Chapter 2 Notes: Cell: The Building Block of Life

The Cell: Basic Unit of Life

• All living organisms are made up of cells — the smallest unit of life
• Unicellular organisms (e.g., bacteria, yeast) consist of only one cell
• Multicellular organisms (e.g., plants, humans) have millions of cells working together
• Cells → Tissues → Organs → Organ Systems

Studying Cells with Microscopes

• The limit of resolution of the human eye is 0.1 mm (viewed from 25 cm)
• Most cells are too small to see with the naked eye
• Robert Hooke (1665) first observed cells in cork using a self-designed microscope and named them 'cells'
• Light Microscope: Uses visible light + lenses (10X, 40X objective); used in school labs
• Electron Microscope: Uses beam of electrons; reveals cell details at nanometre scale (1 nm = 0.000001 mm)

Feature	Light Microscope	Electron Microscope
Medium used	Visible light	Electron beam
Resolution	Moderate	Very high (nanometre scale)
Use	School/basic labs	Advanced research

Cell Membrane (Plasma Membrane)

• A thin boundary (7–10 nm thick) surrounding every cell
• Made of lipids (fats) and proteins
• Selectively permeable — allows some substances to pass through, blocks others
• Cells communicate with surroundings and neighbouring cells through it

Fluid-Mosaic Model

• The membrane has a lipid bilayer: two layers of fat molecules
  • Water-attracting (hydrophilic) heads face outward
  • Water-repelling (hydrophobic) tails face inward
• Proteins are embedded in the bilayer — act as gatekeepers
• Molecules can move sideways, flip and rotate → membrane is fluid
• Proteins arranged like tiles in a mosaic → called mosaic model

Osmosis

• Movement of water through a selectively permeable membrane
• Water moves from dilute solution (more water, less solute) → concentrated solution (less water, more solute)
• Continues until concentrations on both sides become equal

Types of Solutions affecting cells:

Solution Type	Condition	Effect on Cell
Isotonic	Solute concentration same inside and outside	No change in cell size
Hypotonic	Solute concentration outside < inside	Water enters → cell swells
Hypertonic	Solute concentration outside > inside	Water leaves → cell shrinks

• Plasmolysis: In plant cells in hypertonic solution, cell membrane pulls away from cell wall as inner content shrinks

Cell Wall

• Present in plants, fungi, and bacteria; absent in animal cells
• Located outside the cell membrane
• Rigid and permeable (water and dissolved minerals can pass through)
• Made of cellulose in plants (a carbohydrate made of glucose units)
• Functions: Provides structural support, maintains shape, protects against environmental stress
• Because plants are fixed, they need rigidity; animal cells are flexible to allow movement

Types of Cells

Prokaryotic Cells

• Lack a well-defined nucleus and membrane-bound organelles
• Genetic material (DNA) is present in a region called the nucleoid (single circular DNA molecule)
• Most cellular activities occur directly in the cytoplasm
• Example: Bacteria
• (Pro = primitive, karyon = nucleus)

Eukaryotic Cells

• Have a well-defined nucleus and several membrane-bound organelles
• Larger and more complex than prokaryotic cells
• Examples: Plant cells, Animal cells, Fungi
• (Eu = true, karyon = nucleus)

Feature	Prokaryotic	Eukaryotic
Nucleus	Absent (nucleoid)	Present
Membrane-bound organelles	Absent	Present
Size	Smaller	Larger
Example	Bacteria	Plant, Animal cells

Basic Parts of a Cell

1. Cell membrane — selectively permeable boundary
2. Cytoplasm — semi-fluid, jelly-like substance containing organelles
3. Nucleus — controls all cell activities

Cytoplasm

• Semi-fluid, jelly-like substance inside the cell
• Contains organelles and cell inclusions
• Cytoskeleton: Network of fine fibres in eukaryotic cells providing structural support, shape, movement, and internal transport (visible only under electron microscope)
• Cell inclusions: Stored substances like starch (plant cells), calcium oxalate crystals

Acellular Infectious Agents (Not cells!)

