PRELIMS
Cell Organelles and Structure
The eukaryotic cell is a complex system of membrane-bound organelles, each with a specialized function.
Major Organelles and Their Functions
- Mitochondria (Altman, 1886): Powerhouse of the Cell | Site of cellular respiration; synthesizes ATP. Contains own DNA and matrix with folds called Cristae
- Golgi Bodies (Camilo Golgi): Director of Macromolecules | Storage, processing, packaging. In plants, called Dictyosomes
- Endoplasmic Reticulum (ER): Cellular Framework | Network of tubules for material distribution. Rough ER has ribosomes
- Ribosome (Palade): Protein Factory | Site of protein synthesis; made of RNA and protein
- Lysosome (De Duve): Suicidal Bag | Contains hydrolytic enzymes for intracellular digestion. Can digest whole cell
- Centrosome (Boveri): Cell Division Tool | Found only in animal cells. Form spindle fibers during division
- Vacuole: Osmoregulation | Fluid-filled space. Membrane is called Tonoplast. Large in plants, small in animals
- Nucleus: Control Room | Centrally located. Contains nucleolus (rich in RNA/Protein) and chromatin (DNA + Histone)
Plastids (Only in Plant Cells)
- Chloroplasts: Green pigment; Kitchen of the cell. Contains Grana (site of light reaction) and Stroma (site of dark reaction).
- Chromoplasts: Provides various colors to flowers and fruits.
- Leucoplasts: Colorless; stores food as starch, fats, and proteins.
Plant Cell vs. Animal Cell Comparison
Plant Cell:
- Size: Generally larger
- Cell Wall: Present (Cellulose/Chitin)
- Plastids: Present
- Centrosome: Absent
- Vacuoles: One large central vacuole
- Nucleus: Shifted to periphery
Animal Cell:
- Size: Generally smaller
- Cell Wall: Absent
- Plastids: Absent
- Centrosome: Present
- Vacuoles: Many small vacuoles
- Nucleus: Centrally located
Chromosome Counts in Organisms
Organisms have fixed number of chromosomes (Diploid in somatic, Haploid in gametes).
- Ascaris: 1 pair (2n=2) | Cat: 19 pairs (2n=38)
- Mosquito: 3 pairs (2n=6) | Wheat: 21 pairs (2n=42)
- House-fly: 6 pairs (2n=12) | Human: 23 pairs (2n=46)
- Pea: 7 pairs (2n=14) | Chimpanzee: 24 pairs (2n=48)
- Onion: 8 pairs (2n=16) | Horse: 32 pairs (2n=64)
- Tomato: 12 pairs (2n=24) | Dog: 39 pairs (2n=78)
- Frog: 13 pairs (2n=26) | Pigeon: 40 pairs (2n=80)
UPSC Relevance
The "Who's Who" of the cell is frequently tested in Prelims.
- Aliases: Terms like "Suicidal bag" (Lysosome) and "Kitchen of the cell" (Chloroplast) are classic MCQ options.
- Unique Features: Centrosomes in animals and Cell Walls in plants are high-frequency differentiation points.
- Evolutionary Links: Mitochondria and Chloroplasts having their own DNA (Endosymbiotic theory) is a critical S&T concept.
Endosymbiotic Theory (Note)
Proposed by Lynn Margulis. Suggests that Mitochondria and Chloroplasts originated as free-living prokaryotes engulfed by an ancestral eukaryotic cell, forming a symbiotic relationship. Evidence: both organelles have their own circular DNA, 70S ribosomes, and divide independently by fission — similar to bacteria.