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Living systems create order, and appear to break the Second Law. This chapter explains, and resolves, this apparent paradox, drawing on the concept of coupled reactions (as introduced in Chapters 13 and 16), as mediated by ‘energy currencies’ such as ATP and NADH. The chapter then examines the key energy-capturing systems in biological systems – glycolysis and the citric acid cycle, and also photosynthesis. Topics covered include how energy is captured in the conversion of glucose to pyruvate, the mitochondrial membrane, respiration, electron transport, ATP synthase, chloroplasts and thylakoids, photosystems I and II, and the light-independent reactions of photosynthesis.

All biological operations require the input of energy. In contemporary organisms, energy is harvested by mechanisms that are essentially electrical. Energy transduction relies on the circulation of ions across membranes (usually protons), and on a sophisticated rotary ATP synthase to capture the energy and conserve it as ATP. This chapter begins with an overview of chemiosmotic energy transduction and then poses the question, How did LUCA make a living? Current thinking questions the traditional notion that that LUCA lived off prebiotic organic molecules, and favors geochemical processes, such as the reduction of CO2 by hydrogen gas. One possible habitat for such organisms would be mineral honeycombs laid down by hydrothermal vents. If there is any truth to this hypothesis, then chemiosmotic energy transduction will have been part of LUCA's endowment, and may have evolved under the special circumstances that prevail in those vents. Subsequent evolution will have led to the liberation of cells from their mineral cradle, and the progressive expansion of energy metabolism by the invention of both photosynthesis and respiration.

It seems inescapable that at some point primitive cells incorporated chemical reactions related to what we now call metabolism. In all life today, metabolic reactions are driven by sources of chemical or photochemical energy, and each step is catalyzed by enzymes and regulated by feedback systems. There have been multiple proposals for the kinds of reactions that could have been incorporated into early life, but so far there is little consensus about a plausible way for metabolism to begin. This chapter will briefly review the main ideas that are familiar to chemists as solution chemistry but then ask a new question from the epigraph: how can reactions in bulk aqueous solutions be captured in membranous compartments? This question is still virtually unexplored, but I can offer some ideas in the hope of guiding potentially fruitful approaches. Because metabolism is such a complex process, it is helpful to use bullet points to help clarify the discussion. The first is a list of questions that guide the discussion, the second is list of facts to keep in mind, and the third is a list of assumptions that introduce the argument. Questions to be addressed: What are the primary metabolic reactions used by life today? What reactions can occur in prebiotic conditions that are related to metabolism? How can potential nutrient solutes cross membranes in order to support metabolism? How could metabolic systems become incorporated into primitive cellular life? Metabolism can be defined as the activity of catalyzed networks of intracellular chemical reactions that alter nutrient compounds available in the environment into a variety of compounds that are used by living systems. Most of the reactions are energetically downhill, so there is an intimate association between the energy sources available to life and the kinds of reactions that can occur. Here is a summary of energy sources used by life today: Light is by far the most abundant energy source, totaling 1360 watts per square meter as infrared and visible wavelengths. Chemical energy in the form of reduced carbon compounds is made available by photosynthesis.

A more technical chapter in which the author describes the molecular adaptations allowing hyperthermophiles thriving in hell. The chapter starts with reminder of the role of water and carbon in the chemistry of life and the importance of both strong (covalent) and weak chemical bonds in the architecture of macromolecules. The two types of damages affecting these bonds at high temperature, thermodegradation and thermodenaturation, respectively, are discussed, explaining the range of temperatures suitable for life on Earth. The mechanisms protecting proteins, nucleic acids and lipids against thermodegradation and thermodenaturation are described with reminders of the role of these macromolecules in cell physiology. This includes an experiment showing that circular DNA is stable up to 110°C, thanks to topological links between the two DNA strands. RNA is very fragile at high temperature and the author suggests that requirement for messenger RNA stability in eukaryotes could explain why there is no hyperthermophilic eukaryote. Membrane permeability appears to be the Achille’s heel of cells at high temperature and the unique phospholipids of archaea probably explain why most hyperthermophiles belong to this domain. Reading this chapter led you concluding that microbes from hell are not primitive organisms but marvels of adaptation