Ch 04: Cell | Exam Fusion Prep

Robert Hooke

Discovered Cell (1665)

Leeuwenhoek

First living cell (1674)

Schleiden & Schwann

Cell Theory (1838-39)

Rudolf Virchow

Modified Cell Theory (1855)

🔬 Discovery of Cell

Cell is the fundamental unit of life and building block of living organisms
Discovered by Robert Hooke (1665)
Observed dead cells in thin slice of cork (plant bark)
Cells resembled a honeycomb
Coined term 'Cellula' → Small compartment
Cytology → Study of cells
First living cells discovered by Antonie van Leeuwenhoek (1674)

⚠ Exam Trap

Hooke observed dead cells in cork, whereas Leeuwenhoek observed living cells. These are two distinct discoveries and should not be confused.

📖 Cell Theory (1838–1839)

Given by Matthias Schleiden (Botanist) and Theodor Schwann (Zoologist)
All living organisms are made of cells
Cell is the structural and functional unit of life
Modified by Rudolf Virchow (1855)
New cells arise from pre-existing cells (through cell division)
Latin: Omnis cellula-e-cellula

⚡ Quick Recall

Schleiden = Plants | Schwann = Animals — together they proposed the Cell Theory. Virchow added that “new cells arise only from pre-existing cells.”

📐 Size & Shape of Cells

Shape / Type	Example	Special Detail
Irregular / Amoeboid	WBC, Amoeba	—
Cylindrical	Muscle cell, Vessel, Sieve tubes	—
Oval (Largest cell)	Ostrich egg	15×13 cm, 1.4 kg
Tree-like	Nerve cells / Neuron	90–100 cm long
Biconcave	Red Blood Cells (RBC)	7.2 µm
Smallest cell	Mycoplasma PPLO	0.1 µm (Pleuropneumonia Like Organism)

⚠ Exam Trap

Largest cell = Ostrich egg (15×13 cm). Smallest cell = Mycoplasma PPLO (0.1 µm). Longest cell = Neuron (90–100 cm). These are three distinct records.

🧬 Prokaryotic vs Eukaryotic Cell

Feature	Prokaryotic	Eukaryotic
Size	0.1 – 5.0 µm	10 – 100 µm
Nucleus	Nuclear membrane absent Nucleolus absent → Nucleoid / Genophore	Nuclear membrane present Nucleolus present
Membrane-bound organelles (Mito, Plastid, ER, Golgi, Vacuole)	Absent	Present
Ribosome	70S	80S (70S in Mitochondria & Plastids)
Chromosome / DNA	One naked circular DNA Without Histone protein	Multiple, linear DNA With Histone protein
Cellular Respiration	In Mesosome (infoldings of plasma membrane)	In Mitochondria
Examples	Bacteria, Blue-green algae, Mycoplasma	Plants, Fungi, Animals

⚠ Exam Trap

In eukaryotic cells, mitochondria and plastids contain 70S ribosomes, as in prokaryotes, whereas the cytoplasm contains 80S ribosomes. This is a common MCQ trap.

In prokaryotic cells, cellular respiration occurs in the mesosome. The mesosome also assists in DNA replication.

⚡ Quick Recall

Motile bacteria possess flagella, which are thin filamentous extensions from the cell wall.

🌿🐾 Plant Cell vs Animal Cell

Structure / Organelle	Plant Cell	Animal Cell
Cell Membrane	Present (Sterol containing)	Present (Cholesterol containing)
Cell Wall	Present (Cellulose)	Absent
Plastid	Present	Absent
Mitochondria	Present (Less in number)	Present (More in number)
Golgi Complex	Present (Small in size)	Present (Large in size)
Vacuoles	Large central vacuole	Very small or Absent
Ribosome	Present	Present
Centrosome	Absent	Present

⚠ Exam Trap

Centrosome → only in animal cells (forms poles during cell division). Plastid → only in Plant cells. Cell wall → only in plants. These are three major differences.

⚡ Quick Recall

The Golgi bodies of plant cells are called dictyosomes. Vacuoles in animal cells are very small or absent, whereas a plant-cell vacuole occupies 60–90% of the cell volume.

