Slide 1 Recall that in the light-dependent reactions the two photosystems harness the energy in sunlight to produce ATP and NADPH. This ATP and NADPH will be used in the Calvin cycle to build a sugar.
Slide 2 Sometimes it is interesting to learn about how we know what we know about biology. We know about the Calvin cycle because of the work of scientist Melvin Calvin and his colleague Bensen. They essentially built an artificial chloroplast and then they radioactively labelled the carbon in the system.
Slide 3 Chromatography allows scientists to see the movement of the radioactively labelled carbon through the system. After 10 seconds only the first compounds in the cycle are labelled. After 10 minutes more compounds in the cycle are labelled. In this way, Calvin and Bensen were able to determine the order of the compounds in the Calvin cycle.
Slide 4 What they worked out is known as the Calvin cycle - or the light-independent reactions of photosynthesis. The Calvin cycle is a complex biochemical pathway that reduces carbon dioxide to sugar. Notice that there are three phases to this cycle - carbon fixation, reduction, and RuBP regeneration.
Slide 5 The molecules circled in red are the molecules you need to know in this cycle. Carbon dioxide is the input to the cycle - it is brought into the cycle by being added to a 5-carbon compound called RuBP. This is known as fixation - the carbon dioxide gets fixed to an organic molecule. The enzyme that catalyzes this reaction is known as Rubisco. It is thought to be the most abundant protein on earth because there are so many photosynthetic organisms on earth. The resulting 6-carbon compound is highly unstable and immediately breaks down into two 3-carbon compounds. It is these 3-carbon compounds that are reduced to sugar. This reduction process requires both energy and electrons. The energy comes from the ATP that was produced during the light-dependent reactions and the electrons come from the NADPH that was produced during the light-dependent reactions.
Slide 6 The sugar that is produced is known as G3P - it is a 3-carbon sugar that will be used to produce glucose and other organic molecules. The cycle requires a constant supply of RuBP so the next stage is RuBP regeneration. This stage also requires energy in the form of ATP that was produced during the light-dependent reactions. Once the RuBP has been regenerated the cycle can continue and carbon fixation can begin again.
Slide 7 So, to review how to make a sugar. The first stage is fixation when CO2 is added to the 5-carbon compound (ribulose bisphosphate; RuBP) by the enzyme Rubisco. An unstable 6C molecule forms (momentarily) and that 6C molecule splits into 2, 3-carbon molecules. During the reduction phase the 3C molecule is phosphorylated by ATP and forms an activated molecule and the phosphorylated 3C molecule is reduced to glyceraldehyde 3-phosphate (G3P) by NAPDH.
Slide 8 The G3P has two possible fates: It can be used to produce various 6-carbon sugars such as glucose or It can be used to produce more RuBP during the regeneration phase so the Calvin cycle can continue to run.
Slide 9 The light-dependent reactions and the Calvin cycle make life as we know it possible. The light-dependent reactions provide energy for the Calvin cycle and also release oxygen as a by-product. We have already discussed how this oxygen changed the world and created an aerobic environment on earth. The Calvin cycle builds the sugars that serve as the base of the food chain that supports larger organisms such as ourselves.