Cell: The Unit of Life — NEET notes

Cell Theory & Cell Types

The cell theory states all living organisms are made of cells, cells are the basic unit of life, and new cells arise from pre-existing cells. Cells are broadly classified as prokaryotic (no membrane-bound nucleus) or eukaryotic (membrane-bound nucleus and organelles).

Cell Theory

Proposed by Schleiden and Schwann (1838-39) and later modified by Virchow. The foundation stone of modern biology.

The cell theory is one of the most fundamental ideas in biology. It has three key postulates that took decades to formulate. It started with Robert Hooke observing dead cork cells in 1665 and wasn't fully complete until Rudolf Virchow added the critical third postulate in 1855.

• Matthias Schleiden (1838) — studied plants, concluded all plants are made of cells
• Theodor Schwann (1839) — studied animals, concluded all animals are made of cells
• Together they proposed: All living organisms are composed of cells and products of cells
• Rudolf Virchow (1855) — added: Omnis cellula-e cellula (all cells arise from pre-existing cells)
• This third postulate was the missing piece — it demolished the idea of spontaneous generation

Exceptions to Cell Theory

Not everything fits neatly into the cell theory. Viruses lack cells, mature RBCs lack a nucleus, and some organisms are multinucleate or acellular.

The cell theory is powerful but not without exceptions. Several biological entities challenge one or more of its postulates. NEET uses these exceptions as trick questions.

• Viruses — not made of cells, no metabolism outside host, yet can reproduce inside host cells. Are they alive? Cell theory says no.
• Mature RBCs (mammals) — lack a nucleus; are they really 'complete' cells?
• Multinucleate organisms — coenocytic hyphae in fungi (many nuclei, no cell walls between them)
• Mitochondria and chloroplasts — have their own DNA and ribosomes, can self-replicate — are they cells within cells?
• Despite these exceptions, the cell theory remains the cornerstone of biology

Prokaryotic vs Eukaryotic Cells

All cells fall into two categories based on whether they have a membrane-bound nucleus. This distinction affects EVERYTHING about how the cell is organized.

Every cell on Earth is either prokaryotic (Greek: *pro* = before, *karyon* = nucleus) or eukaryotic (*eu* = true). The key difference? Prokaryotes have a nucleoid (naked DNA, no membrane). Eukaryotes have a true nucleus enclosed by a double membrane (nuclear envelope).

Plant Cell vs Animal Cell

Both are eukaryotic, but plants have cell walls, plastids, and a large central vacuole that animals lack. Animals have centrioles and lysosomes that are prominent/absent in plants.

Plant and animal cells are both eukaryotic — they share the nucleus, ER, Golgi, mitochondria, and ribosomes. But they differ in several key structures that reflect their different lifestyles: plants are autotrophic and sessile, animals are heterotrophic and motile.

• Both have: nucleus, ER, Golgi, mitochondria, ribosomes, plasma membrane
• Only plants: cell wall, plastids, large central vacuole, plasmodesmata, glyoxysomes
• Only animals: centrioles (for cell division), prominent lysosomes
• Exception: lower plants (some algae, bryophytes) DO have centrioles

Cell Envelope & Modifications

The cell envelope includes the plasma membrane (universal to all cells), the cell wall (plants, fungi, prokaryotes), and surface modifications like glycocalyx. The plasma membrane's fluid mosaic model explains how its lipid bilayer with embedded proteins controls what enters and exits the cell.

Plasma Membrane

Present in ALL cells (prokaryotic and eukaryotic). A selectively permeable lipid bilayer with embedded proteins that controls transport in and out of the cell.

The plasma membrane (cell membrane) is the outermost boundary of animal cells and lies just inside the cell wall in plant/prokaryotic cells. It's made of lipids and proteins (with small amounts of carbohydrates). The ratio of protein to lipid varies — human RBC membrane is roughly 52% protein and 40% lipid.

