Dark reactions in plants and steps of it's cycle

Dark reactions
Location of dark reaction
Fuel of dark reactions
Steps of calvin cycle
Importance of dark reaction

1. Dark reactions in plants Submitted by : Syeda Fatima Shahbaz

2. introduction Dark reaction is also called carbon-fixing reaction or calvin cycle. It is a light-independent process. sugar molecules are formed from the carbon dioxide and water molecules.

3. Location of dark reaction The dark reactions occur within the chloroplast , a specialized organelle found in plant cells . The chloroplast is a miniature factory, compartmentalizing the light-dependent reactions in flattened, sac-like structures called thylakoids and the dark reactions in the fluid-filled region surrounding the thylakoids, known as the stroma. Imagine the thylakoids as solar panels capturing light energy, while the stroma acts as the workshop where the captured energy is used to build essential molecules.

4. Essential ingredients' : fuel of dark reaction Carbon dioxide (CO₂ ): The raw material, the carbon source plants capture from the atmosphere to build organic molecules. 2. ATP (adenosine triphosphate ): The energy currency of cells, providing the necessary power to drive the reactions forward. Imagine ATP as tiny energy packets that fuel the machinery of the dark reactions. 3. NADPH (nicotinamide adenine dinucleotide phosphate ): An electron carrier, acting as a vital reducing agent, supplying the electrons needed for specific steps in the cycle. Think of NADPH as an electron shuttle, transferring electrons to power specific chemical transformations.

5. Calvin cycle A three step symphony

6. Calvin cycle

7. General diagram of cycle

8. Carbon fixation It is the first step of Calvin cycle. CO 2 molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA ). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco.

9. Details: In plants, atmospheric   CO 2  enters the mesophyll layer of leaves by passing through pores on the leaf surface called stomata . It can then diffuse into mesophyll cells, and into the stroma of chloroplasts, where the Calvin cycle takes place.

10. Continue…. In the first step of the cycle, an enzyme nicknamed rubisco (RuBP carboxylase-oxygenase) catalyzes attachment of to a five-carbon sugar called ribulose bisphosphate (RuBP ). The resulting 6-carbon molecule is unstable, however, and quickly splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA ). Thus, for each CO2 that enters the cycle, two 3-PGA molecules are produced.

11. Continue….. The actual molecular description is shown below.

12. Reduction In the second stage, ATP and NADPH are used to convert the 3-PGA molecules into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate ( G3P ). This stage gets its name because NADPH donates electrons to, or  reduces , a three-carbon intermediate to make G3P. 

13. Continue…. The reduction stage of the Calvin cycle, which requires ATP and NADPH, converts 3-PGA (from the fixation stage) into a three-carbon sugar . This process occurs in two major steps:

14. . 1 ST STEP each molecule of 3-PGA receives a phosphate group from ATP, turning into a doubly phosphorylated molecule called 1,3-bisphosphoglycerate (and leaving behind ADP as a by-product ). 2 ND STEP the 1,3-bisphosphoglycerate molecules are reduced (gain electrons). Each molecule receives two electrons from NADPH and loses one of its phosphate groups, turning into a three-carbon sugar called glyceraldehyde 3-phosphate (G3P). This step produces NADP and phosphate as by-products.

15. Actual chemical structure and reaction shown below;

16. . The ATP and NADPH used in these steps are both products of the light-dependent reactions (the first stage of photosynthesis ). That is, the chemical energy of ATP and the reducing power of NADPH, both of which are generated using light energy, keep the Calvin cycle running. Reciprocally , the Calvin cycle regenerates ADP and NADP , providing the substrates needed by the light-dependent reactions.

17. Regeneration some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor . Regeneration requires ATP and involves a complex network of reactions which may be called “ carbohydrate scramble “.

18. Continue….. In order for one G3P to exit the cycle (and go towards glucose synthesis), three molecules must enter the cycle, providing three new atoms of fixed carbon . When three molecules enter the cycle, six G3P molecules are made. One exits the cycle and is used to make glucose, while the other five must be recycled to regenerate three molecules of the RuBP acceptor.

19. Summary of calvin cycle reactants and products Carbon     combine with   RuBP acceptors, making   molecules of glyceraldehyde-3-phosphate (G3P).   G3P molecule exits the cycle and goes towards making glucose.   G3P molecules are recycled, regenerating   RuBP acceptor molecules ATP   ATP are converted to   ADP (  during the reduction step,   during the regeneration step). NADPH   NADPH are converted to   NADP  (during the reduction step).

20. Continue…. A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule . It would take : six turns of the cycle 6CO2       18 ATP   12 NADPH to produce one molecule of glucose.

21. IMPORTANCE OF DARK REACTIONS Convert CO₂ into organic molecules. Provide plants with energy (glucose). Support the growth and development of plants. Foundation for the food chain.

22. Thank you. .