What Is Photosynthesis?

Plants, like every other organism, need energy to live, grow and repair. Unlike heterotrophs – animals that consume food to synthesize energy – plants or autotrophs are self-reliant — they can make their own food, and therefore energy, by using the resources available in their surroundings. The resources include sunlight, water and carbon dioxide, and this incredible process is called photosynthesis.

mimosa pudica

The ever-famous Touch-Me-Not. (Image Source: Wikimedia)

The process is known as photosynthesis because, while water and carbon dioxide are the major ingredients required to cook the food, it is light that ignites the stove, and sunlight is the most abundant light that illuminates the planet.

The Photosystem

A plant is essentially a highly efficient solar panel. It is replete with molecular structures that have evolved to soak up and absorb as much sunlight as possible. The cellular structure, called the chloroplast, consists of a dense grid of chlorophyll molecules, pigments that absorb light of certain wavelengths and reflect the rest.

This is how pigments or colors are radiated. Plants (most of them) are green because they reflect wavelengths that we associate with green while absorbing the other wavelengths. However, because the energy of light is inversely proportional to its wavelength, from their color, we can infer that plants absorb a lot of energy. Why don’t plants absorb low-energy red wavelengths and instead reflect orange or yellow light? We believe that underwater plants that evolved this trait were at a greater advantage than plants who didn’t, as the high-energy light of smaller wavelengths could penetrate the water to greater depths to reach and rejuvenate them.

Chloroplast Photosystem

Chlorophyll molecules embedded in the thylakoid membrane. (Photo Credit : Ollin / Wikimedia Commons)

Solar energy is used by the chloroplast to trigger a chemical reaction between the two reactants – water and carbon dioxide.

The Reaction

A plant obtains water from its roots. The roots are embedded in a patch of soil that is wringing with water, which they acquire through osmosis. From there, the water ascends through the stem and is transported to every part of the plant that requires it for a myriad of processes other than photosynthesis. To ensure that the systems don’t dry out, plants also evolved to incorporate pores, which allowed them to respire. However, the crucial development of pores came at the expense of losing water unnecessarily.

The pores on leaves are called stomata, and they enable a plant to exchange gases with their surroundings. The stomata inhale carbon dioxide exhaled by animals, which reacts with water in the presence of sunlight to create sugar (glucose), their food. However, the reaction unfolds in two parts.

Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant with stomata.

Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant (Photo Credit : At09kg / Wikimedia Commons)

The first part is called a light-dependent or simply, a light reaction, in which light breaks down water to produce oxygen molecules. These molecules are the same oxygen molecules that we breathe. They are exhaled through the stomata and dispersed into the air. The light energy absorbed by a pigment can be either simply dissipated as heat or be converted into another form of energy. We witness the latter in plants. The light reaction converts solar energy into chemical energy; the reaction also produces ATP (Adenosine Tri-Phosphate) and NADP+ (Nicotinamide Adenine Dinucleotide Phosphate), organic compounds that become sources of energy for subsequent metabolic processes.

One of these processes is the next part of the reaction itself. The two sources of energy fuel the light-independent or dark reaction. The energy breaks down the carbon dioxide molecules and reorganizes the constituents to form a molecule of glucose. The chloroplast then harvests energy by breaking down that glucose, just how mitochondria in animal cells produce energy by breaking down the food they consume. Photosynthesis as a combination of the two reactions can be summarized with this expression:

Energy harvest synthesis formulas

In this manner, we share a deep and indispensable symbiotic relationship with plants. The byproduct, or put more impolitely, the waste product exhaled by plants gives us life, whereas carbon dioxide, the waste product that we exhale, gives plants their life. American biologist and one of my favorite science communicators, Lynn Margulis, called this innocuous act of breathing, spirituality.

However, she believes that “the connection doesn’t stop at the exchange of gases in the atmosphere… The fact that we are connected through space and time shows that life is a unitary phenomenon, no matter how we express the fact.”


  1. Estrella Mountain Community College
  2. Smithsonian Institution
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About the Author:

Akash Peshin is an Electronic Engineer from the University of Mumbai, India and a science writer at ScienceABC. Enamored with science ever since discovering a picture book about Saturn at the age of 7, he believes that what fundamentally fuels this passion is his curiosity and appetite for wonder.

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