Cell: The Unit of Life — Biology Class 11 Notes (CBSE & HBSE)
Free NCERT Biology notes for Cell: The Unit of Life (Class 11) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Cell: The Unit of Life (CBSE & HBSE)
The cell is the basic structural and functional unit of life. This chapter establishes the cell theory, contrasts prokaryotic and eukaryotic cells, and explores cell architecture: the cell membrane (fluid-mosaic model), cell wall, and the full set of organelles — endomembrane system (ER, Golgi, lysosomes, vacuoles), mitochondria, plastids, ribosomes, microbodies, cytoskeleton, cilia/flagella and centrosome — ending with the nucleus and chromosomes. It is conceptually dense and one of the most scoring chapters for CBSE, HBSE and NEET.
Cell Theory and Prokaryotic vs Eukaryotic Cells
The Cell Theory
Building on Robert Hooke's discovery of 'cells' (1665) in cork and Leeuwenhoek's first sight of living cells, the cell theory was framed by Schleiden (1838, plants) and Schwann (1839, animals), and extended by Rudolf Virchow (1855).
The modern cell theory states:
- All living organisms are composed of cells and products of cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells (Omnis cellula-e cellula — Virchow).
Schleiden + Schwann gave the first two ideas but could not explain how new cells form; Virchow supplied the third point.
Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| True nucleus | Absent (nucleoid) | Present (membrane-bound) |
| Membrane organelles | Absent | Present |
| Ribosomes | 70S | 80S (cytoplasm), 70S (organelles) |
| Cell wall | Peptidoglycan (bacteria) | Cellulose (plants)/absent (animals) |
| Size | 1–10 µm | 5–100 µm |
| DNA | Single, circular, naked | Linear, with histones |
| Examples | Bacteria, cyanobacteria, mycoplasma, PPLO | Protists, fungi, plants, animals |
Prokaryotic features worth noting:
- Mesosomes — infoldings of plasma membrane; help in respiration, secretion, DNA replication and cell-wall formation.
- Glycocalyx — outer covering as a loose slime layer or tight capsule.
- Plasmids — extra-chromosomal circular DNA giving traits like antibiotic resistance.
- Bacterial flagellum has three parts: filament, hook, basal body.
- Pili and fimbriae — surface structures for attachment/conjugation.
NEET fact: Mycoplasma is the smallest living cell and lacks a cell wall.
Cell Membrane, Cell Wall and the Endomembrane System
Cell Membrane — Fluid Mosaic Model
The plasma membrane is chiefly phospholipids arranged as a bilayer with proteins. Singer and Nicolson (1972) proposed the fluid mosaic model:
- Lipids are arranged with polar (hydrophilic) heads outward and non-polar (hydrophobic) tails inward.
- Proteins are integral (embedded) or peripheral (on the surface).
- The membrane is quasi-fluid, allowing lateral movement of lipids and proteins → important for growth, endocytosis, cell division.
Transport across the membrane:
| Type | Energy | Direction |
|---|---|---|
| Simple diffusion | No (passive) | High → low conc. |
| Facilitated diffusion | No (passive) | Down gradient via carrier |
| Osmosis | No (passive) | Water, down water potential |
| Active transport | Yes (ATP) | Against gradient (pump) |
Cell Wall
A non-living, rigid outer layer in plants, fungi, algae. In plants it has the middle lamella (calcium pectate, cements cells), primary wall (cellulose) and secondary wall. Functions: shape, support, protection from infection, and cell-to-cell connection via plasmodesmata.
Endomembrane System
Organelles whose functions are coordinated; membranes are continuous or linked via vesicles. Members: ER, Golgi apparatus, lysosomes, vacuoles (mitochondria and chloroplasts are excluded as their functions are independent).
- Endoplasmic Reticulum (ER): RER (with ribosomes) → protein synthesis; SER → lipid/steroid synthesis, detoxification.
- Golgi apparatus: stacked cisternae; cis (forming) face receives, trans (maturing) face dispatches. Packaging, modification and secretion of proteins/lipids; site of glycosylation and formation of lysosomes.
- Lysosomes: membrane sacs rich in hydrolytic (acid) enzymes; intracellular digestion; called 'suicidal bags'.
- Vacuoles: membrane(tonoplast)-bound; in plants occupy up to 90% volume, maintain turgor and store sap.
Trap: Golgi shows cis–trans polarity — material enters at the cis (convex) face and leaves at the trans (concave) face.
Mitochondria, Plastids, Other Organelles and the Nucleus
Mitochondria — 'Power House'
Double-membrane organelle; inner membrane folds into cristae (increase surface area), enclosing the matrix. Site of aerobic respiration producing ATP. Semi-autonomous: has its own circular DNA and 70S ribosomes, divides by fission.
Plastids (plant cells)
- Chloroplasts — green, contain chlorophyll; site of photosynthesis. Have stacked thylakoids (grana) in a stroma; contain DNA and 70S ribosomes.
- Chromoplasts — coloured (yellow/orange/red) pigments.
- Leucoplasts — colourless storage: amyloplasts (starch), elaioplasts (oils), aleuroplasts (proteins).
Other Organelles
| Organelle | Key role |
|---|---|
| Ribosomes | Protein synthesis (70S in pro/organelles, 80S in eukaryotic cytoplasm) |
| Microbodies | Membrane-bound vesicles with enzymes (peroxisomes, glyoxysomes) |
| Cytoskeleton | Microtubules + microfilaments → shape, support, movement |
| Cilia & flagella | Hair-like outgrowths for movement; 9 + 2 axoneme |
| Centrosome/centrioles | 9 + 0 triplet arrangement; form spindle fibres, basal bodies |
The 9 + 2 array describes cilia/flagella (9 peripheral doublets + 2 central); centrioles show 9 + 0 (9 peripheral triplets, no central). A classic NEET discriminator.
Nucleus and Chromosomes
Discovered by Robert Brown (1831). Bounded by a double nuclear envelope with nuclear pores; contains nucleoplasm, one or more nucleoli (ribosomal RNA synthesis) and chromatin.
- Chromatin = DNA + histone proteins; condenses into chromosomes during division.
- A chromosome has a centromere bearing kinetochores; on either side are the arms.
- Based on centromere position: metacentric (middle), sub-metacentric, acrocentric and telocentric (terminal).
- Satellite chromosomes bear a secondary constriction with a small fragment.
Remember: the nucleolus is not membrane-bound and is the site of ribosome (rRNA) formation.
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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Cell: The Unit of Life.