Fundamentals of Biochemistry - A. A. Anisimov 1986

Biological Oxidation and Bioenergetics
Mitochondria as Intracellular Centers of Aerobic Respiration

Cell/35.html">Mitochondria are specialized cellular Organelles, intracellular centers of aerobic Respiration, and The Cell's "power plants." Found in virtually all animal and plant Cells, they typically appear as oval bodies measuring approximately 0.5×3 μm. Mitochondrial shape varies widely across different cell types, ranging from filaments and rods to loops and spheres. Serial sectioning of unicellular organisms reveals that mitochondria can reach giant proportions—nearly matching the size of the cell itself—and exhibit irregular, branched morphologies. Such giant mitochondria have been identified in flagellates, Yeasts, and rat Diaphragm Muscle cells. An ultrathin section through such a giant mitochondrion can easily be mistaken for several smaller organelles.

A single cell may contain anywhere from several dozen to several thousand mitochondria, depending on its type and function. While mitochondria are distributed relatively evenly in many cells, they tend to localize in regions of highest metabolic activity. In certain Tissues, mitochondria are strategically aligned to facilitate ATP delivery to energy-consuming structures. In epithelial cells, for instance, mitochondria are often oriented with their long axes parallel to the direction of secretion. In ciliated Protozoa, they are positioned directly at the base of these specialized locomotory structures.

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Fig. 7.1. Diagram of a mitochondrion. A — cross section; B — inner membrane:

1 — inner membrane, 2 — outer membrane, 3 — cristae, 4 — intermembrane space, 5 — matrix

The ultrastructure of mitochondria has been extensively studied using Electron Microscopy. Structurally, a mitochondrion comprises two membranes—the outer and the inner—an internal space, and an intermembrane space. The internal space is filled with a gel-like, semi-fluid substance known as the matrix. The inner membrane folds inward to form projections called cristae (Fig. 7.1). Mitochondrial membranes are lipoprotein structures measuring 5–7 nm in thickness.

The outer membrane is smooth and highly permeable to molecules with a Molecular Weight of less than 10,000. It is characterized by a high phospholipid-to-protein ratio (approximately 1:1). The outer membrane contains a variety of Enzymes involved in the elongation of saturated fatty acid molecules, as well as Proteins that catalyze non-phosphorylating oxidation reactions. The specific enzymatic Complement of the outer membrane varies among mitochondria from different organisms.

The matrix-facing side of The inner mitochondrial membrane is studded with knob-like protrusions¹, or "elementary particles," which function primarily as ATP synthase (ATPase). Ultrasonic disruption of the inner membrane yields "inside-out particles" in which these stalked spheres face outward. The inner membrane is impermeable to most substances, including H⁺ and OH^- ions; only O₂ and neutral molecules with a molecular weight below 150 can pass through. It features a low phospholipid-to-protein ratio of roughly 1:3. Across all cell types, the inner membrane performs a universal function: coupling Biological Oxidation with ATP synthesis, thereby acting as a coupling membrane. It houses the Enzymes of the Electron Transport Chain and ATP synthesis machinery.

¹ Some researchers suggest that these knob-like protrusions are artifacts resulting from tissue fixation and staining Procedures during electron microscopy preparation.

The intermembrane space of mitochondria contains the enzymes adenylate kinase and nucleoside diphosphate kinase. The mitochondrial matrix houses the β-oxidation system for Fatty acids and the enzymes of The Tricarboxylic Acid Cycle.

Bacteria lack mitochondria; instead, aerobic oxidation and phosphorylation take place in the cytoplasmic membrane and specialized membrane invaginations known as mesosomes. Mesosomes occur in two main forms—lamellar and vesicular—which differ in their membrane packing patterns. Vesicular mesosomes are particularly well developed in members of the family Bacillaceae, whereas lamellar mesosomes are more commonly found in cocci.