Prokaryotic vs. Eukaryotic Cells: Structure, Function, and Differences
Prokaryotic vs. Eukaryotic Cells
Prokaryotic Cells
Prokaryotes are evolutionarily ancient life forms that existed billions of years before eukaryotic cells. Even with the evolution of more complex cells, prokaryotes like bacteria and Archaea remain supremely successful. They lack a nucleus.
Eukaryotic Cells
Eukaryotic cells are more complex, evolving from prokaryote-like predecessors. Most familiar living things, including animals, plants, fungi, and protists, are composed of eukaryotic cells. Eukaryotic organisms can be single-celled or multi-celled and possess a nucleus.
Cell Structures
Cell Membrane
The cell membrane (plasma membrane or plasmalemma) is a biological membrane separating a cell’s interior from its external environment. It surrounds all cells and is selectively permeable, controlling substance movement. It contains various biological molecules, primarily proteins and lipids, involved in cellular processes like cell adhesion.
Cell Wall
The plant cell wall is a rigid structure that provides a significant difference between plant and other eukaryotic cells. It gives plant cells a defined shape and influences their functions. Composed of cellulose fiber, polysaccharides, and proteins, the cell wall is thin and less rigid in new cells.
Prokaryotic Cells: Detailed Characteristics
- Most primitive, earliest form of life
- Do not have a pre-defined nucleus
- Chromosomes are dispersed in the cytoplasm
- Contain no membrane-bound organelles
- Have circular chromosomes and lack histone proteins
- Most metabolically diverse
- Small – typically 0.2-2.0 micrometers in diameter
- Have primitive cytoskeletal structures or lack a cytoskeleton
- Smaller (70S) ribosomes
- Do not undergo meiosis but reproduce sexually by transferring DNA fragments through conjugation
Eukaryotic Cells: Detailed Characteristics
- More complex, evolved organisms
- Contain true nuclei with chromosomes compacted as chromatin
- Contain membrane-bound organelles
- Have linear DNA and contain histone proteins
- Larger – typically 10-100 micrometers in diameter
- Have a complex cytoskeleton
- Larger (80S) ribosomes
- Reproduce sexually using meiosis
Gram Test
We can classify prokaryotic cells into Gram-negative and Gram-positive bacteria based on the Gram staining protocol.
Gram-negative bacteria do not retain crystal violet dye and appear pink or red after staining. They have a cytoplasmic membrane and an outer membrane containing lipopolysaccharide.
Gram-positive bacteria retain crystal violet dye and appear dark blue or violet. They retain the stain due to the high amount of peptidoglycan in their cell wall. They have a cytoplasmic layer.
Endosymbiotic Theory
The endosymbiotic theory explains the origins of mitochondria and plastids (organelles in eukaryotic cells). It proposes that these organelles originated as separate prokaryotic organisms that were engulfed by a host cell as endosymbionts. Mitochondria developed from proteobacteria, and chloroplasts developed from cyanobacteria.
Viruses vs. Cells
Viruses are not considered cells because they cannot replicate independently. They must utilize the machinery of a host cell to replicate, making them intracellular parasites.
Main Parts of Eukaryotic Cells
- Nucleus: The “control center” containing the cell’s DNA (chromosomes).
- Organelles: “Little organs” that carry out specific cell functions.
- Mitochondria: The “energy center” that converts food into usable energy (ATP).
- Ribosomes: Synthesize proteins.
- Golgi Apparatus: Processes, packages, and secretes proteins.
- Lysosome: Contains digestive enzymes, breaks down waste, “suicide sac”.
- Endoplasmic Reticulum: Transport network, “intracellular highway”. Ribosomes on the rough ER synthesize proteins that enter the ER for transport.
- Vacuole: Stores water or other substances. Plant cells have a large central vacuole.
- Chloroplast: Uses sunlight to create food through photosynthesis (only in plant cells).
- Cell Wall: Provides additional support (in plant and bacteria cells).
Mitochondria and Chloroplasts
Mitochondria in animal cells convert energy into usable forms. Chloroplasts in plant cells are essential for photosynthesis, converting sunlight into energy.