Cell

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Cell — Quick Facts

The cell is the fundamental structural and functional unit of all living organisms. All cells arise from pre-existing cells (Rudolf Virchow’s principle: “Omnis cellula e cellula”).

Two Types of Cells:

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	0.1–5 μm	10–100 μm
Nucleus	No true nucleus (nucleoid)	True nucleus with nuclear envelope
DNA	Circular, naked	Linear, associated with histones
Membrane-bound organelles	Absent	Present (mitochondria, ER, Golgi, etc.)
Cell wall	Present (peptidoglycan in bacteria)	Plant cells: cellulose; Animal cells: absent
Mitochondria	Absent (no membrane-bound respiratory system)	Present
Ribosomes	70S (50S + 30S)	80S (60S + 40S) in cytoplasm
Examples	Bacteria, cyanobacteria, mycoplasma	Plants, animals, fungi, protozoa

⚡ Exam tip: 70S ribosomes are found in prokaryotes, chloroplasts, and mitochondria. 80S ribosomes are in the cytoplasm of eukaryotes. (S = Svedberg unit, measures sedimentation rate — not additive!)

⚡ NEET favourite: Mycoplasma is the smallest known cell (0.3 μm) and is the only bacterium lacking a cell wall — hence resistant to penicillin (which targets cell wall synthesis).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Cell — NEET/JEE Study Guide

Plant Cell vs Animal Cell:

Organelle	Plant Cell	Animal Cell
Cell wall	Present (cellulose + pectin + hemicellulose)	Absent
Plastids	Present (chloroplasts, chromoplasts, leucoplasts)	Absent
Vacuole	Large central vacuole (maintains turgor)	Small and numerous
Centrioles	Absent (except in lower plants)	Present (form basal bodies, cilia)
Flagella	Present in gametes	Present in sperm
Lysosomes	Rare	Common

Important Cell Organelles:

1. Nucleus:

   • Nuclear envelope (double membrane with pores)
   • Nucleoplasm with chromatin (DNA + histone proteins)
   • Nucleolus (site of rRNA synthesis) — disappears during cell division
   • Contains genetic information (in eukaryotes)

2. Mitochondria:

   • Double membrane: outer smooth, inner with cristae (increases surface area)
   • Site of aerobic respiration → ATP production (powerhouse of cell)
   • Have their own DNA (circular, prokaryotic-like), ribosomes (70S), and divided independently
   • Semi-autonomous organelle

3. Endoplasmic Reticulum (ER):

   • Rough ER: Has ribosomes → protein synthesis and modification (especially for export)
   • Smooth ER: No ribosomes → lipid synthesis, detoxification (liver), steroid hormone synthesis

4. Golgi Apparatus (Golgi Body / Golgisome):

   • Stack of flattened cisternae (cis face = receiving, trans face = sending)
   • Modifies, packages, and ships proteins from ER
   • Produces lysosomes, secretory vesicles

5. Chloroplast:

   • Double membrane with thylakoid stacks (grana)
   • Site of photosynthesis
   • Contains its own DNA and 70S ribosomes → semi-autonomous
   • Chlorophyll pigment embedded in thylakoid membranes

6. Ribosomes:

   • Site of protein synthesis (translation of mRNA)
   • 70S in prokaryotes, chloroplasts, mitochondria; 80S in eukaryotic cytoplasm
   • Made of rRNA + proteins; two subunits (large + small)

7. Cytoskeleton:

   • Microtubules (tubulin protein, in flagella, centrioles, spindle)
   • Microfilaments (actin protein, in muscle contraction, cell movement)
   • Intermediate filaments (various proteins, provide mechanical strength)

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Cell — Comprehensive Notes

Cell Membrane Structure — Fluid Mosaic Model (Singer & Nicolson, 1972): The plasma membrane is a phospholipid bilayer (7–10 nm thick) with proteins embedded or attached. Phospholipids have a hydrophilic (water-loving) phosphate head and hydrophobic fatty acid tails. Cholesterol in animal cell membranes stabilises the bilayer and reduces permeability.

Membrane proteins are of two types:

• Integral/Intrinsic proteins: Span the bilayer (transmembrane); transport proteins, receptors
• Peripheral/Extrinsic proteins: Attached to surface; enzymes, structural proteins

The “fluid” nature comes from lateral movement of phospholipids and proteins within the bilayer. The “mosaic” refers to the scattered pattern of proteins.

Cell Wall (Plant Cells):

• Primary wall: Cellulose + pectin + hemicellulose, laid down during cell growth
• Middle lamella: Pectin layer between adjacent cells, cements cells together
• Secondary wall: Lignin (for rigidity) deposited inside primary wall in mature cells (xylem, sclerenchyma)

Plastids — Three Types:

1. Leucoplasts: Colourless, store starch (amyloplasts), proteins (proteinoplasts), or fats (elaioplasts)
2. Chromoplasts: Coloured — carotene (red, e.g., tomato), xanthophyll (yellow, e.g., marigold), mixed (e.g., autumn leaves)
3. Chloroplasts: Green — contain chlorophyll a and b; found in mesophyll cells of leaves

Chloroplast structure: Outer membrane → intermembrane space → inner membrane → stroma → thylakoids (stacked = grana) → lamellae connecting grana.

Vacuoles:

• Plant cells have a large central vacuole (up to 90% cell volume in mature cells)
• Tonoplast: Single membrane surrounding vacuole
• Functions: Maintains cell turgidity (osmotic water uptake), storage (water, ions, sugars, pigments), degradation (lytic enzymes)

Nucleus — Chromatin Structure: DNA wraps around histone octamer (2 copies each of H2A, H2B, H3, H4) → nucleosome bead. Nucleosomes coil into 30 nm fibre → loops → coils → metaphase chromosome.

Histone proteins are positively charged (rich in lysine and arginine) and bind to negatively charged DNA (phosphate groups).

Cell Junctions (NEET Focus):

• Tight junctions: Seal adjacent cells (e.g., intestinal epithelial cells) — prevent paracellular passage
• Adhering junctions (desmosomes): Spot welds between cells — mechanical attachment; have cadherin proteins connecting intermediate filaments
• Gap junctions: Channels connecting cytoplasm of adjacent cells — allow ions and small molecules to pass

Endosymbiotic Theory: Mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells (~2 billion years ago). Evidence:

• Both have their own circular DNA (like bacteria)
• Both have 70S ribosomes (prokaryotic type)
• Both divide by binary fission
• Both have double membranes (outer from host, inner from engulfed bacterium)

NEET Pattern Analysis: Cell biology contributes 2–3 questions per year. High-yield areas: differences between prokaryotic and eukaryotic cells, structure of cell membrane (fluid mosaic model), cell organelle functions, semi-autonomous organelles, and cell wall composition. Questions frequently ask students to identify organelles from electron microscope images or match functions.

⚡ NEET 2022 Qn: Which organelle is involved in detoxification of drugs and synthesis of lipids? Answer: Smooth Endoplasmic Reticulum (SER) — abundant in liver hepatocytes and in cells producing steroid hormones.

⚡ NEET 2021 Qn: Which pigment is responsible for absorption of light in photosynthesis? Answer: Chlorophyll (a and b). Chlorophyll a is the primary pigment; chlorophyll b is an accessory pigment.