• Lipid bilayer — two layers of phospholipids with hydrophilic heads facing out and hydrophobic tails facing in
• Also contains cholesterol (in animal cells) — affects membrane fluidity
• Integral/intrinsic proteins — embedded partially or completely through the bilayer (transmembrane proteins)
• Peripheral/extrinsic proteins — loosely attached to the surface, easily removed
• Selectively permeable — allows some molecules through, blocks others
• The lipid bilayer is the reason non-polar molecules pass easily but ions/polar molecules need channels

Fluid Mosaic Model

Proposed by S.J. Singer and G.L. Nicolson (1972). The membrane is a fluid lipid sea with proteins floating in it like icebergs — some anchored deep, some sitting on the surface.

The fluid mosaic model (Singer & Nicolson, 1972) is the currently accepted model of membrane structure. Fluid = the lipids and proteins can move laterally within the bilayer (the membrane isn't rigid, it's like a flowing river). Mosaic = proteins are scattered in the lipid bilayer like tiles in a mosaic pattern.

• Fluid — lipids and proteins can drift laterally (lateral movement is fast; flip-flop between layers is rare and slow)
• Mosaic — proteins are unevenly distributed, creating a mosaic-like pattern
• Integral proteins — partially or fully embedded in the lipid bilayer; some span the entire membrane (transmembrane)
• Peripheral proteins — attached to the surface (inner or outer) by weak bonds; easy to detach
• Membrane fluidity depends on temperature and lipid composition (unsaturated fatty acids = more fluid; cholesterol = buffers fluidity)

Transport Across Membrane

Molecules cross the membrane by passive transport (no energy), active transport (ATP needed), or bulk transport (endocytosis/exocytosis). NEET tests the differences and examples of each.

The plasma membrane is selectively permeable — it decides what gets in and what stays out. Small non-polar molecules (O _2 , CO _2 ) slip right through the lipid bilayer. Ions and large polar molecules need help from proteins.

• Passive transport — down the concentration gradient, NO ATP needed
• Osmosis — movement of WATER through a selectively permeable membrane from high water concentration to low water concentration
• Diffusion — movement of any substance from higher to lower concentration
• Facilitated diffusion — passive but uses membrane proteins (channel or carrier proteins) for molecules that can't cross the lipid bilayer alone
• Active transport — AGAINST the concentration gradient, REQUIRES ATP (e.g., Na ^+ /K ^+ ATPase pump)
• Endocytosis — cell engulfs material (pinocytosis = liquids; phagocytosis = solids)
• Exocytosis — cell expels material (secretion of enzymes, hormones)

Cell Wall

A non-living, rigid structure outside the plasma membrane. Made of cellulose (plants), chitin (fungi), or peptidoglycan (bacteria). Provides structural support and protection.

The cell wall is a rigid, non-living structure that lies outside the plasma membrane. It provides shape, structural support, and protection against mechanical stress and osmotic lysis. Animal cells do NOT have a cell wall — just a plasma membrane.

• Plant cell wall — made of cellulose (a polysaccharide of glucose), plus hemicellulose, pectin, and proteins
• Fungal cell wall — made of chitin (a polymer of N-acetylglucosamine)
• Bacterial cell wall — made of peptidoglycan/murein (sugar + amino acid polymer)
• Primary cell wall — thin, flexible, laid down first by a growing cell
• Secondary cell wall — thick, rigid, deposited INSIDE the primary wall after growth stops (especially in xylem, sclerenchyma)
• Middle lamella — shared layer between adjacent cells, made of calcium pectate — acts as cement holding cells together
• Plasmodesmata — cytoplasmic connections through pores in cell walls linking adjacent plant cells

Glycocalyx

In prokaryotes, glycocalyx refers to the capsule or slime layer outside the cell wall. In animal cells, the glycocalyx is a carbohydrate-rich zone on the outer surface of the plasma membrane.

Glycocalyx is a general term for a carbohydrate-rich layer on the cell surface. In bacteria, it lies outside the cell wall and can be a thick, tough capsule or a loose, diffuse slime layer. In animal cells, it's the sugar coat formed by glycoproteins and glycolipids on the outer surface of the plasma membrane.

