Start with a tree’s mass, then follow light-driven electron transfer and carbon fixation inside the chloroplast. The pathways connect to respiration, but matter and energy take different routes.
INSIDE THE CHLOROPLAST
Follow the energy and the carbon
Photosynthesis joins two linked but different stories: light-driven electron transfer stores energy in carriers, and the Calvin–Benson cycle uses those carriers to incorporate carbon dioxide into organic molecules.
ELECTRON FLOW + CHEMIOSMOSIS
Light Reactions: Move Electrons, Build a Proton Gradient
Across the thylakoid membrane
StromaATP and NADPH become available here for carbon-assimilation reactions.
Thylakoid lumenProtons accumulate here, creating an electrochemical gradient.
Light energy excites electrons at Photosystem II and Photosystem I.
Photosystem IIlight energizes electrons; water oxidation releases O₂
Electron carriersplastoquinone → cytochrome b₆f → plastocyanin; transfer helps build the proton gradient (H⁺)
Photosystem Ia second light-driven excitation raises electron energy
NADPHreceives high-energy electrons on the stromal side
H⁺ gradient across the membraneH⁺ returns through ATP synthasechemiosmosis supports ATP formation
Oxygenreleased from water oxidation at Photosystem II
ATPmade as H⁺ flows from the lumen back to the stroma through ATP synthase
NADPHcarries reducing power to carbon-assimilation reactions
Keep matter and energy separate. Water supplies electrons and oxygen atoms; light supplies energy. Electrons move through carriers, while the H⁺ gradient powers ATP synthesis.
Scope: This is a selected linear-electron-flow model. Cyclic flow around Photosystem I can supply extra ATP without net NADPH or O₂; regulation, alternate routes, and photorespiration are not drawn.
INTERACTIVE MODEL
Follow two things through the same system
Follow a representative route for carbon: from A gas whose carbon atoms can be incorporated into organic molecules during photosynthesis. into organic molecules and, for some carbon, back to A gas whose carbon atoms can be incorporated into organic molecules during photosynthesis..
Trace Carbon. Step 1 of 6: Carbon Dioxide.
enters carbon fixation
net carbon can leave as
can contribute carbon to
may later be used in
can return carbon as
STEP 1 OF 6Air around a leaf
Carbon Dioxide
A tree takes in A gas whose carbon atoms can be incorporated into organic molecules during photosynthesis. through leaf pores called Adjustable pores in a leaf surface that allow gases such as carbon dioxide to move in and water vapor to move out.. This is a representative route, not a single atom tracked continuously: carbon atoms mix among many molecules and can move through water, soil, organisms, and geological stores.
Step 1 of 6
PAUSE AND PREDICT
Check the model
CHECK YOUR UNDERSTANDING
Where do the carbon atoms in new plant carbohydrates enter this model?
CHECK YOUR UNDERSTANDING
Which statement best describes energy across photosynthesis and respiration?
FOLLOW THE SOURCES
Read more and inspect the model’s basis
OpenStax explains plant nutrition and photosynthesis; the NGSS pages show how those broader models relate to standards and where this route adds optional biochemical detail.