Where do plants get the energy to make ATP and use it to power photosynthesis?

Plants use solar energy to make ATP to fuel photosynthesis.
There are two parts to photosynthesis: the Light Dependent reactions (AKA Light Reactions) and the Light Independent reactions (AKA Dark reactions or the Calvin Cycle.)
In the light dependent reactions, solar energy is captured by photosystem II, a collection of proteins and pigments (chlorophyll is the pigment that you've probably heard the most about) in the thylakoid membrane of the chloroplast. The solar energy excites electrons in reaction center of the photosystem to a higher energy level. Those excited electrons are passed through proteins, giving up energy a little bit at a time. That energy is used to pump H+ from the inside of the thylakoid to the outside, creating a higher concentration of H+ in the chloroplast stroma than in the thylakoid space. The H+ will then diffuse though ATP synthase, an enzyme in the thylakoid membrane that makes ATP when H+ move through it. The ATP (and NADPH made by photosystem I) will then be used in the Calvin cycle to "fix" CO2 into organic molecules such as glucose.
I've attached a diagram of photosynthesis from OpenStax, Biology. OpenStax. May 20,2013.

The short answer is the sun. Of course, it's a bit more complex than that!


Photosynthesis is the process plants use to convert unusable energy from the sun into usable chemical energy in the form of organic molecules. The overall chemical reaction for photosynthesis can be summarized as:

6H2O + 6CO2 ----------> C6H12O6+ 6O2

However, photosynthesis is actually a series of chemical reactions. During photosynthesis energy from the sun splits water molecules into hydrogen and oxygen. The oxygen is released into the atmosphere. Since this part of photosynthesis requires light, these reactions are commonly called the light reactions.

Enzymes within plant cells then catalyze hydrogen and carbon dioxide to make a molecule called PGAL (phosphoglyceraldehyde). PGAL is then used to produce glucose, which the plant can use as food. This part of photosynthesis is commonly referred to as the dark reactions, simply to distinguish it from the reactions that require light.

The glucose created from photosynthesis can then be used in cellular respiration to produce ATP--the molecule used as "fuel" by all living things! While cellular respiration is actually a series of reactions, like photosynthesis, take a look at the overall reaction: C6H12O6+ 6O2 ----------> 6H2O + 6CO2 The overall reactions of the two processes are essentially the reverse of one another. Pretty neat!

Al living organisms use the process of respiration for generating ATP molecules (which serve as the energy currency in the living beings) and carrying out various operations.
Plants also use the same process. During respiration, plants consume the sugar molecules (generated during the process of photosynthesis) in the presence of oxygen, to generate energy molecules, carbon dioxide, and water. The chemical reaction that takes place during the process of respiration can be written as:
C_6H_12O_6 + 6O_2 -> 6CO_2 + 6H_2O + ATP
Respiration takes place continuously, unlike photosynthesis, which needs sunlight and hence can take place only during the daytime. In contrast, the respiration continues throughout the life of a plant.
Interestingly, photosynthesis and respiration consume the products of each other. The glucose molecules and oxygen produced from carbon dioxide and water during the photosynthesis process are the raw materials for the process of respiration.
Hope this helps.