• Bacterial capsule — thick, organized glycocalyx; made of polysaccharides (sometimes polypeptides in *Bacillus anthracis*)
• Slime layer — loose, disorganized glycocalyx; easily washed off
• Capsule provides protection from phagocytosis — encapsulated bacteria are more virulent
• Capsule helps in attachment to surfaces (biofilm formation)
• In animal cells, glycocalyx aids in cell-cell recognition, protection, and adhesion
• The glycocalyx determines blood group antigens (A, B, O) on RBC surfaces

Endomembrane System

The endomembrane system is a network of membrane-bound organelles that work together — ER (protein/lipid synthesis), Golgi (processing/packaging), lysosomes (digestion), and vacuoles (storage). They're connected by vesicle traffic, functioning like a cellular postal service.

Endoplasmic Reticulum

A vast network of membrane-bound tubules and cisternae extending from the nuclear envelope. Comes in two types: rough (with ribosomes, makes proteins) and smooth (no ribosomes, makes lipids).

The endoplasmic reticulum (ER) is an extensive network of membrane-bound tubules and flattened sacs (cisternae) that spreads through the cytoplasm. It's continuous with the outer membrane of the nuclear envelope. The ER divides the cell's interior into two compartments — luminal (inside ER) and extra-luminal (cytoplasm).

• ER is the largest membrane-bound organelle in most eukaryotic cells
• RER ribosomes synthesize proteins that are threaded INTO the ER lumen for processing
• SER in liver cells detoxifies drugs and poisons (this is why chronic alcohol drinkers have enlarged SER in liver cells)
• SER in muscle cells is called sarcoplasmic reticulum — stores Ca ^ 2+ ions for muscle contraction
• SER also produces lipids and steroids — abundant in cells that secrete steroid hormones (adrenal cortex, testes, ovaries)

Golgi Apparatus

Discovered by Camillo Golgi (1898). Stacks of flattened membrane sacs (cisternae) with a cis face (receiving) and trans face (shipping). Modifies, sorts, and packages proteins/lipids from the ER.

The Golgi apparatus (also called Golgi complex or Golgi body) consists of stacks of flattened, membrane-bound cisternae (typically 4-8 per stack). It was first described by Camillo Golgi (1898) in nerve cells of a cat. In plant cells, the Golgi is called dictyosome.

• Cis face (forming face) — faces the ER; receives proteins/lipids via transport vesicles from RER
• Trans face (maturing face) — faces the plasma membrane; ships modified products out as vesicles
• Functions: glycosylation (adding sugar chains to proteins → glycoproteins), phosphorylation, sulphation
• Sorts proteins into vesicles destined for: lysosomes, plasma membrane, or secretion outside cell
• Produces lysosomes (primary lysosomes bud off from trans-Golgi)
• In plant cells: important for cell plate formation during cytokinesis and synthesis of cell wall polysaccharides

Lysosomes

Membrane-bound vesicles containing hydrolytic enzymes (about 50 types) that digest worn-out organelles, food particles, and even the entire cell. Called 'suicide bags' by Christian de Duve.

Lysosomes are single membrane-bound vesicles containing powerful hydrolytic (digestive) enzymes — about 50 different types, all working best at acidic pH (~5). They're formed by the Golgi apparatus and were discovered by Christian de Duve (1955), who also coined the term 'suicide bags.'

• Contain acid hydrolases — proteases, lipases, nucleases, glycosidases, phosphatases etc.
• Internal pH maintained at ~5 (acidic) by H ^+ pumps in the lysosomal membrane; cytoplasm is ~7.2
• Autophagy — lysosomes digest the cell's own worn-out organelles (mitochondria, ER fragments)
• Autolysis — lysosomes burst and digest the ENTIRE cell → 'suicide bags' (happens during programmed cell death, metamorphosis)
• Heterophagy — digestion of materials brought into the cell by endocytosis/phagocytosis
• Prominent in WBCs (phagocytic cells) and absent/rare in RBCs

Vacuoles

Membrane-bound sacs for storage. Huge in plant cells (tonoplast-bound central vacuole), small/temporary in animal cells. Also includes food vacuoles, contractile vacuoles, and gas vacuoles.

