Basic Biochemistry for Astrobiology

  • Biochemistry is the study of chemical processes relating to living things.  An understanding of biochemistry is important in astrobiology.  For example, the search for microbial life beyond Earth often includes the search for chemical evidence.  This can include chemical evidence of molecules necessary for or conducive to life, of molecules necessary for chemical reactions of life, and of molecules that result from chemical reactions of life.  As a concrete example, in the Fosdick’s Astrobiology Series article entitled, “Life on Titan with Methane as a Solvent?”, the detection of surface lakes of methane on Titan, moon of Saturn, combined with evidence of low concentrations of elemental hydrogen and acetylene near the surface of Titan, in part led to a hypothesis regarding potential methanogenic microbes living in the lakes of Titan.  This is an example of an astrobiological hypothesis rooted largely in biochemistry – known or hypothetical.
  • Life on Earth is primarily based on six chemical elements: carbon, hydrogen, nitrogen,  oxygen, phosphorus and sulfur.  These are the primarily elements that form the building blocks of most biomolecules (the molecules of living things).
  • There are four basic types of biomolecules: carbohydrates, lipids, proteins and nucleic acids.  All of these are made up mostly of the six mentioned elements, and each type is defined by specific, technical chemical and molecular make-up attributes (the details of which need not be covered here).
  • Many biomolecules are made of up smaller sub-molecular units called monomers that are linked together to form larger biomolecules or macromolecules, called polymers.  Furthermore, various macromolecules may assemble in groups or complexes as needed for different biological requirements.
  • Lipids refer to a variety of relatively water-insoluble or hydrophobic compounds.  Lipids play roles in cellular energy storage and as structural components of cell membranes.  Lipids include fats, such as in normal human diets (oil, cream, etc.).
  • Proteins are macromolecules that have many roles in life, including in structures and as enzymes that catalyze metabolic chemical reactions.  Proteins are essentially chains of amino acids, which are a type of biomolecule composed of carbon, hydrogen, oxygen and nitrogen.  More particularly, peptide-bonded chains of amino acids form polypeptides, and proteins are formed of one or more polypeptides.  Proteins often form as specific three-dimensional structures related to their functions.  Furthermore, proteins often play important roles as part of macromolecular assemblies, which are massive, complex chemical structures that form specifically defined three-dimensional compositions and shapes (such as cellular organelles and cellular membranes).
  • Nucleic acids, prevalent in cell nuclei, are extremely large macromolecules that are necessary for all known life.  They are used in storing and conveying genetic information.  The well-known genetic storage macromolecules deoxyribonucleic acid and ribonucleic acid, or DNA and RNA,  are formed from nucleic acids.  Nucleic acids also play roles in intracellular signaling as well es energy systems, and form the base molecule in adenosine triphosphate, or ATP, used to store energy.
  • Carbohydrates include sugars.  Carbohydrates play many biological roles, but they are notably key in energy storage, and also play an important role in genetic information storage.
  • Carbohydrates contain carbon, hydrogren and oxygen (the name “carbohydrate” stems from the incorporation of carbon (“carbo”) as well as the incorporation of the constituents of water, hydrogen and oxygen (“hydrate”).
  • Carbohydrates are built from monomers called monosaccharides, notably glucose (C6H12O6), as well fructose and dioxyribose.  Most living things use glucose to store energy, and break it down (through oxidation) to access that energy (which may be further stored).
  • In cellular aerobic respiration (which uses elemental oxygen (O2)), glucose + elemental oxygen react to form carbon dioxide + water + energy (which explains why, for example, humans breathe in elemental oxygen and breathe out carbon dioxide).
  • However, some microbes use anaerobic respiration (which does not use elemental oxygen), such as methane-producing microbes (methanogens).  Anaerobic microbes may synthesize their own complex organic compounds (as opposed to getting them from their environment, like humans), some of which is later broken down into glucose and then used for energy.  In particular, some anaerobic microbes, such as may live around hydrothermal vents, use energy from the oxidation of certain inorganic compounds (or methane (CH4), the simplest organic compound) to drive their synthesis of complex organic compounds; this is called chemosynthesis (as contrasted with photosynthesis, in which energy is derived from sunlight).  As a consequence of all this, anaerobic microbes around hydrothermal vents are able to thrive without sunlight, without elemental oxygen, and without complex organic compounds supplied from their environment.
Honey - high in glucose, oxidized for energy by most life on earth, from   bacteria to humans.  Image: public domain, available at
Honey – high in glucose, a sugar.  The simple carbohydrate molecule of glucose is oxidized for energy by most life on earth, from anaerobic chemosynthetic microbes living around hydrothermal vents in the dark depths of the oceans, their kind having been on Earth for millions of centuries, to you and me. Image: public domain, available at:

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