Metabolic activity refers to the set of chemical reactions that maintain life in any organism. Metabolic activity involves the transformation of energy and matter in the body, two elements that must always be present for life to be sustained. Two types of metabolic processes are anabolism and catabolism. Anabolism is constructive metabolism, during which small molecules are formed into larger ones, requiring an input of energy. Catabolism is the opposite process; it necessitates an output of energy, and large molecules are broken into smaller ones.
Human beings are heterotrophs, which means that humans are dependent on organic molecules that have been ingested to meet energy needs. Organic molecules are the building blocks that meet anabolic needs as well. Metabolic activity begins with ingestion, when food is taken into the body. Metabolic enzymes come into play during digestion, when food is broken down in the body through a process called hydrolysis. During hydrolysis, starches become sugars, proteins become amino acids, fats become fatty acids and glycerol, and nucleic acids become nucleotides.
After digestion, metabolic activity continues with absorption into the body and transportation to the cells, where it enters them. Once the food parts are in the cells, the molecules continue to break down until they contain between two and four carbon atoms. The molecules might continue up the metabolic pathways and become building blocks for sugar and amino acids. The new parts of the cell, which include polysaccharides, proteins, fats and nucleic acids, are then assembled.
Another option is that the molecules may be further broken down into inorganic molecules like water, carbon dioxide and ammonia. Large amounts of energy are released during this phase of metabolic activity, making it catabolic. Nutrition processes that involve metabolic activity include autotrophic nutrition and heterotrophic nutrition. Autotrophs include plants, algae and bacteria and rely on sunlight to change inorganic molecules such as water into smaller, organic molecules; this is known as photosynthesis. In heterotrophic nutrition, energy is taken from organic molecules and broken down to form even smaller molecules, and the food molecules come from autotrophs.
The metabolism of an organism defines which substances it will find nutritious and which ones will be poisonous. Metabolic rate also plays a strong factor in determining how much food an organism requires. Internal metabolic pathways are similar, even across species. The similarities are due to the efficiency of the pathways and the fact that they continue to evolve with species.