Cell – The Unit of Life Notes

1. Cell – Introduction & Discovery

What is a Cell?

Cell is the structural and functional unit of all living organisms
All metabolic activities of an organism take place inside the cell
Structure determines function in a cell
Biology is the study of organisms and living processes happening in them

Discovery Timeline

Scientist	Year	Contribution
Robert Hooke	1665	Discovered cells in dead cork cells using self-made microscope. Named them "cells" (empty compartments like little rooms). Described in book Micrographia.
Antonie Van Leeuwenhoek	1674	First described LIVING cells — observed free-living cells in pond water. First to describe protozoans.
Robert Brown	1831	Discovered the Nucleus in plant cells
Schleiden	1838	German botanist — observed plant cells and proposed all plants are made of cells
Schwann	1839	British zoologist — observed animal cells and proposed all animals are made of cells. Also proposed cell membrane (now called plasma membrane). Formulated Cell Theory (with Schleiden).
Rudolf Virchow	1855	Modified Cell Theory — "Omnis cellula-e-cellula" — new cells arise only from pre-existing cells

Cell Theory

Proposed by Schleiden and Schwann (1839):
1. All living organisms are made up of cells and products of cells
2. Cell is the basic unit of life
3. (Modified by Virchow, 1855) New cells arise from pre-existing cells — "Omnis cellula-e-cellula"

Drawback of Cell Theory: Cell theory cannot explain the origin of the first cell. Also does not explain viruses (which are non-cellular).

Q

Who discovered the nucleus? In which year?

Ans: Robert Brown, 1831

Robert Brown in 1831 discovered the nucleus while studying plant cells.
This is a very commonly asked question. Note: Robert Hooke discovered cells (1665) in dead cork, Leeuwenhoek discovered living cells (1674), and Robert Brown discovered the nucleus (1831) — three different scientists for three different discoveries.

Q

Cell theory was proposed by which scientists? What was Virchow's contribution?

Ans: Schleiden & Schwann (1838–39); Virchow added "Omnis cellula-e-cellula"

Schleiden (1838) — studied plants and proposed all plants made of cells.
Schwann (1839) — studied animals and proposed all animals made of cells. Together they proposed cell theory.
Virchow (1855) modified it by adding that new cells arise ONLY from pre-existing cells — "Omnis cellula-e-cellula" (Every cell from a cell). This explained cell reproduction and removed the earlier drawback that didn't explain cell origin.

2. Types of Organisms Based on Cell Number

Type	Definition	Examples
Unicellular	Organism contains only ONE cell. Can live and exist independently.	Bacteria, BGA (Blue-Green Algae/Cyanobacteria), Protista (unicellular eukaryotes), Yeast, Chlamydomonas, Amoeba
Multicellular	Organism contains many cells. Different types of cells perform different functions.	All higher plants and animals

Unicellular organisms can live and exist independently as a single cell. In multicellular organisms, a single cell cannot survive independently — all cells are pooled together.

3. Prokaryotic vs Eukaryotic Cell

Feature	Prokaryotic Cell	Eukaryotic Cell
Nuclear membrane	Absent — no true nucleus (nucleoid region)	Present — well-defined nucleus with nuclear envelope
Membrane-bound organelles	Absent	Present (mitochondria, ER, Golgi, etc.)
Ribosome	70S (50S + 30S)	80S (60S + 40S) in cytoplasm; 70S in mitochondria & chloroplast
Cell size	Generally smaller (1–10 μm)	Larger (10–100 μm)
DNA	Circular, naked (no histone proteins), found in nucleoid	Linear, associated with histone proteins, in nucleus
Cell wall	Present (made of peptidoglycan/murein in bacteria)	Present in plants (cellulose), absent in animals
Centrosome/Centriole	Absent	Present in animal cells (absent in plant cells)
Examples	Bacteria, Cyanobacteria (BGA), Mycoplasma, PPLO	Plants, Animals, Fungi, Protista

Cell Size Facts

Cell / Organism	Size / Shape
Smallest living cell	Mycoplasma / PPLO — 0.3 μm diameter (also has no nucleus)
Largest isolated single cell	Ostrich egg — largest cell visible to naked eye
Longest cell in human body	Nerve cell (Neuron) — long and branched
RBC (Red Blood Cell)	Round, biconcave, no nucleus in humans
WBC	Amoeboid shape
Typical bacteria	1–5 μm length (rod-shaped) to 3–5 μm (round)
E. coli	~1 μm long, divides every 20 min

