Lysosomes (Animals)

• Contain digestive enzymes
• Break down macromolecules
• Autophagy (digest old organelles)
• Apoptosis (programmed cell death)

Vacuoles

Plant central vacuole:

• Storage (water, ions, pigments)
• Maintains turgor pressure
• Waste disposal

Animal vacuoles:

• Smaller, temporary
• Food vacuoles (phagocytosis)
• Contractile vacuoles (osmoregulation)

Energy Organelles

Mitochondria

Structure:

• Double membrane
• Outer membrane (smooth)
• Inner membrane (cristae - folded)
• Matrix (inner space)

Function:

• Cellular respiration
• ATP production
• Own DNA (maternal inheritance)
• Divide independently

Chloroplasts (Plants)

Structure:

• Double membrane
• Thylakoids (stacked in grana)
• Stroma (fluid)

Function:

• Photosynthesis
• Convert light → chemical energy
• Own DNA
• Divide independently

Cytoskeleton

Three types of fibers:

1. Microfilaments (actin):

   • Thinnest (7 nm)
   • Cell shape, movement
   • Muscle contraction
   • Cytoplasmic streaming

2. Intermediate filaments:

   • Medium (8-12 nm)
   • Structural support
   • Nuclear lamina
   • Keratin in hair/nails

3. Microtubules:

   • Thickest (25 nm)
   • Cell shape, organelle movement
   • Chromosome separation (spindle fibers)
   • Cilia and flagella structure

Other Organelles

Ribosomes

• Protein synthesis
• Free ribosomes → cytoplasmic proteins
• Bound ribosomes → secreted/membrane proteins
• NOT membrane-bound

Peroxisomes

• Contain enzymes
• Break down fatty acids
• Detoxify harmful substances
• Produce hydrogen peroxide → water

Centrioles (Animals)

• Pair of cylindrical structures
• Organize microtubules
• Form spindle apparatus in cell division

Key Concepts

1. Nucleus stores DNA and controls cell
2. Endomembrane system makes, modifies, transports proteins
3. Mitochondria produce ATP (cellular respiration)
4. Chloroplasts capture light energy (photosynthesis)
5. Cytoskeleton provides structure and movement
6. Organelles compartmentalize cellular functions

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.