Photosynthesis & Carbon Cycle
Trace light energy into chemical energy and follow carbon from atmospheric CO₂ into molecules that can become food and living material.
See how the pieces connect
- Step 1Photosynthesismoves light energy into organic carbon
- Step 2Chloroplasthouses the membranes and enzymes
- Step 3Light reactionsmake ATP and NADPH; release oxygen
- Step 4Calvin cycleuses CO₂, ATP, and NADPH to make G3P
- Step 5Cellular respirationcan transfer energy from organic molecules
A leaf absorbs light, and the chloroplast uses that energy to drive electron transfer across thylakoid membranes. ATP and NADPH then power carbon fixation in the stroma. The resulting G3P can contribute carbon to sugars and starch; the Calvin cycle does not directly release one glucose molecule each turn.
Read the supporting reference: OpenStax Biology 2e · PhotosynthesisLight energy becomes fixed carbon
Two linked sets of reactions take place in different chloroplast compartments.
Light-dependent reactions
Electron flow across the thylakoid membrane
- Light + chlorophyllChloroplastPigments capture photons and transfer excitation to reaction centers.Open record ↗
- excitation energy reachesPhotosystem IIThylakoid membraneSplits water; releases O₂ and sends energized electrons into the chain.Open record ↗
- passes energized electrons intoElectron transportLumen becomes H⁺-richTransfers electrons and helps build an H⁺ gradient in the lumen.Open record ↗
- delivers electrons toPhotosystem IThylakoid membraneAbsorbs light to re-energize electrons for transfer to ferredoxin.Open record ↗
- re-energizes electrons that help reduceNADPHStromaElectrons and H⁺ reduce NADP⁺; NADPH carries reducing power.Open record ↗
Carbon fixation and sugar building
The Calvin–Benson cycle runs in the stroma
- CO₂StromaAn inorganic carbon source enters the cycle.Open record ↗
- RuBisCO attaches CO₂ toRuBPStromaA five-carbon acceptor is regenerated during the cycle.Open record ↗
- which becomes3-phosphoglycerateStromaThe unstable six-carbon product splits into two three-carbon molecules.Open record ↗
- ATP and NADPH help reduce it toG3PStromaA triose phosphate; most is recycled to regenerate RuBP.Open record ↗
- some G3P can be used to buildCarbohydratesChloroplast and cellExported carbon can support sucrose, starch, and other biosynthesis.Open record ↗
Where the ATP comes from
A separate coupling path links the light reactions to ATP formation
- H⁺ gradientAcross thylakoid membraneProtons accumulate in the thylakoid lumen.Open record ↗
- creates a gradient used byATP synthaseThylakoid membraneProton flow from lumen to stroma powers ATP synthesis.
- to make ATP for theCalvin cycleStromaUses ATP together with NADPH to reduce fixed carbon.Open record ↗
In oxygenic photosynthesis, water supplies electrons and oxygen is released. The Calvin–Benson cycle fixes CO₂ into G3P; three CO₂ yield one net G3P while the rest of the G3P is used to regenerate RuBP. G3P contributes to sugar synthesis; glucose is not the direct product of one cycle turn. Photosynthetic ATP and NADPH are made and consumed in the chloroplast, not exported as the cell’s general ATP supply.
What molecule is the net carbohydrate product that leaves the Calvin cycle after three CO₂ are fixed?
Topics in this branch
Photosynthesis
The capture of light energy and transfer of carbon from carbon dioxide into organic molecules by plants, algae, and some bacteria.
Explore this idea 02Light-dependent Reactions
Thylakoid-membrane reactions that convert absorbed light into ATP and NADPH while oxidizing water and releasing oxygen.
Explore this idea 03Calvin Cycle
A chloroplast-stroma cycle that uses ATP and NADPH to incorporate carbon dioxide into triose phosphate and regenerate its carbon acceptor.
Explore this idea 04Carbon Cycle
The movement of carbon among atmospheric, aquatic, geological, and living reservoirs through biological and physical processes.
Explore this idea 05Carbon Fixation
Incorporation of inorganic carbon, such as CO₂, into organic molecules by an organism.
Explore this idea 06Carbohydrate Biosynthesis
The conversion of triose phosphates from carbon fixation into sucrose, starch, and other carbohydrates.
Explore this idea 07Chloroplast
A plant and algal organelle that houses photosynthetic membranes and carbon-assimilation machinery.
Explore this idea 08Thylakoid
An internal chloroplast membrane sac that carries photosystems, electron-transfer complexes, and ATP synthase.
Explore this idea 09Chloroplast Stroma
The aqueous compartment inside the chloroplast envelope and outside the thylakoid lumen.
Explore this idea 10Chlorophyll
A family of light-absorbing pigments that transfers excitation energy to photosynthetic reaction centers.
Explore this idea 11Photosystem II
A thylakoid protein–pigment complex that uses light energy to extract electrons from water.
Explore this idea 12Photosystem I
A thylakoid protein–pigment complex that re-energizes electrons for reduction of NADP⁺ to NADPH.
Explore this idea 13RuBisCO
The enzyme complex that catalyzes CO₂ addition to ribulose 1,5-bisphosphate in the Calvin cycle.
Explore this idea 14Carbon Dioxide
A carbon atom released during oxidative decarboxylation reactions.
Explore this idea 15Water
Removed from 2-phosphoglycerate by enolase.
Explore this idea 16ATP
Adenosine triphosphate, the cell's main short-term energy carrier.
Explore this idea 17NADPH
Reduced nicotinamide adenine dinucleotide phosphate; it supplies reducing power for biosynthesis and antioxidant defense.
Explore this idea 18Glyceraldehyde 3-phosphate
The three-carbon aldose phosphate that enters the payoff phase. Each glucose yields two.
Explore this idea 19Glucose
A six-carbon sugar and the starting fuel for glycolysis.
Explore this idea 20Molecular Oxygen
The terminal electron acceptor in mitochondrial respiration.
Explore this idea 21Cellular Respiration
How cells transfer energy from fuel molecules into ATP, often using oxygen in the final electron-accepting step.
Explore this idea