Shapes of Bacteria

Shape	Name	Example
Round/Spherical	Coccus (pl. Cocci)	Streptococcus, Diplococcus
Rod-shaped	Bacillus	E. coli, Lactobacillus
Comma-shaped	Vibrio	Vibrio cholerae
Spiral-shaped	Spirillum	Spirillum

Q

What is the smallest living cell? What is special about it?

Ans: Mycoplasma (PPLO) — 0.3 μm

Mycoplasma / PPLO (Pleuro Pneumonia Like Organism) is the smallest living cell with a diameter of about 0.3 μm (some sources say 0.1–0.5 μm).
Special features: It has NO cell wall (only plasma membrane) and is the smallest independently existing cell. It is a prokaryote. It causes diseases like walking pneumonia, urogenital infections.
Contrast: Smallest cell ≠ smallest organism. Viruses are smaller but are non-cellular (not truly living).

Q

What are the fundamental similarities between all prokaryotic cells despite their diverse shapes and sizes?

Ans: All share basic structure, size, organization fundamentally similar

Despite having different shapes (cocci, bacilli, vibrio, spirillum) and sizes, all prokaryotic cells share:
1. Absence of membrane-bound nucleus (nucleoid region instead)
2. Absence of membrane-bound organelles
3. 70S ribosomes
4. Circular naked DNA (no histones)
5. Cell wall present (peptidoglycan in eubacteria)
6. Basic metabolic pathways similar
The diversity is mostly in shape, size, and surface structures (flagella, pili) — but internal organization is fundamentally similar.

4. Prokaryotic Cell – Detailed Structure

Cell Envelope (3 Layers from outside in)

Glycocalyx (outermost): Outermost layer — can be loose (slime layer) or thick (capsule). Protects bacteria from immune system. Helps in bacterial death resistance during desiccation (drying).
Cell Wall: Provides support, shape, and strength. Prevents cell from bursting or collapsing. Made of peptidoglycan/murein in eubacteria.
Cell Membrane: Innermost flexible selectively permeable membrane. Controls what enters and exits the cell.

Gram Staining

Feature	Gram Positive (+)	Gram Negative (−)
Retain Gram stain?	Yes — retains Crystal violet stain (purple/blue)	No — does not retain stain (pink/red after counterstain)
Cell wall	Thick peptidoglycan layer	Thin peptidoglycan + outer lipopolysaccharide layer
Example	Streptococcus, Staphylococcus	E. coli, Salmonella

Nucleoid

Region in prokaryote where DNA is found — not bounded by membrane
DNA is circular, double-stranded, naked (no histone proteins)
Not true nucleus — called nucleoid/nuclear region
DNA is stabilised by polyamine proteins (not histones — those are eukaryotic)

Plasmid

Extra-chromosomal circular DNA present in many bacteria
Small, circular, double-stranded DNA (separate from main chromosomal DNA)
Provides various phenotypic characters (e.g., antibiotic resistance)
Can replicate independently
Used extensively in genetic engineering as vectors

Ribosomes in Prokaryotes

70S ribosomes (50S large subunit + 30S small subunit)
Membraneless organelles
Help in protein synthesis
When multiple ribosomes attached to a single mRNA strand → called Polysome / Polyribosome

Inclusion Bodies

Membraneless structures found in cytoplasm for storage of different materials
Phosphate granules: Store phosphate
Glycogen granules: Store food (glycogen)
Cyanophycean granules: Store pigments
Gas vacuoles: Not true vacuoles — gas bubbles that provide buoyancy (found in BGA)

Flagella in Bacteria

Extension of cell membrane — help in motility
Extend from cell body through cell wall via hook and basal body
Made of flagellin protein
Bacterial flagella structure: Filament + Hook + Basal body (connects to cell membrane)

Pili (Fimbriae)

Pili: Help in transfer of genetic material from one bacterium to another — process called Conjugation. Discovered by Lederberg. Made of pilin protein.
Fimbriae: Small bristle-like structures on bacteria surface. Help bacteria to attach to host tissues. Visible only under electron microscope. Made of fimbrin protein.

