Molecular Biology
The substances of life
- There are 118 elements in the Periodic Table
- Only the first 92 elements occur in Nature ; the rest are artificially-synthesised in laboratories and are very unstable
- Only around 21 elements are required for life
- The rest have no role in sustaining life (some are poisonous 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 elements in biological systems (this means they are found everywhere)
- These 4 make up over 96% of living matter
- Oxygen - 65% of body mass (humans)
- Carbon - 18%
- Hydrogen - 10%
- Nitrogen - 3%
- Other trace elements found in organic compounds are: bromine, calcium, chlorine, chromium, copper, iodine, iron, magnesium, manganese, molybdenum, phosphorus, potassium, selenium, silicon, sodium and sulfur
- There are other trace elements found in certain phyla only e.g. strontium in certain corals (Cnidaria)
The elements of the Periodic Table that form parts of biological molecules
Elements in biology exist mainly in compounds
- Such compounds are mainly covalent compounds
- Electrons are shared between atoms to generate strong bonds within compounds
- For example, elemental carbon only exists 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 are ionic eg. sodium chloride
- Some elements form prosthetic groups with larger organic molecules eg. magnesium in chlorophyll, iron in haemoglobin
All of Biology can be explained at a molecular level
- The molecules in cells, and the elements that go to form them, are the basis of all events that occur in Nature
- Everything that is observed has a molecular explanation
- Imagine an all-powerful 'zoom lens' that could look into any level of detail of life
- Such a lens could start at its most zoomed-out, looking at our biosphere, the Earth
- We assume that alien life does not exist because we haven't found evidence for it yet
- We see habitats, populations, communities and individual organisms coming into view in that order, as we zoom in
- As we zoom in on one organism, we enter its body and see increasing levels of detail, right down to the molecular level
- The zoom model helps understand the important interfaces between chemistry, biology and physics
- We could zoom in further, to look at sub-atomic particles, although that begins to enter into the realms of physics!
We can zoom into any part of the biosphere to identify all of Biology at a molecular (and atomic) level
Examiner Tip
Please note that you do not need to know the specific details of the Periodic Table, it is provided here for context to support your understanding of important biological compounds