Properties of Carbon (SL IB Biology)

Carbon forms covalent bonds

• A covalent bond forms when a pair of electrons are shared between two atoms
• A single covalent bond is represented by a short straight line between the two atoms, e.g. H-H
• Electrons are shared between atoms to generate strong bonds within compounds

Electrons are shared in a covalent bond

Carbon in biological molecules

• Carbon is present in all of the four major categories of biological molecules; this is why life on Earth is often described as "carbon based"
• Carbon is present in:
  • Carbohydrates
  • Lipids
  • Proteins
  • Nucleic acids
• Carbon has four electrons in its outer shell, meaning that each atom can form four covalent bonds
  • Carbon can therefore be a component of large, stable molecules
• Carbon forms millions of different covalently-bonded compounds, mainly with hydrogen and oxygen
• Carbon atoms can arrange themselves to form a huge variety of chemical compounds; it can:
  • Bond to other carbon atoms, or other atoms such as hydrogen, nitrogen, oxygen and sulfur
  • Form molecules with long branched chains such as glycogen
  • Form long straight chain molecules such as cellulose
  • Form molecules containing cyclic single rings such as the pyrimidines (thymine, uracil and cytosine)
  • Form molecules with multiple rings, including starches and the purines (adenine and guanine)
  • Produce a tetrahedral structure which allows the formation of varied carbon compounds which have different 3-D shapes and hence, different biological properties

• Carbon atoms can form up to four single covalent bonds or a combination of double and single bonds, e.g.
  • Carbon dioxide contains two double bonds
  • Methane contains four single covalent bonds

Covalent bonding in carbon-containing molecules diagram

Carbon atoms can form either single or double bonds in a variety of molecules. Carbon dioxide (left) contains double bonds, while methane (right) contains single bonds.

• Double and triple bonds can form with an adjacent carbon atom, allowing unsaturated compounds to form
• Carbon atoms can also form part of many different functional groups that give organic compounds their individual properties, e.g.
  • Hydroxyl groups
  • Carboxyl groups
  • Amino groups
  • Phosphate groups

Functional groups diagram

Functional groups include hydroxyl (OH), amino (NH₂), carboxyl (COOH), and phosphate (H₂PO₄) groups

NOS: Scientific conventions are based on international agreement

• The professional scientific community is global, meaning that scientists from all over the world may work on the same research, and need to be able to communicate clearly with each other 
• Scientific conventions are thereby agreed upon and used internationally 
  • SI (which stands for système international) unit prefixes is one example
    • kilo = 10³
    • centi = 10^-2
    • milli = 10^-3
    • micro = 10^-6
    • nano = 10^-9