Q

What is the difference between pili and fimbriae?

Ans:

Pili: Longer tubular structures on bacteria surface. Main function = transfer of DNA from one bacterium to another during conjugation. Made of pilin protein.
Fimbriae: Short, bristle-like projections on bacteria surface. Main function = help bacteria attach to host tissues (important for pathogenesis). Visible only under electron microscope. Made of fimbrin protein.
Key difference: Pili = DNA transfer (conjugation) | Fimbriae = attachment to host

Q

What is a plasmid? Why is it important in genetic engineering?

Ans:

Plasmid is extra-chromosomal, small, circular, double-stranded DNA found in bacteria (separate from main genomic DNA).
It can replicate independently of chromosomal DNA and provides various phenotypic characters like antibiotic resistance.
In genetic engineering: Plasmids are used as vectors — the foreign gene of interest is inserted into the plasmid, which is then introduced into a bacterial host cell. The bacteria then replicates, producing multiple copies of the foreign gene (cloning). This is the basis of recombinant DNA technology.

5. Eukaryotic Cell – Plant vs Animal Cell

Feature	Plant Cell	Animal Cell
Cell wall	Present (cellulose)	Absent
Central vacuole	Present (large — ~90% of cell volume)	Absent (small contractile vacuoles in some)
Plastids (Chloroplast)	Present	Absent
Centriole	Absent (except lower plants)	Present
Plasma membrane	Present	Present
Mitochondria	Present	Present
Nucleus	Present	Present
ER, Golgi, Ribosomes	Present	Present

Typical plant cell example: Onion peel cell | Typical animal cell example: Human cheek cell

6. Cell Membrane (Plasma Membrane)

Chemical Composition

Made of lipids and proteins (and some carbohydrates on outer surface)
Most abundant lipid = Phospholipid
Phospholipid molecule: Hydrophilic head (polar, faces outward) + Hydrophobic hydrocarbon tails (non-polar, faces inward)
Lipids form a bilayer — tails pointing inward, heads facing outward
Carbohydrates found ONLY on outer surface of membrane (in animal cells) — form glycocalyx
Sterols present in membrane — ergosterol (fungi), cholesterol (animals)

Fluid Mosaic Model

Proposed by Singer and Nicolson (1972)
Most accepted model of plasma membrane structure
Key features:
• Lipid bilayer is fluid — lipids can move laterally (provides fluidity)
• Proteins are embedded in the lipid bilayer like tiles in a mosaic (hence "mosaic")
• Lipid to protein ratio = 40:60 (protein-rich) but varies between cell types
• Proteins can be peripheral (on surface) or integral/intrinsic (buried in membrane)

Type of Protein	Location	Also Called
Peripheral proteins	On surface of lipid bilayer (not buried)	Extrinsic proteins — easily extracted
Integral / Intrinsic proteins	Partially or fully buried in lipid bilayer	Transmembrane proteins (span full bilayer)

Functions of Cell Membrane

Selectively permeable — controls entry and exit of substances
Secretion, endocytosis, exocytosis
Cell growth and cell division
Cell-cell interaction and signalling
Transport across membrane

Transport Across Membrane

Type	Direction	Energy needed?	Examples
Passive Transport (Simple Diffusion)	High conc → Low conc	No (ATP not needed)	O₂, CO₂, small non-polar molecules
Facilitated Diffusion	High conc → Low conc	No, but needs carrier protein	Glucose, amino acids (via protein channels)
Osmosis	High water potential → Low water potential (across semi-permeable membrane)	No	Water movement across cell membrane
Active Transport	Low conc → High conc (against gradient)	Yes (ATP required)	Na⁺/K⁺ pump, mineral absorption by roots

Q

Fluid Mosaic Model was proposed by whom and in which year? What does "fluid" and "mosaic" mean?

