Is Photosynthesis Endothermic or Exothermic? (And Why?)

Photosynthesis is an endothermic process. It requires an input of energy, usually in the form of sunlight, to convert carbon dioxide and water into glucose and oxygen. ^{1 2} This energy is absorbed by chlorophyll in plant cells and used to drive the chemical reactions involved in photosynthesis. ³

Well, this was just a simple answer. But there are few more things to know about this topic which will make your concept super clear.

So let’s dive right into it.

Why is photosynthesis an endothermic process?

Photosynthesis is an endothermic process because it requires an input of energy in the form of sunlight to drive the chemical reactions. The energy from sunlight is absorbed by chlorophyll and other pigments in plant cells, and this absorbed energy is used to convert carbon dioxide and water into glucose and oxygen.

In photosynthesis, plants utilize light energy to convert carbon dioxide and water into glucose and oxygen. This process occurs in specialized organelles called chloroplasts, specifically in the thylakoid membrane where chlorophyll pigments are located. ⁴

The absorbed light energy excites the electrons in chlorophyll, initiating a series of chemical reactions known as the light-dependent reactions. ⁵ These reactions produce energy-rich molecules such as ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which serve as “currency” for the subsequent light-independent reactions. ⁶

During the light-independent reactions, also known as the Calvin cycle, ATP and NADPH are used to convert carbon dioxide into glucose. ⁷ This process requires energy input to form the chemical bonds of glucose and other organic molecules. 

Since the overall process of photosynthesis requires an input of energy, it is considered endothermic. The energy absorbed from sunlight provides the necessary activation energy to drive the synthesis of glucose and other essential organic compounds for the plant’s growth and survival.

Overall, photosynthesis is an endothermic process because it relies on the absorption of light energy to initiate the chemical reactions that convert carbon dioxide and water into glucose and oxygen. The input of energy from sunlight is essential for the synthesis of glucose, which serves as the primary source of energy for both plants and many other organisms in the ecosystem.

Why is photosynthesis not an exothermic process?

Photosynthesis is not an exothermic process because it does not release energy in the form of heat. While photosynthesis involves the production of energy-rich molecules like ATP and NADPH, the overall process requires an input of energy in the form of light, making it endothermic.

Photosynthesis is a complex metabolic process that occurs in the chloroplasts of plants. It involves two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). ^{8 9}

In the light-dependent reactions, light energy is absorbed by pigments, such as chlorophyll, and is used to generate ATP and NADPH. These energy carriers are then utilized in the light-independent reactions to convert carbon dioxide into glucose.

During the light-independent reactions, carbon dioxide molecules are fixed and undergo a series of chemical reactions that result in the formation of glucose.

This process requires the input of energy, specifically in the form of ATP and NADPH, which were produced in the light-dependent reactions. The energy stored in these molecules is utilized to synthesize glucose and other organic compounds.

Although the overall process of photosynthesis involves energy transformations, it does not produce a net release of energy in the form of heat. Instead, photosynthesis is considered an endothermic process since it requires an input of energy (in the form of light) to drive the chemical reactions and build energy-rich molecules like glucose.

Further reading

Is Combustion Endothermic or Exothermic?
Is Melting Ice Endothermic or Exothermic?
Is Sublimation Endothermic or Exothermic?
Is Photosynthesis a Chemical Change?
Is Sublimation a Physical or Chemical Change?

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2. What is Photosynthesis. (2017, April 12). Smithsonian Science Education Center. https://ssec.si.edu/stemvisions-blog/what-photosynthesis
3. The Cell, Respiration and Photosynthesis. (n.d.). The Cell, Respiration and Photosynthesis. https://globalchange.umich.edu/globalchange1/current/lectures/kling/energyflow/PSN_primer.html
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