Bio 1000- Photosynthesis Cont'd Part 3

THE CALVIN CYCLE

The Calvin cycle is the second process that makes up photosynthesis and is often called light independent reactions. Calvin cycle as mentioned earlier is the conversion of carbon dioxide into an organic molecule usually in the form of carbohydrates. The Calvin cycle occurs in the stroma of the chloroplast. The Calvin cycle can be subdivided into three stages:

1. Fixation
2. Reduction
3. Regeneration

Fixation
During this phase, one carbon dioxide molecule is added to 5-carbon molecule called RuBP to form a 6-carbon molecule. The 6-carbon molecule is highly unstable and is immediately broken into two molecules of 3-carbon compound called 3-phosphoglycerate

Reduction
In this phase, each 3-phosphoglycerate molecule gets an phosphate added to it through the breakdown of ATP, producing 1,3-biphospoglycerate. Each of these molecules is then reduced by electros from NADPH and the phosphate added to each molecule is removed to produce glyceraldehyde-3-phosphate or G3P

Regeneration
For each turn of the Calvin cycle, two molecules of G3P are produced, a total of 6 carbon atoms. Ideally, from the 6 carbon atoms produced, 5 of these carbons are rearranged to regenerate a single molecule of RuBP from the breakdown of an ATP molecule while 1 of these carbons is used to generate a glucose molecule or related carbohydrates.

Based on the diagram above, for 3 turns of  the Calvin Cycle:

Reactants	Products
Fixation:
3 CO2 (3 Carbons) 3 RuBP (15 Carbons)	6 3-Phosphoglycerate (18 Carbons)
Reduction:
6 3-Phosphoglycerate (18 Carbons) 6 ATP	6 1,3-Biphosphoglycerate (18 Carbons) 6 ADP
6 1,3-Biphosphoglycerate (18 Carbons) 6 NADPH	6 G3P (18 Carbons) 6 NADP+ 6 Pi
Regeneration
5 G3P (15 carbons) 3 ATP	3 RuBP (15 Carbons)
1 G3P (3 carbons)	Glucose or other related molecules

IMPORTANCE OF RUBISCO OR RuBP

Ribulose-1,5-biphosphate carboxylase oxygenase or Rubisco is the enzyme of the Calvin cycle that catalyzes the fixation of carbon dioxide into its organic form. Rubisco is considered a highly important enzyme as it provides all photoautotrophs with its source of organic carbon molecules, making it the planet's most abundant protein.

PHOTORESPIRATION-WHAT IS IT?

Though Rubisco is a highly important protein, believe it or not, it can also be inefficient. WHY? The active site of the Rubisco enzyme is not specific to carbon dioxide. That is, that an oxygen molecule can bind to the active site and react with RuBP. This reaction results in a product of a two-carbon compound that is exported out of the chloroplast and requires the cell to consume ATP to convert it into carbon dioxide. This wasteful process is what is referred to as PHOTORESPIRATION. Photorespiration occurs in light and similar to cellular respiration in that it consumes oxygen to release carbon dioxide.

When oxygen binds to the active site of Rubisco, the enzyme acts as an oxygenase instead of a carboxylase. A carboxylation reaction leads to a carbon gain whereas an oxygenation reaction results in the cell losing carbon.