Ans: Singer and Nicolson, 1972

Singer and Nicolson proposed the Fluid Mosaic Model in 1972 — it is the most widely accepted model of plasma membrane structure.
"Fluid" = The lipid bilayer is not rigid — lipid molecules can move laterally within the bilayer. This gives the membrane its fluid nature and allows it to function dynamically (fusion, budding, etc.).
"Mosaic" = Proteins are embedded in the lipid bilayer in a non-uniform, mosaic-like pattern — like tiles set in a floor. Some proteins float on the surface (peripheral), some are partially buried, some span the entire bilayer (transmembrane).

7. Cell Wall

First discovered by Robert Hooke in dead cork cells
Provides shape, rigidity and support to the cell
Protects from mechanical injury and chemical infections
Prevents over-expansion (cell bursting) during osmosis
Allows cell-cell interaction and prevents entry of macromolecules

Organism	Cell Wall Composition
Archaebacteria	Pseudopeptidoglycan (no muramic acid)
Eubacteria	Peptidoglycan / Murein
Fungi	Chitin (+ cellulose, galactose, mannans, minerals like CaCO₃)
Plants	Cellulose + hemicellulose + pectin + some proteins
Animals	Absent

Plant Cell Wall Layers

Middle Lamella: Outermost layer — made of calcium pectate. Holds adjacent plant cells together (like cement). Traverses through cell wall via plasmodesmata.
Primary cell wall: Present in young growing cells — thin, elastic, capable of growth
Secondary cell wall: Thick, rigid — formed on inner side of primary wall in older cells. Gives wood its strength.

Plasmodesmata: Channels that pass through cell wall connecting neighbouring plant cells together. Traverse the cell wall via pectate.

8. Endomembrane System

The endomembrane system includes membrane-bound organelles inside the cell that work in a coordinated manner: Endoplasmic Reticulum → Golgi Apparatus → Lysosomes → Vacuoles

Mitochondria, Chloroplast and Peroxisomes are NOT part of endomembrane system — they work independently (semi-autonomous organelles).

Endoplasmic Reticulum (ER)

Network of membranous structures spread throughout the cytoplasm
Continuous with the outer nuclear envelope
Seen using electron microscope with precision
Has lumen (cisternal space inside ER) and extraluminal space (rest of cytoplasm)

Type	Feature	Function
Rough ER (RER)	Has ribosomes on surface (looks rough)	Protein synthesis and secretion. Helps in formation of cell membrane.
Smooth ER (SER)	No ribosomes (smooth appearance)	Lipid synthesis, detoxification, glycogen metabolism, drug detoxification

Golgi Apparatus

Discovered by Camillo Golgi (1898)
Densely stained reticular structure near nucleus — more close to ER
Made of flat, disc-like sacs called cisternae arranged concentrically (like stack of plates)
Cisternae are distinct but connected
In plants, Golgi apparatus units are called Dictyosomes
Has convex cis face (receiving/forming face — faces ER) and concave trans face (maturing face — faces cell membrane)

Functions of Golgi Apparatus:
• Processing, packaging and secretion of proteins and lipids
• Glycosylation of proteins (adding sugar chains) → glycoproteins
• Modification of proteins from ER before secretion
• Formation of lysosomes
• Principal site of carbohydrate synthesis

Lysosomes

Membrane-bound organelles containing hydrolytic enzymes (acid hydrolases)
Work in acidic pH (pH ~5) — maintained by H⁺ pump
Called "Suicidal bags" of the cell — can destroy the cell itself by releasing enzymes (autolysis)
Enzymes include: proteases, lipases, nucleases, carbohydrases
Function: Intracellular and extracellular digestion (phagocytosis, autophagy)

Vacuoles

In Plants: Central vacuole occupies ~90% of cell volume. Called storehouse of the cell. Membrane surrounding it = Tonoplast. Stores water, ions, minerals, proteins, amino acids, pigments (anthocyanin = blue/red colour of flowers).
In Animals: Small, temporary. Contractile vacuole in Amoeba — for osmoregulation and excretion. Water expelled by contraction.
Food vacuoles: Formed in Amoeba during phagocytosis

Q

Why are lysosomes called suicidal bags of the cell?

Ans: Because they can digest the entire cell content including the cell itself.

