NCERT Solutions for Class 11 Biology Chapter 8 Cell: The Unit of Life
NCERT Solutions for Class 11 Biology Chapter 8 Cell: The Unit of Life provides comprehensive notes and detailed answers on cell theory, prokaryotic and eukaryotic cell structures, organelle functions, and chromosome organization.
Overview of Topics & Subtopics (Click to Jump)
1. What is a Cell & Cell Theory
Short Note:
• Definition: The cell is the fundamental structural and functional unit of all living organisms. Anton von Leeuwenhoek first saw and described a live cell.
• Cell Theory Formulation:
- Matthias Schleiden (1838): Observed that all plants are composed of different kinds of cells.
- Theodore Schwann (1839): Reported that animal cells have a thin outer layer (plasma membrane) and stated that plant cells uniquely possess a cell wall.
- Rudolf Virchow (1855): Modified cell theory by adding "Omnis cellula-e cellula" (all cells arise from pre-existing cells).
• Modern Cell Theory Postulates:
1. All living organisms are composed of cells and products of cells.
2. All cells arise from pre-existing cells through cell division.
2. Prokaryotic Cells
Short Note:
• Represented by Bacteria, Blue-green algae, Mycoplasma (PPLO - Pleuro Pneumonia Like Organisms). Multiply rapidly and lack a membrane-bound nucleus or membrane-bound organelles.
• Cell Envelope: Consists of a tightly bound 3-layered structure: outermost Glycocalyx (slime layer/capsule), middle Cell Wall, and innermost Plasma Membrane.
• Mesosomes: Special membranous structures formed by extensions of plasma membrane into the cell. Function in cell wall formation, DNA replication, respiration, and secretion.
• Genetic Material: Single circular genomic DNA not enclosed by a nuclear membrane (Naked DNA/Nucleoid). Many bacteria also have small circular extrachromosomal DNA called Plasmids (confer resistance to antibiotics).
• Ribosomes & Inclusion Bodies: 70S ribosomes (50S + 30S subunits) site of protein synthesis. Reserve materials stored as non-membrane bound inclusion bodies (e.g., phosphate granules, cyanophycean granules, glycogen granules).
3. Eukaryotic Cell Structure & Organelles
Short Note:
• Characterized by extensive compartmentalization of cytoplasm through membrane-bound organelles and a well-organized nucleus.
• Plasma Membrane: Based on the Fluid Mosaic Model (Singer and Nicolson, 1972). Phospholipid bilayer with embedded proteins (integral and peripheral) and carbohydrates. Shows quasi-fluid nature allowing lateral movement of proteins.
• Endomembrane System: Coordinated organelles including:
- Endoplasmic Reticulum (ER): RER (bearing ribosomes, protein synthesis) and SER (lipid and steroid synthesis).
- Golgi Apparatus: Cis and trans faces; functions in packaging, modification, and secretion of proteins/lipids.
- Lysosomes: Hydrolytic enzyme-rich vesicles ("suicide bags") active at acidic pH.
- Vacuoles: Membrane-bound space (tonoplast) storing water, sap, and excretory products.
• Energy Transducers:
- Mitochondria: Double-membraned "powerhouse of the cell", site of aerobic respiration producing ATP. Contains 70S ribosomes and circular DNA.
- Plastids: Chloroplasts (photosynthesis, contains chlorophyll), Chromoplasts (fat-soluble pigments like carotene/xanthophyll), Leucoplasts (colorless storage plastids like Amyloplasts, Elaioplasts, Aleuroplasts).
• Ribosomes: 80S type (60S + 40S subunits) in eukaryotic cytoplasm, non-membrane bound, discovered by George Palade.
4. Nucleus & Chromosomes
Short Note:
• First described by Robert Brown (1831). Stained material named Chromatin by Flemming.
• Structure: Double-layered nuclear envelope with nuclear pores, nucleoplasm, nucleolus (site of active rRNA synthesis), and chromatin network.
• Chromosome Classification (based on Centromere position):
1. Metacentric: Centromere in the middle forming two equal arms.
2. Sub-metacentric: Centromere slightly away from middle, forming one shorter arm and one longer arm.
3. Acrocentric: Centromere situated close to its end, forming one extremely short and one very long arm.
4. Telocentric: Terminal centromere at the tip of chromosome.
Intext & Conceptual Questions
(b) Schleiden and Schwann formulated the cell theory.
(c) Virchow explained that cells arise from pre-existing cells.
(d) A unicellular organism carries out its life activities within a single cell.
Answer: All options listed above are scientifically correct statements. (If formatted as an option error question: "Schleiden and Schwann modified the cell theory" would be incorrect as Virchow gave the final shape).
A mesosome is a characteristic folded structure in prokaryotic cells formed by the extensions of the plasma membrane into the cytoplasm (in the form of vesicles, tubules, and lamellae).
Functions of Mesosomes:
• Helps in cell wall formation.
• Aids in DNA replication and distribution of chromosomes to daughter cells.
• Increases the surface area of plasma membrane and enzymatic content.
• Plays an active role in cellular respiration and secretion processes.
Textbook Chapter End Exercise Questions
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nuclear Envelope | Absent (Genetic material in Nucleoid). | Present (Well-defined nucleus). |
| Membrane-Bound Organelles | Absent (No ER, Golgi, Mitochondria, etc.). | Present (Mitochondria, ER, Golgi present). |
| Ribosome Type | 70S type (50S + 30S). | 80S type in cytoplasm (70S in organelles). |
| DNA Structure | Circular, naked DNA without histones. | Linear DNA associated with histone proteins. |
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (composed of cellulose/pectin). | Absent. |
| Plastids/Chloroplasts | Present. | Absent. |
| Vacuole | Large central permanent vacuole. | Small, temporary, or absent. |
| Centrosome/Centrioles | Absent in higher plants. | Present (aids in cell division). |
Read More & Chapter Resources
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