Chapter 12: Cellular Energetics

The text details the catabolism of glucose starting with glycolysis in the cytosol, a ten-step anaerobic pathway converting glucose into pyruvate while yielding a net gain of ATP and NADH, regulated by key enzymes like phosphofructokinase-1. In the absence of oxygen, cells utilize fermentation to regenerate NAD+, whereas aerobic conditions facilitate the transport of pyruvate into the mitochondrion. The chapter offers an in-depth look at mitochondrial structure, including the double-membrane system, cristae architecture, and the dynamic nature of the organelle through fusion, fission, and mitophagy, as well as its evolutionary origin via endosymbiosis. Within the mitochondrial matrix, the citric acid cycle (Krebs cycle) and fatty acid beta-oxidation oxidize acetyl-CoA to carbon dioxide, reducing electron carriers NAD+ and FAD to NADH and FADH2. These high-energy electrons feed into the electron transport chain embedded in the inner mitochondrial membrane, comprising multiprotein complexes I through IV and mobile carriers like ubiquinone and cytochrome c. As electrons flow down their reduction potential gradient toward molecular oxygen, protons are pumped into the intermembrane space, establishing a voltage and concentration gradient. This proton-motive force powers ATP synthase (F0F1 complex), which employs a rotational binding-change mechanism to phosphorylate ADP to ATP. The narrative then shifts to chloroplasts and photosynthesis in plants, describing how light energy is captured by chlorophyll pigments in Photosystems II and I located within thylakoid membranes. The linear electron flow drives the photolysis of water to release oxygen, the generation of a proton gradient for photophosphorylation, and the reduction of NADP+ to NADPH. Finally, the chapter covers the carbon fixation reactions of the Calvin cycle in the stroma, where the enzyme Rubisco assimilates carbon dioxide into organic sugars, and addresses the inefficiencies of photorespiration alongside evolutionary adaptations like the C4 pathway found in plants adapted to hot, dry environments.