Discovery, Cell Theory & Microscopy
The Origin & Cell Theory
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Life likely originated in water, potentially in hot springs similar to where modern thermophilic bacteria thrive.
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Robert Hooke first observed and named “cells” in 1665 using a slice of cork.
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The Cell Theory: Formulated by Schleiden (1838) and Schwann (1839), and expanded by Virchow (1855). It states: All living things are made of cells, the cell is the basic structural unit, and all cells arise from pre-existing cells.
Microscopy & Resolution
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Human Eye: Limit of resolution is 0.1 mm (viewed from 25 cm).
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Light/Electron Microscopes: Light microscopes use lenses to magnify. Electron microscopes use electron beams to reveal structures at the nanometre scale (1 nm = one-billionth of a metre).
Cell Boundaries & Osmosis
The Plasma Membrane & Cell Wall
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Cell Membrane: A 7-10 nm thick, selectively permeable boundary. The Fluid-Mosaic Model describes it as a lipid bilayer embedded with mobile transport proteins.
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Cell Wall: A rigid layer outside the membrane found in plants (cellulose), fungi, and bacteria. It provides support and prevents bursting.
Diffusion & Osmosis
Diffusion: Movement of particles from higher to lower concentration.
Osmosis: Diffusion of water across a selectively permeable membrane.
Fluid-Mosaic Membrane Model
Top Layer: Hydrophilic Heads (Attract water)
Middle Layer: Hydrophobic Tails (Repel water)
Embedded: Large Protein Channels (Gatekeepers)
Bottom Layer: Hydrophilic Heads (Attract water)
Osmosis Effects on Cell
ISOTONIC: Equal solute inside and outside ➔ No size change.
HYPOTONIC: Lower solute outside ➔ Water enters ➔ Cell swells.
HYPERTONIC: Higher solute outside ➔ Water leaves ➔ Cell shrinks.
Cell Types & The Control Center
Prokaryotic vs. Eukaryotic Cells
Prokaryotes (1-10 µm): Unicellular. Lack membrane-bound organelles; genetic material is loose in the nucleoid.
Eukaryotes (10-100 µm): Unicellular or multicellular. Possess a membrane-bound nucleus and specialized organelles.
The Nucleus (Control Center)
Surrounded by a double-layered nuclear membrane with pores.
Contains the nucleolus (synthesizes ribosomes).
Houses DNA: As tangled chromatin when resting, or condensed chromosomes when dividing.
Cell Types Overview
Prokaryote (Bacteria): Outer cell wall ➔ Floating Ribosomes ➔ Free-floating Nucleoid ➔ Tail (Appendage).
Eukaryote (Plant/Animal): Cell membrane ➔ Well-defined Nucleus ➔ Mitochondria ➔ Endoplasmic Reticulum.
Nucleus to DNA Pathway
Nucleolus (Dark center) ➔ Surrounded by Chromatin (Fuzzy threads) ➔ Condenses into Chromosomes (X-shape) ➔ Unravels into DNA (Double helix).
The Endomembrane System
Endoplasmic Reticulum (ER)
A membrane network continuous with the nucleus.
Rough ER (RER): Studded with ribosomes; synthesizes/secretes proteins.
Smooth ER (SER): Lacks ribosomes; synthesizes lipids and hormones.
Golgi Apparatus & Lysosomes
Golgi Apparatus: Flattened sacs that modify, sort, and package ER products into vesicles.
Lysosomes: The cellular clean-up system; sacs of digestive enzymes that break down waste.
Protein Processing Pathway
1. Nucleus: Sends instructions.
2. Rough ER: Ribosomes synthesize proteins.
3. Golgi Apparatus: Modifies, sorts, and packages the proteins.
4. Vesicles: Transport the final products to the Cell Membrane.
Energy, Synthesis & Storage
Mitochondria (The Powerhouse)
Supply ATP energy via cellular respiration.
Feature a highly folded inner membrane (cristae) and have their own DNA/ribosomes.
Plastids (Plant Organelles)
Chloroplasts: Contain green chlorophyll for photosynthesis; sugars stored in fluid stroma.
Chromoplasts: Yellow/orange/red pigments for fruits/flowers.
Leucoplasts: Colourless; store starch, oils, proteins.
Vacuoles
Storage sacs. Plant cells have one massive central vacuole (provides firmness); animal vacuoles are tiny.
Mitochondria vs. Chloroplast
Mitochondrion: Smooth Outer Membrane ➔ Finger-like Inner Folds (Cristae) ➔ Floating Ribosomes & DNA.
Chloroplast: Smooth Outer Membrane ➔ Stacks of Chlorophyll Discs ➔ Fluid Stroma ➔ Floating Ribosomes & DNA.
Cell Growth & Division
Why Cells Divide
For organism growth, to replace dead/damaged cells, and for reproduction.
Mitosis (Growth & Repair)
One parent cell divides once to produce two genetically identical daughter cells (same chromosome count).
Meiosis (Reproduction)
Occurs in reproductive organs to produce gametes.
A two-step division producing four daughter cells, each with half the original chromosomes (creates genetic diversity)
Mitosis vs. Meiosis Pathways
Mitosis Pathway:
Parent Cell (46 Chromosomes) ➔ Splits once ➔ Two Identical Daughter Cells (46 & 46).
Meiosis Pathway:
Parent Cell (46 Chromosomes) ➔ Splits ➔ Two Intermediate Cells (23 & 23) ➔ Splits again ➔ Four Gamete Cells (23, 23, 23, 23).
Internal Support & Cellular Lifespan
Internal Support Structures
Cytoskeleton: Fibrous network providing structural support and movement in eukaryotes.
Cell Inclusions: Stored non-living substances (e.g., starch, silica).
Lifespan, Cancer & Cell Death
Contact Inhibition: Normal animal cells stop dividing when touching neighbors.
Cancer: Abnormal cells lose contact inhibition, dividing uncontrollably to form tumours.
Programmed Cell Death (PCD): Genetically regulated cell destruction, vital for immunity and embryonic development.
Observing Cell Division in Onion Root
Resting Cells: Box-like plant cell with a solid, perfectly round nucleus inside.
Dividing Cells: Box-like plant cell where the dark genetic material is being pulled apart to opposite ends of the cell.
