SCIENCE · INVESTIGATE AND EXPLAIN

Audit an uphill transport cycle before claiming a mechanism

Moving a neutral ideal solute from lower to higher concentration requires a positive free-energy inputRTln(cdestination/csource) per mole. The logarithm uses a dimensionless concentration ratio. This supplied thermodynamic expression evaluates a state change; it does not specify the protein motions or rate of a transport cycle.

Learning outcome: Compare ratio11 and21 at drive20 and30kJ/mol cycles. Locate a case where the specified drive is insufficient. Explain why adding an ionic charge would require an additional electrical contribution rather than reuse of the same neutral ledger.

Starting recap

Connect calibrated dimensions, cell organization, material balance and membrane energetics. State the model boundary before interpreting a changed condition.

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Try

At298K and destination/source ratio11, one-solute cost isRTln11≈5.941kJ/mol. Three-solute cost≈17.823kJ/mol cycles. With30kJ/mol cycles of drive, combined change≈−12.177kJ/mol cycles: favorable within the ideal neutral-solute assumptions.

Open the experiment

Predict

Can a favorable energy ledger prove that a particular membrane protein couples the driving reaction to transport?

Your prediction and reason

Experiment

Compare ratio11 and21 at drive20 and30kJ/mol cycles. Locate a case where the specified drive is insufficient. Explain why adding an ionic charge would require an additional electrical contribution rather than reuse of the same neutral ledger.

Audit a neutral-solute uphill transport budget

Worked teaching model · synthetic values · Ideal neutral dilute solute at298K, three solutes transported uphill per cycle. Cost=3RTln(ratio), R0.008314kJ/(molK). Compare with supplied driving energy magnitude; membrane voltage, dissipation, kinetic barriers and leakage omitted. Favorable balance permits but does not prove transport.

Destination/source concentration ratio: 11 ratio Available driving free energy per cycle: 30 kJ/mol cycles

Reset keeps your prediction, captures and practice history.

The stated ideal coupled cycle is energetically favorable, but transport still requires a coupling mechanism.

ΔGcombined=3RTln(cdestination/csource)−available driving magnitude. Neutral solute deliberately excludes an electrical term. Ions require electrochemical accounting; negative free-energy change does not determine rate, specificity or leakage.

Calculated quantities under stated assumptions
Quantity	Value	Unit
Uphill cost per mole solute	5.940958	kJ/mol solute
Three-solute uphill cost per cycle	17.82287	kJ/mol cycles
Available driving energy	30	kJ/mol cycles
Combined free-energy change	-12.17713	kJ/mol cycles
Strictly favorable stated cycle	1	0 no / 1 yes

Retain prior predictions, settings and quantities in the notebook. These are original supplied/calculated comparisons, not physical specimen observations or laboratory participation.

The stated ideal coupled cycle is energetically favorable, but transport still requires a coupling mechanism.

Observation to attach to a capture

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Understand

1. Orient: Audit an uphill transport cycle before claiming a mechanism

2. Explain the mechanism and boundary

The model moves three solutes uphill per cycle and compares their total cost with a supplied driving free-energy magnitude. The combined change is3RTln(ratio) minus the driving magnitude. A negative value favors the stated coupled cycle thermodynamically, but only a mechanism that actually couples the processes can use the driving reaction to support transport.

Primary active transport couples directly to an energy-releasing process such as ATP hydrolysis. Secondary transport can use a pre-existing electrochemical gradient generated elsewhere. The gradient is an energy store, not free energy created by the carrier. For ions, an electrical term must be included; our neutral-solute calculation deliberately excludes membrane voltage.

Available driving energy depends on conditions, and real cycles also dissipate energy and can leak. At zero combined change there is no net thermodynamic preference, not a prediction of rapid pumping. Negative free energy does not prove selectivity, establish stoichiometry experimentally or determine flux. Keep energetic feasibility separate from mechanistic evidence.

3. Work a quantitative example

4. Predict and investigate

Predict first: Can a favorable energy ledger prove that a particular membrane protein couples the driving reaction to transport?

Retain your initial prediction, control settings and all relevant displayed evidence in the lesson notebook. The schematic and calculated records are original synthetic teaching material, not measurements of a physical specimen.

5. Transfer and explain

Apply the mechanism to a changed case

At298K, a cycle transfers two neutral solutes against a10-fold concentration ratio and has20kJ/mol cycles of available drive. UsingR0.008314kJ/(molK), estimate combinedΔG and interpret its scope.

Retain your calculation, units and assumptions in the lesson notebook before opening this public practice response.

Compare a worked response

Two-solute cost=2×0.008314×298×ln10≈11.41kJ/mol cycles, so combinedΔG≈−8.59kJ/mol cycles. The ideal coupled cycle is favorable, but coupling, rate, leakage and any electrical terms still need evidence.

6. Reflect and retain evidence

Explain the link from cellular structure or membrane process to the modeled observation. State a competing explanation, an omitted variable and one independent comparison that could challenge your conclusion. Do not turn a simulated observation into a claim of laboratory participation or independently demonstrated practical skill.

Scientific sources and continuation

Supporting scientific reference, checked30October2026. Original prose, controls, datasets and diagrams; no source figure or exercise is reproduced. Return to General Biology I for the course-specific mixed responses, retained project evidence and remaining module status.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

At298K, a cycle transfers two neutral solutes against a10-fold concentration ratio and has20kJ/mol cycles of available drive. UsingR0.008314kJ/(molK), estimate combinedΔG and interpret its scope.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

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Previous: Test a finite-capacity carrier without equating saturation with active transport Next: Interpret organelle enrichment using independent marker controls

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