Cell Structure and Function: Prokaryotes vs. Eukaryotes

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Cell Structure: Prokaryotes vs. Eukaryotes

Overview of Cell Types

Prokaryotic and eukaryotic cells represent two fundamental organizational types in microbiology. Understanding their structural differences is essential for grasping microbial physiology and classification.

Prokaryotes: Include bacteria and archaea; lack membrane-bound organelles.
Eukaryotes: Include fungi, protozoa, algae, plants, and animals; possess membrane-bound organelles.

SEM and LM images of prokaryotic and eukaryotic cells

External Structures of Prokaryotic Cells

Prokaryotic cells possess several external structures that contribute to their survival and pathogenicity.

Flagella: Long, whip-like appendages used for locomotion. Arrangements include monotrichous, lophotrichous, amphitrichous, and peritrichous.
Pili: Rigid, hollow tubes for attachment and DNA transfer.
Fimbriae: Short, hair-like fibers for attachment, especially in biofilms.
Glycocalyx: Includes slime layer (loose, protective, aids in attachment) and capsule (firmly attached, protects from phagocytosis).

Structure of a prokaryotic cell with labeled parts Diagram of prokaryotic cell showing external and internal structures

Prokaryotic Cell Envelope and Wall

The cell envelope consists of the cell wall, plasma membrane, and, in some cases, an outer membrane.

Cell Wall: Provides shape, structural support, and protection from lysis. Composed mainly of peptidoglycan.
Gram-Positive: Thick peptidoglycan layer, teichoic acids, stains purple.
Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), stains pink.

Comparison of Gram-positive and Gram-negative cell walls Gram-positive cell wall structure Gram-negative cell wall structure

Table: Comparative Characteristics of Gram-Positive and Gram-Negative Bacteria

Characteristic	Gram-Positive	Gram-Negative
Gram Reaction	Retains crystal violet, stains blue/purple	Can be decolorized, stains red/pink
Peptidoglycan Layer	Thick, multilayered	Thin, single-layered
Teichoic Acid	Present	Absent
Outer Membrane	Absent	Present
LPS Content	Low	High
Flagellar Structure	2 rings in basal body	4 rings in basal body

Internal Structures of Prokaryotic Cells

Prokaryotic cells contain several internal components essential for their function.

Cytoplasm: Gelatinous solution, site of biochemical reactions.
Nucleoid: Region containing a single, circular DNA molecule.
Ribosomes: 70S type, site of protein synthesis.
Inclusions: Reserve deposits of chemicals.
Endospores: Highly resistant structures formed by some bacteria for survival under harsh conditions.
Plasmids: Small, circular DNA molecules conferring traits like drug resistance.
Cytoskeleton: Protein fibers involved in cell shape and division.

Internal structures of a prokaryotic cell Endospore formation diagram

Eukaryotic Cell Structure

External Structures of Eukaryotic Cells

Eukaryotic cells have more complex external structures.

Glycocalyces: Less organized than prokaryotic capsules; function in cell recognition and protection.
Cell Wall: Present in fungi, algae, plants, and some protozoa; absent in animal cells.

Eukaryotic Cytoplasmic Membrane

Fluid Mosaic Model: Composed of phospholipids and proteins; contains steroid lipids for fluidity.
Membrane Rafts: Regions of lipids and proteins for signaling and transport.

Internal Structures of Eukaryotic Cells

Eukaryotic cells contain numerous membrane-bound organelles.

Nucleus: Contains most of the cell’s DNA; surrounded by nuclear envelope.
Endoplasmic Reticulum (ER): Rough ER (with ribosomes) and Smooth ER (without ribosomes); functions in protein and lipid synthesis.
Golgi Body: Processes and packages molecules for export.
Lysosomes, Peroxisomes, Vacuoles: Store and transfer chemicals; lysosomes digest macromolecules, peroxisomes degrade toxins.
Mitochondria: Site of ATP production; contains its own DNA and 70S ribosomes.
Chloroplasts: Found in photosynthetic eukaryotes; site of photosynthesis.
Cytoskeleton: Network of fibers for cell shape, movement, and organelle anchoring.
Ribosomes: 80S type, larger than prokaryotic ribosomes.

Structure of a typical eukaryotic cell

Comparative Summary: Prokaryotes vs. Eukaryotes

Key Differences

Prokaryotes: No membrane-bound organelles, nucleoid region, 70S ribosomes, cell wall with peptidoglycan.
Eukaryotes: Membrane-bound organelles, nucleus, 80S ribosomes, cell wall (if present) lacks peptidoglycan.

Comparison of prokaryotic and eukaryotic cells

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

Small aerobic prokaryotes became internal symbionts within larger anaerobic prokaryotes.
These symbionts evolved into mitochondria and chloroplasts.

Take Home Points

Know the major prokaryotic structures and their functions.
Know the major eukaryotic structures and their functions.
Be able to compare and contrast structures between eukaryotic and prokaryotic cells.

Additional info:

Prokaryotic cell walls are a primary target for antibiotics, as disruption leads to cell lysis.
Gram-negative bacteria are generally more resistant to antimicrobial agents due to their outer membrane.
Endospores are highly resistant to heat, desiccation, chemicals, and radiation.