The unicellular origins of complex physiology

Published on November 30, 2016   19 min

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My name is John Torday, I'm a Professor of Evolutionary Medicine at UCLA. The title of this lecture is "The Unicellular Origins of Complex Physiology".
Sterols and membrane fluidity in prokaryotes and eukaryotes. Sterols play a fundamental role in membrane fluidity in determining interactions between the external and internal environments of the cell. Bacteria express hopanoids, which determine the structure of the cell membrane. Eukaryotes express cholesterol, which plays the same role that hopanoids do in the cell membrane.
Ancient atmospheric oxygen levels, shown in frames A and B of the slide, had to have been high enough to allow the biosynthesis of cholesterol. It requires 11 atoms of oxygen to synthesize 1 molecule of cholesterol. The insertion of cholesterol into the eukaryotic cell membrane fostered the basic characteristics of vertebrate evolution, metabolism, respiration, and locomotion. Subsequently, the cell membrane cholesterol formed lipid rafts that formed the sites for receptors that mediate cell-cell interactions. Ultimately, cholesterol was the substrate for the steroid hormones and vitamin D, which are a large part of the endocrine system that integrates and orchestrates physiologic functions.
The polycyclic hydrocarbons that were contained within asteroids, snow balls, that pelted the early Earth, devoid of oxygen to burn them up upon entry into the atmosphere, were suspended in the early nascent oceans. Lipid emulsions in an aqueous medium, spontaneously generate micelles, semi permeable membrane bound spheres. Within these spheres chemiosmosis, the partitioning of positively and negatively charged ions, would have generated energy to reduce the endogenous entropy of the protocell. Homeostatic control of the system would have sustained the negative entropy of the cell far from equilibrium. The subsequent rise in oceanic calcium due to the increased amounts of carbon dioxide in the atmosphere dissolving in water to form carbonic acid, which leached calcium from the ocean floor, threatened the existence of early life on Earth, since calcium ions come up lipids, proteins, and nucleotides, the essence to life. In response, lipids were employed to form calcium channels to regulate the flow of calcium in and out of the cell. Subsequently, peroxisomes evolved to buffer the deleterious effects of increased cytoplasmic calcium caused by physiologic stress. The epistatic balancing of calcium by lipids has evolved over biologic evolution as the fundament of vertebrate physiology and pathophysiology.

The unicellular origins of complex physiology

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