“Cell Biology: Cell Structure and Organelles”

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your WAEC exam.

Cell Theory (Modern Formulation):

1. All living organisms are composed of cells
2. The cell is the basic unit of life
3. All cells arise from pre-existing cells (Rudolf Virchow, 1858)
4. Cell contains genetic material (DNA) that is passed to daughter cells

Two Main Cell Types:

Feature	Prokaryotic	Eukaryotic
Size	0.1-10 μm	10-100 μm
Nucleus	No (nucleoid region)	Yes (membrane-bound)
Organelles	Few (ribosomes only)	Many (mitochondria, etc.)
DNA	Circular, naked	Linear, with histones
Cell wall	Yes (peptidoglycan)	Plants (cellulose), Fungi (chitin)
Example	Bacteria, Blue-green algae	Plants, Animals, Fungi

Cell Organelles - Quick Reference:

Organelle	Function	Found In
Nucleus	Contains DNA, controls cell activities	All eukaryotes
Mitochondria	Aerobic respiration, ATP production	All eukaryotes
Ribosome	Protein synthesis	All cells
RER	Protein synthesis and transport	Eukaryotes
SER	Lipid synthesis, detoxification	Eukaryotes
Golgi apparatus	Modifies, packages, secretes proteins	Eukaryotes
Lysosome	Contains digestive enzymes	Animal cells
Chloroplast	Photosynthesis	Plant cells
Cell wall	Provides structure and support	Plants, bacteria
Cell membrane	Controls what enters/exits cell	All cells

⚡ WAEC Tip: Animal cells have lysosomes and centrioles; plant cells have chloroplasts, large vacuoles, and cell walls. Know which organelles are unique to which cell type.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Detailed Structure and Function:

Nucleus:

• Nuclear envelope (double membrane with pores)
• Nucleoplasm (jelly-like matrix)
• Chromatin (DNA + proteins = chromosomes)
• Nucleolus (site of ribosomal RNA synthesis)

Mitochondria:

• Double membrane: outer smooth, inner folded (cristae)
• Matrix contains enzymes for Krebs cycle
• Cristae have enzymes for electron transport chain
• Site of aerobic respiration → ATP production
• Contains its own DNA (semi-autonomous)

Endoplasmic Reticulum (ER):

Rough ER:

• Ribosomes attached on outer surface
• Synthesizes proteins destined for secretion or membrane
• Proteins enter ER lumen, are folded and processed

Smooth ER:

• No ribosomes
• Synthesizes lipids and steroids
• Detoxifies harmful substances (liver cells)
• Stores calcium ions (muscle cells = sarcoplasmic reticulum)

Golgi Apparatus:

• Cis face (receiving) and trans face (shipping)
• Modifies proteins (glycosylation, phosphorylation)
• Packages proteins into vesicles
• Produces lysosomes

Lysosomes:

• Membrane-bound vesicles with hydrolytic enzymes
• pH ~4.5-5 (acidic)
• Enzymes digest: worn-out organelles (autophagy), food particles (phagocytosis)
• Responsible for cell death when released

Chloroplast (Plant Cells):

• Double membrane with intermembrane space
• Stroma (fluid matrix with enzymes for Calvin cycle)
• Thylakoids (stacked = grana)
• Contains chlorophyll (green pigment)
• Site of photosynthesis

⚡ Common Mistake: Confusing the function of smooth and rough ER. Remember: Rough = Ribosomes = proteins (which often get exported or put in membrane). Smooth = no ribosomes = lipids and steroids.

Cell Membrane Structure - Fluid Mosaic Model:

• Phospholipid bilayer (hydrophilic heads outward, hydrophobic tails inward)
• Proteins float in the lipid layer (mosaic pattern)
• Proteins move laterally (fluid)
• Cholesterol provides stability (in animal cells)

Membrane Transport:

Type	Mechanism	Example
Diffusion	High → Low concentration	O₂, CO₂ entering cells
Osmosis	Water across semipermeable membrane	Water in plant roots
Facilitated diffusion	Channel/carrier proteins	Glucose into cells
Active transport	Carrier protein + ATP	Na⁺/K⁺ pump
Endocytosis	Membrane engulfs material	Phagocytosis, pinocytosis
Exocytosis	Vesicle fuses with membrane	Secretion of hormones

Osmosis Examples:

• Plant cells in hypertonic solution → plasmolysis (cell membrane pulls away from cell wall)
• Plant cells in hypotonic solution → turgidity (cell wall prevents bursting)
• Animal cells in hypertonic → crenation
• Animal cells in hypotonic → lysis

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for serious exam preparation.