Vacuoles are membrane-bound compartments filled with water, sap, or other materials. In plant cells, the vacuole can occupy up to 90% of cell volume — it's bounded by a membrane called the tonoplast. In animal cells, vacuoles are small and temporary (if present at all).

• Plant cell vacuole — large central vacuole bound by tonoplast; contains cell sap (water, sugars, salts, pigments, waste products)
• Maintains turgor pressure — keeps the plant cell turgid and the plant upright
• Tonoplast facilitates transport of ions and other materials against concentration gradients
• Food vacuole — formed in protists (Amoeba) during phagocytosis; fuses with lysosomes for digestion
• Contractile vacuole — in freshwater protists (Amoeba, Paramecium); pumps out excess water for osmoregulation
• Gas vacuoles — found in some prokaryotes (cyanobacteria); provide buoyancy to float in water

The Endomembrane System

ER, Golgi, lysosomes, and vacuoles are linked by vesicle traffic and membrane flow. They function as an integrated system, not as independent organelles.

The endomembrane system includes organelles whose membranes and functions are interconnected — ER, Golgi, lysosomes, and vacuoles. They communicate via vesicle transport: small membrane-bound bubbles bud off from one organelle and fuse with another, carrying cargo between them.

• The pathway: RER → transport vesicles → cis-Golgi → trans-Golgi → lysosomes/plasma membrane/secretory vesicles
• All these organelles share membrane components — their membranes are derived from each other
• Mitochondria, chloroplasts, and peroxisomes are NOT part of the endomembrane system — they have independent origins
• Mitochondria and chloroplasts have their own DNA and make some of their own proteins — they're semi-autonomous
• The endomembrane system coordinates: protein synthesis (RER) → modification (Golgi) → disposal/use (lysosomes/secretion)

Mitochondria & Plastids

Both are double membrane-bound, semi-autonomous organelles with their own circular DNA and 70S ribosomes. Mitochondria are the 'powerhouse' (aerobic respiration, ATP synthesis). Plastids are unique to plant cells — chloroplasts (photosynthesis), chromoplasts (color), leucoplasts (storage).

Mitochondria

Double membrane-bound organelles where aerobic respiration occurs. The inner membrane is highly folded into cristae to maximize surface area for ATP production.

Mitochondria (singular: mitochondrion) are the sites of aerobic respiration — they generate most of the cell's ATP. They're found in nearly all eukaryotic cells (absent in mature RBCs of mammals). Shape varies from sausage-shaped to round, typically 1-4 μm in length.

• Outer membrane — smooth, porous, contains porins (allow molecules up to 5 kDa to pass)
• Inner membrane — highly folded into cristae (singular: crista); contains enzymes for electron transport chain (ETC) and ATP synthase
• Intermembrane space — narrow space between outer and inner membranes; accumulates H ^+ ions during ETC
• Matrix — gel-like interior containing enzymes for Krebs cycle, circular DNA, 70S ribosomes, RNA
• Cristae increase surface area → more space for oxidative phosphorylation → more ATP
• Number varies: very high in metabolically active cells (muscle, liver) — can be 1000-2000 per cell

Semi-Autonomous Nature

Mitochondria have their own circular DNA, 70S ribosomes, and can self-replicate by fission. This supports the endosymbiotic theory — they were once free-living bacteria.

Mitochondria are called semi-autonomous organelles because they have their own genetic machinery — but they still depend on the nuclear genome for most of their proteins. They can make some proteins themselves but import the majority from the cytoplasm.

• Have their own circular DNA (similar to bacterial DNA, no histones)
• Have 70S ribosomes (same as bacteria, NOT the 80S ribosomes of eukaryotic cytoplasm)
• Can self-replicate by binary fission (like bacteria)
• Mitochondrial DNA encodes only ~13 proteins (out of ~1500 needed) — the rest come from nuclear DNA
• Endosymbiotic theory (Lynn Margulis) — mitochondria were once free-living aerobic bacteria that were engulfed by an ancestral eukaryotic cell
• Inherited maternally — mitochondria in the zygote come from the egg (sperm contributes almost no mitochondria)

Plastids

Found only in plant cells. Three types: chloroplasts (green, photosynthesis), chromoplasts (colored, attract pollinators), and leucoplasts (colorless, store starch/lipids/proteins).

