What this calculator is estimating
Cellular respiration converts chemical energy in glucose into ATP, primarily by transferring electrons
to carriers (NADH, FADH2) and then using the electron transport chain to drive ATP synthase.
This calculator provides an ATP yield estimate and a simplified stoichiometry scale-up for any amount of glucose.
Main takeaway: In aerobic respiration, most ATP comes from oxidative phosphorylation (ATP synthase driven by a proton gradient),
not from the “direct ATP” steps.
Overall reaction (aerobic)
A simplified overall equation for complete oxidation of one glucose is:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
This is a “big-picture” summary. Real biochemistry includes intermediates, proton balances, and cellular compartment effects.
For calculator purposes, it is a useful stoichiometric scaling rule.
Stage-by-stage bookkeeping per 1 glucose
The estimator splits ATP yield into substrate-level phosphorylation (direct ATP/GTP) and
oxidative phosphorylation (ATP produced from NADH/FADH2 via P/O ratios).
Totals per glucose (aerobic): direct ATP = 4 (2 from glycolysis + 2 from Krebs), and carriers = 10 NADH + 2 FADH2.
How carrier counts become ATP: P/O ratios
The calculator uses adjustable “P/O ratios” to convert carriers to ATP:
\[
\text{ATP}_{\text{oxphos}} =
(\#\text{NADH})\cdot(\text{P/O}_{\text{NADH}})
+
(\#\text{FADH}_2)\cdot(\text{P/O}_{\text{FADH}_2})
\]
Common teaching defaults:
- Modern estimate: P/O(NADH) ≈ 2.5, P/O(FADH2) ≈ 1.5
- Classic estimate: P/O(NADH) ≈ 3.0, P/O(FADH2) ≈ 2.0
These are estimates. ATP yield depends on proton leak, membrane transport costs, and which shuttle transfers glycolysis NADH into mitochondria.
Shuttle choice: “higher yield” vs “lower yield”
Glycolysis produces 2 NADH in the cytosol. To contribute to oxidative phosphorylation, their electrons must enter mitochondria via a shuttle.
This calculator uses a simple dropdown:
-
Higher yield: glycolysis NADH is counted like mitochondrial NADH (uses P/O(NADH)).
This resembles the malate–aspartate shuttle.
-
Lower yield: glycolysis NADH is counted like FADH2 (uses P/O(FADH2)).
This resembles the glycerol phosphate shuttle.
Only the 2 NADH from glycolysis are affected by the shuttle selection. The NADH and FADH2 produced inside mitochondria are unchanged.
Total ATP per glucose (example with modern estimates)
Using P/O(NADH)=2.5 and P/O(FADH2)=1.5 with the higher yield shuttle:
\[
\begin{aligned}
\text{Direct ATP} &= 4 \\
\text{ATP from NADH} &= (10)\cdot 2.5 = 25 \\
\text{ATP from FADH}_2 &= (2)\cdot 1.5 = 3 \\
\text{Total ATP per glucose} &= 4 + 25 + 3 = 32
\end{aligned}
\]
If the lower yield shuttle is chosen, the 2 glycolysis NADH are treated like FADH2, typically reducing the total.
Stoichiometry scaling with glucose amount
If the amount of glucose is scaled by \(n_{\text{glucose}}\) (in mol), the simplified aerobic stoichiometry scales linearly:
\[
n_{CO_2}=6\,n_{\text{glucose}},\qquad
n_{O_2}=6\,n_{\text{glucose}},\qquad
n_{H_2O}=6\,n_{\text{glucose}}
\]
If you enter glucose in grams, the calculator first converts to mol using the molar mass of glucose,
then applies the scaling above.
How to interpret the graphs
-
Stacked ATP bar (by stage): compares contributions from glycolysis, pyruvate oxidation, Krebs cycle, and oxidative phosphorylation.
Hover to see exact values; zoom changes the vertical scale for readability.
-
Sankey-style flow: summarizes how glucose energy becomes carrier energy (NADH/FADH2) and then becomes ATP.
The graphs are designed to emphasize the key learning outcome: oxidative phosphorylation usually dominates total ATP yield in aerobic respiration.
Common pitfalls
- ATP yields are estimates: P/O ratios are not universal constants.
- Anaerobic toggle: when aerobic is off, the calculator uses a simplified “glycolysis-only” case (2 ATP per glucose).
- Stoichiometry is simplified: the overall equation is a useful scaling tool, not a step-by-step mechanistic pathway.
