Cell Biology and Cell Structure
🟢 Lite — Quick Review (1h–1d)
Rapid summary for last-minute revision before your exam.
Cell Biology — Key Facts for Sri Lanka A/L Examination
Cell Theory (Key for exam):
- All living organisms are composed of cells
- The cell is the basic unit of life
- All cells arise from pre-existing cells (Rudolf Virchow, 1855)
Types of Cells:
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Chloroplasts | Present | Absent (except some) |
| Vacuoles | Large central vacuole | Small, numerous |
| Centrioles | Absent (in most) | Present |
| Shape | Rectangular, fixed | Round, irregular |
Key Organelles:
- Nucleus: Contains DNA, controls cell activities
- Mitochondria: “Powerhouse” - produces ATP
- Endoplasmic Reticulum (ER): Protein and lipid synthesis
- Golgi Apparatus: Packaging and secretion
- Chloroplasts: Site of photosynthesis (plants only)
⚡ A/L Exam Tip: For plant cell diagrams, always include cell wall, cell membrane, nucleus, chloroplasts, vacuole, and mitochondria!
🟡 Standard — Regular Study (2d–2mo)
Standard content for students with a few days to months.
Cell Biology — Detailed Study Guide
Prokaryotic vs Eukaryotic Cells
Prokaryotic Cells (Bacteria, Cyanobacteria):
- No membrane-bound nucleus (nucleoid region)
- No membrane-bound organelles
- Smaller (1-10 μm)
- Cell wall present (peptidoglycan)
- DNA: Single circular chromosome
- Examples: Bacteria, Archaea
Eukaryotic Cells (Plants, Animals, Fungi, Protists):
- Membrane-bound nucleus
- Membrane-bound organelles
- Larger (10-100 μm)
- Cell wall in plants (cellulose), absent in animals
- DNA: Linear chromosomes in nucleus
- Examples: Plant cells, animal cells, fungi
Cell Membrane Structure:
- Fluid Mosaic Model (Singer & Nicolson, 1972)
- Phospholipid bilayer with embedded proteins
- Functions: Selective permeability, cell signaling, transport
⚡ Common Mistake: Students confuse prokaryotic cells with eukaryotic plant cells. Remember - prokaryotes have no nuclear membrane and no membrane-bound organelles!
Plant Cell Organelles
Cell Wall:
- Composed of cellulose (β-1,4-glucan fibers)
- Provides structural support and protection
- Primary wall (young cells) and secondary wall (mature cells)
- Middle lamella (pectin layer between cells)
Chloroplasts:
- Double membrane organelle
- Contains thylakoids (stacked as grana)
- Site of photosynthesis
- Contains own DNA and ribosomes (endosymbiotic theory evidence)
- Chlorophyll: Green pigment in thylakoid membranes
Mitochondria:
- Double membrane organelle
- Site of cellular respiration (Krebs cycle, electron transport chain)
- Produces ATP
- Contains own DNA and ribosomes
Nucleus:
- Contains genetic material (DNA)
- Nuclear envelope (double membrane with pores)
- Nucleolus: Site of ribosomal RNA synthesis
- Chromatin: DNA + histone proteins
Vacuoles:
- Central vacuole: Large, fills 90% of plant cell volume
- Contains cell sap (water, ions, sugars, pigments)
- Maintains turgor pressure
- In animals: Smaller food vacuoles, contractile vacuoles
🔴 Extended — Deep Study (3mo+)
Comprehensive coverage for students on a longer study timeline.
Cell Biology — Complete Notes for A/L Sri Lanka
Cell Division
Mitosis (Somatic cells):
| Phase | Key Events |
|---|---|
| Prophase | Chromatin condenses, nuclear envelope breaks |
| Metaphase | Chromosomes align at cell equator |
| Anaphase | Sister chromatids separate, move to poles |
| Telophase | Nuclear envelope reforms, chromosomes decondense |
| Cytokinesis | Cell cytoplasm divides |
Meiosis (Gamete formation):
- Two divisions: Meiosis I and Meiosis II
- Meiosis I: Prophase I (crossing over), Metaphase I, Anaphase I, Telophase I
- Meiosis II: Like mitosis (sister chromatids separate)
- Result: 4 haploid cells (n) from 1 diploid cell (2n)
⚡ A/L PYQ: “What is the importance of crossing over in Prophase I of Meiosis?” Answer: Creates genetic variation by exchanging genetic material between homologous chromosomes.
