Cell: The Unit of Life is one of the most reliable scoring chapters in NEET Biology. It bridges biochemistry, cell division, and physiology. Because NTA crafts questions directly from NCERT sentences, mastering subtle organelle distinctions guarantees 12 to 16 marks.
1. The Cell Envelope & Prokaryotic Nuances
Prokaryotic cells (bacteria) feature a tightly bound three-layered cell envelope:
- Glycocalyx: Outermost layer. Loose sheath = slime layer; thick and tough = capsule (provides immune protection against phagocytosis).
- Cell Wall: Middle layer made of peptidoglycan; determines cell shape and prevents osmotic bursting.
- Plasma Membrane: Selectively permeable inner lipid bilayer, biochemically similar to eukaryotes.
Mesosomes: Membranous infoldings of the plasma membrane in the form of vesicles, tubules, and lamellae. Crucial functions: cell wall synthesis, DNA replication and distribution to daughter cells, respiration, and increasing surface area for enzymatic content.
2. The Endomembrane System
While eukaryotic organelles are distinct, the Endomembrane System consists of those whose functions are closely coordinated:
| Organelle | Structure | Core Function & NCERT Specifics |
|---|---|---|
| Endoplasmic Reticulum (ER) | Network of tubular membranes; divides intracellular space into luminal & extraluminal compartments | RER: Studded with 80S ribosomes; protein synthesis and secretion. SER: Major site for lipid synthesis; in animal cells, synthesises steroid hormones (estrogen, testosterone). |
| Golgi Apparatus | Parallel flat cisternae (0.5 μm to 1.0 μm diameter) with convex cis (forming) and concave trans (maturing) faces | Packaging of proteins delivered from ER; site of formation of glycoproteins (glycosylation of proteins) and glycolipids. |
| Lysosomes | Membrane-bound vesicular structures formed by packaging in the Golgi apparatus | Rich in hydrolytic enzymes (lipases, proteases, carbohydrases) optimally active at acidic pH (pH ~ 5). |
| Vacuoles | Bound by single membrane called the tonoplast; occupies up to 90% volume of plant cells | Tonoplast transports ions against concentration gradient into the vacuole; sap has higher solute concentration than cytoplasm. |
3. Autonomous Organelles: Mitochondria & Chloroplasts
Both organelles are semi-autonomous due to endosymbiotic origin:
- Double-membrane envelope. Inner membrane of mitochondria folds into cristae (increases surface area for ATP synthase complexes).
- Matrix contains: single circular double-stranded DNA molecule, a few RNA molecules, 70S ribosomes, and components for protein synthesis.
- Chloroplast thylakoids are arranged in stacks called grana connected by flat membranous tubules called stroma lamellae. Stroma contains enzymes required for dark reactions (RuBisCO) and carbohydrate synthesis.
4. Cytoskeleton: Cilia, Flagella, and Centrioles
- Cilia & Flagella: Core is the axoneme with 9 doublets of radially arranged peripheral microtubules and 1 central pair of microtubules (9 + 2 array). Both emerge from centriole-like structures called basal bodies.
- Centrosome & Centrioles: Centrosome contains two cylindrical centrioles lying perpendicular to each other. Shows a cartwheel pattern with 9 peripheral evenly spaced triplets of tubulin fibrils with no central microtubule (9 + 0 array). Form the basal body of cilia/flagella and spindle apparatus during animal cell division.