PHOTOSYNTHESIS

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Redox Reactions: transfer of electrons between substances (LeO GeR)

• Reduction Reaction: a substance gains electrons

• Oxidation Reaction: a substance loses electrons

Light Dependent Reaction: 1st stage of Photosynthesis

• Where: 

  • Thylakoid membrane in the Chloroplast

Thylakoid (highlighted) in the Chloroplast.

• Structures:

  • Photosystems (1&2): have pigments (chlorophyll) which capture light > excite/energize electrons

    • Oxygen Evolving Complex (OEC): enzyme that breaks water down. Breaks 2 water molecules (H2O) at a time.

  • Electron Transport System: group of proteins that transfers electrons.

  • ATP synthase: membrane enzyme that creates ATP by joining inorganic phosphate with ADP

  • NADP+: electron carrier (accept electrons and drop it off)

    • NADPH is the "reduced" form

Structures of the Light Dependent Reactions

• Reactants: 

  • light energy + H2O

• Process: 

  • 1. Photosystem 2 absorbs light energy > ‘excites/energizes’ 2 electrons > excited electrons go through ETC

• 2. Replace electrons in photosystem 2

  • Photolysis: 2 water molecules are oxidized + split by OEC, resulting in protons (H+), oxygen, and electrons

    • Protons: contribute to proton gradient

    • Oxygen: byproduct/waste product, released outside (what us humans use to breathe)

    • Electrons: 4 are donated to photosystem 2 to replace electrons in photosystem 2

      • Since photolysis releases 4 electrons, the light dependent reaction can run 2 full cycles before another photolysis is needed

• 3. Excited electrons in ETC lose energy when moving through ETC > Energy is used by the proton pump to establish a proton gradient

  • Proton gradient: a concentration gradient (difference in concentrations between 2 areas) made of protons

  • Proton pump: a protein pump that is part of the ETC. Pump proteins from stroma > lumen.

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• 4. Chemiosmosis: movement of protons from high to low (along the proton gradient, from lumen to stroma) across ATP synthase to make ATP (ADP + inorganic phosphate).

  • ADP + inorganic phosphate are loosely attached to ATP synthase. The proton flow across ATP synthase causes ATP synthase to rotate binding them together.

• 5. Low-energy electrons at ETC end up at photosystem 1. Photosystem 1 absorbs light energy > re-exciting electrons

• 6. Excited Electrons travel through a shorter ETC to be picked up by NADP+ > NADPH (carries these electrons to the Calvin cycle)

• Product: 

  • NADPH + ATP (used in the Calvin cycle), oxygen (byproduct)

Calvin Cycle: 2nd stage of Photosynthesis

• Where: 

  • Stroma in the Chloroplast

Stroma (highlighted) in the Chloroplast

• Reactants: 

  • 6 NADPH + 9 ATP (from the Light Dependent Reactions), 3 CO₂ (Carbon Dioxide)

    Note:  Each full turn of the Calvin Cycle uses 1 CO₂, but the process is often described when 3 turns occur because it produces a product of 1 G3P molecule. That’s why you’ll often see "3 CO₂" instead of just 1.

• Process:  CO2 produces sugar

  • 1. Carbon Fixation: RuBisCO fixes CO₂

    • To make CO₂ usable, RuBisCO (enzyme) combines 3 CO₂ (1 carbon molecule) with 3 RuBP (5-carbon molecule)

    • = 3 6-carbon molecules > unstable so splits in half = 6 3-PGA (3 carbon molecules)

  

  • 2. Carbon Reduction: Reducing 3-PGA > G3P

    • 6 ATP + 6 NADPH (from light dependent reaction) are used to convert 6 3-PGA > 6 G3P (3 carbon molecule)

      • ATP: donates phosphate, making 3-PGA more reactive and ready to accept electrons from NADPH.

      • NADPH: donates electrons, reducing (gain electrons) 3-PGA > G3P

  

  • 3. The Regeneration of RuBP: Regenerating RuBP + Making Sugar

    • 5 G3P regenerate 3 RuBP using 3 ATP (allow cycle to continue).

      • G3P has 3 carbon molecules and RuBP has 5 carbon molecules.

        • 5 G3P: 5 x 3 (carbon molecules) = 15 carbon molecules

        • 3 RuBP: 3 x 5 (carbon molecules) = 15 carbon molecules

    • Last 1 G3P exit the cycle to SOON become sugar (glucose, cellulose, etc.) for the plant.

      • For example, glucose requires 2 G3P molecules, so the Calvin Cycle needs to occur TWICE.

