Introduction and Cell Theory
- Cell: The fundamental structural and functional unit of all living organisms NEET 2019.
- Robert Hooke: Discovered and termed the 'cell' in dead cork tissue.
- Anton Von Leeuwenhoek: First saw and described a live cell.
- Robert Brown (1831): Discovered the nucleus NEET 2024 Re.
- Cell Theory Formulators: Matthias Schleiden (1838, Botanist) observed plant cells; Theodore Schwann (1839, Zoologist) observed animal cells, reported the plasma membrane, and concluded the cell wall is unique to plants AIPMT 1993.
- Rudolf Virchow (1855): Modified the theory with Omnis cellula-e cellula (new cells arise from pre-existing cells) NEET 2019.
- Exception to Cell Theory: Viruses do not have a cellular organization AIPMT 1993.
- Microscopy: Electron microscopes have high resolving power due to the very low wavelength of the electron beam AIPMT 1992, 1990. Resolution is the ability to distinguish two close points AIPMT 1991.
An Overview of Cell
- Cytoplasm: Semi-fluid matrix that occupies the cell volume; the main arena for cellular activities and metabolic reactions in both plants and animals AIPMT 2010.
- Size & Shape:
- Mycoplasma: Smallest cells (0.3 µm length), lack a cell wall, can pass through <1 micron filter NEET 2022, 2016.
- Bacteria: Typically 3 to 5 µm.
- Ostrich egg: Largest isolated single cell.
- Eukaryotes vs. Prokaryotes: Eukaryotes have a membrane-bound nucleus and organelles (Amoeba, Paramoecium, Saccharomyces, Euglena). Prokaryotes lack them (Escherichia coli, Streptococcus, Oscillatoria, Nostoc) NEET 2018, AIPMT 2011, 2010.
Prokaryotic Cells
General Characteristics and Cell Envelope
- Genetic material is naked (not enveloped by a nuclear membrane); consists of a single circular DNA (genomic DNA) AIPMT 1998.
- Plasmids: Small, extra-chromosomal circular DNA that confers unique phenotypic characters (e.g., antibiotic resistance) and is used to monitor bacterial transformation.
- Cell Envelope: Tightly bound three-layered structure:
- Glycocalyx: Outermost layer. Can be a loose slime layer or a thick, tough capsule. Provides a sticky character to the cell NEET 2017.
- Cell Wall: Determines shape and provides structural support (made of peptidoglycan) NEET 2016. Difference in Gram +ve and Gram -ve bacteria resides in their cell wall/envelope AIPMT 1990.
- Plasma Membrane: Semi-permeable; structurally similar to eukaryotes.
Surface Structures and Appendages
- Mesosome: Invagination of the plasma membrane (vesicles, tubules, lamellae). Helps in cell wall formation, DNA replication, distribution to daughter cells, respiration, and secretion NEET 2024 Re, 2023, 2015, 2014, AIPMT 1997.
- Chromatophores: Membranous extensions into the cytoplasm containing pigments (found in cyanobacteria) NEET 2015.
- Flagella: Used for motility; composed of three parts: filament (longest portion), hook, and basal body NEET 2014. Structurally different from eukaryotic flagella AIPMT 2004, 1998.
- Pili and Fimbriae: Surface structures not involved in motility NEET 2016. Fimbriae are bristle-like fibers that help attach the bacteria to rocks or host tissues NEET 2015.
Ribosomes and Inclusion Bodies
- Ribosomes (70S): Attached to the plasma membrane (50S + 30S subunits).
- Polyribosome (Polysome): Several ribosomes attached to a single mRNA chain to synthesize multiple copies of a polypeptide simultaneously NEET 2018, 2016, AIPMT 1989.
- Inclusion Bodies: Non-membrane-bound reserve material lying free in the cytoplasm (e.g., phosphate, cyanophycean, and glycogen granules) NEET 2020, 2015. Gas vacuoles are found in blue-green, purple, and green photosynthetic bacteria NEET 2020, 2016.
