Chemical Elements Required for Life (College Board AP Biology)

• There are 118 elements in the Periodic Table
• Only the first 92 elements occur in Nature; the others are synthesized artificially in laboratories and are very unstable
• Only around 21 elements are required for life
  • Around 25 for animals, 17 for plants
  • The remaining 71 have no direct role in sustaining life 
    
    • In fact, some are toxic eg, arsenic
  • Some elements can be used in medicine eg. titanium for skeletal implants, thanks to its inertness, lightness and strength
• There are 4 ubiquitous (found everywhere) elements in biological systems 
• These 4 elements make up over 96% of living matter
  
  • Oxygen - 65% of body mass (humans)
  • Carbon - 18%
  • Hydrogen - 10%
  • Nitrogen - 3%
• Two other elements also commonly found in biological macromolecules
  • Phosphorus; as phosphate in DNA, RNA, ATP, phospholipids
  • Sulfur; in proteins as part of the two sulfur-containing amino acids (cysteine and methionine)

Trace Elements

• Other trace elements found in organic compounds are: bromine, calcium, chlorine, chromium, copper, iodine, iron, magnesium, manganese, molybdenum, potassium, selenium, silicon, sodium
• There are other trace elements found in certain phyla only e.g. strontium in certain corals (Cnidaria)

The Elements Found in Biological Systems Diagram

The elements of the Periodic Table that are found in biological molecules

Elements in Biology Exist Mainly as Compounds

• Biological compounds are mainly covalent compounds
  
  • Electrons are shared between atoms to generate strong bonds within compounds
  • For example, elemental carbon exists in nature only as graphite and diamond, which are of no direct use to organisms
  • Carbon forms millions of different covalently bonded compounds, mainly with hydrogen and oxygen
  • Oxygen is absorbed in elemental form but is quickly converted to its compounds during transportation and respiration
• Some compounds are ionic eg. sodium chloride
• Some elements form prosthetic groups with larger organic molecules eg. magnesium in chlorophyll, iron in hemoglobin

Exchange of Elements

• Organisms must exchange matter with the environment to grow, reproduce and maintain their organization
• Exchange with the environment often includes the processes of 
  
  • Eating
    
    • An animal acquires elements from its food
    • For example, an elephant eating bamboo leaves gets its carbon from eating plants
  • Drinking
    
    • Water includes hydrogen and oxygen but also carries important mineral ions needed in the diet
  • Breathing
    
    • Absorption of oxygen and excretion of carbon dioxide produced from the process of respiration
  • Photosynthesis
    
    • The process by which inorganic carbon dioxide from the air is first incorporated into biological molecules by organisms
  • Excretion
    
    • Removal of waste products elements to the environment
    • For example, urea returns nitrogen to the soil so that the nitrogen can be used by other organisms
  • Decay
    
    • Decomposition of dead biological matter returns all its chemical elements to the environment that can be utilized by other organisms