![]() The interior of the bacterial cell (or the mitochondrial matrix) is relatively alkaline, whereas the exterior periplasmic space (or the mitochondrial intermembrane space) is relatively acidic. Mitochondria generate a proton gradient across the inner mitochondrial membrane. All prokaryotic cells (bacteria and archaea) maintain a proton gradient (pH gradient) across their plasma membranes. The proton-motive force is created by a large (1000-fold) difference in proton concentrations across a membrane. A proton-motive force across a membrane provides the energy for ATP synthase (a molecular machine) to make ATP from ADP and inorganic phosphate. Oxidative phosphorylation synthesizes the bulk of a cell’s ATP during cellular respiration. However, only a small amount of ATP is made this way in cells undergoing respiration. A couple of the enzymes in glycolysis make ATP through substrate-level phosphorylation, as well as an enzyme in the citric acid cycle. Substrate-level phosphorylation means that a phosphate is transferred to ADP from a high-energy phosphorylated organic compound. How do cells make ATP? Cells can make ATP in either of two ways: either by substrate-level phosphorylation of ADP, or by oxidative phosphorylation of ADP. We find that all cells – Bacteria, Archaea, Eukarya – use the energy released via ATP hydrolysis (ATP –> ADP + Pi Pi = inorganic phosphate deltaG = -7.3 kcal/mol) to perform most of the cellular work. Explain how cells exploit the proton motive force to make ATP.Compare and contrast aerobic and anaerobic respiration.Explain how proton gradients are generated across membranes.Explain the difference between substrate-level phosphorylation and oxidative phosphorylation.Identify whether an organism is a heterotroph, photoautotroph or chemoautotroph based on their sources of energy and organic carbon.Explain the role of NAD+/NADH as an electron shuttle.Identify what molecule is oxidized, and what molecule is reduced in a redox reaction.This page is lightly modified from my original March 2010 blog post: “Evolutionary perspective on energy metabolism”. What I prefer is an evolutionary approach that begins with concepts and processes fundamental to all living cells, that must have been present in the last universal common ancestor (LUCA). Moreover, the standard textbook version of how this elaborate metabolic network evolved beginning with glycolysis is probably wrong, accordingly to the compelling essay by Lane et al. The standard freshman biology textbook presentation focuses narrowly on glucose metabolism by animal cells, barely touches on fats and amino acids, and ignores most of the metabolic diversity of life.
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