• Viruses: Genetic material + protein coat; no cells
• Viroids: Genetic material only, no protein coat
• Prions: Misfolded proteins, no genetic material

Nucleus — The Control Centre

• Surrounded by double-layered nuclear membrane with nuclear pores (allow transfer of material between nucleus and cytoplasm)
• Contains nucleolus — dense round body where ribosomal subunits are synthesised
• Contains chromosomes (visible as rod-shaped structures when cell divides)
  • Made of DNA + specific proteins
  • DNA contains genes — functional segments carrying genetic information
• In non-dividing cells, DNA exists as chromatin material (entangled thread-like mass)
• Mature RBCs in humans have no nucleus → more space for haemoglobin → transport more oxygen → lifespan ~120 days

Ribosomes — Protein Factories

• Tiny structures, free in cytoplasm or attached to ER
• Site of protein synthesis
• Present in both prokaryotic and eukaryotic cells

Endoplasmic Reticulum (ER) — Manufacturing Factory

• Network spreading through cytoplasm, continuous with outer nuclear membrane
• Two types:
  • RER (Rough ER): Has ribosomes on surface → looks rough → involved in protein synthesis and secretion (e.g., pancreatic cells)
  • SER (Smooth ER): No ribosomes → looks smooth → involved in synthesis and storage of fats and hormones

Golgi Apparatus — Packaging & Shipping Centre

• Stacks of flattened, sac-like structures
• Modifies, sorts, and packages proteins/lipids into vesicles for transport, secretion, or lysosome formation
• Acts like the cell's post office
• Functionally linked to ER and cell membrane

Lysosomes — The Clean-Up System

• Single membrane-bound sacs filled with digestive enzymes
• Break down unwanted proteins, carbohydrates, fats, and damaged organelles
• Products released into cytoplasm for reuse
• Human sperm cells contain lysosomal enzymes to break outer layer of egg during fertilisation

Mitochondria — Powerhouse of the Cell

• Surrounded by two membranes:
  • Outer membrane: smooth and porous
  • Inner membrane: folded into cristae (finger-like projections) → increase surface area for chemical reactions
• Site of cellular respiration — glucose broken down to release energy
• Energy stored as ATP (Adenosine Triphosphate) — energy currency of the cell
• Contain their own DNA and ribosomes → can make some of their own proteins
• Many small mitochondria > one giant mitochondrion (more total surface area for reactions)

Plastids — Food Synthesis in Plant Cells

(Present only in plant cells)

Type	Pigment	Function
Chloroplasts	Chlorophyll (green)	Photosynthesis
Chromoplasts	Yellow/Orange/Red pigments	Colour in flowers/fruits; attract pollinators
Leucoplasts	None (colourless)	Store starch, oils, or proteins

• Chloroplast structure: Double membrane; inside has semi-fluid stroma + disc-shaped membrane structures containing chlorophyll; starch granules stored in stroma
• Chloroplasts have their own DNA and ribosomes (like mitochondria) → evolutionary link to bacteria

Vacuoles — Storage and Support

• Plant cells: One large central vacuole surrounded by a single selectively permeable membrane; filled with cell sap (water, minerals, sugars, waste)
  • Maintains turgor pressure → keeps plant firm
  • When plant lacks water → vacuole shrinks → plant wilts
• Animal cells: Vacuoles present but smaller and temporary

Why Do Cells Divide?

• Growth of organisms (cells divide, not just get bigger)
• Repair of damaged tissues (e.g., skin healing after a cut)
• Replacement of old/dead cells (hundreds of billions of cells replaced daily in humans)
• Reproduction

Cell Cycle

• Controlled, orderly manner in which eukaryotic cells divide
• Both prokaryotic and eukaryotic cells divide, but eukaryotic cells have a more regulated process

Mitosis — Normal Growth and Repair

• Most common type of cell division
• Produces two genetically identical daughter cells from one parent cell
• Daughter cells have the same DNA and same number of chromosomes as the parent
• Occurs in: skin cells, root tip cells, all body (somatic) cells
• Important for: normal growth, repair, maintenance, and asexual reproduction

