0.29 Bis2a 07.5 the calvin cycle or using light energy to make  (Page 17/14)

Intro into carbon fixation

The general principle of carbon fixation is that some cells under certain conditions can take inorganic carbon, CO ₂ (also referred to as mineralized carbon) and reduce it to a usable cellular form. Most of us are aware that green plants can take up CO ₂ and produce O ₂ in a process known as photosynthesis. We have already discussed photophosphorylation, the ability of a cell to convert light energy to chemical energy in the form of a high energy electron that then enters the electron transport chain to produce ATP and NADPH, see module as described in Module 6.3. In photosynthesis, the plant cells use the ATP and NADPH formed during photophosphorylation, the light reactions, to reduce CO ₂ to sugar, (as we will see, specifically G3P) in what is called the dark reactions. While we all familiar with this process in green plants, I want to point out that this process had its origins in the bacterial world. ATP and NADPH can be made during anoxygenic photophosphorylation and CO ₂ into reduced sugars for the cell. In this module we will go over the general reactions of the Calvin Cycle, a reductive pathway that incorporates CO ₂ into cellular material. In many ways it is the reverse of the oxidative pentose phosphate pathway. One exercise to keep in mind is to look for the similarities between these two pathways.

A note from the instructor

Fixing carbon into biological molecules

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage. The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds, whereas the products of the light-independent reactions (carbohydrates and other forms of reduced carbon) can survive for hundreds of millions of years. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? It comes from carbon dioxide, the gas that is a waste product of respiration in microbes, fungi, plants, and animals.