1.5.4: Mitochondria

A mitochondrion (plural = mitochondria) is an oval membranous structure that generates the majority of ATP within the cell. It consists of an outer and an inner lipid bilayer membrane (Figure 25). The outer membrane serves as a barrier and maintains structural integrity. The space between the outer and inner membranes is where various metabolic processes occur, facilitating communication between the cytoplasm and the mitochondrial matrix. The inner membrane is folded to form structures called cristae, which harbor an array of proteins, enzymes, and molecules that orchestrate the biochemical reactions of cellular respiration, such as the citric acid cycle (Krebs cycle) and oxidative phosphorylation. These reactions convert the potential energy contained in nutrient molecules like glucose into ATP, a form of cellular energy that can be readily utilized. High-energy demand cells, such as muscle cells, necessitate copious amounts of ATP for sustaining muscle contractions. Hence, they house a considerable quantity of mitochondria. Similarly, neurons require substantial ATP to power the activity of their sodium-potassium exchange pumps, prompting their possession of over a thousand mitochondria per cell. In contrast, a bone cell might possess only a few hundred mitochondria, as its metabolic activity is comparably lower ³³ , ³⁴ . Mitochondria have their own genetic material—specifically circular DNA and bacterial-like ribosomes—reflecting their evolutionary origin from an ancient aerobic bacterium that was engulfed by a precursor eukaryotic cell.³⁵

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