VII The chemistry of life
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7.17.1 TheThe originorigin ofof thethe elementselements
The big bang created H, He and traces of other low-atomic number elements. Every other element used by life has been created by stars:
Core collapse supernovae (CCSN): caused when the core of a >8 M star runs-out of nuclear fuel and collapses to a black hole or sun neutron star. A shock wave is produced by the collapse which ejects the outer layers.
Type Ia supernovae (SNIa): caused when a white dwarf accretes enough material from a companion to pass the Chandrasekhar limit (~1.4 M ) and collapses to form a neutron star. sun
The high energies created cause fusion to occur producing large quantities of heavy elements (most Fe is from SNIa).
(See PHY111)
7.17.1 TheThe originorigin ofof thethe elementselements
Planetary nebulae (PNe): at the end of the red giant phase of stellar evolution a star will shed its outer layers leaving a bare white dwarf. As it sheds its outer layers, it also releases CNO that have been produced by nuclear fusion.
Planetary nebulae have nothing to do with planets, they just appeared non-point(star)-like in early telescopes, in the same way as planets do.
7.17.1 TheThe originorigin ofof thethe elementselements
Terrestrial life is based on 4 main elements: the CHON group.
HYDROGEN: from the big bang (primordial nucleosynthesis)
OXYGEN: originates in CCSN, mainly via alpha capture C + He -> O
NITROGEN: expelled by PNe, created as part of the CNO fusion cycle.
CARBON: less clear, probably mainly from massive stars (superwinds and/or CCSN), but may be significant from the PNe phase?
Note that it takes time to produce O, N and C, the levels building-up within galaxies as stars form and evolve.
7.17.1 TheThe originorigin ofof thethe elementselements
Is it any surprise life is based on the 4 most abundant reactive elements?
7.17.1 TheThe originorigin ofof thethe elementselements
Note that alpha-elements are very common: C, O, Ne, Mg, Si, P, Ar, K, Ca...
Produced by adding alpha-particles (He nuceli) together.
Other processes (R-process, S-process etc.) occur in supernovae and hot H-burning to make intermediate elements. E.g. N is made by adding two protons to a C (12C + p -> 13C -> 13N + e- ; 13N + p -> 14N).
7.27.2 BiochemistryBiochemistry
Biochemistry is the study of the chemistry of living organisms.
Terrestrial life is based around a number of organic molecules made primarily from CHON (the other major components are S and P):
Lipids, Carbohydrates, Amino acids and Proteins, Nucleic acids.
Together these molecules are the basis for life on Earth.
7.27.2 WaterWater
The other major molecule vital for life on Earth is water (typically ~70% of the mass of organisms).
Water is a polar molecule. This means that water molecules can form bonds with each-other, and with other elements/molecules.
Polar molecules will dissolve in water (hydrophilic), non-polar molecules will not (hydrophobic).
Water also has the unusual property that its solid phase is less dense than the liquid phase, causing ice to float (so insulating the lower liquid layers).
(The other probably common liquids are methane and ethane which are not polar.)
7.27.2 CarbohydratesCarbohydrates (sugars)(sugars)
The most common biological molecules. Carbon-chain molecules with many hydroxyl (OH) groups – hydroxyls are polar and this causes carbohydrates to be highly soluble in water. Carbohydrates fold into a ring from a chain when dissolved in water (think of sugar grains in tea). Below are the three basic monosaccharides C H O (structural 6 12 6 isomers):
7.27.2 CarbohydratesCarbohydrates (sugars)(sugars)
Monosaccharides can be linked together to form complex carbohydrates. They are linked by removing an H from one and an OH from another and forming a bond releasing H O in the process (a 2 peptide bond). This process can continue forming long-chain polysaccaride carbohydrates.
7.27.2 LipidsLipids (fatty(fatty acids)acids)
Lipids are long chain hydrocarbon molecules of the general form: CH (CN ) COOH (typically n=12-24) 3 2 n they have a hydrophobic (-CH ) and hydrophilic (-COOH) end, and 3 are generally insoluble in water, for this reason they are a good way to make cell membranes. They are also a long-term way to store energy.
7.27.2 AminoAmino acidsacids
There are 20 amino acids vital for cells (of which 10 humans can synthesise ourselves and 10 that must be gained from food).
All but one of these amino acids consist of a carboxylic acid (-COOH) and amino (-NH2) group attached to a carbon atom: eg. Glycine:
As with carbohydrates, amino acids can be linked together into long chains – PROTEINS – by peptide/amide bonds.
7.27.2 ProteinsProteins
Proteins have varied uses in biochemistry. They can form the basis of structure in specialised cells (e.g. Hair) or act as catalysts in various biochemical reactions (such proteins are known as enzymes).
CHIRALITY (handedness): most amino acids and proteins have chirality, that is two mirror-image forms. When randomly created the two chiralities should have equal likelihood.
However, life on Earth hugely favours left-handed amino acids and right- handed carbohydrates for some reason.
(Right handed amino acids tend to taste Sweet, while left are tasteless).
It has recently been suggested that chirality has an astrophysical origin from the polarisation of UV light in planetary nebulae. Maybe?
7.27.2 NucleicNucleic acids:acids: DNADNA andand RNARNA
Life uses two nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These molecules carry genetic information (see next lecture).
DNA and RNA have a (carbon) 'backbone' along which the four nucelobases are arranged.
DNA consists of a double helix – two nucleic acid strands connected by hydrogen bonds between pairs of bases. RNA has only one strand (although parts of the strand may connect back to other parts with hydrogen bonds).
7.27.2 NucleicNucleic acidsacids
The 4 bases in DNA are A, G, T and C (in RNA T is replaced by Uracil (U)).
SummarySummary ofof biochemistrybiochemistry
Terrestrial life is based on the CHON elements.
The main molecules of Earth life are: Water: liquid basis for life acting as a solvent and transport medium. Lipids: insoluble molecules used in cell walls and as an energy store. Carbohydrates: Basic energy source and store of life. Amino acids: A sub-set of 20 are used by cells to construct proteins, dominated by left-handed molecules. Proteins: Chains of amino acids linked by peptide bonds. Proteins perform many different biochemical functions. Nucleic acids: The information storage system within the cell, storing information on how to build particular proteins required by the cell.
SummarySummary ofof biochemistrybiochemistry
Possibly very importantly, Terrestrial life is based on the most common reactive elements in the Universe.
We know that CHON are common in the Universe, and we know that CHON can be used as the basis for life.
Is CHON the only (biochemical) way to make life?
Is CHON (by far?) the most common way to make life?