Light Dependent Reactions

Biology • Photosynthesis and Plant Energy

Written by STEM Calculators Team Published January 7, 2026 Updated February 24, 2026

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Light-Dependent Reactions — ATP, NADPH, and O₂ planner

Simplified stoichiometry model for non-cyclic vs cyclic photophosphorylation, with an ATP:NADPH ratio planner.

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Mode

Choose the question you want to answer. Visualizations appear after Calculate.

Amount unit

Used for ATP, NADPH, O₂, and electron-pair amounts.

Model reminder
Non-cyclic flow produces NADPH, ATP, and O₂ (from water splitting). Cyclic flow produces ATP only (no NADPH, no O₂). Coefficients are editable below.

Assumptions (editable)

ATP per NADPH (non-cyclic)

Default reflects a common teaching ratio (~1.5 ATP per NADPH) for linear flow.

ATP per cyclic electron pair

A “cyclic electron pair” means 2 e⁻ cycling through PSI.

O₂ per NADPH (non-cyclic)

Default: 1 O₂ per 2 NADPH ⇒ 0.5 O₂ per NADPH.

Photon estimate

Include photons Optional simplified photon bookkeeping.

Photons per electron (non-cyclic)

Common simplification: ~2 photons per e⁻ (PSII + PSI).

Photons per electron (cyclic)

Common simplification: ~1 photon per e⁻ (PSI only).

Demand inputs

NADPH needed

Use the chosen amount unit (e.g., mmol).

ATP needed

Use the chosen amount unit (e.g., mmol).

Optional: Calvin helper (turn CO₂ into ATP and NADPH demand)

Preset

Applies typical Calvin bookkeeping: per CO₂ → 3 ATP and 2 NADPH (editable below).

CO₂ fixed (amount)

Used only when preset is “Given CO2 fixed”.

ATP per CO₂

NADPH per CO₂

Optional: ATP:NADPH ratio planner

Target ATP:NADPH ratio

If used, ATP needed = ratio × NADPH needed.

Use ratio target Override ATP needed from ratio

Keeps NADPH input and computes ATP automatically.

Batch / CSV scenarios (paste or upload)

Paste scenarios as CSV. Columns supported: mode (demand/flows/compare), ATP, NADPH, NC_pairs, CYC_pairs, pairs. Amounts are interpreted in the selected unit.

CSV input

You can use comma or tab separators.

Upload CSV file

Batch results appear in the Results table (scrollable on small screens).

Results

Run Calculate to see outputs, diagrams, and steps.

Z-scheme style flow (interactive)

Hover for values. Mouse wheel zooms. Drag to pan. Use Reset view if needed.

ATP supply split (stacked bars, interactive)

Hover to read segment values. Mouse wheel zooms. Drag to pan.

ATP:NADPH ratio gauge (target vs achieved)

Hover for values. Mouse wheel zooms gauge range. Reset to restore.

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Light-dependent reactions: what this calculator models

The light-dependent reactions occur in the thylakoid membrane of chloroplasts. Light energy drives electron transfer through a PSII → PSI “Z-scheme”, producing: ATP (via proton motive force and ATP synthase), NADPH (reducing power for the Calvin cycle), and O₂ (from water splitting at PSII).

This calculator is a teaching/planning tool: it uses simplified stoichiometry to relate electron flow (non-cyclic vs cyclic) to ATP, NADPH, and O₂ production, and to help plan an ATP:NADPH supply ratio.

Important: Real chloroplast bioenergetics depends on organism, light intensity, proton leaks, and regulation. The coefficients are editable because the “ATP per NADPH” is not a universal constant.

Core bookkeeping relationships used by the calculator

The calculator works in “electron pairs” (2 electrons). In the simplified model: one non-cyclic electron pair reduces one NADP⁺ to one NADPH.

\[ \begin{aligned} n_{\text{NADPH}} &\approx n_{\text{NC}} \\ n_{\text{ATP,NC}} &= \left(\text{ATP per NADPH}\right)\cdot n_{\text{NADPH}} \\ n_{\text{ATP,cyc}} &= \left(\text{ATP per cyclic pair}\right)\cdot n_{\text{cyc}} \\ n_{\text{ATP,total}} &= n_{\text{ATP,NC}} + n_{\text{ATP,cyc}} \\ n_{\mathrm{O_2}} &= \left(\mathrm{O_2\ per\ NADPH}\right)\cdot n_{\text{NADPH}} \\ \text{ATP:NADPH} &= \dfrac{n_{\text{ATP,total}}}{n_{\text{NADPH}}}\quad (\text{if }n_{\text{NADPH}}>0) \end{aligned} \]

Default oxygen coefficient: 0.5 O₂ per NADPH, meaning 1 O₂ for every 2 NADPH (typical classroom bookkeeping tied to water splitting).

Planning cyclic flow to meet an ATP demand

If a target ATP demand exceeds ATP produced by non-cyclic flow (given the chosen “ATP per NADPH”), the calculator adds cyclic flow to supply the difference:

\[ \begin{aligned} n_{\text{ATP,extra}} &= \max\left(0,\ n_{\text{ATP,need}}-n_{\text{ATP,NC}}\right) \\ n_{\text{cyc}} &= \dfrac{n_{\text{ATP,extra}}}{\text{ATP per cyclic pair}} \end{aligned} \]

Interpretation: Cyclic photophosphorylation increases ATP without producing NADPH or O₂. In vivo, cyclic flow also contributes to regulation and photoprotection; here it is used purely as an “ATP booster”.

