All living organisms are made of cells. Cells are classified into two fundamental types based on their internal organisation: prokaryotic cells (pro = before, karyon = nucleus) which lack a membrane-bound nucleus, and eukaryotic cells (eu = true, karyon = nucleus) which have a well-defined, membrane-bound nucleus. Bacteria are prokaryotes; plants, animals, and fungi are eukaryotes.
Prokaryotic: no membrane-bound nucleus; DNA in nucleoid region.
Eukaryotic: true membrane-bound nucleus; DNA linear + histones.
Prokaryotes: 70S ribosomes; Eukaryotes: 80S ribosomes.
Prokaryotes lack membrane-bound organelles; eukaryotes have them.
Prokaryote size: 1–10 µm; Eukaryote size: 10–100 µm.
Examples: Prokaryotes — bacteria, cyanobacteria; Eukaryotes — plants, animals, fungi.
Prokaryotes divide by binary fission; eukaryotes by mitosis/meiosis.
Plasmids (extra-chromosomal DNA) found in prokaryotes, not eukaryotes.
Feature | Prokaryotic Cell | Eukaryotic Cell Nuclear membrane | Absent (no nuclear envelope) | Present (distinct nucleus) DNA form | Circular, not associated with histones | Linear, associated with histone proteins Nucleolus | Absent | Present Cell size | Small (1–10 µm) | Large (10–100 µm) Organelles | No membrane-bound organelles | Have membrane-bound organelles Mitochondria | Absent | Present (eukaryotes) Chloroplasts | Absent (have thylakoids but no plastids) | Present (in plant cells) Endoplasmic Reticulum | Absent | Present Golgi Apparatus | Absent | Present Ribosomes | 70S (smaller) | 80S (in cytoplasm); 70S in mitochondria/chloroplasts Cell wall | Present in bacteria (peptidoglycan) | Plants: cellulose; Fungi: chitin; Animals: absent Flagella | Simple (lacks 9+2 arrangement) | Complex (9+2 microtubule arrangement) Cell division | Binary fission | Mitosis/Meiosis Histones | Absent (DNA naked) | Present Examples | Bacteria, Cyanobacteria (Blue-green algae) | Plants, Animals, Fungi, Protists
Prokaryotic cells are simpler and more primitive:
No membrane-bound nucleus: • DNA is located in a region called the nucleoid (not enclosed by membrane) • DNA is typically circular (loop-shaped) • Not associated with histone proteins
No membrane-bound organelles: • No mitochondria, ER, Golgi, vacuoles • Energy production occurs at the cell membrane (mesosome)
Size: • Typically 1–10 micrometres (µm)
Ribosome size: • 70S ribosomes (S = Svedberg unit) • Made of 30S + 50S subunits
Extra-chromosomal DNA: • May contain small circular DNA called plasmids (not present in eukaryotes)
Examples of prokaryotes: • Bacteria: E. coli, Staphylococcus, Streptococcus, Mycobacterium tuberculosis • Archaea: Thermophiles, Halophiles, Methanogens • Cyanobacteria (Blue-green algae): Nostoc, Anabaena
Eukaryotic cells are complex and evolved:
True membrane-bound nucleus: • Double nuclear membrane (nuclear envelope) with pores • Contains linear DNA associated with histone proteins • Nucleolus present (site of ribosome synthesis)
Membrane-bound organelles: • Mitochondria: 'powerhouse' — produces ATP • Endoplasmic Reticulum: protein and lipid synthesis • Golgi Apparatus: packaging and secretion • Lysosomes: digestion of cellular waste • Vacuoles: storage, regulation
Size: • Typically 10–100 µm — much larger than prokaryotes
Ribosome size: • 80S ribosomes (60S + 40S subunits) in cytoplasm • Note: mitochondria and chloroplasts have 70S ribosomes (evidence for endosymbiotic theory)
Cell division: • Through mitosis (for growth/repair) and meiosis (for sexual reproduction)
Examples of eukaryotes: • Animals: humans, tigers, fish • Plants: mango tree, grass, algae (most) • Fungi: mushrooms, yeast, Aspergillus • Protists: Amoeba, Paramecium, Euglena
The main difference is the presence/absence of a membrane-bound nucleus. Prokaryotic cells lack a nuclear membrane — DNA lies free in the nucleoid region. Eukaryotic cells have a true, membrane-enclosed nucleus. Eukaryotes also have membrane-bound organelles (mitochondria, ER, Golgi) which prokaryotes lack. Examples: Bacteria (prokaryotes); Plants and Animals (eukaryotes).
70S ribosomes: found in prokaryotes (bacteria), also in mitochondria and chloroplasts. Made of 30S + 50S subunits. Smaller. 80S ribosomes: found in eukaryotic cytoplasm. Made of 40S + 60S subunits. Larger. The difference in ribosome type supports the endosymbiotic theory of mitochondrial/chloroplast origin.
Prokaryotic organisms: E. coli, Staphylococcus, Nostoc (cyanobacteria), Mycobacterium tuberculosis, Archaebacteria. Eukaryotic organisms: Animals (humans, fish), Plants (mango tree, grass), Fungi (mushrooms, yeast), Protists (Amoeba, Paramecium).
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