Mitochondria are the powerhouses of cells, producing most of their energy through a process called oxidative phosphorylation. These organelles have a unique structure with two membranes, allowing them to efficiently generate ATP, the cell's energy currency.

The endosymbiotic theory explains how mitochondria and chloroplasts evolved from ancient bacteria. These organelles retain some bacterial features, like their own DNA and ribosomes, but have become integral parts of eukaryotic cells, working together to meet energy needs.

Mitochondria and Cellular Energy Production

Structure and function of mitochondria

• Mitochondria are double-membrane organelles found in eukaryotic cells
  • Outer membrane is smooth and permeable to small molecules (ions, nutrients)
  • Inner membrane is highly folded, forming cristae, which increase surface area for energy production
    • Inner membrane contains electron transport chain (ETC) components (protein complexes I-IV) and ATP synthase
• Matrix, the innermost compartment, contains enzymes for the citric acid cycle (Krebs cycle) (pyruvate dehydrogenase, citrate synthase)
• Mitochondria are the primary site of cellular respiration and ATP production
  • Glucose is broken down through glycolysis (cytosol), citric acid cycle (matrix), and oxidative phosphorylation (inner membrane)
  • Mitochondria produce the majority of a cell's ATP (90-95%) through oxidative phosphorylation

Process of oxidative phosphorylation

• Oxidative phosphorylation occurs in the inner mitochondrial membrane
• Electron transport chain (ETC) consists of a series of protein complexes (I, II, III, and IV) and electron carriers (ubiquinone and cytochrome c)
  1. NADH and FADH2, produced during glycolysis and the citric acid cycle, donate electrons to the ETC
  2. Electrons are passed through the complexes, releasing energy used to pump protons (H+) from the matrix into the intermembrane space
  3. This creates an electrochemical gradient, known as the proton motive force ($\Delta p$)
• ATP synthase, a protein complex, uses the proton motive force to generate ATP
  1. Protons flow down their concentration gradient through ATP synthase, driving the rotation of the enzyme's subunits ($F_0$ and $F_1$)
  2. This rotation catalyzes the phosphorylation of ADP to form ATP ($ADP + P_i \rightarrow ATP$)
• Oxidative phosphorylation is the primary source of ATP in aerobic organisms, producing the majority of a cell's energy currency (36-38 ATP per glucose molecule)

Endosymbiotic Theory and Organelle Structure

Endosymbiotic theory for organelles

• Endosymbiotic theory proposes that mitochondria and chloroplasts originated from ancient prokaryotic cells that were engulfed by larger eukaryotic cells
  • Mitochondria likely evolved from aerobic bacteria (α-proteobacteria), while chloroplasts evolved from photosynthetic cyanobacteria
• Evidence supporting the endosymbiotic theory includes:
  • Mitochondria and chloroplasts have their own DNA (circular), ribosomes (70S), and the ability to synthesize proteins
  • These organelles replicate independently of the cell through binary fission, similar to bacterial cell division
  • The inner mitochondrial membrane and chloroplast thylakoid membrane have similarities to bacterial cell membranes (cardiolipin phospholipids)
• Over time, the engulfed prokaryotic cells developed a symbiotic relationship with their host cells
  • The host cell provided protection and nutrients, while the endosymbionts supplied energy (ATP) or organic compounds (sugars through photosynthesis)
• Many genes from the endosymbionts were transferred to the host cell's nuclear genome (gene transfer), leading to the integration of these organelles into eukaryotic cell function

Mitochondria vs chloroplasts in eukaryotes

• Similarities between mitochondria and chloroplasts:
  • Both are double-membrane organelles with their own DNA, ribosomes (70S), and the ability to synthesize proteins
  • Both play crucial roles in energy production for the cell (ATP or sugars)
  • Both are believed to have originated through endosymbiotic events (α-proteobacteria for mitochondria, cyanobacteria for chloroplasts)
• Differences between mitochondria and chloroplasts:
  • Mitochondria are involved in cellular respiration and ATP production, while chloroplasts are the site of photosynthesis (light-dependent and light-independent reactions)
  • Mitochondria have a smooth outer membrane and a highly folded inner membrane with cristae, while chloroplasts have a smooth outer membrane and a thylakoid membrane system
  • Chloroplasts contain additional compartments:
    • Stroma, the fluid surrounding the thylakoid membranes, contains enzymes for the Calvin cycle (RuBisCO)
    • Thylakoid membranes, the site of light-dependent reactions of photosynthesis, contain chlorophyll and other photosynthetic pigments (carotenoids, phycobilins)
  • Chloroplasts are typically larger than mitochondria (5-10 μm vs 1-2 μm) and are found primarily in plant and algal cells, while mitochondria are present in nearly all eukaryotic cells (except mature mammalian red blood cells)