Plastids are double membrane-bound organelles found only in plant cells (and some protists). They're the largest organelles in a plant cell. Like mitochondria, they have their own DNA and 70S ribosomes — they're semi-autonomous.

• Chloroplasts — contain chlorophyll (green pigment); site of photosynthesis
• Chromoplasts — contain carotenoids (red, orange, yellow pigments); found in fruits, flowers, autumn leaves
• Leucoplasts — colorless; found in non-green parts (roots, underground stems). Sub-types:
• • Amyloplasts — store starch (most common leucoplast)
• • Elaioplasts/Oleoplasts — store lipids/oils
• • Proteinoplasts/Aleuroplasts — store proteins
• Plastids can interconvert: chloroplasts → chromoplasts (green tomato turns red), leucoplasts → chloroplasts (potato turning green in sunlight)

Chloroplast Structure

The chloroplast has a double membrane, an internal thylakoid membrane system (where light reactions occur), and a gel-like stroma (where the Calvin cycle occurs).

The chloroplast is the organelle of photosynthesis — it converts light energy into chemical energy (glucose). It's typically lens-shaped (biconvex), about 5-10 μm long. Its internal structure is beautifully organized for capturing light and fixing carbon.

• Outer membrane — smooth, permeable to small molecules
• Inner membrane — less permeable, encloses the stroma
• Stroma — gel-like matrix; contains enzymes for Calvin cycle (dark reactions/carbon fixation), circular DNA, 70S ribosomes, starch granules
• Thylakoids — flattened membrane-bound sacs within the stroma; site of light reactions
• Grana (singular: granum) — stacks of thylakoids piled like a stack of coins (typically 10-20 thylakoids per granum)
• Stroma lamellae (fret membranes) — flat tubular connections linking grana to each other
• Thylakoid lumen — internal space of thylakoids where H ^+ accumulates during light reactions

Ribosomes, Cytoskeleton & Centrioles

Ribosomes are non-membrane-bound protein factories (70S in prokaryotes, 80S in eukaryotes). The cytoskeleton is a protein scaffold (microfilaments, microtubules, intermediate filaments) that gives shape and enables movement. Centrioles organize the spindle apparatus and form the base of cilia/flagella (9+2 arrangement).

Ribosomes

Found in ALL cells (prokaryotic and eukaryotic). Made of rRNA + proteins. 70S in prokaryotes and organelles; 80S in eukaryotic cytoplasm. The universal protein synthesis machine.

Ribosomes are the only non-membrane-bound organelles (some textbooks don't even call them organelles — they're 'cell inclusions'). They're found in ALL living cells — prokaryotic and eukaryotic. Their sole job: translate mRNA into proteins (translation).

• Made of ribosomal RNA (rRNA) + proteins in roughly equal amounts
• 70S ribosomes — found in prokaryotes, mitochondria, and chloroplasts. Two subunits: 50S (large) + 30S (small)
• 80S ribosomes — found in eukaryotic cytoplasm. Two subunits: 60S (large) + 40S (small)
• 'S' = Svedberg unit (sedimentation coefficient) — NOT additive (50S + 30S = 70S, not 80S)
• Free ribosomes in cytoplasm → make proteins used WITHIN the cell
• Bound ribosomes on RER → make proteins for EXPORT (secretory proteins) or membrane insertion
• Polyribosomes (polysomes) — multiple ribosomes translating the same mRNA simultaneously = faster protein production

Cytoskeleton

A network of protein filaments in the cytoplasm of eukaryotic cells. Three types: microfilaments (actin), microtubules (tubulin), and intermediate filaments (various proteins).

The cytoskeleton is a network of protein fibers extending throughout the cytoplasm of eukaryotic cells. It's NOT a static skeleton — it's dynamic, constantly being assembled and disassembled. It provides mechanical support, cell shape, and enables movement (cell locomotion, organelle transport, cell division).

