Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Electron Transport, Oxidative Phosphorylation, and Regulation of ATP Synthesis
Electron transfer energy is also utilized for other purposes

The crucial role of Electron transport is, of course, to provide energy for ATP synthesis during Oxidative Phosphorylation. However, the energy derived from electron transport can also be utilized for other biological Functions (Fig. 17-20), such as thermogenesis. Newborn infants, the offspring of hairless mammals, and certain hibernating animals possess a specialized adipose tissue known as brown fat, located in the neck region and upper back. Its primary function is to generate heat through fat oxidation. This adipose tissue appears brown because it is exceptionally rich in Cell/35.html">Mitochondria, which contain high concentrations of reddish-brown Cytochromes. Specialized brown fat mitochondria (Fig. 17-21) typically do not synthesize ATP. Instead, the Free energy of electron transport is dissipated as heat, thereby maintaining the core body Temperature of young animals at an appropriate level. The inner membranes of brown fat mitochondria feature specialized pores for H⁺ ions. Protons pumped out of the mitochondria during electron transport flow back into the matrix through these pores, bypassing F₀F₁-ATPase. Consequently, the free energy of electron transport is harnessed for heat production rather than ATP synthesis.

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Fig. 17-20. The transmembrane H⁺ gradient supplies energy for various cellular activities.

Fig. 17-21. Cytology/practical/72.html">Cross section of a mitochondrion from rat brown adipose tissue. This mitochondrion contains numerous long, densely packed cristae with a high cytochrome content, resulting in extremely high respiratory activity. Nearly all the energy generated by electron transport in brown fat mitochondria is converted into heat because the extruded H⁺ ions return to the matrix not via ATP synthase, but through open H⁺ channels.

The H⁺ concentration gradient generated by electron transport is also utilized to drive the uptake of Ca²⁺ ions from the Cytosol into the mitochondria of animal Cells (Fig. 17-22). The influx of Ca²⁺ into mitochondria is counterbalanced by an efflux pathway, a regulatory mechanism by which mitochondria help maintain the characteristically low cytosolic Ca²⁺ concentration (approximately 10^-7 M). Free Ca²⁺ serves as a vital intracellular second messenger that regulates numerous cellular functions. An elevation in Ca²⁺ concentration triggers or accelerates processes such as Muscle contraction (Section 14.14), Glycogen breakdown (Section 25.7), and Pyruvate oxidation (Section 16.10), whereas a decrease in Ca²⁺ concentration slows or halts these processes.

Fig. 17-22. Influx and efflux of Ca²⁺ ions in mitochondria. Both processes are energy-dependent. The inner mitochondrial membrane contains two transport systems for Ca²⁺, which appear to be proteinaceous in nature. One system mediates Ca²⁺ influx, while the other mediates its efflux. The cytosolic Ca²⁺ concentration is maintained at a very low level, determined by the relative rates of Ca²⁺ influx and efflux.