Eukaryotic Cells
Cell Membrane
- Fluid Mosaic Model: Proposed by Singer & Nicolson (1972) AIPMT 2012. "Protein iceberg in a sea of phospholipid".
- Phospholipid Bilayer: Arranged with polar (hydrophilic) heads outwards and non-polar (hydrophobic) tails inwards. Provides fluidity to the membrane AIPMT 2012.
- Proteins (52% in RBCs): Extrinsic (peripheral) and Intrinsic (integral). Proteins can undergo lateral movement within the lipid bilayer. Both lipids and proteins can flip-flop (lipids frequently, proteins rarely) AIPMT 2008, 2005.
- Carbohydrates: Glycolipids and glycoproteins on the outer surface are crucial for cell-to-cell recognition and adhesion AIPMT 1993.
- Transport:
- Passive: Simple diffusion (neutral solutes) and Osmosis (water) move along the concentration gradient without energy AIPMT 1993.
- Active: Moves molecules against the concentration gradient utilizing ATP (e.g., Na+/K+ pump).
Cell Wall
- Non-living, rigid boundary found in fungi, algae, and plants. Gives shape and protects from mechanical damage.
- Composition: Algae (cellulose, galactans, mannans, CaCO3). Plants (cellulose, hemicellulose, pectins, proteins).
- Middle Lamella: Mainly calcium pectate; acts as a glue holding neighboring cells together AIPMT 2009.
- Plasmodesmata: Cytoplasmic connections traversing the cell wall and middle lamella, facilitating transport between adjacent cells AIPMT 2010, 2009.
- Growth: Occurs by intussusception (addition of materials within the existing wall) and apposition AIPMT 1998.
Endomembrane System
Organelles whose functions are coordinated: Endoplasmic Reticulum, Golgi complex, Lysosomes, and Vacuoles NEET 2021, 2019, AIPMT 2011.
Exception: Mitochondria, Chloroplasts, and Peroxisomes are not part of this system NEET 2023, AIPMT 2011.
| Organelle | Key Features & Functions | PYQ Relevance |
|---|---|---|
| Rough ER (RER) | Bears 80S ribosomes. Active in protein synthesis, folding, secretion, and cleavage of signal peptides. | NEET 2018, 2015, AIPMT 1997 |
| Smooth ER (SER) | Lacks ribosomes. Major site for lipid and steroidal hormone synthesis. | NEET 2020, 2015 |
| Golgi Apparatus | Cisternae arranged near nucleus. Cis face (forming) receives from ER; Trans face (maturing) secretes. Modifies ER proteins. Important site for formation of glycoproteins & glycolipids. | NEET 2024 Re, 2024, 2021, 2020, AIPMT 2011, 2000, 1994 |
| Lysosomes | Membrane-bound vesicles packaged in Golgi. Rich in hydrolytic enzymes active at acidic pH (~5). Become inactive in alkaline pH. | NEET 2022, 2019, 2018, 2016, AIPMT 2012, 1996, 1993 |
| Vacuoles | Bound by a single membrane (tonoplast). In plants, occupies up to 90% of cell volume. Contractile vacuoles in Amoeba handle osmoregulation. | AIPMT 1998, 1991 |
Mitochondria & Plastids
| Feature | Mitochondria | Chloroplast (Plastid) |
|---|---|---|
| Membranes | Double. Outer is permeable (porins) NEET 2019. Inner is relatively less permeable NEET 2024 and infolded into cristae NEET 2024, 2019, AIPMT 1994. | Double. Inner is relatively less permeable NEET 2024. Encloses the stroma. |
| Internal Structures | Matrix contains single circular DNA, RNA, 70S ribosomes NEET 2019, 2012. Cristae contain F0-F1 particles (Oxysomes) for ATP synthesis AIPMT 1992. | Stroma contains single circular DNA, 70S ribosomes, and dark reaction enzymes NEET 2024. Thylakoids (flattened sacs) stack into grana NEET 2015, 2012. |
| Function | Powerhouse of the cell. Site of aerobic respiration and ATP production NEET 2017. | Site of photosynthesis. Chlorophyll located in thylakoids/grana AIPMT 2005. |
| Genetics | Semi-autonomous. Contains extranuclear DNA NEET 2016. | Semi-autonomous. Contains extranuclear DNA NEET 2016, AIPMT 2000. |
- Types of Plastids:
- Leucoplasts (colorless): Amyloplasts (starch), Elaioplasts (oils/fats), Aleuroplasts (proteins).
