Photosynthesis and Carbon Fixing
How it works.
Life cannot exist without an energy input from outside itself. That could be organic food (products of earlier life), energy from sunlight, thermal vents, etc. Obviously organic food is not an option for the very early species, which appear in the geologic record at almost earliest possible moment after the earth had cooled to a "livable" temperature, at around 3.8 Ga (Billion Years Ago)—a very hot temperature, perhaps about that of pasteurization (140-150°F)M.50.
The organic carbon found as early as 3.8 Ga, and the fossils of what appear to be cyanobacteria (or their ancestors) around 3.45 Ga indicate that photosynthesis, harvesting of energy from sunlight, appeared with the first life, or very soon thereafter.M.51
Photosynthesis is an incredibly elaborate process, and the basic parts of it are essentially the same for all photosynthetic life. The cyanobacteria conduct photosynthesis in folded membranes called thylakoids ("light reaction" in the following figure). Plants conduct photosynthesis in Chloroplasts as show in the Figure.M.52
A number of complex molecules, some actually nano-sized motors, are involved in photosynthesis. There are two stages: the light reaction which harvests energy from light, and the dark reaction (Calvin Cycle in the following diagram), which uses the products of the light process to fix carbon and make sugars. The dark reaction does not harvest light but exchanges products with the light reaction.
The following (deceptively simple) chemical equation describes the action of photosynthesis:
6H2O+6CO2+light energy -> C6H12O6+6O2
Water + Carbon dioxide + light energy -> sugar + (waste) oxygen.
The Light Reaction.
The light process centers around chlorophyll: molecules that harvest light with the help of a central magnesium atom. There are two forms of chlorophyll which work cooperatively but have, remarkably, opposite characteristics—one is a strong oxidizer, and the other is a strong reducer—each the best known design for its particular task to be found in all of nature.
Photosystem II, involving P680 Chlorophyll: absorbs yellow (680 nm) light. This is the strongest known biological oxidizer. It oxidizes water to produce protons (H+ = P) and oxygen (O2) as a waste product in the Oxygen-evolving complex. It is "II" because it was discovered after Photosystem I. The protons are used in the manufacture of ATP, the universal "energy battery" used, in particular, in the dark process. A summary of its action is:
H2O + light energy -> 2 H+ + O2
water + light energy -> protons + oxygen.
Photosystem I, involving chlorophyll P700: absorbs orange (700 nm) light. This is the strongest known biological reducer. It uses light energy to prepare a precursor molecule NADPH for sugar production in the dark process.
The protons of Photosystem II supply protons (H+) to Photosystem I to make NADPH, and also propel a marvelous and complex rotary proton nanomotor, ATP synthase, which prepares ATPM.53 (see figure).
The Dark Reaction.
The dark reaction fixes carbon and then uses it to form glucose, C6H12O6, used by the cell to form starch, amino acids and other sugars. The following remark emphasizes the importance of carbon fixing.
"Carbon is essential to life. All of our molecular machines are built around a central scaffolding of organic carbon. Unfortunately, carbon in the earth and atmosphere is locked in highly oxidized forms, such as carbonate minerals and carbon dioxide gas. In order to be useful, this oxidized carbon must be "fixed" into more organic forms, rich in carbon-carbon bonds and decorated with hydrogen atoms.M.54"
This is done by a complex motor molecule, RuBisCO. It is the most common protein on earth—about half of all the protein. The RuBisCo molecule is large (consisting of two subunits), very slow (it can fix about 3 CO2 molecules per second) and very inefficient, because it spends about 20% of its time "fixing" oxygen instead of carbon: "a wasteful process"M.55 called photorespirationM.56. One author called the molecule "dim-witted." Nonetheless, despite many billions spent seeking an improved RuBisCO, no practical substitute has been found. The RuBisCO active site uses a Magnesium atomM.57 to perform its function. The overall reaction can be summarized as:
CO2 + H2O + ATP → CH2O + O2
carbon dioxide + water + ATP -> sugar (fragment) + oxygen + ADP + P+++
where ATP is the energy "battery" with three phosphates, one of which is released generating energy, ADP and a phosphate ion. This is repeated three times in the Calvin cycle.
RuBisCO discriminates against the isotope 13C in preference to 12C. This arises due to slight differences in kinematics and binding energy in the CO2 molecule. Thus organic carbon is slightly deficient in 13C compared with inorganic carbon.