Where ATP and NADPH Get Used in Photosynthesis

After light reactions make ATP, NADPH, and O2 in the chloroplasts:

  • O2 diffuses out 🌬️
  • ATP and NADPH power food (sugar) making! This is the biosynthetic phase.

💡 Fun fact: This phase doesn’t need direct light but does need ATP + NADPH (from light reactions) + CO2 + H2O. If you turn off the light, it keeps running briefly before stopping!

Meet the Calvin Cycle! 🔁

This is how plants use ATP + NADPH to turn CO2 into sugar. It happens in the stroma and has 3 stages:

StageWhat HappensKey Player
1. CarboxylationCO2 attaches to a 5-carbon sugar called RuBP (Ribulose bisphosphate). This makes two 3-PGA molecules (a 3-carbon acid).Enzyme: RuBisCO (RuBP carboxylase-oxygenase)
2. ReductionATP and NADPH turn 3-PGA into sugars! 🍬 (This step uses energy & electrons).ATP → ADP + P
NADPH → NADP+
3. RegenerationSome sugars are used to rebuild RuBP so the cycle can restart. More ATP is used here!ATP → ADP + P

✨ Cool discovery: Melvin Calvin found the first CO2 fixation product (3-PGA) using radioactive carbon (14C)! The cycle is named after him.

Energy Bill for Making Sugar 💪

To make one glucose molecule (C6H12O6), the cycle runs 6 times because:

  • 1 glucose = 6 carbon atoms
  • Each CO2 adds 1 carbon

Total energy needed per glucose:

  • 18 ATP → 18 ADP
  • 12 NADPH → 12 NADP+

Not All Plants Are the Same! 🌾 vs 🌽

Plants fix CO2 in different ways:

TypeFirst CO2 ProductExample PlantsSpecial Features
C3 Plants3-carbon PGARice, WheatStandard Calvin cycle
C4 Plants4-carbon OAA (Oxaloacetic acid)Maize, Sorghum
  • Have Kranz anatomy (wreath-shaped bundle sheath cells 🥨)
  • No photorespiration
  • More efficient in hot/sunny climates ☀️

NEET Power Concepts ⚡

  1. ATP & NADPH role: They’re used in the Calvin cycle (biosynthetic phase) to reduce CO2 → sugar.
  2. Calvin Cycle inputs/outputs: 6 CO2 + 18 ATP + 12 NADPH → 1 glucose + 18 ADP + 12 NADP+.
  3. RuBisCO: Key enzyme fixing CO2 to RuBP in carboxylation (Stage 1).
  4. C3 vs C4: First stable product (PGA vs OAA), Kranz anatomy in C4, photorespiration absence in C4.
  5. Kranz anatomy: Bundle sheath cells with many chloroplasts – no gas exchange, found in C4 plants.

Keep shining like sunlight on a leaf! You’ve got this 🌟