotrichous (tuft at one end), amphitrichous (one at each end), peritrichous (all over).

• H Antigen: Flagellar proteins serve as antigens for immune recognition.

• Movement: Bacteria move by rotating flagella; movement is directed by chemotaxis (chemical stimuli) or phototaxis (light stimuli).

• Endoflagella: Found in spirochetes; located within the periplasmic space, causing a corkscrew motion.

Example: Escherichia coli is peritrichous; Treponema pallidum has endoflagella.

Fimbriae and Pili

These are hair-like appendages on bacterial surfaces.

• Fimbriae: Short, numerous; used for attachment to surfaces.

• Pili: Longer, fewer; involved in conjugation (DNA transfer) and sometimes motility.

Example: Fimbriae help Neisseria gonorrhoeae adhere to host tissues.

Cell Wall: Structure and Function

The bacterial cell wall provides shape, protection, and prevents osmotic lysis.

• Peptidoglycan: Main component; consists of repeating disaccharides (NAM & NAG) cross-linked by polypeptides.

• Lysozyme: Enzyme that breaks the glycosidic bonds in peptidoglycan.

• Penicillin: Inhibits peptide cross-linking, weakening the cell wall.

Example: Gram-positive bacteria are more susceptible to lysozyme and penicillin.

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Lipopolysaccharide (LPS)	Absent	Present
Lipid A (Endotoxin)	Absent	Present
O Antigen	Absent	Present

• Lipid A: Endotoxin component of LPS; triggers strong immune responses.

• O-polysaccharide: Serves as the O antigen for immune recognition.

Atypical Cell Walls

Some bacteria have cell walls that differ from the typical Gram-positive or Gram-negative structure.

• Mycoplasma: Lack cell walls; have sterols in the plasma membrane.

• Mycobacterium: Contain mycolic acid (waxy lipid) in cell wall.

• Archaea: May lack peptidoglycan; have pseudopeptidoglycan or proteinaceous walls.

Example: Mycobacterium tuberculosis resists desiccation and antibiotics due to its waxy cell wall.

Plasma (Cell) Membrane

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins.

• Functions: Controls entry/exit of substances, site of metabolic processes (e.g., respiration in prokaryotes).

Cytoplasm and Internal Structures

The cytoplasm contains all materials inside the plasma membrane.

• Nucleoid Region: Contains the bacterial chromosome (DNA); not membrane-bound.

• Plasmids: Small, circular DNA molecules; often carry antibiotic resistance genes.

• Ribosomes: Sites of protein synthesis; prokaryotic ribosomes are 70S (subunits 50S + 30S).

• Streptomycin: Antibiotic that inhibits 70S ribosomes, blocking protein synthesis.

• Inclusions: Reserve deposits (e.g., glycogen, polyphosphate, sulfur granules, gas vacuoles).

Endospores

Endospores are highly resistant, dormant structures formed by some bacteria for survival under harsh conditions.

• Producers: Bacillus and Clostridium species.

• Destruction: Require autoclaving (high pressure and temperature) for destruction.

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

Bacteria vs. Archaea

Feature	Bacteria	Archaea
Cell Wall	Peptidoglycan	No peptidoglycan
Membrane Lipids	Ester-linked	Ether-linked
Ribosomes	70S	70S (different proteins/rRNA)
Pathogenicity	Some pathogenic	None known

Eukaryotic Cell Structures & Functions

Differences Between Eukaryotic and Prokaryotic Cells

Eukaryotic cells are generally larger, have a true nucleus, and possess membrane-bound organelles.

• Nucleus: Present in eukaryotes, absent in prokaryotes.

• Organelles: Eukaryotes have mitochondria, ER, Golgi, etc.

• Cell Division: Eukaryotes divide by mitosis/meiosis; prokaryotes by binary fission.

Flagella and Cilia

Eukaryotic flagella and cilia are complex, membrane-bound structures used for movement.

• Flagella: Long, few; move in a wave-like motion.

• Cilia: Short, numerous; move in a coordinated, beating fashion.

Example: Cilia move mucus in the human respiratory tract.

Cell Wall and Glycocalyx

• Cell Wall: Present in plants (cellulose), fungi (chitin), and some protists; absent in animals.

• Glycocalyx: Carbohydrate-rich layer outside the plasma membrane; involved in cell recognition and protection.

Plasma Membrane

• Structure: Phospholipid bilayer with proteins and sterols (e.g., cholesterol).

• Function: Selective permeability, cell signaling, endocytosis/exocytosis.

• Difference from Prokaryotes: Eukaryotic membranes contain sterols; prokaryotic membranes usually do not (except Mycoplasma).

Cytoplasm and Organelles

• Cytosol: Fluid portion of cytoplasm.

• Cytoskeleton: Network of protein filaments for shape and movement.

• Ribosomes: 80S (60S + 40S subunits); free or bound to ER.

• Nucleus: Contains DNA; surrounded by nuclear envelope.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; protein synthesis.

  • Smooth ER: Lipid synthesis, detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins/lipids.

• Mitochondria: Site of ATP production; contains its own DNA.

• Chloroplasts: Site of photosynthesis in plants/algae; contains its own DNA.

• Lysosomes: Contain digestive enzymes.

• Peroxisomes: Break down fatty acids and detoxify hydrogen peroxide.

• Centrosomes: Organize microtubules during cell division.

• Vacuoles: Storage of substances; large in plant cells.

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Both organelles have double membranes, their own circular DNA, and 70S ribosomes.

Example: Mitochondria are thought to have evolved from aerobic bacteria.

Additional info: Academic context and examples have been added to expand on the brief points in the original outline. Tables have been constructed to compare cell wall types and differences between bacteria and archaea.