What is Photosynthesis?

Photosynthesis is the process by which plants convert light energy into carbohydrates they use as building blocks and energy stores. Light-absorbing pigments in plants do the heavy lifting, collecting light energy and passing it along to chemical processes inside the leaves that convert light, water, and carbon dioxide (CO2) into carbohydrates. The overall reaction looks like this:

Water (H2O) + Light (hv) + Carbon Dioxide (CO2) = Carbohydrates + Oxygen (O2)

Photosynthesis can be broken down into two sub-processes: light-dependent reactions and light-independent reactions.

Light-Dependent Reactions: These reactions occur in the thylakoid membranes of chloroplasts and require sunlight. Here, light energy is absorbed by chlorophyll and other pigments, which triggers the splitting of water molecules into oxygen and hydrogen. The energy released during this process is used to create ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are energy-rich molecules necessary for the next stage.

Light-Independent Reactions (Calvin Cycle): In this stage, ATP and NADPH produced in the light-dependent reactions are utilized to convert carbon dioxide into glucose. This cycle takes place in the stroma of chloroplasts and involves a series of enzyme-mediated reactions. The resulting glucose can be used by the plant for energy, growth, and various metabolic processes.

It might be simpler to consider the light-dependent reactions “water-side” reactions because they use light energy to break up a water molecule (H2O). The energy produced from these reactions is captured in two compounds that fuel the rest of the photosynthetic process: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). Below is the simplified light-dependent/water-side reaction:

Water (H20) + Light (hv) = ATP + NADPH + Oxygen (O2)

Light-independent reactions are more commonly called “dark reactions,” though they don’t only occur at night as their name might imply. They just don’t require light, so it’s better to call them light-independent reactions.

In these reactions, CO2 is converted into carbohydrates with the aid of the energy-building blocks produced by the light-dependent reactions, ATP and NADPH. Because these reactions consume CO2, they can be considered “carbon fixation” reactions. Simplified, the light-independent/carbon fixation reaction looks like this:

ATP + NADPH + Carbon Dioxide (CO2) = Carbohydrates + Oxygen (O2)