CBSE Class 11 Biology Unit 3: Cell Structure and Functions

Biology involves studying living organisms through their diversity in cellular organization.
Reductionist Biology utilizes a physico-chemical approach to explain biological processes in molecular terms.
Living tissues are analyzed for elements and compounds to determine the molecular basis of physiological processes and disease conditions.

Founded the Madras school of conformational analysis of biopolymers.
Key contributions include the discovery of the triple helical structure of collagen (published in Nature, 1954) and the development of the Ramachandran plot.
Academic history: Graduated at the top of the B.Sc. (Honors) Physics course at the University of Madras (1942) and earned a Ph.D. from Cambridge University (1949).
Influenced by Linus Pauling’s work on $\alpha$ -helix and $\beta$ -sheet models.

The cell is the basic unit of life; unicellular organisms are capable of independent existence and performing essential life functions.
Antonie Von Leeuwenhoek first observed a live cell; Robert Brown discovered the nucleus.
Historical development:
- Matthias Schleiden (1838, German botanist): Observed all plants are composed of cells.
- Theodore Schwann (1839, German zoologist): Concluded animal and plant bodies are composed of cells; identified the plasma membrane and plant cell walls.
- Rudolf Virchow (1855): Formalized the theory with Omnis cellula-e cellula, stating all cells arise from pre-existing cells.
Modern Cell Theory: (i) All living organisms are composed of cells and their products. (ii) All cells arise from pre-existing cells.

Represented by bacteria, blue-green algae, mycoplasmas ( $0.3\,\mu m$ ), and PPLO ( $0.1\,\mu m$ ).
Lacks a membrane-bound nucleus; genetic material is "naked" genomic DNA (single circular chromosome) and frequently contains small circular plasmids.
Cell Envelope: Outermost glycocalyx (slime layer or capsule), cell wall, and plasma membrane.
Mesosomes: Infoldings of the plasma membrane that aid in cell wall formation, DNA replication, and respiration.
Structures for Movement: Flagella (consisting of filament, hook, and basal body), pili, and fimbriae (for attachment).
Ribosomes: $70S$ variety ( $50S$ and $30S$ subunits); site of protein synthesis.

Fluid Mosaic Model (Singer and Nicolson, 1972): Describes the lipid bilayer as quasi-fluid, allowing lateral movement of proteins.
Membrane Composition: Phospholipids, cholesterol, proteins, and carbohydrates. Human erythrocyte membranes contain $52\%$ protein and $40\%$ lipids.
Transport Mechanisms:
- Passive Transport: Simple diffusion, osmosis (water movement).
- Active Transport: Energy-dependent process utilizing ATP, e.g., the $Na^+/K^+$ Pump.
Cell Wall: Provides shape and protection; in plants, it consists of cellulose, hemicellulose, pectins, and proteins. Algae also contain $CaCO_3$ .

Endoplasmic Reticulum (ER): Divided into Rough ER (ribosomes attached; protein synthesis) and Smooth ER (lipid/sterol synthesis).
Golgi Apparatus: Discovered by Camillo Golgi (1898); functions in packaging materials into vesicles and forming glycoproteins/glycolipids.
Lysosomes: Vesicular structures containing hydrolytic enzymes (lipases, proteases) active at an acidic $pH$ .
Vacuoles: Bound by the tonoplast; facilitates ion transport and osmoregulation.

Mitochondria: Bound by double membranes; inner membrane forms cristae to increase surface area for aerobic respiration; contains $70S$ ribosomes and circular DNA.
Plastids: Exclusive to plants and euglenoids.
- Chloroplasts: Contain chlorophyll for photosynthesis; consist of stroma, thylakoids (grana), and circular DNA.
- Storage Plastids: Amyloplasts (starch), Elaioplasts (oils/fats), and Aleuroplasts (proteins).
Ribosomes (George Palade, 1953): Eukaryotic ribosomes are $80S$ ( $60S$ and $40S$ subunits).
Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments for mechanical support and motility.
Cilia and Flagella: Specialized $9+2$ array of microtubules (axoneme) emerging from basal bodies.
Centrosome: Contains two centrioles with a $9+0$ triplet arrangement; forms the spindle apparatus.

Nuclear Envelope: Double membrane with perinuclear space ( $10$ to $50\,nm$ ) and nuclear pores for RNA/protein transport.
Chromatin (Flemming): Contains DNA, histones, and RNA. Human cells contain approximately $2\,m$ of DNA across $46$ chromosomes.
Chromosome Classification (by centromere position):
- Metacentric: Middle centromere; equal arms.
- Sub-metacentric: Off-center centromere; one short and one long arm.
- Acrocentric: Centromere near the end; extremely short arm.
- Telocentric: Terminal centromere.

Match the following:
- (a) Cristae: (ii) Infoldings in mitochondria.
- (b) Cisternae: (iii) Disc-shaped sacs in Golgi apparatus.
- (c) Thylakoids: (i) Flat membranous sacs in stroma.
Function of Nuclear Pores: Passages for movement of RNA and protein molecules in both directions between the nucleus and the cytoplasm.
Living Status of Enucleated Cells: Discussion on whether erythrocytes and sieve tube cells, which lack a nucleus as mature cells, are considered 'living'.
Mesosome Functions: Cell wall formation, DNA replication, distribution to daughter cells, respiration, and secretion processes.