🧱 1. Cell Wall

Only present in plant cells
Made up of Cellulose
Thick and flexible or rigid
Permeable, helps in protection

🟦 2. Cell Membrane / Plasma Membrane

Found in all cells
Composition: Protein (50%) + Lipid (40%) + Carbohydrates (10%)
Very thin and flexible
Selectively permeable membrane

🔀 Transport Across Cell Membrane

Passive Transport (No energy needed)

Diffusion: High → Low concentration
Rate: Gas > Liquid > Solid
CO₂, O₂ transport
Osmosis: Water movement across semipermeable membrane
Absorption of water in roots

Active Transport (Energy needed)

Endocytosis: Engulfing material INTO cell
Phagocytosis — solid food intake
Pinocytosis — liquid food intake
Exocytosis: Removing waste OUT of cell ("cell vomiting")

💧 Osmosis — Solution Types

Solution Type	Condition	Effect on Cell
Hypotonic	Conc. of solution < cell	Cell swells
Isotonic	Conc. of solution = cell	No change
Hypertonic	Conc. of solution > cell	Cell shrinks

⚡ Quick Recall

Aquaporins = specialised channel proteins that facilitate water transport across cell membrane in response to osmotic gradient.

⚠ Exam Trap

Phagocytosis = Solid food | Pinocytosis = Liquid food. Both are types of endocytosis. Endocytosis occurs in Amoeba.

🧫 3. Protoplasm

Term coined by J.E. Purkinje (1839), Discovered by Felix Dujardin
"Physical basis of life" — Huxley
Semi-solid jelly-like matrix / Living material
Site of many metabolic activities

Cytoplasm:Protoplasm inside cell membrane (excluding nucleus) Nucleoplasm:Protoplasm present inside nucleus

🧠 A. Nucleus

Discovered by Robert Brown (1831)
Largest cell organelle (except vacuoles in plant cell)
Control centre / Brain of cell
Controls all vital functions; regulates cell division & inheritance

Parts of Nucleus

Nuclear membrane: Double membrane with nuclear pores
Nucleoplasm: Basal matrix
Nucleolus: Discovered by Fontana; synthesises r-RNA
Chromatin: Thread-like; condenses into chromosomes during cell division

Chromosome Facts

Made of DNA + Histone Protein
Functional segment of DNA = Gene
Human chromosomes: 23 pairs
22 pairs = Autosomes (physical traits)
1 pair = Allosome / Sex chromosome
XY = Male | XX = Female

⚠ Exam Trap

Safranin is used to stain the nucleus. The nucleus is the largest organelle, although a vacuole may be larger in a plant cell; remember this exception.

🕸️ B. Endoplasmic Reticulum (ER)

Discovered by Keith R. Porter, Albert Claude & Ernest Fullam (1945)
Network of single membrane-bound tubules and flattened sheets
Contains Cisternae and Vesicles
Provides cytoskeleton (structural framework)
Forms network of channels for material transport
Helps form cell membrane and nuclear membrane

Rough ER (RER)

Ribosomes present on surface
Synthesis of Protein / Enzymes

Smooth ER (SER)

Ribosomes absent
Synthesis of Lipid / Steroids
Detoxification

⚪ C. Ribosome

Discovered by G.E. Palade (1953)
Membrane less organelle
Site of protein synthesis

70S Ribosome

= 50S + 30S subunits
In Bacteria, Plastids, Mitochondria

80S Ribosome

= 60S + 40S subunits
In Nucleus, Cytoplasm, Rough ER

⚡ Quick Recall

A chain of ribosomes attached to m-RNA = Polysome → helps in protein synthesis.

📦 D. Golgi Body

Discovered by Camillo Golgi (1898)
In plant cells called Dictyosomes
Single membrane-bound parallel flattened vesicles, tubules and cisternae
Cis face (convex) → towards nucleus
Trans face (concave) → towards cell membrane
Secretion, storage, packaging & processing of ER products
Modification: Simple sugar → Complex sugar
Formation of Lysosome and Acrosome of sperm
"Traffic police" / "Post office" of cell

⚠ Exam Trap

Golgi body = "Post office / Traffic police" of cell. Both lysosomes and the acrosome of sperm are formed by the Golgi body. The cytoplasmic area around the Golgi is the Zone of exclusion (no organelles).