Lysosomes contain powerful hydrolytic enzymes that can break down all types of biological macromolecules — proteins, lipids, carbohydrates, nucleic acids.
These enzymes work at an acidic pH (~5) maintained by a proton pump (H⁺ ATPase).
In normal conditions, the lysosomal membrane keeps these enzymes contained. But when the membrane ruptures — either in damaged/dead cells or during programmed cell death (apoptosis) — these enzymes are released and digest the entire cell content, including the organelles.
This self-digestion process = autolysis. This ability to destroy their own cell earns lysosomes the name "suicidal bags."

Q

What is tonoplast? What does it store?

Ans: Tonoplast = membrane surrounding central vacuole in plant cells

Tonoplast is the membrane that surrounds the central vacuole in plant cells.
The central vacuole (bounded by tonoplast) stores: water, inorganic ions, minerals, proteins, amino acids, and pigments like anthocyanin (responsible for blue, red, and purple colour of flowers and fruits).
The central vacuole occupies up to 90% of the cell volume in mature plant cells, pushing the nucleus to the periphery.

9. Mitochondria

Called "Powerhouse of the cell" — produce ATP through aerobic respiration
Shape: Cylindrical / sausage-shaped (most common) — but can vary
Number: Depends on physiological activity — more in cells with high energy requirements
Size: 0.2–1.0 μm (width) × up to few μm (length) — larger than prokaryotic cells
Can divide by fission (semi-autonomous organelle)

Structure

Part	Details
Outer membrane	Smooth, freely permeable to small molecules (porin channels)
Inner membrane	Less permeable than outer membrane. Has inward folds called cristae — increase surface area for ATP synthesis. Contains ATP synthase enzymes.
Intermembrane space	Space between outer and inner membranes
Matrix (mitochondrial matrix)	Semifluid interior — contains own circular DNA, 70S ribosomes, enzymes of Krebs cycle

Semi-Autonomous Nature (Endosymbiotic Theory)

Mitochondria have own circular DNA (like bacteria)
Have 70S ribosomes (like bacteria)
Can synthesise some of their own proteins
Can divide by binary fission
Supported by Endosymbiotic Theory — mitochondria evolved from ancient free-living bacteria that were engulfed by a host cell
Methionine is the initiator amino acid in mitochondria (same as bacteria — uses f-Met)

Mitochondria are both similar to bacteria AND to eukaryotic organelles. They have 70S ribosomes (like bacteria) but are found in eukaryotic cells. Most proteins are encoded by nuclear DNA and imported into mitochondria.

Q

Why is mitochondria called a semi-autonomous organelle?

Ans: Because it has own DNA, ribosomes, and can make some of its own proteins, but still depends on nuclear DNA.

Mitochondria are called semi-autonomous because:
1. They have own circular DNA (like prokaryotes)
2. They have 70S ribosomes (like bacteria)
3. They can synthesize some of their own proteins
4. They can divide by binary fission
However, they are NOT fully autonomous because: Most of their proteins (>95%) are encoded by nuclear DNA and must be imported from the cytoplasm. They cannot live independently outside the cell.
This semi-autonomous nature supports the Endosymbiotic Theory proposed by Lynn Margulis.

10. Plastids

Found in plants and some unicellular eukaryotes (e.g., Euglena)
Not found in animal cells
Plastids can interconvert into each other
Also semi-autonomous (own circular DNA + 70S ribosomes)

Type	Colour	Pigment/Content	Function/Location
Chloroplast	Green	Chlorophyll + Carotenoids	Photosynthesis — leaves and green parts
Chromoplast	Yellow, Orange, Red	Carotenoids (carotene, xanthophyll)	Attract pollinators — flowers and fruits
Leucoplast	Colourless	Stores starch/oils/proteins	Storage — seeds, roots

⚡ Leucoplast types

Amyloplast → stores starch | Elaioplast → stores oils/fats | Aleuroplast → stores proteins

Chloroplast — Detailed Structure

Number per cell: 1 in Chlamydomonas (20–40 in most plant cells)
Size: 4–6 μm (length) — larger than mitochondria
Shape varies: cup-shaped in Chlamydomonas, spiral in Spirogyra, ribbon-shaped in some

Part	Details
Outer membrane	Permeable to small molecules
Inner membrane	Less permeable — selective transport
Stroma (matrix)	Fluid-filled interior — contains own circular DNA, 70S ribosomes, enzymes for Calvin cycle (dark reactions)
Grana	Stacks of disc-like thylakoids (like stack of coins). Each stack = granum (pl. grana). Site of light reactions / photosystems.
Thylakoids	Flat membranous sacs arranged in grana. Contain chlorophyll and carotenoids. Site of light-dependent reactions.
Stroma lamellae	Membranous connections between grana stacks

Q

What are chromoplasts? Where are they found and what is their significance?

