Download The stuff of life

VII
The chemistry of
life
http://sgoodwin.staff.shef.ac.uk/phy229.html
7.1 The origin of the elements
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
Msun star runs-out of nuclear fuel and collapses to a black hole or
neutron star. A shock wave is produced by the collapse which ejects
the outer layers.
Type Ia supernovae (SNIa):
(SNIa) caused when a white dwarf accretes
enough material from a companion to pass the Chandrasekhar limit
(~1.4 Msun) and collapses to form a neutron star.
The high energies created cause fusion to occur producing large
quantities of heavy elements (most Fe is from SNIa).
(See PHY111)
7.1 The origin of the elements
Planetary nebulae (PNe):
(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.
7.1 The origin of the elements
Terrestrial life is based on 4 main elements: the CHON group.
HYDROGEN:
HYDROGEN from the big bang (primordial nucleosynthesis)
OXYGEN:
OXYGEN originates in CCSN, mainly via alpha capture C + He -> O
NITROGEN:
NITROGEN expelled by PNe, created as part of the CNO fusion
cycle.
CARBON:
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.
7.1 The origin of the elements
Is it any surprise life is based on the 4 most abundant reactive elements?
7.1 The origin of the elements
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).
7.2 Biochemistry
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.
7.2 Water
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.)
7.2 Carbohydrates (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 C6H12O6 (structural
isomers):
7.2 Carbohydrates (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 H2O in the process (a
peptide bond). This process can continue forming long-chain
polysaccaride carbohydrates.
7.2 Lipids (fatty acids)
Lipids are long chain hydrocarbon molecules of the general form:
CH3 (CN2)n COOH (typically n=12-24)
they have a hydrophobic (-CH3 ) and hydrophilic (-COOH) end, and
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.
7.2 Amino acids
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.
7.2 Proteins
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 righthanded 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?
7.2 Nucleic acids: DNA and RNA
Life uses two nucleic acids: deoxyribonucleic acid (DNA)
DNA and
ribonucleic acid (RNA).
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).
7.2 Nucleic acids
The 4 bases in DNA are A, G, T and
C (in RNA T is replaced by Uracil
(U)).
Summary of biochemistry
Terrestrial life is based on the CHON elements.
The main molecules of Earth life are:
Water:
Water liquid basis for life acting as a solvent and transport medium.
Lipids:
Lipids insoluble molecules used in cell walls and as an energy store.
Carbohydrates:
Carbohydrates Basic energy source and store of life.
Amino acids:
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:
acids The information storage system within the cell, storing
information on how to build particular proteins required by the cell.
Summary of biochemistry
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?