• Microfilaments (actin filaments) — thinnest (7 nm diameter), made of actin protein
• • Function: muscle contraction, amoeboid movement, cytokinesis (cleavage furrow), cell shape
• Microtubules — thickest (25 nm diameter), hollow tubes made of tubulin protein (α and β tubulin dimers)
• • Function: form spindle fibers (cell division), structure of centrioles/cilia/flagella, intracellular transport (tracks for motor proteins)
• Intermediate filaments — medium (10 nm diameter), made of various proteins (keratin, vimentin, desmin etc.)
• • Function: mechanical strength, resist stretching, anchor nucleus in place
• Microtubules are dynamic — they grow and shrink rapidly (dynamic instability) especially during mitosis

Centrioles

Cylindrical structures made of 9 triplets of microtubules (9+0 arrangement). Found in animal cells and lower plant cells. Organize the mitotic spindle and form the basal body of cilia and flagella.

Centrioles are cylindrical organelles found in the centrosome (cell center) of animal cells. They come in pairs — the two centrioles are oriented perpendicular (at right angles) to each other. Each centriole is made of 9 triplets of microtubules arranged in a ring — the 9+0 pattern (9 triplets, nothing in the center).

• Structure: 9 sets of 3 microtubules each = 9 triplets in a cartwheel arrangement (9+0 pattern)
• Come in pairs — perpendicular to each other within the centrosome
• Function: organize the spindle apparatus during cell division (aster formation)
• Form the basal body — when a centriole migrates to the cell surface and nucleates a cilium/flagellum
• Present in animal cells and lower plant cells (some algae, bryophyte sperm)
• Absent in higher plant cells — they use other mechanisms to organize spindle fibers

Cilia & Flagella

Hair-like projections for movement. Cilia are short and numerous; flagella are long and few. Both have the same internal structure: axoneme with 9+2 arrangement of microtubules.

Cilia and flagella are hair-like projections from the cell surface used for locomotion or moving substances past the cell. Both arise from basal bodies (which are structurally identical to centrioles). Their internal structure — the axoneme — has a beautiful 9+2 arrangement.

• Axoneme — the core of a cilium/flagellum; made of microtubules in the 9+2 arrangement
• 9 peripheral doublets — 9 pairs of microtubules arranged in a ring at the periphery
• 2 central singlets — 2 single microtubules in the center (connected by bridges)
• Peripheral doublets connected to each other by dynein arms (motor protein — uses ATP for movement)
• Radial spokes connect peripheral doublets to the central sheath around the 2 central singlets
• Basal body at the base — has 9+0 triplet arrangement (same as centriole)

Nucleus

The nucleus is the cell's command center — it houses DNA (organized as chromatin/chromosomes), controls gene expression, and directs protein synthesis. Bounded by a double nuclear envelope with pores, it contains the nucleoplasm, chromatin, and the nucleolus (rRNA factory).

Nuclear Envelope

A double membrane that separates the nucleus from the cytoplasm. Studded with nuclear pores that regulate traffic of RNA and proteins between nucleus and cytoplasm.

The nuclear envelope is a double membrane system that encloses the nucleus. The outer membrane is continuous with the rough ER (and often has ribosomes on its surface). The inner membrane is smooth. Between them is the perinuclear space (10-50 nm wide).

• Double membrane — outer and inner, each a lipid bilayer
• Outer membrane is continuous with RER; may have ribosomes attached
• Nuclear pores — about 3000-4000 per nucleus; formed where the two membranes fuse
• Nuclear pores are lined by the nuclear pore complex (NPC) — a protein assembly that regulates transport
• Small molecules and ions pass freely; large molecules (mRNA, ribosomal subunits, proteins) need active transport through pores
• Nuclear lamina — a meshwork of intermediate filaments (lamins) lining the inner surface; provides structural support

Nucleoplasm & Nucleolus

Nucleoplasm is the gel-like matrix of the nucleus. The nucleolus is a dense, non-membrane-bound structure within the nucleus that makes rRNA and assembles ribosomal subunits.