Optional photon estimate (simplified)

If photon bookkeeping is enabled, the calculator estimates photon usage from a user-selected “photons per electron” coefficient for non-cyclic and cyclic flow:

\[ \begin{aligned} \text{Electrons per pair} &= 2 \\ n_{\text{photons}} &\approx p_{\text{lin}}\cdot\left(2\cdot n_{\text{NC}}\right) + p_{\text{cyc}}\cdot\left(2\cdot n_{\text{cyc}}\right) \end{aligned} \]

This is a deliberately simple estimate meant for intuition and comparisons, not for detailed photochemical quantum yield calculations.

Calculator modes

Meet Calvin demand (given ATP and NADPH): Uses NADPH demand to set non-cyclic flow, then adds cyclic flow only if ATP is short.
Given flows (non-cyclic + cyclic pairs): Directly computes ATP, NADPH, and O₂ from the entered electron-pair flows.
Compare cyclic vs non-cyclic (same electron-pair budget): Computes two extreme scenarios: all pairs as non-cyclic vs all pairs as cyclic.

Calvin helper (optional preset)

The calculator includes a practical helper that converts a common Calvin “product target” into ATP and NADPH demand:

1 G3P (3 CO₂): 9 ATP and 6 NADPH
1 glucose (6 CO₂): 18 ATP and 12 NADPH
Given CO₂ fixed: uses editable per-CO₂ coefficients (default 3 ATP and 2 NADPH per CO₂)

How to interpret the visualizations

Z-scheme diagram: shows the direction of electron flow and summarizes outputs. Hover boxes to read exact computed values.
ATP split bars: compares the ATP contribution from non-cyclic vs cyclic flow (stacked).
ATP:NADPH gauge: shows a target marker (triangle) and the achieved ratio (needle). If NADPH = 0, the ratio is undefined.

Batch CSV scenarios

You can paste CSV (comma or tab) or upload a file. The calculator supports mixed modes per row.

Column	Meaning	Used in mode	Example
mode	demand / flows / compare	All	demand
ATP	ATP amount (in the selected unit)	demand	18
NADPH	NADPH amount (in the selected unit)	demand	12
NC_pairs	non-cyclic electron pairs (in the selected unit)	flows	12
CYC_pairs	cyclic electron pairs (in the selected unit)	flows	3
pairs	electron-pair budget (in the selected unit)	compare	12

Assumptions and limitations

Simplified mapping: \(n_{\text{NADPH}} \approx n_{\text{NC}}\) (one non-cyclic electron pair gives one NADPH).
ATP yield is adjustable: “ATP per NADPH” varies in real systems due to the effective H⁺/ATP and pathway details.
O₂ estimate is proportional: using “O₂ per NADPH” as a teaching coefficient.
Photon estimate is rough: it does not model absorption spectra, photochemical efficiency, or wavelength effects.
Not a kinetics model: time, light intensity, and enzyme regulation are not included.

Worked example (quick)

Suppose the Calvin cycle demand is 12 mmol NADPH and 18 mmol ATP. With defaults 1.5 ATP per NADPH and 1.0 ATP per cyclic pair:

\[ \begin{aligned} n_{\text{NC}} &= 12\ \text{mmol} \Rightarrow n_{\text{ATP,NC}} = 1.5\cdot 12 = 18\ \text{mmol} \\ n_{\text{ATP,extra}} &= \max(0,\ 18-18)=0 \Rightarrow n_{\text{cyc}}=0 \\ n_{\mathrm{O_2}} &= 0.5\cdot 12 = 6\ \text{mmol} \end{aligned} \]

If you raise ATP demand (for example, to 22 mmol), the calculator will add cyclic flow to supply the extra 4 mmol ATP (with the chosen “ATP per cyclic pair”).

Frequently Asked Questions

What is the difference between cyclic and non-cyclic electron flow in this calculator?

Non-cyclic (linear) flow is modeled as producing NADPH, ATP, and O2, while cyclic flow is modeled as producing ATP only. The tool uses these simplified differences to plan how much cyclic flow is needed when ATP demand exceeds what non-cyclic flow supplies.

What does an electron pair mean in the light-dependent reactions planner?

An electron pair represents 2 electrons moving through the photosystems. In the simplified model used here, one non-cyclic electron pair is treated as producing about one NADPH.

Why is O2 often shown as 0.5 per NADPH by default?

A common classroom bookkeeping link is 1 O2 produced for every 2 NADPH formed, which is 0.5 O2 per NADPH. The coefficient is editable because different teaching conventions may be used.

How does the calculator turn CO2 fixation into ATP and NADPH demand?

The Calvin helper applies preset bookkeeping (such as per G3P or per glucose) or uses editable per-CO2 coefficients. This converts a CO2 fixation amount into the ATP and NADPH inputs used by the demand mode.

How are photon estimates computed in this calculator?

When photon estimate is enabled, the tool multiplies the number of electrons implied by electron pairs by a photons-per-electron setting for non-cyclic and cyclic flow. This provides a simplified photon count for planning rather than a full biophysical model.

Light-Dependent Reactions — ATP, NADPH, and O2 planner

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