Ans: Chromoplasts are coloured plastids containing carotenoid pigments.

Chromoplasts contain carotenoid pigments (carotene = orange/yellow, xanthophyll = yellow), lycopene (red in tomatoes), and other pigments.
Found in: petals of flowers, fruits (like ripe tomatoes, red peppers, carrot root), and some roots.
Significance: Their bright colours attract pollinators (bees, butterflies) and seed dispersers (birds, animals) — important for reproduction and seed dispersal.
Chloroplasts can convert to chromoplasts as fruits ripen (green tomato → red tomato = chloroplast → chromoplast conversion).

11. Nucleus

Structure of Nucleus

Part	Details
Nuclear Envelope	Double membrane (outer + inner nuclear membrane) with nuclear pores. Outer membrane continuous with ER. Nuclear pores allow bidirectional transport — mRNA goes OUT, proteins come IN.
Nucleoplasm	Semifluid material inside nucleus — contains nucleolus, chromatin, and nuclear proteins
Nucleolus	Dense, darkly staining, membraneless structure. Site of rRNA synthesis and ribosome assembly. Size 0.5–1 μm. Cells making large amounts of protein → larger nucleolus. Can be one or more per nucleus. Continuous with nucleoplasm. Named by Flemming.
Chromatin	Network of DNA + histone proteins in non-dividing cells. Stained by basic dyes. Named by Flemming. During cell division → condenses to form chromosomes.

Chromosomes

Condensed form of chromatin during cell division
Stained with basic dyes
Have primary constriction = Centromere
Centromere: Constriction where spindle fibre attaches during cell division. Present in ALL chromosomes (46 in humans → all have centromere)
DNA in chromosomes = double-stranded linear DNA (associated with histone proteins — rich in arginine and lysine)

Types of Chromosomes Based on Centromere Position

Type	Centromere Position	Shape at Anaphase
Metacentric	Middle — equal arms	V-shape
Sub-metacentric	Slightly off centre — unequal arms	L-shape
Acrocentric	Near one end — very unequal arms	J-shape
Telocentric	At the very end — only one arm	I-shape (rod)

Satellite chromosomes = chromosomes with secondary constriction (NOR — Nucleolus Organiser Region). Not present in all chromosomes — only in some. Human chromosomes 13, 14, 15, 21, 22 have satellites (SAT chromosomes).

Q

What is the nucleolus? What is its function? Why is it larger in some cells?

Ans: Nucleolus is site of rRNA synthesis and ribosome assembly.

Nucleolus is a dense, staining, membraneless spherical structure inside the nucleus, found in both plant and animal cells.
Function: Site of rRNA (ribosomal RNA) synthesis and ribosome subunit assembly. The rRNA genes (in nucleolus organiser region = NOR) are transcribed here, and ribosomal proteins sent from cytoplasm assemble with rRNA to form ribosome subunits.
Why larger in some cells: Cells that synthesize large amounts of protein (e.g., secretory cells, pancreatic cells) need more ribosomes → more nucleolus activity → larger nucleolus. Cells with low protein synthesis have smaller nucleolus.
Named by Flemming. Can be one or more per nucleus.

12. Cytoskeleton, Cilia, Flagella & Centrosome

Cytoskeleton

Complex network of filamentous proteinaceous structures in eukaryotic cells
Provides mechanical support, shape to the cell
Helps in motility (cilia, flagella movement), organelle movement, cell division
Made of fibrous proteins

Type	Diameter	Protein
Microtubules	25 nm (largest)	Tubulin protein
Intermediate filaments	10 nm	Various proteins (keratin, vimentin, desmin)
Microfilaments (Actin filaments)	7 nm (smallest)	Actin protein

Cilia and Flagella

Hair-like extensions of cell membrane — help in movement
Cilia: Short, numerous, coordinated movement (like oars). Found on surface of cells.
Flagella: Long, fewer (1–2 per cell), whip-like movement
Both made of tubulin protein (unlike bacterial flagella made of flagellin)
Both have 9+2 arrangement of microtubules — 9 peripheral doublets + 2 central singlets
Central pair covered by central sheath, connected to peripheral doublets by radial spokes

Bacterial flagella = made of flagellin protein. Eukaryotic cilia/flagella = made of tubulin protein. Different proteins — different structure!