Parent cell (e.g., 11 chromosomes)
         ↓ Mitosis
Daughter cell 1 (11 chromosomes) + Daughter cell 2 (11 chromosomes)
[Genetically identical to parent]

Meiosis — For Sexual Reproduction

• Occurs only in reproductive organs (testes, ovaries in animals; anthers, ovaries in plants)
• Produces four daughter cells (gametes) each with half the number of chromosomes
• A two-step division process:
  1. First division: Parent cell → 2 daughter cells with half chromosomes
  2. Second division (like mitosis): Each daughter cell → 2 cells → total 4 gametes with half chromosomes and half DNA
• During fertilisation, two gametes combine → original chromosome number restored
• Creates genetic variation → children resemble parents but are not identical

Parent cell (11 chromosomes)
    ↓ First division
2 cells (5-6 chromosomes each)
    ↓ Second division
4 gametes (half chromosomes)

Feature	Mitosis	Meiosis
Daughter cells	2	4
Chromosome number	Same as parent	Half of parent
Genetic identity	Identical to parent	Different (variation)
Where it occurs	All body cells	Reproductive organs only
Purpose	Growth, repair, asexual reproduction	Sexual reproduction, genetic diversity

Errors in Cell Division

• Errors in mitosis: Uncontrolled division → tumours, abnormal chromosome numbers
• Errors in meiosis: Genetic disorders, developmental problems, early pregnancy loss, reduced fertility
• Contact inhibition: In many animal cells, division stops when cells touch neighbours — cancer cells lose this control

Cell Culture

• Growing plant/animal cells outside the body in nutrient-rich medium under controlled conditions (temperature, pH, moisture, sterile)
• Used for: studying cells, producing medicines, vaccines, biochemicals
• Totipotency: Special ability of plant cells — even a mature plant cell can develop into a complete plant if given suitable nutrients and conditions (basis of Plant Tissue Culture Technology)

Cell Theory — The Unifying Principle of Biology

Historical Development:

• 1665: Robert Hooke — first observed cells in cork using self-designed microscope; named them 'cells'
• 1838: Matthias Schleiden (German botanist) — all plants are made of cells
• 1839: Theodor Schwann (German zoologist) — all animals are made of cells
• 1855: Rudolf Virchow — new cells arise only from pre-existing cells

Classical Cell Theory (3 principles):

1. All living organisms are made up of one or more cells
2. The cell is the basic unit of structure and function in living beings
3. All cells arise from pre-existing cells

This unifies all biology — from bacteria to humans — and explains life's continuity through cell division.

Cell Life and Death

• Cells grow, divide, carry out functions, and eventually die — every cell has a definite life span
• Dead cells are replaced by new cells through mitosis
• Contact inhibition: Division stops when animal cells touch neighbours → Cancer cells lose this → uncontrolled division → tumours
• Programmed Cell Death (PCD): Genetically controlled, organised cell death essential for normal development
  • Example: PCD eliminates cells between fingers in an embryo → forms separate fingers (without PCD, we'd have webbed hands)

Synthetic Biology — J. Craig Venter's Experiment (2010)

• Scientists synthesised an exact copy of bacterial DNA (from Mycoplasma mycoides) in a laboratory
• Inserted this synthetic DNA into a bacterium whose own DNA was removed
• The cell grew and divided following the synthetic DNA's instructions
• Conclusion: DNA controls the structure and activities of a cell
• Note: Only the DNA was synthetic; the rest of the cell (cytoplasm, membrane) came from an existing cell — not a fully new cell from scratch

Totipotency and Plant Tissue Culture

• Gottlieb Haberlandt (1902, Austrian botanist) proposed that any living plant cell can develop into a complete plant
• This ability = Totipotency
• Led to the development of Plant Tissue Culture Technology

Summary: Key Differences — Plant vs Animal Cells

Feature	Plant Cell	Animal Cell
Cell wall	Present (cellulose)	Absent
Chloroplasts	Present	Absent
Plastids	Present (all types)	Absent
Central vacuole	Large, prominent	Small or absent
Lysosomes	Rarely found	Present
Shape	Fixed, box-like	Irregular, flexible