Morning sunlight poured through the greenhouse window.
"The green is glowing," Kana touched a leaf.
Milia smiled. "The moment photosynthesis begins."
"Photosynthesis?"
Rei held the leaf up to the light. "Converting light energy to chemical energy."
"How?"
"Inside chloroplasts," Milia opened her notebook.
Kana peered in. "Chloroplasts, the green things?"
"Chlorophyll is green. Absorbs red and blue light, reflects green."
Rei supplemented. "That's why plants appear green."
"Where does the absorbed light go?" Kana asked.
"Photosystem II. That's where the reaction starts," Milia explained.
"II? Why is II first?"
Rei laughed. "They were discovered in reverse order."
"When light hits, chlorophyll electrons get excited," Milia continued.
"Excited?"
"They enter a high-energy state. Those electrons are passed to the electron transport chain."
Kana wrote in her notebook. "Electron transport... similar to mitochondria?"
"Similar. But in reverse direction," Rei said.
"Reverse?"
"In mitochondria, electrons go down. In chloroplasts, electrons go up."
Milia drew a diagram. "Electrons lost at Photosystem II are replenished from water."
"From water?" Kana was surprised.
"H₂O breaks down into electrons, H⁺, and oxygen."
Rei added. "This oxygen is what we breathe."
Kana looked out the window. "Plants are making oxygen?"
"Every day, trillions of water molecules are broken down," Milia said quietly.
"Where do the electrons go after?"
Rei explained. "Through plastoquinone and cytochrome b₆f complex to Photosystem I."
"Light again?"
"Excited once more. A two-stage light reaction."
Milia continued. "Finally, NADPH is made."
"NADPH?"
"Reducing power. A molecule carrying electrons."
Rei supplemented. "In the process, H⁺ accumulates inside the membrane."
"A gradient again?" Kana understood.
"Yes. ATP synthase makes ATP."
Kana organized. "Light → electron movement → ATP and NADPH."
"These are the light reactions," Milia acknowledged.
"Light reactions? Are there dark reactions too?"
Rei nodded. "The Calvin cycle. Proceeds even without light."
"But ATP and NADPH are needed."
Milia drew a diagram. "CO₂ is fixed. An enzyme called RuBisCO works."
"Fixed?"
"Inorganic carbon becomes an organic compound."
Rei explained. "CO₂ combines with ribulose 1,5-bisphosphate to become 3-phosphoglycerate."
"Long names..." Kana looked confused.
"What matters is that carbon is captured," Milia said.
"After that, using ATP and NADPH, glucose is made."
Kana asked. "Glucose?"
"Eventually. First, G3P, glyceraldehyde 3-phosphate."
Rei calculated. "Six CO₂ molecules make one glucose molecule."
"What's the efficiency?" Kana asked.
"About 6% of visible light," Milia answered.
"Low?"
"But if all plants on Earth do it, that's enormous energy."
Rei looked out the window. "Fossil fuels are also products of photosynthesis."
Kana was moved. "Sunlight became oil?"
"Over hundreds of millions of years," Milia said quietly.
"This morning's light will also become energy someday."
Rei touched the leaf. "Carbon fixed by this leaf might return to soil and become coal someday."
Kana pondered. "What if there were no photosynthesis?"
"No oxygen, no food," Milia answered.
"Animals couldn't exist," Rei acknowledged.
Kana gazed at the leaf. "This green supports the world."
"Thirty-five hundred million years since cyanobacteria began photosynthesis."
Milia added. "The atmosphere changed, life changed."
Rei closed the notebook. "Photosynthesis is the most important invention in Earth's history."
The morning sun grew stronger.
"Right now, hundreds of millions of photons are reaching the chloroplasts," Kana murmured.
Milia and Rei smiled.
"And life continues."
The three quietly gazed at the leaf. Photosynthesis begins again today.