Is Photosynthesis Endergonic or Exergonic?

All plants use photosynthesis to make their own food. Photosynthesis is an endergonic process, but what does this mean? In endergonic reactions, energy is absorbed and creates more energy. On the other hand, exergonic reactions release energy instead of consuming it. Plants use the energy from light and other sources to synthesize it into glucose, the sugar they need to survive.

Sunlight, soil, water and fertilizer do not actually "feed" plants; instead, the plants use all that plus carbon dioxide in the air to make glucose. This process is called photosynthesis. Endergonic reactions store outside energy, and photosynthesis falls right into this category. While some might think that photosynthesis is an exergonic reaction, it is not.

Endergonic reactions create new chemical bonds (anabolic reactions), which store that energy until the bonds are eventually broken. Reactions that break bonds to instead release energy are catabolic reactions. Photosynthesis is at the bottom of our energy pyramid and provides what humans and other higher organisms need to survive. We get our energy from breaking down fats and sugars, which is how that stored energy becomes available.

The chemical reaction for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O_2. The reactants are six carbon dioxide molecules and six water molecules. Through the process of photosynthesis and with the addition of light energy via chlorophyll, these reactants are converted to their products, a sugar molecule and six oxygen molecules.

Exergonic reactions release free energy instead of consuming it. These reactions can occur on their own without outside factors being involved. In exergonic reactions, the free energy released is negative, but in endergonic reactions, positive changes in free energy mean that it has been stored.

One example of exergonic reactions could be when sodium and chlorine are mixed together, which creates table salt. This of course requires an outside factor, like a human mixing the two together. Another is the digestion of food. Metabolism breaks down the complex food molecules into simpler ones, releasing the energy. Cellular respiration also qualifies as exergonic because glucose breaks down to release usable energy for cells.

Photosynthesis is endergonic, but how does it actually work? The majority of life on Earth depends on photosynthesis in one way or another. Essentially, photosynthesis is a process that captures energy from sunlight and uses it to produce oxygen and chemical energy that is stored in glucose.

The plants also absorb water and carbon dioxide (CO2) from the soil and air. Then, the water loses its electrons (it becomes oxidized) within plant cells. The CO2 gains electrons, which transform the water into oxygen; it also turns the carbon dioxide into glucose. Then, the plant returns oxygen into the air and stores that energy inside glucose molecules.

Plant cells also contain chloroplasts that store sunlight energy, and inside the chloroplasts are light-absorbing pigments better known as chlorophyll. This absorbs energy and gives plants their green color. In photosynthesis, light-dependent reactions involve chlorophyll absorbing energy from light waves and converting them into chemical energy. The light-independent stage takes place deep inside plants when energy is converted into glucose.