70S Ribosomes: Prokaryotic ribosomes are smaller than their eukaryotic counterparts, with a sedimentation coefficient of (composed of and subunits). They are the site of protein synthesis and are found scattered throughout the cytoplasm.
Functional Significance: The structural difference between and ribosomes is a key target for many antibiotics. These drugs can inhibit bacterial protein synthesis without affecting the ribosomes of the human host cells.
Lack of Endomembrane System: Unlike eukaryotes, prokaryotes do not have a rough endoplasmic reticulum. Ribosomes translate mRNA directly in the cytoplasm, and proteins are either used locally or secreted via the plasma membrane.
Murein Cell Wall: The cell wall is a rigid structure composed of murein (also known as peptidoglycan), a polymer of sugars and amino acids. It provides mechanical strength, maintains cell shape, and prevents the cell from bursting due to osmotic pressure.
Plasma Membrane: Located just inside the cell wall, the plasma membrane is a phospholipid bilayer that controls the entry and exit of substances. In some species, it may fold inward to form structures for specialized processes like photosynthesis.
Capsule (Slime Layer): Some bacteria possess an outer layer called a capsule, made of mucilaginous polysaccharides. This layer protects the bacterium from desiccation and helps it evade the host's immune system by masking surface antigens.
Flagella: These are long, whip-like appendages used for motility. Unlike eukaryotic flagella, prokaryotic flagella are made of the protein flagellin and rotate like a propeller to move the cell through liquid media.
Pili and Fimbriae: Pili are hair-like structures used for attachment to surfaces or other cells. Specialized 'sex pili' are involved in the transfer of genetic material (plasmids) between bacterial cells.
Infolding of Membranes: While lacking organelles, some prokaryotes have complex infoldings of the plasma membrane. these increase surface area for metabolic enzymes, supporting processes like aerobic respiration or nitrogen fixation.
| Feature | Prokaryotic Cells | Eukaryotic Cells |
|---|---|---|
| Size | Small () | Large (up to ) |
| DNA | Circular, naked, in cytoplasm | Linear, protein-bound, in nucleus |
| Organelles | None membrane-bound | Many (Mitochondria, Golgi, etc.) |
| Ribosomes | ||
| Cell Wall | Murein (Peptidoglycan) | Cellulose (plants) or Chitin (fungi) |
| Division | Binary fission | Mitosis or Meiosis |
Identify the 'Always' vs. 'Sometimes': Always remember that all prokaryotes have a cell membrane, cytoplasm, ribosomes, and circular DNA. Structures like capsules, flagella, and plasmids are 'optional' and vary between species.
Terminology Precision: Use the term murein or peptidoglycan specifically for prokaryotic cell walls. Avoid using 'cellulose' as that is strictly for plant cells.
DNA Description: When describing prokaryotic DNA, always use the keywords circular and not associated with proteins. This distinguishes it from the linear, histone-bound DNA of eukaryotes.
Common Pitfall: Do not confuse plasmids with the main circular DNA. Plasmids are much smaller and carry 'extra' genes, while the main circular DNA carries the essential genome.
