This is a short description of the process of photosynthesis and how the carbon dioxide molecule gives us life as well as heat

Plants very probably draw through their leaves some part of their nourishment from the air…may not light, also, by freely entering surfaces of leaves and flowers, contribute much to ennobling the principles of vegetables?

Stephen Hales, ‘Vegetable Staticks,’ 1727

We take this for granted these days and note that all life on Earth depends upon CO2, first for providing some of the heating that allows life to be sustained and for providing the carbon that all organic matter contains.

Photosynthesis can be described by one simplified chemical equation:

xCO2 + xH2O + light ® (H2CO)x + xO2 

The reactants are carbon dioxide and water and by a remarkable series of reactions in leaves light is harvested and the associated energy is used to force the reaction forwards to give the products which are sugars (carbohydrates) which are used in plant structures and gaseous oxygen. The reaction has to be forced in the direction as written because the reverse reaction, the burning of the organic material is a spontaneous process and occurs easily either by burning or by the use of vegetation as foodstuff.

Now for some science!

Blackman’s principle (1905) is ‘When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factors.’

This has been updated by chemists to read ‘the rate of a chemical reaction that consists of more than one step is determined by the rate of the slowest step. Thus we have the idea of the rate-determining step.

This applies to photosynthesis as may be seen from the diagram which show some experimental results obtained from the study of the growth of sugar beet leaves.

 

The graphs show how the rate of photosynthesis varies with irradiance, the three experiments differing only in the concentration of CO2. The lowest graph is that for a CO2 concentration of 0.03%, much as it is at this time [actually ~385 ppmv or 0.0385%]. In the early part of the graph the rate of the reaction is related almost directly to the amount of irradiance, but the rate falls off at higher irradiances and eventually levels off. This tells us that the rate determining process at low irradiance is the intensity of the light. At higher irradiances it is the concentration of CO2 that limits the reaction rate, there is sufficient light falling on the leaves, but the requirement for more CO2 is the limiting factor.

            The centre graph shows the results for 0.84% CO2, an increase by a factor of 2.8 and again the light intensity is rate determining at low intensities but the supply of CO2 becomes rate determining at the higher light intensities.

The top graph for 0.132% CO2 shows a little improvement in rate at higher light intensities, but there is another factor limiting growth, presumably one of the essential elements.

Growers of tomatoes in real greenhouses know that raising the CO2 concentration by a factor of three allows four crops instead of three every year. The CO2 levels are raised by having kerosene burning in the greenhouse.

In the Earthly greenhouse the raised levels of CO2 are making the surface greener and crop yields are rising. There is a possible negative feedback operating in that a greener surface reflects more solar radiation than a brown one does. Thus, a greener planet is contributing to its own cooling by having more efficient photosynthesis.

We strongly object to CO2 being regarded as a pollutant; it is the giver of life and we all depend upon its chemical and radiative properties.