Lecture 38 May 2, 2003 R. Jones Chapter 6 and 7

1. We know that the net result of photosynthesis is the production of sugars and these are formed during the dark reactions (more accurately called the light-independent reactions). These reactions were solved at UC by the chemist Melvin Calvin and the reactions that he discovered with his colleague Andy Benson are called the Calvin-Benson Cycle. Calvin received the Nobel Prize in Chemistry in 1961.

2. Calvin and Benson solved this problem because they were in the right place (UCB) at the right time (1940s-60s). The Lawrence Radiation Lab (now LBNL) was one of the few places in the world producing radioactive compounds such as radiolabelled carbon that allowed Calvin and Benson to solve how carbon was incorporated into sugars.

3. It was known that the end product of photosynthesis was a 6-carbon sugar such as glucose, but it was not understood how it was formed. Was it simply the building up of C-C sequentially from CO₂ to give 2-carbon, then 3- carbon, 4-carbon, etc. molecules or was it by some other mechanism?

4. Calvin and Benson set up a simple system (using the single celled algal protist Chlorella) and radioactive CO₂. They found that among the first products of photosynthesis were compounds such as phophoglycerate, phosphoglyceraldehyde (both 3-carbon compounds), glucose (6 carbon), etc. They rationalized the data by showing that a five carbon acceptor molecule, ribulose 1:5 diphosphate accepted CO₂ and formed 2 molecules of phosphoglycerate (PGA). PGA is a 3-carbon molecule and this type of photosynthesis is called C₃ photosynthesis.

5. PGA is reduced to phophoglyceraldehyde (PGAL) using the ATP and NADPH formed in the light dependent reactions of photosynthesis. They showed that the enzyme that catalyzed this reaction called Rubisco (ribulose bisphosphate carboxylase/oxygenase) was found in the stroma of the chloroplasts where carbon fixation occurs. Rubisco is the most abundant enzyme in the biosphere. It catalyzes the incorporation of CO₂ into ribulose bisphosphate but it is also sensitive to O₂ and O₂ prevents CO₂ fixation by Rubisco (see point 8 below).

6. The Calvin Benson Cycle is referred to as a Cycle because some of the PGAL is used to regenerate ribulose 1:5 bisphosphate. The scheme for the formation of one glucose molecule can be written in the abbreviated form this way:

6CO₂ + 12H₂O + 18ATP + 12 NADPH ----> C₆H₁₂O₆ + 6O₂ + 6H₂O + 18 ADP + 18 Pi + 12 NADP.

7. In other words, to make one glucose molecule 18 ATP and 12 NADPH molecules made during the light reactions must be consumed. A very endergonic reaction!

8. Not all plants fix (i.e., trap) CO₂ by the Calvin-Benson pathway. Some plants first trap CO₂ into four carbon organic acids in specialized cells then transport these organic acids to the cells that carry out the Calvin-Benson Cycle using Rubisco. These plants are called C₄ plants because they trap CO₂ using a 4-C compound. The advantage of this method of C fixation is that the enzyme that catalyzes it is insensitive to the presence of oxygen, unlike Rubisco that is sensitive to oxygen. Many of our most productive crops such as corn and sugar cane are C₄ plants.

8. The Carbon Cycle is a very important issue today and the carbon economy of the biosphere is now a hot political topic. The atmosphere today contains about .036% (360 ppm) CO₂. In 1850 at the start of the industrial revolution, it was 270 ppm. The increase is due to the burning of fossil fuels and the heating of rocks to make cement. Both components have dramatically increased atmospheric CO₂ concentrations. CO₂ in the upper atmosphere prevents the escape of heat from the surface of the earth (mostly in the form of infra-red light) causing a rise in global temperatures. This often called the green house effect.

10.Cellular Respiration. Heterotrophic organisms exploit the energy captured by photosynthesis to survive. Heterotrophic organisms metabolize compounds such as sugars, fats and proteins by the process of cellular respiration, a series of reactions that occur in the cytoplasm and mitochondria of eukaryotic cells. Energy in molecules such as glucose is finally extracted as ATP.