Mechanisms of Photosynthesis

June 27, 2026

Let’s talk about something that should be a fairly frequent topic going forward: photosynthesis. As most of us know, photosynthesis is the process by which plants turn sunlight into usable energy. Photosynthesis is actually a quantum mechanical process, leading it to be an incredibly effecient energy transformation phenomenon, but that isn’t what we’ll be talking about today. Today, we’ll be discussing the basic mechanisms of photosynthesis.

Chemical Equations

Photosynthesis is the process through which carbon dioxide ( $\ce{CO_2}$ ) and water ( $\ce{H_2O}$ ) into carbohydrates and oxygen ( $\ce{O_2}$ ). It can be split into dark and light reactions.

Light Reaction

The light reaction is the reaction that involves light in itsexecution. Light is used to split water into oxygen, protons, and electrons. The chemical equation for the light reaction is

\ce{2H_2O +light->O_2 +4H^+ + 4e^- (\Delta G^\degree=+317 kJ\cdot mol^{-1})}

Dark Reaction

The dark reaction is the reaction that does not need light to happen. In this reaction, protons and electrons are used to reduce carbon dioxide into a carbohydrate (a generic carbohydrate formula, $\ce{CH_2O}$ is used here). The chemical equation for the dark reaction is

\ce{CO_2 +4H^+ +4e^–>CH_2O + H_2O (\Delta G^\degree=+162 kJ\cdot mol^{-1})}

Overall Reaction

When combined, these reactions make the whole photosynthetic reaction, characterized by the equation

\ce{H_2O +light + CO_2->CH_2O + O_2 (\Delta G^\degree=+479 kJ\cdot mol^{-1})}

Standard Gibb’s Free Energy

These equations may look a bit different than the equations you remember from an introductory chemistry class, specifically that bit on the end. The symbol $\ce{\Delta G^\degree}$ is what is called the standard Gibb’s free energy, or the standard free energy change of the reaction. In these equations, it is positive, meaning that the reactions require energy to be performed. The way we get this is by taking the standard Gibb’s free energy of formation, wich is essentially how much energy something takes to be made, of the products and subtracting it from that of the reactants. This concept is really important, especially in biochemistry, but is a fairly complex topic that I will go deeper into in a different post. For now, just think of it as the energy needed to do a certain reaction, or the energy releasing in a reaction.

Now, if you remember some intro science classes, you may be a bit confused as to why the Gibb’s free energy is negative. Photosynthesis is supposed to produce energy for the plant, not use it up, right? Well, it does, just not directly. The energy that is given to the plant is in the form of the carbohydrate that gets made. Corbohydrates are the primary energy storage molecules in most living things. What happens is that the plant produces the carbohydrate, sends it to wherever it needs to go, then that carbohydrate gets broken down. When the carbohydrate gets broken down, the high amounts of energy that are stored in the chemical bonds are used for whatever the plant needs. That reaction would have a positive Gibb’s free energy associated with it.

How Does It Work On a Molecular Level

The primary location where photosynthesis occurs is in the leaves, at least for terrestrial plants. Technically, it can happen in other parts, but the leaves were engineered by nature to do this. Going a bit smaller, the actual reactions happen in the chloroplasts of the plant cells. The light reaction happens in the thylakoid, a part of the chloroplast that is thought to be one large membrane that goes throughout the center. The dark reaction happens in the stroma, which is what the thylakoid is in.

Now, embedded in the thylakoid, there are various pigments. The most well known one is chlorophyll, but there are others, like carotenoids and bilins. These pigments are structured in such a way that they absorb various wavelengths of light. I promised I would skip the quantum talk this time, but this is where it would go. There are what are called light-harvesting complexes (LHCs), which are where all of the pigments are located. Most of these are used as antennae that gather the light, and transfer the energy to reaction centers. In these reaction centers, the energy is able to be transfered from a pigment to a different type of molecule, allowing it to be used elsewhere.

In the dark reaction, the hidden products of the light reaction, being ATP and NADPH, are consumed and formed back into ADP, P_i and NADP⁺. There is a lot of work done during this reaction to undo some things that are done. There are many chemicals formed that are then used to remake something that was used as a different part of the reaction. I won’t go into too much detail here, as it can be better explained by other people, but I may revisit it in the future. I also promised to go over the basic mechanisms, and going in depth here would be throwing a lot of complex words in here that I cannot fully explain properly.

The actual conversion of carbon dioxide into a carbohydrate is fairly simple. It is all done by a single enzyme called Rubisco. It does, however, take a lot of carbon to do this, and so plants need to have various mechanisms to concentrate high amounts of carbon.