Chapter Five

Chemistry, photosynthesis and respiration, the Calvin cycle and the Krebs cycle, Cyanobacteria and Mitochondria, the engines of life

1.1   The reason Cyanobacteria and the danger they posed is at the heart of the story of the saving of Lake Annecy, is because of they carry out one of the most miraculous processes in all Nature  - photosynthesis.   Photosynthesis maintains atmospheric oxygen levels and supplies all of the organic compounds and most of the energy necessary for life on Earth.

1.2  Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by water splitting is used in the creation of two further compounds: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.

1.3   In plants, algae and cyanobacteria, sugars are produced by a subsequent sequence of light-independent reactions called the Calvin cycle, as shown in the above diagram.  In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate. Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose.

1.5.   Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about three times the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 thousand million metric tonnes of carbon into biomass per year.

1.6  Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is an endothermic redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and the electrons needed to convert carbon dioxide into a carbohydrate via a reduction reaction. The addition of electrons to a chemical species is called a reduction reaction.

1.7 In general outline and in effect, photosynthesis is the opposite of cellular respiration.   Photosynthesis is the process which absorbs energy from the sun into living creatures, cellular respiration is process by which that energy is released.   Cyanobacteria invented  the machinery to carry out photosynthesis and then by combining symbiotically with other organisms such as algae, and later, plants, enabled life on earth to draw energy from the Sun.  Mitochondria invented the machinery to carry out respiration and then by combining symbiotically with other organisms (in fact in every living cell!) enabled life on earth to burn that energy, to  grow, move, and reproduce. 

1.8   The general equation for photosynthesis as first proposed by Cornelius van Niel is: CO2 + 2H2O + photons → [CH2O] + O2 + H2O  or in words:  carbon dioxide + water + light energy → carbohydrate + oxygen + water.  Its more elaborate formulation is know as the Calvin cycle.

1.9  The general equation for aerobic respiration is C6H12O6 + 6O2 → 6CO2 + 6H2O (+ energy) or in words:  glucose + oxygen → carbon dioxide + water (+ energy).  Its more elaborate formulation is known as the Krebs cycle.

1.10  It was a bit of luck for all of us that a couple of bacteria two billion years ago managed to work all this out for themselves - such an elegant dance! -  before chemistry professors were around to explain it to them.

1.11  But Photosynthesis does not just involve hydrogen, oxygen, carbon and nitrogen from water and the air as shown by the simple formulae above.  There is one other key element.  Phosphorus.

1.12  As it became clear during world wide research into the problem of eutrophication in the years following the pioneering work at Lake Annecy, phosphorus was identified as the key limiting factor in the growth of cyanobacterial blooms which in turn polluted lakes.  Phosphorus not just in human sewage, but also, as it was later identified, in modern detergents.   Building the sewer network around Lake Annecy intercepted both these critical sources of phosphate, and so enabled the lake to recover its prior oligotrophic state relatively quickly.

1.13 How does phosphate enter into the photosynthetic picture?  In biochemistry, the pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt) is a metabolic pathway parallel to glycolysis that generates NADPH and pentoses (5-carbon sugars) as well as Ribose 5-phosphate, a precursor for the synthesis of nucleotides. While it does involve oxidation of glucose, its primary role is anabolic rather than catabolic.

1.14   There are two distinct phases in the pathway. The first is the oxidative phase, in which NADPH is generated, and the second is the non-oxidative synthesis of 6-carbon sugars. For most organisms, the pentose phosphate pathway takes place in the cytosol; in plants, most steps take place in plastids.

1.15   In photosynthetic organisms, NADPH is produced by ferredoxin-NADP+ reductase in the last step of the electron chain of the light reactions of photosynthesis. It is used as reducing power for the biosynthetic reactions in the Calvin cycle to assimilate carbon dioxide. It is used to help turn the carbon dioxide into glucose. It is also needed in the reduction of nitrate into ammonia for plant assimilation in nitrogen cycle.The major source of NADPH in animals and other non-photosynthetic organisms is the pentose phosphate pathway. So phosphate is a key element in these fundamental life processes, and it was phosphate that turned out to be the main contributor to the problem of eutrophication in Lake Annecy, Lake Washington and pretty much most urban lakes and rivers around the world.  But, again, not a lot of local citizens  were entirely up to speed with this when Dr Servettaz began raising his concerns all those years ago.

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