The Cytoskeleton:

Component	Structure	Function
Microfilaments	Actin filaments (7nm)	Cell movement, division, muscle contraction
Microtubules	Tubulin dimers (25nm)	intracellular transport, cilia, flagella, mitotic spindle
Intermediate filaments	Various proteins (8-10nm)	Mechanical strength, cell shape

Centrioles and Basal Bodies:

• Each centriole = 9 triplets of microtubules arranged in a cylinder
• Two centrioles perpendicular = centrosome
• Form spindle fibres during cell division
• Basal bodies anchor cilia and flagella (same structure, different name)

Cilia and Flagella:

• 9+2 arrangement of microtubules
• Dynein arms cause bending (ATP-powered)
• Cilia: short, many per cell, wave-like motion
• Flagella: long, usually one per cell, undulating motion

Cell Adhesion and Junctions:

Junction	Function	Structure
Tight junction	Seal cells together	Proteins fuse outer leaflets
Adherens junction	Attach to actin	Cadherin proteins
Desmosomes	Mechanical strength	Cadherin + intermediate filaments
Gap junction	Cell-cell communication	Connexin proteins (pores)

Specialised Cells:

Cell Type	Specialisation	Adaptations
RBC	Carry oxygen	Biconcave shape, no nucleus, lots of Hb
Nerve cell	Conduct impulses	Long axon, myelin sheath, dendrites
Muscle cell	Contract	Many mitochondria, actin/myosin
Palisade cell	Photosynthesis	Many chloroplasts, thin cell wall
Root hair cell	Absorb water	Long projection, large vacuole

Cell Cycle:

1. G₁ Phase (First gap): Cell grows, organelles duplicate
2. S Phase (Synthesis): DNA replicates
3. G₂ Phase (Second gap): Final preparations for division
4. M Phase (Mitosis): Nuclear division
5. Cytokinesis: Cytoplasm divides

Total cell cycle: ~24 hours in human cells Mitosis itself: ~1 hour

Stages of Mitosis:

Stage	Key Events
Prophase	Chromatin condenses, nuclear envelope breaks, centrioles move apart
Metaphase	Chromosomes align at equator, spindle fibres attach to centromeres
Anaphase	Sister chromatids separate, move to poles
Telophase	Nuclear envelopes reform, chromosomes decondense
Cytokinesis	Cell membrane pinches (animal) or cell plate forms (plant)

Cancer and the Cell Cycle:

• Cancer = uncontrolled cell division
• Causes: mutations in genes controlling the cell cycle
• Oncogenes promote cell division
• Tumour suppressor genes (e.g., p53) inhibit division
• When p53 is mutated, cells divide without restraint

⚡ WAEC Previous Year Pattern:

Year	Question	Concept
2023	Identify organelle from electron micrograph	Mitochondria structure
2022	Osmosis in plant cells	Plasmolysis
2021	Mitosis stages	Chromosome movement

Microscope Calculations:

Magnification = Drawing size / Actual size Actual size = Drawing size / Magnification

Always convert to same units (mm → μm: multiply by 1000)

Stem Cells:

• Undifferentiated cells that can become any cell type
• Embryonic stem cells: pluripotent (can become any cell)
• Adult stem cells: multipotent (limited range)
• Used in medicine for treating diseases and injuries

⚡ Exam Strategy: For microscope questions, always convert to micrometres (μm). 1 mm = 1000 μm. Be careful: Plant cells have chloroplasts and cell walls (cellulose); animal cells have centrioles and lysosomes. Mitochondria are found in BOTH.

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