Introduction and Learning Objectives
The last tutorial explored the light-dependent reactions of photosynthesis. These reactions begin with the absorption of light energy by pigments, and end with the production of stored chemical energy in the form of NADPH and ATP. Now we will follow the NADPH and ATP molecules as they enter the Calvin cycle. Their stored energy will be used to make sugar from carbon dioxide. These anabolic reactions are endergonic (+ΔG), and therefore require energy (from ATP and NADPH). The basic relationship between the Calvin cycle and the light-dependent reactions is summarized in Figure 1.
Figure 1. The relationship between the light-dependent reactions and the Calvin cycle. (Click image to enlarge)
These reactions are sometimes called the "dark reactions" because they can occur in the dark (as long as ATP and NADPH are available). All of the processes of photosynthesis (both light-dependent and light-independent reactions) occur within the chloroplast. By the end of this tutorial you should have a basic working understanding of:
- The energetics of photosynthesis
- The basic workings of the Calvin cycle (including the fixation, reduction, and regeneration phases)
- Illustrate the importance of redox reactions in photosynthesis
- Review that relationship between the light dependent and light independent reactions
- Describe the inputs and outputs for the light independent reactions
Before we begin our study of the Calvin cycle, let's look at the energetics of sugar synthesis.
We have already studied the catabolism (breakdown) of glucose during cellular respiration. You know that the process is exergonic and releases about 686 kcal of energy. Thus, the ΔG for the overall reaction is -686 kcal/mole.
- glucose + O2 --> CO2 + H2O + ATP
If 686 kcal of energy per mole are released in the process of respiration, then it follows that 686 kcal of energy (minimum) are required to produce one mole of glucose. (Remember the first law of thermodynamics?) The Calvin cycle is the process by which glucose is made, and it requires all of that energy. Where does the energy come from? The light reactions of photosynthesis produce ATP, which provides the Calvin cycle with the necessary energy. In addition, the NADPH produced by the light reactions provides the reducing power to put glucose together.
- sunlight + CO2 + H2O --> O2 + glucose
Overview of the Calvin Cycle
The light-dependent reactions of photosynthesis produce ATP and NADPH, which are then used in glucose synthesis during the Calvin cycle (Fig. 2). As you should know from studying the Krebs cycle, metabolic cycles involve inputs and outputs, and some molecules are recycled to complete the cycle.
In the case of the Calvin Cycle, the input molecules are carbon dioxide, ATP, and NADPH. The output molecules are sugar, ADP, NADP+, and inorganic phosphate (Pi). The recycled molecule is ribulose bisphosphate (RuBP). Examine figure 2 and locate these molecules in the cycle.
Figure 2. The Calvin cycle. (Click image to enlarge)
Scientific knowledge is gained through observations and controlled experiments, but how does one study a process that can't be seen directly?
About 50 years ago, Melvin Calvin tried to do just that. He was working in a laboratory at the University of California, Berkeley during WWII. The study of radioactive elements had become an important new field in chemistry during the war. Among these newly discovered radioactive elements was carbon14. On the day in 1945 that the Japanese surrendered, a friend and colleague told Calvin, "Now is the time to do something useful with radioactive carbon." Calvin turned his focus to the study of photosynthesis, and 16 years later he won a Nobel Prize in chemistry and a metabolic process was named after him.
Calvin knew that photosynthesis could only occur in living organisms. Thus, the study of the chemical process was a difficult one. He devised a method whereby he could raise algae (Chlorella pyrenoidosa) in a lollipop-shaped disk (see Fig. 3). He set up a stream of air that could be controlled. He could inject radioactive carbon as carbon dioxide into the air stream for a set period of time. Then he would kill the algae with boiling methanol to stop the process of photosynthesis. He ran the experiment multiple times, each time killing the algae at different lengths of time after injecting carbon14.
Figure 3. The Calvin-Benson apparatus. (Click image to enlarge)
Calvin analyzed the dead algae to determine which molecules had incorporated the carbon14. The technique he used is called chromatography. By comparing the molecules that contained carbon14 after each time period, he found a sequence of compounds that revealed the path of carbon dioxide as it was turned into glucose (you can see an example of the data in Figure 4). In the years that followed, other researchers have discovered the enzymes and other compounds that also function in the Calvin cycle.
Figure 4. Autoradiograms showing the labeling of carbon compounds in the alga Chlorella after exposure to 14CO2. (Click image to enlarge)
Carbon Dioxide Fixation Yields Two, 3-Carbon Compounds
The top of Figure 5 shows the carbon fixation phase of the Calvin cycle. The five-carbon compound, ribulose bisphosphate (RuBP), is joined with one molecule of carbon dioxide to produce a highly unstable 6-carbon compound. The six-carbon compound that results immediately splits into two molecules, each with three carbons. The enzyme rubisco (which is the most abundant protein on Earth), catalyzes this reaction.