• Product: 

  • 1 G3P

Questions (Calvin Cycle)

1. Where does the Calvin Cycle occur?

2. What is the name of the enzyme that "fixes" CO₂?

3. What is the number of CO₂ molecules needed to make 1 G3P molecule?

4. What are the reactant(s) of the Calvin Cycle (to make 1 G3P)? How much of each?

5. What are the product(s) of the Calvin Cycle?

6. What happens in Carbon Fixation?

7. What happens in Carbon Reduction?

8. What happens in the Regeneration of RuBP?

9. What stage is G3P formed?

10. What molecule is CO₂ attached to by RuBisCO to form a 6-carbon molecule?

11. What happens to the 6-carbon molecule formed in Carbon Fixation?

12. How much of the 6 G3P molecules are used to regenerate 3 RuBP molecules?

13. How much ATP is used to regenerate G3P to RuBP?

14. What does ATP give to G3P to turn it into RuBP?

15. How much ATP molecules are used to turn 6 3-PGA molecules to 6 G3P molecules?

16. How much NADPH molecules are used to turn 6 3-PGA molecules to 6 G3P molecules?

17. What does NADPH donate to 3-PGA to reduce it to G3P?

18. What does ATP donate to 3-PGA to make it more reactive?

19. What stage is 3-PGA made?

20. Where does NADPH and ATP come from?

21. Is NADPH oxidized or reduced when it donates electrons to 3-PGA?

    
    

Answers (Calvin Cycle)

1. In the stroma of the chloroplast

2. RuBisCO

3. 3 CO₂ molecules

4. 3 CO₂, 9 ATP, 6 NADPH

5. G3P

6. 3 CO₂ is fixed to 3 RuBP by RuBisCO = 6 carbon molecule that splits, creating 6 3-PGA molecules

7. 6 3-PGA is converted into 6 G3P, using 6 ATP and 6 NADPH

8. 5 of the 6 G3P regenerate 3 RuBP and the last 1 is used to make sugar

9. Carbon Reduction

10. RuBP (5 carbon molecule)

11. It splits into two 3-carbon molecules (3-PGA) because it is chemically unstable.

12. 5 of the 6 G3P molecules

13. 3 ATP

14. A phosphate group

15. 6 ATP

16. 6 NADPH

17. Electrons

18. A phosphate group

19. Carbon Fixation

20. The Light-dependent reactions in the thylakoid membrane

21. Oxidized

    
    

Questions (Light Dependent Reactions)

1. Where does the Light Dependent Reactions occur?

2. What is the purpose of the proton gradient in the Light Dependent Reactions?

3. What happens to the low-energy electrons at the end of the first ETC in the Light Dependent Reactions?

4. What is the final electron acceptor in the Light Dependent Reaction?

5. What event replaces the lost electrons in photosystem 2?

6. What are the products of the Light-Dependent Reaction?

7. What is photolysis in the Light-Dependent Reaction?

8. What is chemiosmosis in the Light-Dependent Reaction?

9. What are the reactants in the Light-Dependent Reaction?

10. What is the role of ATP synthase in the Light-Dependent Reaction?

11. If an anonymous protein undergoes a reduction reaction, will it lose or gain electrons?

12. Is photolysis a reduction or oxidation reaction?

13. NADP+ and NADPH. Which one of these is the oxidized form and which is the reduced form?

14. What do photosystems contain that allows them to absorb sunlight?

15. What is the purpose of the ETC in the Light-Dependent Reaction?

16. What products from the Light Dependent Reaction are going to be used in the Calvin Cycle?

17. How much water molecules does the Oxygen Evolving Complex (OEC) split?

18. What is the function of Oxygen Evolving Complex (OEC) in the Light-Dependent Reaction?

19. Where do the excited electrons go after being excited by photosystem 1?

20. What happens to electrons when photosystems absorb light energy?

    
    
    
    
    
    

Answers (Light Dependent Reactions)

1. In the thylakoid membrane of the chloroplast.

2. To power the making of ATP as protons flow along the concentration gradient (from high to low) through ATP synthase. This action is called Chemiosmosis.

3. The low-energy electrons will arrive at photosystem I, where they get re-excited.

4. NADP⁺ accepts the final electrons. It becomes NADPH after accepting 2 electrons and a proton.

5. Photolysis: water is oxidized, releasing electrons that will replace the lost electrons in Photosystem 2.

6. ATP, NADPH, and oxygen.

7. Where water is split, resulting in electrons, protons, and oxygen. (2H₂O → 4H⁺ + 4e⁻ + O₂)

8. The movement of protons along the concentration gradient through ATP synthase to make ATP.

9. Light energy and H₂O.

10. To make ATP by binding ADP and inorganic phosphate.

11.  It will gain electrons. (GeR - Gain electrons = Reduction)

12. Oxidation because water is losing electrons.

13. NADP+ is the oxidized form, and NADPH is the reduced form.

14. Pigments called chlorophyll.

15. It transfers electrons and uses the energy from the electrons to pump protons from the stroma into the lumen to build the proton gradient.

16. ATP and NADPH.

17. 2 water molecules.

18. Oxidize and splits water, producing electrons, protons, and oxygen.

19. They go through a shorter ETC to reduce NADP+ to NADPH.

20. They are excited or energized.