Cell Biology — Cell Structure, Organelles, Mitosis, Meiosis, Prokaryotic vs Eukaryotic

Cell biology is a core science topic for Kerala PSC. Understanding cell structure, organelles, and cell division is essential.

The Cell — Basic Unit of Life

Aspect	Details
Discovery	Robert Hooke (1665) — observed dead cork cells under microscope; coined the term “cell”
Cell Theory	Proposed by Schleiden and Schwann (1838–39): all living organisms are made of cells; cell is the basic unit of life
Cell Theory extended	Rudolf Virchow (1855): “Omnis cellula e cellula” — all cells arise from pre-existing cells
Smallest cell	Mycoplasma (0.1 micrometer)
Largest cell	Ostrich egg (among animal cells); Caulerpa — longest single-celled organism
Longest human cell	Nerve cell (neuron)

Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic	Eukaryotic
Nucleus	No true nucleus (nucleoid region)	True membrane-bound nucleus
Size	0.1–5 micrometers	10–100 micrometers
Membrane-bound organelles	Absent	Present (mitochondria, ER, Golgi, etc.)
DNA	Circular, naked (no histones)	Linear, wrapped around histones
Ribosomes	70S (50S + 30S)	80S (60S + 40S)
Cell wall	Present (peptidoglycan in bacteria)	Present in plants (cellulose), fungi (chitin); absent in animals
Cell division	Binary fission	Mitosis and meiosis
Examples	Bacteria, Archaea, Cyanobacteria	Plants, animals, fungi, protists

Plant Cell vs Animal Cell

Feature	Plant Cell	Animal Cell
Cell wall	Present (cellulose)	Absent
Chloroplasts	Present	Absent
Vacuole	Large central vacuole	Small or absent
Centrioles	Absent (in most)	Present
Shape	Fixed (rectangular)	Variable (round/irregular)
Lysosomes	Rare/absent	Present
Food storage	Starch	Glycogen

Cell Organelles

Organelle	Structure	Function
Cell membrane (Plasma membrane)	Phospholipid bilayer with proteins	Selectively permeable; controls entry and exit of substances
Nucleus	Double membrane with nuclear pores; contains chromatin	Controls cell activities; stores genetic material (DNA)
Mitochondria	Double membrane; inner membrane folded into cristae	Powerhouse of the cell — ATP production through cellular respiration
Endoplasmic Reticulum (ER)	Network of membranes; two types: Rough ER (ribosomes) and Smooth ER	RER: protein synthesis; SER: lipid synthesis, detoxification
Golgi Apparatus (Golgi body)	Stack of flattened membrane sacs (cisternae)	Packaging, modification, and secretion of proteins; forms lysosomes
Ribosomes	RNA + protein; no membrane	Protein synthesis (protein factory of the cell)
Lysosomes	Single membrane; contains hydrolytic enzymes	Suicide bags of the cell — digest waste, old organelles, foreign material
Chloroplast	Double membrane; contains thylakoids with chlorophyll	Photosynthesis — converts light energy to chemical energy (only in plants)
Vacuole	Membrane-bound sac (tonoplast)	Storage of water, nutrients, waste; maintains turgor pressure in plants
Centrioles	Cylindrical structures near nucleus	Help in cell division — form spindle fibres
Peroxisomes	Single membrane	Contain enzymes for detoxification; break down fatty acids and hydrogen peroxide

Important Discoveries

Scientist	Discovery
Robert Hooke	Coined “cell” (1665)
Anton van Leeuwenhoek	First to observe living cells (bacteria) under microscope
Robert Brown	Discovered nucleus (1831)
Camillo Golgi	Discovered Golgi apparatus (1898)
Christian de Duve	Discovered lysosomes (1955)

Cell Division

Mitosis (Somatic Cell Division)

Phase	Events
Prophase	Chromatin condenses into chromosomes; nuclear membrane begins to dissolve; centrioles move to poles; spindle fibres form
Metaphase	Chromosomes align at the equatorial plate (metaphase plate); spindle fibres attach to centromeres
Anaphase	Centromeres split; sister chromatids pulled to opposite poles
Telophase	Nuclear membrane reforms; chromosomes uncoil; two nuclei formed
Cytokinesis	Cytoplasm divides — cell plate in plants, cleavage furrow in animals

Key features of mitosis:

• Produces 2 daughter cells, genetically identical to parent
• Chromosome number remains same (diploid to diploid, 2n to 2n)
• Occurs in somatic (body) cells
• Purpose: growth, repair, replacement

Meiosis (Reduction Division)

Phase	Key Events
Meiosis I (Reduction)
Prophase I	Crossing over occurs (exchange of genetic material between homologous chromosomes); most complex and longest phase
Metaphase I	Homologous pairs align at equator
Anaphase I	Homologous chromosomes separate (NOT sister chromatids)
Telophase I	Two haploid cells formed
Meiosis II (Similar to mitosis)
Prophase II to Telophase II	Sister chromatids separate
Result	4 daughter cells, each haploid (n)

Key features of meiosis:

• Produces 4 daughter cells, genetically different from parent
• Chromosome number halved (diploid to haploid, 2n to n)
• Occurs in reproductive cells (germ cells)
• Purpose: gamete (egg/sperm) formation; introduces genetic variation

Mitosis vs Meiosis Comparison

Feature	Mitosis	Meiosis
Daughter cells	2	4
Chromosome number	Same (2n)	Halved (n)
Genetic identity	Identical to parent	Different from parent
Crossing over	No	Yes (Prophase I)
Occurs in	Somatic cells	Germ cells (reproductive)
Divisions	1	2 (Meiosis I + Meiosis II)
Purpose	Growth, repair	Gamete formation

Cell Transport Mechanisms

Type	Direction	Energy	Examples
Diffusion	High to low concentration	No (passive)	Oxygen exchange in lungs
Osmosis	Water from low solute to high solute concentration	No (passive)	Water absorption by roots
Active transport	Low to high concentration	Yes (ATP)	Sodium-potassium pump
Endocytosis	Into cell	Yes	Phagocytosis (cell eating), pinocytosis (cell drinking)
Exocytosis	Out of cell	Yes	Secretion of hormones

Osmosis Terms

Term	Definition
Isotonic	Equal solute concentration inside and outside cell
Hypotonic	Lower solute outside; water enters cell (cell swells)
Hypertonic	Higher solute outside; water leaves cell (cell shrinks/plasmolysis)

PSC-Focused Quick Recall

Question Pattern	Answer
Who coined the term “cell”?	Robert Hooke (1665)
Cell theory proposed by	Schleiden and Schwann
Powerhouse of the cell	Mitochondria
Suicide bags of the cell	Lysosomes
Protein factory	Ribosomes
Kitchen of the cell	Chloroplast
Discovered nucleus	Robert Brown (1831)
Mitosis produces	2 identical cells
Meiosis produces	4 different haploid cells
Crossing over occurs in	Meiosis (Prophase I)
Longest human cell	Nerve cell (neuron)
Smallest known cell	Mycoplasma
Cell wall of plants is made of	Cellulose
Cell wall of fungi is made of	Chitin
Prokaryotic ribosome size	70S
Eukaryotic ribosome size	80S
Cytokinesis in plants occurs by	Cell plate formation
Cytokinesis in animals occurs by	Cleavage furrow