1.9 Prokaryotic and Eukaryotic Cells
Two primary types of structurally different cells have evolved: prokaryotic cells and eukaryotic cells. Prokaryotic cells include organisms like archaea, bacteria, and cyanobacteria, while all other forms of life (plants, animals, fungi, protists) are composed of eukaryotic cells.
Comparison of Prokaryotic and Eukaryotic Cells
| Feature | Prokaryotic Cells | Eukaryotic Cells |
|---|---|---|
| Nucleus & Organelles | A true nucleus and other membrane-bound organelles are absent. | A true nucleus and other membrane-bound organelles (e.g., mitochondria, ER) are present. |
| Organism Complexity | Always unicellular. | Can be unicellular or multicellular. |
| Genetic Material (DNA) | DNA is located in the cytoplasm in a region called the nucleoid. It consists of one primary circular chromosome and may have smaller DNA rings called plasmids. DNA is naked (not associated with histones). | DNA is stored within the membrane-bound nucleus. It is organized into multiple, linear, double-stranded chromosomes associated with histone proteins to form chromatin. |
| Cell Wall | Present in bacteria; composed of peptidoglycan (murein). | If present: cellulose in plants, chitin in fungi; absent in animal cells. |
| Ribosomes | Ribosomes are 70S in size, composed of and subunits. | Ribosomes are larger, 80S in size, composed of and subunits. |
| Respiratory Enzymes | Located on the plasma membrane or its infoldings called mesosomes (since mitochondria are absent). | Located within the mitochondria. |
| Locomotive Structures | If present, flagella are composed of repeated flagellin protein, a hook, and a motor complex. | If present, cilia and flagella are more complex, composed of microtubules in a '9+2' arrangement, dynein motor proteins, and are covered by the plasma membrane. |
| Cell Division | Divide by a simpler process called binary fission. | Divide by more complex processes: mitosis (for growth and repair) and meiosis (for sexual reproduction). |
Summary Table
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Nucleus | Absent | Present |
| Organelles | Absent (membrane-bound) | Present (membrane-bound) |
| DNA Form | Circular chromosome + plasmids; naked | Linear chromosomes; associated with histones |
| Cell Wall | Peptidoglycan (bacteria) | Cellulose (plants) / Chitin (fungi) / Absent (animals) |
| Ribosome Size | ( + ) | ( + ) |
| Respiration Site | Plasma membrane / Mesosomes | Mitochondria |
| Division | Binary Fission | Mitosis & Meiosis |
Biological Significance: The prokaryotic/eukaryotic distinction is a fundamental division in biology, forming the basis for the classification of life into Domains (Bacteria, Archaea, and Eukarya). This structural difference dictates the complexity and functional capabilities of an organism.