All life depends on energy for survival. This energy can come from a variety of sources ranging from sunlight to thermal energy to the breakdown of organic molecules. Life is extremely versatile in the types of energy it can utilize. This diversity allows life to exist in almost every niche on Earth. The sum of all of the reactions necessary for life within an organism, those that use or release energy, is called metabolism. As we will see, there are many different reactions that contribute to the overall process we call metabolism
In the most basic of terms, metabolism can be thought of as the combination of two phases. One phase, anabolism, utilizes free energy to create larger molecules from smaller ones. A simple example of this is photosynthesis in plants. Plants capture light energy and use it to make sugar molecules. These sugar molecules are just one way of storing energy for later use. The opposite of anabolism is catabolism, the breakdown of molecules and, most often, release of energy. In plants and animals, an example of a catabolic reaction is respiration. Plants use the sugar that they made during photosynthesis, break apart the chemical bonds in the sugar molecules, and release the energy stored there from the sun. In this way, we can view metabolism as just a cycle of energy within living organisms.
Since there are so many reactions available for use by organisms, it is helpful to classify organisms according to the types of reactions that are characteristic to that organism. In general, we can use energy source to classify organisms. If an organism gets its energy from the sun, we use the prefix “photo-“ to describe. On the flip side, if an organism gets its energy from electron-donating compounds, like sugars, then we use the prefix “chemo-“. We use the suffix “troph” to complete our new names for organisms. For example, plants, since they get utilize sunlight for energy, are considered phototrophs. Humans receive their energy by breaking down the food they eat, therefore they are known as chemotrophs.
Life as we know it is carbon based. However, organisms differ in how they obtain carbon to make life’s molecules. Consequently, we can group organisms according to carbon source as opposed to energy source. If the carbon source of an organism, like a plant, is primarily carbon dioxide, we use the prefix “auto-“. But if an organism, like a human, primarily obtains its carbon from organic molecules, we use the prefix “hetero-“.
Together the two classifications above, energy and carbon source, represent an organism’s nutrition. We have now established the vocabulary to talk about the metabolic diversity of organisms on Earth. For example, a plant gets its carbon from carbon dioxide and receives its energy from the sun, so it is called a photoautotroph. Humans, however, receive their carbon from organic molecules and their energy from the breakdown of food, so they are considered chemoheterotrophs. Many bacteria such as Methanosomonas, Hydrogenomonas, Nitrosomonas, and Thiobacillus are examples of chemoautotrophs because they get their carbon from carbon dioxide, but their energy from the breakdown of organics. Finally, an example of a photoheterotroph would be some green and purple photosynthetic bacteria.
As we see above, just utilizing four simple prefixes allow us to group organisms according to their metabolic requirements. We have only used very large brush strokes to take a brief look at the metabolic diversity on Earth. Discovery of another life form beyond Earth may expand how we think about metabolism. No matter how exotic the life form, scientists agree that extraterrestrial life will have the same requirement of the utilization of energy. The broadness of the classification system we have developed should allow us to include this new life and relate it, somehow, to life on Earth.
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