The mitochondrion is a double-membrane organelle known as the powerhouse of the cell, responsible for aerobic respiration and ATP production. It is important for NEET Biology because NTA frequently tests the Krebs cycle, electron transport chain, F0-F1 ATP synthase mechanism, and the chemiosmotic hypothesis.
Outer Membrane
Smooth, permeable outer boundary containing porins that allow passage of molecules up to 5000 daltons. Similar in composition to other cellular membranes.
Inner Membrane
Highly folded, selectively permeable membrane rich in cardiolipin. Contains the electron transport chain complexes (I-IV) and ATP synthase. Impermeable to most ions and molecules.
Cristae
Inward folds of the inner membrane that greatly increase surface area for oxidative phosphorylation. The number of cristae correlates with the metabolic activity of the cell.
Matrix
Gel-like interior containing enzymes of the Krebs cycle (TCA cycle), mitochondrial DNA, ribosomes, and other metabolites. Site of pyruvate oxidation and fatty acid beta-oxidation.
Intermembrane Space
Narrow space between the outer and inner membranes. Accumulates protons (H+) pumped by the ETC, creating the proton-motive force that drives ATP synthesis.
Ribosomes
70S ribosomes in the matrix, smaller than cytoplasmic 80S ribosomes. Synthesize some inner membrane proteins, supporting the endosymbiotic theory of mitochondrial origin.
Mitochondrial DNA
Circular, double-stranded DNA in the matrix encoding 13 ETC proteins, 22 tRNAs, and 2 rRNAs. Maternally inherited and lacks histones, similar to bacterial DNA.
F0-F1 Particles
ATP synthase complex embedded in the inner membrane and cristae. F0 is the transmembrane proton channel; F1 is the catalytic head that synthesizes ATP from ADP and Pi using the proton gradient.
Krebs Cycle (Matrix)
NTA frequently tests the location and products of the Krebs cycle. Remember: it occurs in the matrix, produces 2 GTP, 6 NADH, 2 FADH2, and 4 CO2 per glucose molecule. Acetyl CoA is the entry molecule. Know the key enzymes — isocitrate dehydrogenase is the rate-limiting enzyme.
ETC and Oxidative Phosphorylation (Inner Membrane)
The electron transport chain (Complexes I-IV) is located on the inner mitochondrial membrane. NADH enters at Complex I, FADH2 at Complex II. The terminal electron acceptor is oxygen. Know the sequence: Complex I → CoQ → Complex III → Cytochrome c → Complex IV.
F0-F1 ATP Synthase
ATP synthase has two parts — F0 (membrane-embedded proton channel) and F1 (catalytic headpiece in the matrix). Protons flow from the intermembrane space through F0 back to the matrix, and the energy drives F1 to synthesize ATP. Expect questions on its structure and mechanism.
Chemiosmotic Hypothesis
Proposed by Peter Mitchell. The ETC pumps H+ ions from the matrix to the intermembrane space, creating an electrochemical gradient (proton-motive force). ATP is synthesized when protons flow back through ATP synthase. This coupling of electron transport to ATP synthesis is called oxidative phosphorylation.
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