Inside the nuclear envelope is the nucleoplasm (also called nuclear sap or karyoplasm) — a gel-like, slightly acidic matrix that contains dissolved enzymes, nucleotides, proteins, and the chromatin. Suspended within the nucleoplasm is the nucleolus — a dense, spherical body.

• Nucleoplasm — contains ions, enzymes (DNA/RNA polymerase), nucleotides, and chromatin fibers
• Nucleolus — NOT membrane-bound; appears as a dense, darkly-staining region
• Nucleolus is the site of rRNA synthesis and ribosomal subunit assembly
• A cell can have one or more nucleoli (some cells have 2-3)
• Nucleolus is formed around specific chromosomal regions called Nucleolar Organizer Regions (NORs) — these contain rRNA genes
• Nucleolus disappears during cell division (prophase) and reappears after division (telophase)
• Cells actively making proteins (secretory cells) have large, prominent nucleoli

Chromatin & Chromosomes

Chromatin is the loose, extended form of DNA (during interphase). Chromosomes are the condensed, compact form (during cell division). Same material, different packaging levels.

DNA in the nucleus exists in two forms depending on the cell cycle stage. During interphase (non-dividing), DNA is loosely packed as chromatin — a complex of DNA wrapped around histone proteins. During cell division, chromatin condenses (coils tightly) into visible chromosomes.

• Chromatin = DNA + histone proteins + non-histone proteins + RNA
• Euchromatin — loosely packed, lightly stained, transcriptionally active (genes are being expressed)
• Heterochromatin — tightly packed, darkly stained, transcriptionally inactive (genes are silenced)
• During cell division: euchromatin + heterochromatin both condense → visible chromosomes
• Nucleosome — the basic unit of chromatin. DNA (146 bp) wraps ~1.65 turns around a histone octamer (2 each of H2A, H2B, H3, H4)
• Histone H1 — linker histone; holds nucleosomes together and helps further compaction
• Prokaryotes: DNA is NOT associated with histones (they have nucleoid-associated proteins instead)

Chromosome Structure

A metaphase chromosome has two sister chromatids joined at the centromere. The position of the centromere determines the chromosome type: metacentric, submetacentric, acrocentric, or telocentric.

A metaphase chromosome (most condensed, best for studying) consists of two identical sister chromatids joined at the centromere (primary constriction). Each chromatid has a short arm (p arm — *petit*) and a long arm (q arm). The tips are called telomeres.

• Centromere (primary constriction) — holds sister chromatids together; kinetochore forms here (attachment point for spindle fibers)
• Telomeres — protective caps at chromosome ends; prevent deterioration and fusion with neighboring chromosomes
• Secondary constriction — an additional constriction; the region beyond it is called a satellite (SAT chromosome = satellite chromosome)
• NOR (Nucleolar Organizer Region) — located at the secondary constriction; contains rRNA genes

Prokaryotic Cell (Detailed)

Prokaryotic cells (bacteria, cyanobacteria) have a complex cell envelope (glycocalyx + cell wall + plasma membrane), unique flagella (flagellin, not microtubules), pili/fimbriae for attachment, a nucleoid (naked circular DNA), plasmids (extra-chromosomal DNA), and mesosomes (membrane infoldings).

Prokaryotic Cell Envelope

The bacterial cell envelope consists of an outermost glycocalyx (capsule/slime layer), a cell wall (peptidoglycan), and the innermost plasma membrane. Gram staining differentiates two major cell wall types.

The prokaryotic cell envelope is a multi-layered protective system. From outside to inside: glycocalyx (if present) → cell wall → plasma membrane. The cell wall and membrane together are the minimum — glycocalyx may or may not be present.