Special Cell Types in Plants
Xylem Elements:
| Element | Function | Features |
|---|---|---|
| Tracheids | Water conduction | Elongated, tapered ends |
| Vessel elements | Water conduction | Short, wide, perforated |
| Xylem parenchyma | Storage | Living cells |
| Xylem fibers | Support | Thick walls, dead |
Phloem Elements:
| Element | Function | Features |
|---|---|---|
| Sieve tube elements | Food conduction | Living, lack nucleus |
| Companion cells | Support sieve tubes | Living, nucleus controls |
| Phloem parenchyma | Storage | Living cells |
| Phloem fibers | Support | Thick walls, dead |
Sieve Tubes:
- Long tubes formed by sieve tube elements
- End walls have sieve plates (sieve areas)
- Adjacent companion cells provide metabolic support
- Transport dissolved sugars (mainly sucrose)
Cell Membrane Transport
Types of Transport:
| Type | Mechanism | Energy | Example |
|---|---|---|---|
| Diffusion | High to low concentration | Passive (no ATP) | CO₂, O₂ movement |
| Osmosis | Water across semi-permeable membrane | Passive | Water in plant roots |
| Active transport | Against concentration gradient | Active (ATP) | Ion pumps |
| Facilitated diffusion | Via carrier proteins | Passive | Glucose transport |
Osmosis in Plant Cells:
- Turgid cell: Water enters, cytoplasm pushes against cell wall (turgor pressure)
- Plasmolysed cell: Water leaves, cytoplasm shrinks from cell wall
- Flaccid cell: No net water movement
⚡ A/L Important: Turgor pressure is essential for maintaining plant structure. Wilting occurs when turgor pressure drops!
Cell Energetics
ATP Synthesis:
- Glycolysis: Glucose → 2 Pyruvate + 2 ATP (cytoplasm)
- Krebs Cycle: Acetyl-CoA → CO₂ + H + 2 ATP (mitochondria matrix)
- Electron Transport Chain: H+ → ATP synthase → 32-34 ATP (inner mitochondrial membrane)
- Total: ~36-38 ATP per glucose molecule
Photosynthesis:
| Stage | Location | Input | Output |
|---|---|---|---|
| Light reactions | Thylakoid membranes | H₂O, light | ATP, NADPH, O₂ |
| Dark reactions (Calvin) | Stroma | CO₂, ATP, NADPH | Glucose |
Light Reactions:
- Photolysis: 2H₂O → 4H+ + O₂ + 4e- (releases oxygen)
- Non-cyclic electron transport: PSII → electron transport chain → PSI → NADP+
- Cyclic electron transport: Only PSI, produces only ATP
Calvin Cycle (C₃ Pathway):
- Carbon fixation: CO₂ + RuBP → 2 × 3-PGA (enzyme: Rubisco)
- Reduction: 3-PGA → G3P → glucose
- Regeneration: G3P → RuBP
⚡ A/L Key Point: Rubisco is the most abundant enzyme on Earth! It catalyzes the first step of carbon fixation.
GCE A/L Sri Lanka Past Paper Tips
Common Questions:
- Draw and label plant cell diagram (6-8 marks)
- Differences between plant and animal cells (5 marks)
- Explain fluid mosaic model (5 marks)
- Describe process of mitosis with labeled diagrams (10 marks)
- Explain structure and function of chloroplast (8 marks)
Drawing Guidelines:
- Use pencil (HB or 2H)
- Clean, continuous lines
- Labels with straight lines (no arrows)
- Label on right side of drawing
- Give magnification when asked
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