"Everything should be made as simple as possible, but not one bit simpler." ...Albert Einstein

This is an overall equation for photosynthesis that still appears in school text books.

6CO₂ +6H₂O = C₆H₁₂O₆ + 6O₂

is an overall equation for photosynthesis that still appears in school text books. In it, C₆H₁₂O₆ is usually identified as glucose.

It may also be simplified to

CO₂ + H₂O = CH₂O + O₂

and the CH₂O then sometimes confined in square brackets or italicised in order to emphasise the fact that [CH₂O] or CH₂O is an empirical representation of any old carbohydrate or "carb" (rather than glucose as such).

Written either way, it may lead a reader to conclude that some of the O₂ evolved is derived from CO₂ whereas it is a well established fact that all of the oxygen comes from H₂O.

To avoid this implication it has become customary to introduce a second molecule of water on the left of the equation. This now becomes

 CO₂ + 2H₂O = CH₂O + O₂ + H₂O

So far so good but this now invites the thoughtful student to ask what happens to the molecule of water on the right of this equation. "Where does it come out?"

The short answer to that question is "It doesn't". Remember that the second molecule of water was arbitrarily introduced into this equation simply to avoid the any implication that photosynthesis involves the release of oxygen from CO₂. Better, you might think, to arrive at a more satisfactory equation in the first place. One that accurately summarises the total of all of the known partial reaction involved such as this.

3 CO₂+ 2H₂O + H₃PO₄ = CH₂OH.CO.CH₂OPO(OH) ₂ + 3O₂

In this equation, the most important immediate end-product of photosynthetic carbon assimilation is not glucose but a triose phosphate (glyceraldehyde-3-phosphate). I am reliably informed that some students (possibly those not overly familiar with chemical terminology) find this too much to contemplate and might run screaming from the lecture theatre. However, it can also be written, more simply, as

3 CO₂+ 2H₂O + phosphate = triose phosphate + 3O₂

This is surely not too complicated to constitute the proverbial "take home lesson"? It doesn't insist that the end product of photosynthesis is glucose, or that oxygen is released from carbon dioxide. Nor does it obscure several important facts, most importantly that the end-products of photosynthestic carbon assimilation are triose phosphates and that these feed directly back into the Benson-Calvin Cycle not only to replenish the carbon dioxide acceptor (ribulose1,5 bisphosphate) but also to permit the amount of this acceptor to be increased as necessary. This allows the cycle to respond to plant growth and to more favourable environmental conditions (such as increases in light intensity or temperature) in an essential autocatalytic fashion. In many leaves, excess triose phosphate is also converted to starch within the chloroplast as a temporary storage product but most is transported through the outer envelopes of the chloroplast, to the cytosol, There they are joined end to end to form hexose bisphosphates prior to further conversion to sucrose. This latter conversion releases phosphate. This re-enters the chloroplast (via the phosphate translocator) and, following incorporation into ADP to give ATP, allows the Benson-Calvin Cycle to continue unchecked.

These are matters that are easily overlooked if we insist that illuminated chloroplasts make free sugars, such as glucose. When they make glucose polymers such as starch, chloroplasts make it from glucose phosphate that is derived, in turn, from triose phosphates. No free glucose is involved in this synthesis. Sucrose, the principal transport metabolite (to growing tissues, fruits, seeds, tubers etc) in many plants, is synthesised in the cytosol. The fact that glucose and, perhaps more importantly, maltose are released from starch (during the mobilisation of chloroplast starch in the dark) should not be allowed to detract from the fact that sugar phosphates are the products of photosynthetic carbon assimilation in illuminated chloroplasts).

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With acknowledgements to Sir Walter Scott ('Marmion' 6, XVII) Oxygraphics and 'Like Clockwork' (pages 80 to 83).

David Alan Walker

                                 Robert Hill Institute

                              University of Sheffield

Saturday, September 16, 2006