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Microorganisms obtain energy through the catabolism and oxidation of electron donors in their environment such as sulfur, carbohydrates, lipids, and proteins. There are several stages of catabolism including glycolysis, beta-oxidation, amino acid catabolism, and the citric acid cycle which generate ATP and electron carriers like NADH. Green plants and photosynthetic bacteria are able to use chlorophyll and sunlight to capture energy and produce oxygen and sugars through photosynthesis. Some anoxygenic purple bacteria lack the ability to produce oxygen and must generate ATP through catabolism without generating NADH.