CELL: THE UNIT OF LIFE
NCERT Masterclass & NEET Biology Study Module
1. Cell Theory
Formulated by Matthias Schleiden (Botanist, 1838) and Theodore Schwann (Zoologist, 1839). Schwann reported that cells have a thin outer layer (plasma membrane) and that plant cells uniquely possess a cell wall.
2. Prokaryotic Cells
Includes bacteria, blue-green algae, mycoplasma, and PPLO. They lack a membrane-bound nucleus and organelles. Genetic material is naked (Nucleoid).
- Cell Envelope: Consists of tightly bound 3-layered structure: Outer Glycocalyx (Slime layer or tough Capsule), middle Cell wall, and inner Plasma membrane.
- Mesosomes: Infoldings of the plasma membrane (vesicles, tubules, lamellae). Help in cell wall formation, DNA replication, respiration, and secretion.
- Plasmids: Small, circular, extrachromosomal DNA that confers unique phenotypic characters (like antibiotic resistance).
- Surface Structures: Flagella (for motility; composed of filament, hook, basal body). Pili (tubular, for mating) and Fimbriae (bristle-like, for attachment to rocks/host tissues).
- Ribosomes: 70S type (50S + 30S subunits). Site of protein synthesis.
3. Eukaryotic Cells: Cell Membrane & Cell Wall
Eukaryotes include Protists, Fungi, Plants, and Animals. They possess organized nuclei with a nuclear envelope and complex compartmentalization.
"Protein icebergs floating in a sea of lipids." The quasi-fluid nature of lipids enables lateral movement of proteins.
- Lipids: Arranged in a bilayer. Polar (hydrophilic) heads outwards, non-polar (hydrophobic) tails inwards (protects from aqueous environment).
- Proteins: Integral (partially or totally buried) and Peripheral (lie on the surface).
Cell Wall: Non-living rigid structure.
- Algae: Cellulose, galactans, mannans, and minerals like $CaCO_3$.
- Plants: Cellulose, hemicellulose, pectins, and proteins.
- Middle Lamella: Made of calcium pectate, glues neighboring cells together. Plasmodesmata connect the cytoplasm of adjacent cells.
4. The Endomembrane System
Organelles whose functions are coordinated: ER, Golgi complex, Lysosomes, and Vacuoles.
- Endoplasmic Reticulum (ER): Network of tiny tubular structures. Rough ER (RER) has ribosomes (protein synthesis). Smooth ER (SER) lacks ribosomes (lipid and steroid hormone synthesis).
- Golgi Apparatus: Discovered by Camillo Golgi. Consists of flat, disc-shaped sacs (cisternae). Important site of formation of glycoproteins and glycolipids. Modifies and packages materials. Has a forming (cis) face and maturing (trans) face.
- Lysosomes: Membrane-bound vesicular structures containing hydrolytic enzymes (lipases, proteases, carbohydrases) optimally active at acidic pH.
- Vacuoles: Bound by a single membrane called tonoplast. In plant cells, it can occupy 90% of the volume. Tonoplast facilitates transport of ions against concentration gradients. In Amoeba, the contractile vacuole helps in excretion.
5. Mitochondria and Plastids
Both are double membrane-bound and semi-autonomous (contain their own circular DNA and 70S ribosomes).
- Mitochondria: Powerhouses of the cell. Inner membrane forms infoldings called cristae to increase surface area. Produce ATP.
- Plastids: Found in plant cells and euglenoids.
- Chloroplasts: Contain chlorophyll. Inner space is stroma. Thylakoids are arranged in stacks called grana.
- Chromoplasts: Contain fat-soluble carotenoid pigments (carotene, xanthophylls) giving yellow/orange/red colors.
- Leucoplasts: Colorless plastids for storage. Amyloplasts (carbohydrates/starch), Elaioplasts (oils/fats), Aleuroplasts (proteins).
6. Nucleus & Chromosomes
Nucleus was discovered by Robert Brown. Material inside named Chromatin by Flemming. Has a double-layered nuclear envelope with nuclear pores.
Nucleolus: Non-membranous, spherical body inside the nucleus. It is the site for active ribosomal RNA (rRNA) synthesis.
- Cilia and Flagella: Core is called axoneme. Has 9+2 array of microtubules. Base is called basal body.
- Centrosome and Centrioles: Non-membranous. Made of 9 evenly spaced peripheral fibrils (triplets). Array is 9+0. Arranged like a cartwheel. Form the spindle apparatus during cell division in animal cells.
- Satellite: Some chromosomes have a non-staining secondary constriction at a constant location, giving the appearance of a small fragment called the satellite.
🚀 NEET CELL UNIT OF LIFE MEGA QUIZ (100 MCQ)
Solve the 5 parts below to master Cell Theory, Organelles, and Chromosomes.
Post a Comment