The carbon dioxide used in the Calvin cycle can get to the chloroplast in several different ways. In algae it simply diffuses through membranes from the surrounding water. In plants the carbon dioxide comes in through pores (stomata) in the leaves. These same pores are where oxygen, produced in the light reactions, escapes into the atmosphere.
Figure 5. The Calvin Cycle. (Click image to enlarge)
Reduction of the Two, Three-Carbon Compounds
In the first phase of the Calvin cycle, carbon dioxide is fixed into a 6-carbon molecule, which splits into two, 3-carbon molecules. In the second phase (shown in Figure 6), the 3-carbon molecules are reduced to glyceraldehyde 3-phosphate (G3P), a 3-carbon molecule. Remember, reduction means that electrons are added to the molecule. NADPH, produced during the light-dependent reactions, provides the high-energy electrons for this process. ATP is also used in this anabolic reaction. Thus, the Calvin cycle is energetically tied to the light-dependent reactions of photosynthesis.
At the end of the reduction phase, some of the G3P leaves the cycle to become sugar, however, most of it is used to regenerate RuBP. Notice that glucose is not produced by the Calvin cycle; rather it is the sugar G3P. This G3P can then be used by the cell as an immediate energy source (to be metabolized in cellular respiration) or used to build glucose molecules that will be used by the plant to build cell walls, starches, and other polysaccharides.
Figure 6. The Calvin Cycle. (Click image to enlarge)
Regeneration of G3P to RuBP
As shown in Figures 7 and 8, the Krebs cycle and the Calvin cycle have some general similarities. Remember, the Krebs cycle regenerates oxaloacetate at the end of one cycle to begin the next. In much the same way, the Calvin cycle regenerates RuBP to begin the next cycle. For every three carbon dioxide molecules that are fixed, three molecules of RuBP were needed. Thus, at the end of the cycle there must be three molecules of RuBP or the cycle would get out of balance.
Figure 7. The Calvin Cycle. (Click image to enlarge)
Figure 8. The Krebs Cycle. (Click image to enlarge)
The three molecules of RuBP that began the cycle had a total of three carbons multiplied by five molecules, or 15 atoms of carbon. Three molecules of carbon were then fixed for a surplus of three carbons in the cycle. Those three carbons are expelled from the cycle as one molecule of G3P. The remaining 15 carbons are still in the form of G3P. Therefore, they must be converted back to RuBP to start the process again. More ATP, as well as many steps involving enzymes, are necessary for this regeneration.
The Calvin Cycle and the Global Climate
The earth’s climate has changed many times over the course of its history (for example, in class we discussed the change from an anaerobic to an aerobic environment that took place about 2 billion years ago as a result of increased photosynthetic activity). Currently, a lot of attention is being given to the topic of global climate change. The amount of carbon dioxide, and other so-called “greenhouse gases”, is increasing in the atmosphere which has resulted in an increase in the mean global temperature. In an attempt to better understand the factors that play a role in global warming, scientists are attempting to learn more about both the natural and anthropogenic inputs and outputs of greenhouse gases.
Atmospheric carbon dioxide is constantly being fixed into sugars (and other macromolecules), which, in turn, are oxidized back into CO2. This relationship, on a global scale, is termed the carbon cycle. Through the burning of fossil fuels we are generating CO2 at a faster rate than plants can fix them back into sugars and other carbon molecules.
Plants and other photosynthetic organisms play an important role in determining the amount of carbon dioxide in the atmosphere. They are both consumers and producers of carbon dioxide.
- What organelle and cellular process enables plants to consume carbon dioxide?
- What organelle and cellular process enables plants to produce carbon dioxide?
We have finished our discussion of photosynthesis by showing the major anabolic pathway that results in the reduction of carbon dioxide to G3P which can be used to synthesize various sugars (including glucose). The formation of sugars is energetically unfavorable, as can be predicted from the +ΔG for the formation of glucose from carbon dioxide and water. The energy and reducing power for this process comes from the light-dependent reactions, which were presented in the last tutorial.
Carbon dioxide enters the Calvin cycle by being added to the 5-carbon compound called ribulose bisphosphate (RuBP). This reaction, involving the enzyme rubisco, results in a 6-carbon compound that is very unstable and is rapidly split into two, 3-carbon molecules. The resulting 3-carbon compound is activated by phosphorylation (via ATP obtained from the light-driven reactions of photosynthesis), then reduced by NADPH (again, obtained from the light-driven reactions of photosynthesis). After reduction, a 3-carbon molecule, glyceraldehyde 3-phosphate (G3P) exits the cycle. G3P has two fates. Some molecules go on to form more molecules of RuBP (which requires additional ATP; again derived from the light-driven photosynthetic reactions). Some molecules exit the Calvin cycle to form sugars (including glucose).
After reading this tutorial, you should have a working knowledge of the following terms:
- Calvin cycle
- carbon cycle
- glyceraldehyde 3-phosphate (G3P)
- ribulose bisphosphate (RuBP)
- stoma (pl. stomata)
Questions? Send your instructor a message through ANGEL!