The sum total of biochemical reactions occurring in living organisms by which energy is made available to the organism. Metabolism consists of catabolism, the chemical breakdown of large molecules into their smaller molecular components (e.g., from proteins to amino acids), and anabolism, the reconstruction or chemical synthesis of large molecules. In physiology, metabolism also describes regulated sequences of chemical reactions (physiological pathways), such as protein metabolism or urea metabolism. In ecology, the metabolism of lakes or ponds is the sum of the chemical reactions taking place between the inhabitants and the environment.