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BIOL: Lecture Notes-Sept. 5: Mitochindria&Chloroplasts

Endosymbiotic origin of mitochondria and chloroplasts

  • Most eukaryotic cells contain mitochondria; mature red blood cells (RBCs) are an exception and lack mitochondria, illustrating that not every cell has these organelles.
  • Chloroplasts are present in plants and photosynthetic algae; green parts of plants and photosynthetic algae carry chloroplasts.
  • Central question raised: where did chloroplasts originate? Answer hinted: some bacteria possessed photosynthetic pigments and capabilities.
  • Endosymbiotic theory (inferred from transcript):
    • Mitochondria originated from a free-living proteobacterium that entered a primitive eukaryotic ancestor and became a symbiotic organelle.
    • Chloroplasts originated from photosynthetic cyanobacteria entering a primitive eukaryote.
  • DNA evidence for endosymbiosis (mentioned in transcript): organelles (mitochondria and chloroplasts) contain their own DNA similar in organization to bacterial DNA.
  • Comparative DNA/rRNA evidence: similarities between organelle genomes and bacterial genomes support a bacterial origin.
  • Structural and functional parallels:
    • Double membranes around mitochondria and chloroplasts (outer from host cell, inner from engulfed bacterium).
    • Ribosomes within organelles resemble bacterial 70S ribosomes rather than eukaryotic 80S ribosomes.
    • Replication by binary fission-like processes within the organelles.
  • Practical implications of endosymbiosis:
    • Explains why some cells lack mitochondria (evolutionary variation).
    • Helps understand the distribution of photosynthetic capacity across plant and algal lineages.
  • Photosynthesis formula (contextual):
    6 \mathrm{CO2} + 6 \mathrm{H2O} \xrightarrow{\text{light}} \mathrm{C6H{12}O6} + 6 \mathrm{O2}
  • Broader significance: endosymbiotic events are foundational to eukaryotic cell evolution, enabling aerobic respiration and photosynthesis.

Mitochondria: function, energy production, and ROS

  • Primary role: generate ATP through oxidative phosphorylation via the electron transport chain (ETC) located in the inner mitochondrial membrane.
  • Oxidative metabolism produces energy and reactive byproducts, such as reactive oxygen species (ROS).
  • Hydrogen peroxide production and detoxification:
    • Peroxisomes also generate hydrogen peroxide (H₂O₂) via oxidation reactions, serving roles in lipid metabolism and detoxification.
    • Peroxisomes contain catalase to convert H₂O₂ to water and oxygen:
      \mathrm{2\,H2O2 \rightarrow 2\,H2O + O2}
  • Mentions that oxidation can occur within organelles, highlighting ROS as byproducts of metabolism and the need for detoxifying systems.
  • RBCs lack mitochondria, illustrating cellular diversity in energy strategies:
    • RBCs rely on glycolysis for ATP production rather than oxidative phosphorylation in mitochondria.
    • Glycolysis yield (context): net ATP per glucose from glycolysis is 2, under anaerobic-like conditions in RBCs.
      \text{Net ATP from glycolysis per glucose} = 2

Chloroplasts: photosynthesis in plants and algae

  • Chloroplasts are the site of photosynthesis in plants and photosynthetic algae.
  • Plants and algae both contain chloroplasts, enabling capture of light energy and conversion to chemical energy.
  • Endosymbiotic origin (again): chloroplasts likely arose from ancient photosynthetic bacteria (cyanobacteria) that entered a eukaryotic host.
  • Evidence supporting chloroplast endosymbiosis includes circular DNA, 70S-like ribosomes, double membranes, and photosynthetic pigments.
  • Photosynthetic capacity extends beyond plants to algae, explaining green parts of plants and the presence of photosynthetic organisms in aquatic environments.

Peroxisomes: oxidation and hydrogen peroxide detoxification

  • Peroxisomes host oxidation reactions, including fatty acid beta-oxidation and detoxification processes.
  • Byproducts include hydrogen peroxide (H₂O₂), a reactive oxygen species requiring detoxification.
  • Catalase within peroxisomes decomposes H₂O₂ to water and oxygen:
    \mathrm{2\,H2O2 \rightarrow 2\,H2O + O2}
  • Peroxisomes thus play a crucial role in managing cellular redox balance and protecting the cell from oxidative damage.

Evidence for endosymbiosis: DNA, ribosomes, membranes

  • Key lines of evidence cited in the transcript:
    • Circular DNA in mitochondria and chloroplasts similar to bacterial genomes.
    • 70S ribosomes within organelles, resembling bacteria rather than eukaryotic cytosolic ribosomes.
    • Enzymatic and functional parallels with certain bacteria (e.g., oxidative metabolism, photosynthesis in chloroplast ancestors).
    • Double membranes around both organelles, consistent with an engulfment event.
    • Reproduction by binary fission-like processes inside organelles.
  • Implication: these features collectively support the endosymbiotic origin of mitochondria and chloroplasts.

Intracellular transport and organelle dynamics in long cells

  • In elongated cells, such as neurons, organelles and cargo are transported along cytoskeletal tracks.
  • Movement is not simple or linear: long structures require back-and-forth movement and active transport mechanisms.
  • Growth cones in developing neurons illustrate dynamic transport and cytoskeletal remodeling:
    • Growth cone features a motor-driven navigation system to extend processes toward targets.
    • The movement of cargos (e.g., mitochondria, vesicles) is mediated by motor proteins that traverse microtubules.

Neuronal growth cone: actin structures and motility

  • The growth cone is the dynamic tip of a growing axon, guiding neurite outgrowth.
  • Actin-based structures within the growth cone:
    • Filopodia: slender, stick-like protrusions made of tightly bundled actin filaments; extend and explore the environment.
    • Lamellipodia: broad, wave-like projections consisting of a branched actin network; drive forward movement and membrane advance.
  • Movement mechanics described (interpretation of transcript):
    • Motor proteins in neurons attach to cargo and “hold on” to one side while moving along microtubule tracks on the other side, enabling directed movement toward target regions.
    • Growth cone extension involves coordinated cycles of actin polymerization in filopodia and lamellipodia and reorganization of the cytoskeleton.
  • The overall process reflects a balance between forward protrusion (actin assembly) and retraction, regulated by guidance cues and intracellular signaling.

Connections to foundational principles and real-world relevance

  • Endosymbiotic theory connects to broader themes in evolution and cellular biology: complex organelles can arise from simpler ancestral partners.
  • Energy metabolism: mitochondria provide ATP for cellular activities; RBCs rely on glycolysis due to lack of mitochondria.
  • ROS management: ROS are byproducts of oxidative metabolism; detoxification systems (e.g., catalase in peroxisomes) are essential for cellular health.
  • Cytoskeletal dynamics and motor proteins underpin fundamental cell biology topics: intracellular transport, neuron development, and growth cone navigation.
  • Real-world implications:
    • Disorders affecting mitochondria, peroxisomes, or cytoskeletal transport can impact energy metabolism, lipid processing, and nervous system development.
    • Understanding endosymbiosis informs studies of genetic inheritance, organelle evolution, and ancient host–symbiont interactions.