• Glycocalyx — outermost layer. If thick and tough: capsule. If loose: slime layer. Made of polysaccharides (sometimes polypeptides)
• Cell wall — made of peptidoglycan (murein) — a polymer of NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid) cross-linked by short peptides
• Gram-positive bacteria — thick peptidoglycan layer (20-80 nm); retain crystal violet stain → appear purple/violet
• Gram-negative bacteria — thin peptidoglycan layer (7-8 nm) + additional outer membrane (with lipopolysaccharides/LPS); lose crystal violet, take counterstain → appear pink/red
• Plasma membrane — similar to eukaryotic (lipid bilayer + proteins) but lacks sterols (except in Mycoplasma)

Prokaryotic Flagella

Bacterial flagella are made of flagellin protein (NOT tubulin), have NO membrane covering, and rotate like a propeller. Structurally and functionally unrelated to eukaryotic flagella.

Prokaryotic flagella are thin, helical filaments used for locomotion. They look superficially similar to eukaryotic flagella but are completely different in structure, composition, and mechanism. This is one of NCERT's key points.

• Made of flagellin protein (NOT tubulin like eukaryotic flagella)
• No membrane covering — the filament is naked protein
• No 9+2 axoneme — just a hollow tube of flagellin subunits
• Movement: rotary motion (spins like a propeller); driven by proton motive force, NOT ATP
• Three parts: filament (long helical part) → hook (curved connector) → basal body (anchored in cell envelope)
• Based on flagella number and arrangement: monotrichous (1), amphitrichous (2, one at each end), lophotrichous (cluster at one end), peritrichous (all around)
• Basal body has rings that anchor in the cell wall and membrane

Pili & Fimbriae

Short, hair-like structures on the surface of (mainly Gram-negative) bacteria. Fimbriae help in attachment to surfaces; pili are involved in bacterial conjugation (DNA transfer).

Besides flagella, many bacteria (especially Gram-negative) have short, hair-like surface appendages called fimbriae and pili. They don't help in movement — they serve completely different functions.

• Fimbriae — short, bristle-like fibers; help bacteria attach to host cells and surfaces (biofilm formation)
• Made of pilin protein (same as pili, different subtypes)
• Present in large numbers (hundreds per cell)
• Pili (singular: pilus) — longer than fimbriae, fewer in number (1-10 per cell)
• Sex pili — specialized pili used in bacterial conjugation (transfer of plasmid DNA from donor to recipient bacterium)
• Pili form a conjugation bridge between two bacteria through which DNA passes
• Both are found mainly in Gram-negative bacteria

Nucleoid & Plasmids

The nucleoid is the region where the single circular chromosome resides (no membrane). Plasmids are small, extra-chromosomal circular DNA molecules that confer special abilities like antibiotic resistance.

Prokaryotes don't have a membrane-bound nucleus. Their genetic material is concentrated in a region called the nucleoid. The DNA is a single, circular, double-stranded chromosome — not wrapped around histones (unlike eukaryotic DNA).

• Nucleoid — not membrane-bound; DNA is in direct contact with cytoplasm
• Single circular chromosome — typically one per cell (haploid)
• DNA is naked — not associated with histone proteins (associated with nucleoid-associated proteins instead)
• Plasmids — small, circular, extra-chromosomal DNA molecules; NOT essential for normal cell survival
• Plasmids replicate independently of the chromosomal DNA
• Plasmids carry genes for: antibiotic resistance (R-plasmid), toxin production, heavy metal resistance, nitrogen fixation
• Plasmids are the basis of genetic engineering — used as vectors to carry foreign genes into cells

Mesosomes

Extensions of the plasma membrane that fold into the cell. Their exact function is debated, but NCERT mentions roles in cell wall formation, DNA replication, respiration, and secretion.

Mesosomes are specialized infoldings of the plasma membrane into the cytoplasm of prokaryotic cells. They appear as vesicles, tubules, and lamellae (membrane layers). Think of them as prokaryotic cells trying to compensate for having no internal membrane-bound organelles.

• Formed by infolding (invagination) of the plasma membrane
• Function in cell wall formation during cell division — help in septum formation (dividing wall between daughter cells)
• Function in DNA replication and distribution — chromosome is attached to mesosome, ensuring each daughter cell gets a copy
• Function in respiration — contain respiratory enzymes (like a primitive mitochondrion substitute)
• Also involved in secretion and increasing surface area for enzymatic activity
• More developed in Gram-positive bacteria than Gram-negative