Uphill reactions are those that require an input of energy to proceed. In thermodynamic terms, any reaction with a positive change in Gibbs free energy (ΔG greater than zero) is considered uphill. The products end up with more stored energy than the reactants started with, which means the reaction will not happen on its own without an outside push.
What Makes a Reaction “Uphill”
The uphill and downhill labels come from an energy landscape analogy. Picture a ball sitting at the bottom of a hill. To move it to a higher point, you have to push it. That push is the energy input an uphill reaction demands. The technical name for these reactions is endergonic, meaning energy enters the system. The key marker is a positive ΔG value: the products contain more free energy than the reactants, so the difference has to come from somewhere.
Downhill reactions are the opposite. They have a negative ΔG, release free energy, and proceed spontaneously. When a reaction both releases heat and increases disorder (entropy), it will always be spontaneous regardless of temperature. Conversely, a reaction that absorbs heat and decreases entropy will always be uphill, no matter how hot or cold the environment gets. Some reactions fall in between, where temperature tips the balance one way or the other.
One critical point: “uphill” does not mean “impossible.” It means the reaction needs energy delivered to it. Cells, industrial systems, and even sunlight routinely supply that energy. The reaction just won’t happen by itself.
Photosynthesis: The Classic Uphill Reaction
The most familiar uphill reaction in biology is photosynthesis. Plants take carbon dioxide and water and convert them into glucose and oxygen. The overall ΔG for this process is +2,870 kJ/mol, making it enormously uphill. Sunlight provides the energy to drive it.
The process works in two stages. In the light reactions, photons strike chlorophyll molecules at the core of two protein complexes called Photosystem II and Photosystem I. When a photon hits the chlorophyll pair in Photosystem II (which absorbs light near 680 nm), it loosens an electron, creating a high-energy, electron-hungry molecule. That molecule is so reactive it can rip electrons from water, splitting water molecules and releasing oxygen gas. The electrons travel through a chain of carriers, and a second round of photon absorption in Photosystem I (near 700 nm) boosts their energy again, ultimately producing hydrogen carriers and ATP. This light-driven stage alone accounts for about +2,640 kJ/mol of uphill energy storage.
In the second stage, the light-independent reactions, those hydrogen carriers and ATP molecules power the conversion of carbon dioxide into sugar. The energy stored during the light reactions is spent to push carbon atoms into the high-energy bonds of glucose. The entire process is a textbook example of using one energy source (sunlight) to force an uphill reaction that would never occur spontaneously.
How Cells Power Uphill Reactions With ATP
Your cells run uphill reactions constantly: building proteins from amino acids, copying DNA, pumping ions across membranes. None of these happen spontaneously. The trick cells use is coupling, linking an uphill reaction to a downhill one so the total energy change is negative.
The most common energy currency for this coupling is ATP. When ATP breaks apart into ADP and a phosphate group, it releases energy (a downhill reaction). Building ATP from ADP and phosphate is itself uphill, requiring about +7 kcal/mol of energy. Cells invest energy from food to create ATP, then spend that ATP to drive other uphill processes. By pairing ATP breakdown with an otherwise unfavorable reaction, the combined process becomes downhill overall, and the uphill reaction proceeds.
Enzymes coordinate this coupling, but they have a strict limitation. An enzyme speeds up a reaction by lowering the activation energy, the initial energy barrier that must be overcome for any reaction to start. However, an enzyme lowers that barrier equally in both directions, forward and reverse. It does not change the ΔG of the reaction at all. An enzyme cannot, by itself, make an uphill reaction go. It can only make a coupled reaction happen faster. As one molecular biology textbook puts it, enzymes “cannot make water run uphill.”
Uphill Reactions in Industry
Water electrolysis is a straightforward industrial example. Splitting water into hydrogen and oxygen gas is uphill, with a ΔG of +237.2 kJ/mol. This translates to a minimum voltage of 1.23 V that must be applied across the electrodes at 25°C before the reaction can even begin. Below that voltage, water simply will not split. In practice, real electrolysis cells operate above 1.48 V (the thermoneutral voltage) to account for heat losses and inefficiencies. Every hydrogen fuel cell or green hydrogen facility is essentially forcing this uphill reaction with electrical energy.
Synthetic chemistry also deals with uphill reactions. Researchers have demonstrated that certain chemical fuels can ratchet reactions uphill by small increments. In one example published in Nature Synthesis, a chemical coupling agent pushed a reaction uphill by an additional +0.63 kJ/mol beyond its natural equilibrium, storing that energy in the product. These are small numbers compared to photosynthesis, but they illustrate the same principle: energy in, higher-energy products out.
Quick Way to Identify Uphill Reactions
If you’re looking at a reaction and trying to classify it, here’s what to check:
- Positive ΔG: The defining feature. If ΔG is greater than zero, the reaction is uphill (endergonic) and non-spontaneous.
- Products have more energy than reactants: Energy is stored in the products rather than released.
- Requires energy input: The reaction needs light, heat, electrical energy, or coupling to another reaction to proceed.
- Absorbs heat or decreases entropy (or both): Reactions that take in heat make ΔG more positive. Reactions that decrease disorder in the system also push ΔG positive.
Common examples you’ll encounter in textbooks include photosynthesis, protein synthesis, DNA replication, ATP formation from ADP, and water electrolysis. Any process that builds complex, energy-rich molecules from simpler, lower-energy ones is running uphill. The energy has to come from somewhere, whether that’s sunlight, a battery, or the breakdown of another molecule.