(a) Mitochondria:

Structure:

• Double membrane:
  • Outer membrane: smooth, permeable
  • Inner membrane: highly folded into cristae (increases surface area)
• Matrix: fluid-filled interior space
  • Contains enzymes for Krebs cycle
  • Own circular DNA (mtDNA)
  • 70S ribosomes
• Size: 1-10 μm

Function:

• Cellular respiration → ATP production
• Krebs cycle (matrix)
• Electron transport chain (inner membrane)
• ATP synthase in cristae
• ~36-38 ATP per glucose molecule

(b) Chloroplasts:

Structure:

• Double membrane (outer and inner)
• Thylakoids: flattened membrane sacs
  • Stack to form grana (singular: granum)
  • Contains chlorophyll and photosystems
• Stroma: fluid-filled space surrounding thylakoids
  • Contains enzymes for Calvin cycle
  • Own circular DNA
  • 70S ribosomes
• Size: 5-10 μm

Function:

• Photosynthesis → glucose production
• Light reactions (thylakoid membrane)
• Calvin cycle (stroma)
• Converts light energy to chemical energy (glucose)

(c) Endosymbiotic Theory:

Hypothesis: Mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells in an endosymbiotic relationship.

Timeline:

1. ~2 billion years ago: aerobic bacterium engulfed → mitochondria
2. ~1.5 billion years ago: photosynthetic cyanobacterium engulfed → chloroplasts

Evidence Supporting Theory:

1. Double membrane:

   • Inner membrane from original prokaryote
   • Outer membrane from host cell's food vacuole

2. Own DNA:

   • Circular DNA (like bacteria)
   • No histones (prokaryotic feature)
   • Can replicate independently

3. 70S ribosomes:

   • Same size as bacterial ribosomes
   • Different from eukaryotic 80S ribosomes
   • Similar rRNA sequences to bacteria

4. Binary fission:

   • Divide independently of cell
   • Similar to bacterial reproduction

5. Gene similarity:

   • mtDNA similar to α-proteobacteria

• Chloroplast DNA similar to cyanobacteria

1. Double membrane structure:
   • Consistent with phagocytosis model

$\boxed{\text{Endosymbiotic theory: organelles = former prokaryotes}}$

Modern examples: Corals with zooxanthellae, paramecium with algae show similar relationships.

Protein Secretion Pathway (Endomembrane System):

(a) Organelles in order:

1. Ribosome (on rough ER)
2. Rough Endoplasmic Reticulum (RER)
3. Transport vesicle
4. Golgi apparatus (cis → medial → trans)
5. Secretory vesicle
6. Plasma membrane

(b) Step-by-step process:

Step 1: Translation begins (Ribosome)

• mRNA binds to free ribosome in cytoplasm
• Signal sequence (first amino acids) synthesized
• Begins: NH₂-Met-...signal peptide...

Step 2: ER targeting (Signal Recognition)

• Signal Recognition Particle (SRP) recognizes signal sequence
• SRP binds to ribosome, pauses translation
• SRP-ribosome complex binds to SRP receptor on RER

Step 3: Synthesis in RER

• Ribosome docks on ER membrane
• Polypeptide fed through translocon channel into ER lumen
• Translation resumes
• Signal peptidase cleaves signal sequence
• Protein folds in ER lumen with help of chaperones
• Glycosylation begins (adds carbohydrate groups)

Step 4: Quality control

• Properly folded proteins continue
• Misfolded proteins → ER-associated degradation (ERAD)

Step 5: Vesicle formation

• COPII-coated vesicles bud from ER
• Carry proteins toward Golgi
• Vesicles fuse to form vesicular-tubular clusters

Step 6: Golgi processing

• Cis face (receiving side): vesicles fuse
• Proteins move through Golgi stack:
  • Cis → medial → trans cisternae
• Modifications:
  • Further glycosylation
  • Phosphorylation
  • Proteolytic cleavage
  • Sorting signals added

Step 7: Sorting at Trans Golgi Network (TGN)

• Proteins sorted by destination
• Packaged into specific vesicles
• Molecular tags determine fate

Step 8: Secretory vesicle

• Clathrin-coated or other vesicles bud from TGN
• Move along cytoskeleton (microtubules) using motor proteins
• Approach plasma membrane

Step 9: Exocytosis

• Vesicle fuses with plasma membrane
• v-SNARE (vesicle) + t-SNARE (target) proteins mediate fusion
• Protein released outside cell
• Vesicle membrane becomes part of plasma membrane

(c) Role of vesicles:

Functions:

1. Transport: Move proteins between organelles
2. Protection: Shield proteins from cytoplasm
3. Compartmentalization: Maintain separation
4. Membrane expansion: Add membrane to plasma membrane
5. Regulation: Allow controlled, timed secretion

Types of secretion:

Constitutive secretion:

• Continuous, unregulated
• Default pathway
• Example: antibodies from plasma cells

Regulated secretion:

• Stored in vesicles until signal received
• Triggered by Ca²⁺ or other signals
• Example: insulin from pancreatic β-cells, neurotransmitters

Vesicle formation mechanisms:

• Coat proteins (COPII, COPI, clathrin) deform membrane
• Cargo receptors select specific proteins
• Vesicles pinch off using dynamin (GTPase)
• Coats removed before fusion

$\boxed{\text{Ribosome → RER → vesicle → Golgi → vesicle → plasma membrane → EXPORT}}$

Time: Entire process takes ~30-120 minutes depending on protein and cell type.