💣 E. Lysosome

Discovered by Christian de Duve (1955)
Single membrane-bound organelle
Waste disposal system of cells
Contains digestive (hydrolytic) acidic enzymes
Digests: complex food → simple components
Also digests: foreign particles, dead/damaged organelles, bacteria, viruses, waste
"Suicidal bag" — rupture causes self-digestion of cell

⚠ Exam Trap

Lysosome = "Suicidal bag". This nickname is frequently tested directly in MCQs. Christian de Duve discovered it in 1955.

⚡ F. Mitochondria

Discovered by Albert von Kolliker (1857)
Double membrane bound
Outer membrane = Porous
Inner membrane = Folded into finger-like projections = Cristae
"Powerhouse of cell"
Forms ATP (Adenosine Triphosphate) — energy currency
Via: Cellular respiration, Kreb's cycle, ETS
Semi-autonomous organelle: Has its own circular naked DNA, RNA and 70S ribosomes
Also participates in cell death (Apoptosis)

⚠ Exam Trap

Mitochondria = "Powerhouse of cell". They are semi-autonomous and possess their own DNA, RNA and 70S ribosomes. They also play a role in apoptosis (cell death). Inner membrane = Cristae.

🌿 G. Plastids

Discovered by Ernst Haeckel and Schimper
Present only in Plant cells
Semi-autonomous, double membrane-bound

🟢 Chloroplast

Green colour
Pigment: Chlorophyll
Performs Photosynthesis
"Kitchen of cell"

🟠 Chromoplast

Colourful (except green)
Cause of different colours in fruits & flowers

⚪ Leucoplast

White / Colourless
Amyloplast → Starch
Elaioplast → Oil
Aleuroplast → Protein

⚡ Quick Recall

Chloroplast = "Kitchen of cell" | Mitochondria = "Powerhouse of cell" | Lysosome = "Suicidal bag" | Golgi = "Post office" — these nicknames are frequently tested in examinations.

💧 H. Vacuole

Single membrane (Tonoplast) bound sacs
Storage house OR dustbin of cell
Stores: water, food, pigment (anthocyanin), excretory/waste materials
Absent or very small in animal cells
Large in plant cells — covers 60–90% of cell volume
Regulates cell turgidity (by storing water and cell sap)

📌 Important Additional Facts

Centrosome forms poles during cell division (animal cells)
Centrioles: 9 + 0 structure of triplet microtubules
Peroxisomes in liver: Convert harmful H₂O₂ → H₂O + O₂ (detoxification)
Zone of exclusion: Cytoplasmic area around Golgi — no organelles here

🔄 Cell Cycle — Phases

Interphase (Metabolically active)

G₁ phase: Comparatively longest; cell increases in size after division
G₀ phase: Quiescent phase (cells exit active division — non-dividing state)
S phase: Replication of DNA & centrosome; synthesis of Histone protein
G₂ phase: Cell prepares for mitosis; protein/organelle synthesis; DNA damage check

M-Phase (Division)

Karyokinesis: Nuclear division
Cytokinesis: Cytoplasmic division
In plant cells: Phragmoplast formed during cytokinesis (cell wall construction)

⚖️ Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Occurs in	Somatic cells	Germ cells
Phases	Single phase	Two phases (Meiosis I & II)
Chromosome number	Same as parent (2n → 2n)	Half of parent (2n → n)
Progeny cells	2 identical cells	4 haploid different cells
Purpose	Growth & repair of cells	New genetic variations in species
Phases (Karyokinesis)	Prophase → Metaphase → Anaphase → Telophase	Same stages ×2
Disorder / Disease	Uncontrolled Mitosis = Cancer	Turner's syndrome, Klinefelter's syndrome (chromosome segregation errors)

⚠ Exam Trap

Uncontrolled Mitosis = Cancer. Turner's syndrome & Klinefelter's syndrome = Meiosis errors. Mitosis discoverer = Walther Flemming.

⚡ Quick Recall

During the leptotene stage of Prophase I in Meiosis I, chromosomes condense. Walther Flemming discovered mitosis through the study of chromosomes.

🔬 Chapter 04 — Cell