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  Photosynthesis & Respiration

In this section we focus on two complementary processes in plants. Photosynthesis which removes CO2 from the atmosphere and replaces it with O2; and respiration which does the reverse.

Using chlorophyll, plants and some algae and bacteria use sunlight energy to enable a reaction which combines carbon dioxide (CO2) and water (H2O) to form sugars or other carbohydrates which store energy and release oxygen (O2) as a product, back into the atmosphere. This is represented as an equation like the following based on a sugar e.g. glucose:
Image of chemical equation
Image of chemical equation Whilst land plants, and rainforests in particular might produce much of the earth’s oxygen through photosynthesis, at least half is produced by the same process in the tiny phytoplankton of the oceans.

Plants and photosynthesising algae and bacteria use some of the carbohydrates they make to release energy for various functions, whilst the rest is used to increase their bulk (biomass), in a process chemically similar to burning. This reaction returns carbon dioxide (CO2) and water (H2O) to the atmosphere as well as releasing energy and can be represented by an equation like the following based on a sugar e.g. glucose:
Image of chemical equation
Image of chemical equation
Though the processes of respiration and photosynthesis appear to cancel each other out, the fact that some of the carbohydrate remains in the biomass of the plant means that the carbon in this enters other components of the earth system e.g. eaten by animals which decompose and eventually enter the geosphere in some form. Over millions of years, this has built up an imbalance with more oxygen being put into the atmosphere and less carbon dioxide. Some of this geospheric carbon ends up in rocks like limestone or in organic derivatives like coal, oil and gas.

Plant respiration and certainly the impact of increased CO2 on it is arguably less well studied than photosynthesis; it is also often treated only in the context of photosynthesis and so there are far fewer useful resource links for it as a separate topic. We now know an enormous amount about the biochemistry of photosynthesis in terms of ATP, ADP, NAD, NADH & NADPH, the light and dark reactions (Calvin cycle) and photosystems I & II. More details can be found in the links.

Even with our increased knowledge at the micro level, with human activity altering the balance of CO2 in the atmosphere further, we cannot be sure what the net result will be, since even at the macro level these interdependencies are so complex. For example, the amount of CO2 in the atmosphere alters the temperature but how much CO2 dissolves in rivers or the sea also varies with temperature. The growth of plants varies with the amount of CO2 in the atmosphere but also with temperature etc. Recent research on respiration with higher levels of CO2 appears to be species dependent and not necessarily reduced!
  Photosynthesis: How Life Keeps going
Photosynthesis: comprehensive biobook
Photosynthesis: wikipedia entry
Photosynthesis: animation (SWF)
Photosynthesis: simplified details
Photosynthesis: BBC Bitesize revision
Photosynthesis: internet resources
Photosynthesis puzzle solved
Photosynthesis Berkeley webcast
Photosystems I and II Imperial College
Relationship between Photosystems I and II

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