Bio Tutorial 29 Q/a

Front 1

Review the Krebs Cycle. Why is it called a cycle? Compare the Krebs Cycle with the Calvin Cycle

Back 1

The Krebs Cycle occurs during cellular respiration in the mitochondria. The Krebs cycle is a collection of enzymes which convert acetyl CoA (a product of glycolysis) (and producing a small amount of ATP directly) and generate the highly reduced compounds NADH and FADH2, that in turn, send their electrons through the electron transport chain in the mitochondrial membrane where they are typically donated eventually to oxygen. The Krebs Cycle and the Calvin Cycle are both called “cycles” because they involve inputs and outputs, but some molecules are recycled full-circle. Krebs cycle regenerates oxaloacetate at the end of one cycle to begin the next. 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).

Front 2

What molecules are produced by the light reactions of photosynthesis that are used in the Calvin Cycle?

Back 2

ATP and NADPH.

Front 3

Recall the reaction of the breakdown of glucose. What was the ΔG and what products and reactants were involved? Is this an anabolic or catabolic reaction?

Back 3

glucose + O2 → CO2 + H2O + ATP

This catabolic reaction is exergonic and releases about 686 kcal of energy. Thus, the ΔG for the overall reaction is -686 kcal/mole.

Front 4

What would the ΔG of the synthesis of glucose be? What would the reaction be? What products and reactants are involved? Is this a catabolic or anabolic reaction?

Back 4

unlight + CO2 + H2O → O2 + glucose

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. Therefore, this is an anabolic reaction with the ΔG = 686 kcal/mole.

Front 5

If the synthesis of glucose is endergonic, what does it require? Where does it get this?

Back 5

The synthesis of glucose requires energy, which can obtained from ATP produced in the light reaction of photosynthesis.

Front 6

What is the reaction for the light reaction of photosynthesis?

Back 6

light + water → NADPH + ATP, with oxygen as a byproduct.

Front 7

Briefly describe the process that Melvin Calvin used to determine this cycle.

Back 7

He devised a method whereby he could raise algae in a lollipop-shaped disk. 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. Calvin analyzed the dead algae to see which molecules had incorporated the carbon14 by 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.

Front 8

Describe what happens to carbon dioxide after it enters the Calvin Cycle. To what does it join, and what happens to that molecule? What enzyme makes this fixation possible?

Back 8

Carbon dioxide is initially fixed (i.e., taken out of the gas phase) by joining to a five-carbon compound (RuBP). An enzyme, rubisco, helps join one molecule of incoming carbon dioxide with one molecule of RuBP. The resulting six-carbon compound is so unstable that it very quickly breaks into two, three-carbon compounds.

Front 9

How do different organisms obtain CO2?

Back 9

In algae, carbon dioxide simply diffuses through membranes from the surrounding water. In plants, the carbon dioxide comes in through pores (stomata) in the leaves and uses one of several mechanisms to get to the chloroplasts. These same pores are where oxygen, produced in the light reactions, escapes into the atmosphere.

Front 10

Describe the form of the carbon-containing molecules that are generated through the Calvin Cycle beginning at carbon fixation and ending at the end of the reduction phase.

Back 10

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, the 3-carbon molecules are reduced (i.e. electrons are added) to glyceraldehyde 3-phosphate (G3P), another 3-carbon molecule. At the end of the reduction phase, some of the G3P leaves the cycle to become sugar.

Front 11

What is the most reduced carbon compound in the Calvin Cycle? What are the possible fates of these reduced carbon compounds?

Back 11

Glyceraldehyde 3-phosphate (G3P). At the end of the reduction phase, some of the G3P leaves the cycle to become sugar. However, most of it gets regenerated into RuBP.

Front 12

Explain how the cycle is able to regenerate the reactants that will then bind with CO2. Where does the energy come from and what enzymes are involved in this process? Keep a tally of how many carbon atoms enter the cycle, and how many were added via fixation, how many

Back 12

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. 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 over. More ATP, as well as many steps involving enzymes, are necessary to do this regeneration.

Front 13

14.) Consider the different enzymes, reactants, reducing agents, and sources of energy involved in the Calvin Cycle. Draw the Calvin Cycle including all enzymes, reactants, etc.

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Back 13

14.) Consider the different enzymes, reactants, reducing agents, and sources of energy involved in the Calvin Cycle. Draw the Calvin Cycle including all enzymes, reactants, etc.

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Front 14

Why is the Calvin Cycle also called the dark reactions of photosynthesis? Do you think that this is misleading?

Back 14

The Calvin Cycle is also called the dark reaction because it can occur in the absence of light, that is, without the use of photons or pigments. However, this name could be misleading because the Calvin Cycle can occur in the presence of light. It does not need to occur in the dark.

Front 15

What is the global carbon cycle? How have humans allegedly put the global carbon cycle out of balance?

Back 15

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. However, humans are burning fossil fuels at a faster rate than plants can fix them back into sugars and other carbon molecules, so many scientists claim that the global carbon cycle is out of balance.

Front 16

What are two views on global warming? Do these views necessarily oppose one another?

Back 16

Some scientists believe that humans are burning fossil fuels at a faster rate than plants can fix them back into sugars and other carbon molecules, so humans have caused the global carbon cycle to become out of balance. Other scientists think that the current rise in CO2 levels is part of a natural cycle (i.e., rising and falling CO2 levels) that has been going on for millions of years. Thus, there are cycles within cycles, each interacting with and affecting the others. It is possible that both of these processes are occurring at the same time. The viewpoints do not necessarily conflict.