- Chromoplasts: Fat-soluble carotenoid pigments (carotene, xanthophyll).
Ribosomes & Cytoskeleton
- Ribosomes: Non-membrane bound granular structures (discovered by George Palade) composed of rRNA and proteins NEET 2015, 2012, AIPMT 1995, 1991. Site of protein synthesis AIPMT 1992. Found in cytoplasm, ER, nucleus, chloroplasts, and mitochondria AIPMT 1997. Eukaryotic = 80S, Prokaryotic = 70S.
- Cytoskeleton: Elaborate network of proteinaceous filaments (Microtubules, Microfilaments, Intermediate filaments) maintaining cell shape and motility NEET 2023, AIPMT 2010, 2009.
- Microtubules: Hollow tubes of tubulin. Constituents of spindle fibres, centrioles, cilia, and flagella NEET 2016.
- Microfilaments: Solid, 6 nm diameter, made of a single type of monomer (actin) NEET 2014.
Cilia, Flagella, and Centrioles
- Cilia & Flagella: Core is the axoneme with a 9+2 array of microtubules (9 peripheral doublets + 2 central singlets) NEET 2024, AIPMT 2006. Emerge from centriole-like basal bodies.
- Centrosome & Centrioles: Consists of two perpendicular cylindrical structures. Exhibits a cartwheel pattern NEET 2024 with a 9+0 array (9 peripheral triplets, central hub, no central microtubules). Help form the spindle apparatus during cell division AIPMT 1992.
Nucleus and Chromosomes
- Nucleus: Discovered by Robert Brown (1831). Material named chromatin by Flemming NEET 2024 Re.
- Nuclear Envelope: Double membrane with perinuclear space forming a barrier NEET 2021. Nuclear pores allow RNA/protein movement in both directions NEET 2021.
- Nucleolus: Non-membrane bound, spherical structure inside the nucleoplasm. Site of active ribosomal RNA (rRNA) synthesis NEET 2020, 2018, 2012.
- Exception: Nucleus is absent in mature mammalian erythrocytes (RBCs) and mature sieve tube elements NEET 2021, AIPMT 2009.
- Hammerling’s Experiment: Using the green alga Acetabularia, proved the nucleus controls heredity and cellular differentiation AIPMT 1992, 1989.
- Chromosomes: Highly condensed form of chromatin. Contains DNA, basic histones, non-histone proteins, and RNA.
- Centromere (Primary Constriction): Holds two chromatids together. Required for chromosome segregation/poleward movement AIPMT 1998.
- Secondary Constriction (NOR): Constant location, non-staining, gives the appearance of a satellite NEET 2024.
- Chromosome Types based on Centromere Position NEET 2024 Re, 2022, 2021, 2018:
- Metacentric: Middle centromere; two equal arms.
- Sub-metacentric: Slightly away from the middle; one shorter (p) arm and one longer (q) arm NEET 2019.
- Acrocentric: Situated close to the end; one extremely short, one very long arm.
- Telocentric: Terminal centromere.
- Polytene Chromosomes: Giant chromosomes found in insect salivary glands. Balbiani rings (puffs) are sites of active RNA and protein synthesis NEET 2015, AIPMT 1993. They appear attached at the chromocentre AIPMT 1995, and somatic pairing occurs between homologous chromosomes AIPMT 1992.
Microbodies
- Membrane-bound minute vesicles containing enzymes, present in both plant and animal cells NEET 2021.
- Peroxisomes: Associated with photorespiration in plants AIPMT 1993.