An overview: Harvesting energy from glucose
Both eukaryotic and prokaryotic cells can harvest energy from glucose using different combinations of the following metabolic pathways:
Under aerobic conditions, when O2 is available as the final electron acceptor, four pathways operate (Figure 9.4A). Glycolysis is followed by the three pathways of cellular respiration: pyruvate oxidation, the citric acid cycle (also called the Krebs cycle or the tricarboxylic acid cycle), and electron transport/ATP synthesis (also called oxidative phosphorylation).
Figure 9.4 Energy-Yielding Metabolic Pathways Energy- yielding reactions can be grouped into five metabolic pathways: glycolysis, pyruvate oxidation, the citric acid cycle, the respiratory chain/ATP synthesis, and fermentation. (A) The three lower pathways occur only in the presence of O2 and are collectively referred to as cellular respiration. (B) When O2 is unavailable, glycolysis is followed by fermentation. Activity 9.1 Glycolysis and Fermentation
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ac9.1 In eukaryotes and many prokaryotes, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation do not function under anaerobic conditions. The pyruvate produced by glycolysis is further metabolized by fermentation (Figure 9.4B). Some prokaryotes, however, are able to harvest energy in pathways involving oxidative phosphorylation even in the absence of oxygen (anaerobic respiration; see Key Concept 9.3).
The five pathways shown in Figure 9.4 occur in different locations in the cell (Table 9.1).
| Eukaryotes | Prokaryotes |
|---|---|
| In cytoplasm | In cytoplasm |
| Glycolysis | Glycolysis |
| Fermentation | Fermentation |
| Citric acid cycle | |
| Inside mitochondrion | On cell membrane |
| Matrix | Pyruvate oxidation |
| Citric acid cycle | Respiratory chain |
| Pyruvate oxidation | |
| Inner membrane | |
| Respiratory chain |
Activity 9.2 Energy Pathways in Cells
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