Centrosome and Centriole

Centrosome: Present in animal cells — absent in higher plant cells and prokaryotes. Contains 2 centrioles at 90° to each other + pericentriolar material
Centriole: Membraneless structure made of microtubules arranged in 9 triplets (9+0) — 9 peripheral triplets, no central pair (unlike cilia/flagella 9+2)
Centriole helps in animal cell division — forms spindle fibres during mitosis
Basal body = centriole-like structure at base of cilia/flagella — organises the axoneme (9+2 structure)
Centriole has cartwheel-like pattern at proximal end

Microbodies (Peroxisomes and Glyoxysomes)

Small, membrane-bound structures found in both plant and animal cells
Peroxisomes: Contain enzyme catalase (breaks H₂O₂ → H₂O + O₂). Found in both plant and animal cells. Involved in photorespiration (plants).
Glyoxysomes: Found in seeds of plants (especially fatty seeds). Contain glyoxylate cycle enzymes. Convert fats → carbohydrates during seed germination. Discovered by Robert Brown.

Q

What is the 9+2 arrangement in cilia/flagella? How is it different from centriole structure?

Ans: Cilia/flagella = 9+2 (9 doublets + 2 central singlets); Centriole = 9+0 (9 triplets, no central pair)

Cilia and Flagella (9+2):
— 9 peripheral doublets of microtubules arranged in a ring
— 2 central singlet microtubules in the centre (covered by central sheath)
— Peripheral doublets connected to central pair by radial spokes
— Made of tubulin protein

Centriole (9+0):
— 9 peripheral triplets of microtubules (each set of 3)
— NO central pair (hence 9+0)
— Has cartwheel pattern at proximal end
— Made of tubulin protein

Key: Both use tubulin, but cilia/flagella have DOUBLETS and CENTRAL PAIR (9+2), while centrioles have TRIPLETS and NO CENTRAL PAIR (9+0).

13. Quick Revision — All Key Points

Key Fact	Detail
Cell discovered by	Robert Hooke (1665) — dead cork cells — book Micrographia
Living cell first described by	Antonie Van Leeuwenhoek (1674)
Nucleus discovered by	Robert Brown (1831)
Cell Theory proposed by	Schleiden and Schwann (1838–39)
Cell Theory modified by	Rudolf Virchow (1855) — "Omnis cellula-e-cellula"
Smallest living cell	Mycoplasma / PPLO (~0.3 μm)
Largest cell (naked eye)	Ostrich egg
Longest human cell	Nerve cell (Neuron)
Plasma membrane model	Fluid Mosaic Model — Singer & Nicolson (1972)
Most abundant lipid in membrane	Phospholipid
Powerhouse of cell	Mitochondria (ATP synthesis)
Suicidal bags	Lysosomes
Golgi apparatus discovered by	Camillo Golgi (1898)
Golgi units in plants	Dictyosomes
Tonoplast	Membrane surrounding central vacuole in plant cells
Mitochondria ribosome	70S (like prokaryotes)
Eukaryotic cytoplasmic ribosome	80S (60S + 40S)
Prokaryotic ribosome	70S (50S + 30S)
Chromatin named by	Flemming
Cilia/Flagella arrangement	9+2 (tubulin protein)
Centriole arrangement	9+0 triplets
Peroxisome enzyme	Catalase (breaks H₂O₂)
Glyoxysomes	In fatty seeds — convert fat → carbohydrate. Discovered by Robert Brown.
Middle lamella made of	Calcium pectate
Cell wall of bacteria	Peptidoglycan / Murein
Pili made of	Pilin protein — for conjugation
Fimbriae made of	Fimbrin protein — for attachment
Bacterial flagella made of	Flagellin protein

Cell – The Unit of Life – Complete Notes