Question
Describe the ultrastructure of a mitochondrion with a labelled diagram. Why is the mitochondria called the “powerhouse of the cell”? Mention any two pieces of evidence supporting its endosymbiotic origin.
(NCERT Class 11, Cell: The Unit of Life)
Solution — Step by Step
A mitochondrion has a double membrane system:
- Outer membrane: Smooth, permeable to small molecules due to porins (channel proteins). Separates mitochondrial contents from cytoplasm.
- Inner membrane: Highly folded into projections called cristae (singular: crista). This membrane is selectively permeable and houses the electron transport chain (ETC) and ATP synthase.
The space between the two membranes is called the intermembrane space.
The matrix is the gel-like substance inside the inner membrane. It contains:
- Enzymes of the Krebs cycle (citric acid cycle)
- Mitochondrial DNA (circular, 16.5 kb in humans)
- 70S ribosomes (smaller than cytoplasmic 80S ribosomes)
- Calcium ions, ATP, ADP, and various metabolites
The matrix is where pyruvate is oxidised through the Krebs cycle, producing NADH and FADH₂.
The mitochondria produces the majority of ATP in the cell through oxidative phosphorylation:
- Krebs cycle (in matrix) produces NADH and FADH₂
- ETC (on inner membrane/cristae) transfers electrons from NADH/FADH₂ to O₂
- This pumps H⁺ into the intermembrane space, creating a proton gradient
- ATP synthase uses this gradient to synthesise ATP (chemiosmosis)
One glucose molecule generates approximately 30-32 ATP through mitochondrial respiration (compared to just 2 ATP from glycolysis in the cytoplasm). Since the mitochondria accounts for ~95% of a cell’s ATP production, it rightly earns the title “powerhouse.”
- Circular DNA: Mitochondrial DNA is circular like bacterial DNA, not linear like nuclear DNA
- 70S ribosomes: Same size as bacterial ribosomes (not 80S like cytoplasmic ones)
- Double membrane: The inner membrane may be the original bacterial membrane, while the outer membrane came from the host cell’s engulfing vesicle
- Binary fission: Mitochondria divide by fission, similar to bacteria
- Size: Similar to bacteria (~0.5-1 μm diameter)
Why This Works
The key to understanding mitochondrial function is the concept of chemiosmosis — Peter Mitchell’s Nobel Prize-winning idea. The inner membrane acts as a dam. The ETC pumps protons across it (like filling a reservoir). When protons flow back through ATP synthase (the turbine), their potential energy is converted to chemical energy (ATP).
The cristae increase the surface area of the inner membrane by 5-10 times. More surface area = more ETC complexes = more ATP synthase molecules = more ATP production. Cells with high energy demands (like muscle cells, liver cells) have mitochondria with densely packed cristae.
Alternative Method
For a quick recall framework, remember the mitochondria has four compartments: outer membrane, intermembrane space, inner membrane (cristae), and matrix. Each compartment has a specific role in cellular respiration.
NEET loves matching-type questions: match the reaction with its location. Krebs cycle = matrix. ETC = inner membrane. Oxidative phosphorylation = inner membrane. Fatty acid oxidation = matrix. The intermembrane space is where H⁺ ions accumulate. Get these locations right for easy marks.
Common Mistake
Students often say “mitochondria produces energy.” Mitochondria does NOT produce energy — energy cannot be created (first law of thermodynamics). It converts chemical energy from food molecules (glucose, fatty acids) into a usable form (ATP). The distinction matters in board exams where examiners look for precise language. Write “site of ATP synthesis” or “converts energy into ATP” — not “produces energy.”