Chapter 4- Light Dependent Reactions of Photosynthesis

Question 1

Light dependent reactions

Answer 1

• take place in thylakoid membrane
• harness light energy to produce ATP and NADPH (electron carrier)
• ATP generated here is consumed in the Calvin cycle! (NOT used to power the cell)

Question 2

Photosystems

Answer 2

• contain special pigment chlorophyll and carotenoids, that absorb photons
• Photosystem II (P680) & Photosystem I (P700) are used in photosynthesis

Question 3

Reaction center

Answer 3

-pair of chlorophyll molecules in the center of the proteins

Question 4

Non-cyclic photophosphorylation (6 steps)

Answer 4

• light dependent reactions.
• Steps 1-6:
  1. Photolysis (water is split), passing electrons to photosystem II and releasing protons into thylakoid lumen.
  2. Photons excite electrons in the reaction center of photosystem II, passing the electrons to a primary electron acceptor.
  3. The primary electron acceptor sends excited electrons to the ETC. Protons are pumped from the stroma to the thylakoid lumen during the redox reactions in ETC. The electrons are then deposited into photosystem I.
  4. Photons excite pigments in photosystem I, energizing the electrons in reaction center to pass to another primary electron acceptor.
  5. Electrons are sent to a short ETC that terminates with NADP+ reductase (enzyme that reduces NADP+ to NADPH).
  6. The accumulation of protons in the thylakoid membrane generates an electrochemical gradient that makes ATP using ATP synthase, as H+ moves from the lumen back into the stroma.

Question 5

Cyclic photophosphorylation

Answer 5

• occurs when photosystem I passes electrons back to the 1st ETC instead of 2nd ETC, causing more proton pumping & more ATP production
• no NADPH is generated here unlike non-cyclic.

Question 6

Calvin Cycle

Answer 6

6 CO₂ + 18 ATP + 12 NADPH + H⁺ —-> 18 ADP + 18 Pᵢ +12 NADP⁺ + 1 glucose

• does NOT directly use light energy but can only occur when ATP and NADPH are produced from L.d reactions.
• takes place in the chloroplast stroma of plant mesophyll cells.
• Fixes carbon dioxide that enters the stomata.

Step 1-4:

1. Carbon fixation- CO2 combines with five-carbon ribulose-1,5-biphosphate (RuBP) to form six-carbon molecule, which breaks down into three-carbon phosphoglycerates (PGA). Catalyzed by RuBisCo.
2. Reduction- PGA is phosphorylated by ATP and reduced by NADPH to form (G3P).
3. Regeneration- Most of G3P is converted back to RuBP.
4. Carbohydrate synthesis- leftover G3P is used to make glucose

Question 7

Photorespiration (C2 photosynthesis)

Answer 7

• process in which RuBisCo causes oxygen to bind to RuBP.
• Occurs in the stroma under HOT and DRY conditions, stomata closes to minimze water loss.
• Produces phosphoglycolate (2-carbon) that is taken to peroxisome and mitochondria for conversion into PGA.
• Fixed carbon is lost as CO2 in the process.
• Net loss of fixed carbon atoms and no new glucose is made.