What Happens To The Sun’S Energy After Photosynthesis?

What happens to the sun's energy after photosynthesis?

Photosynthesis is the process plants use to convert sunlight into chemical energy they can use for growth. During photosynthesis, plants use chlorophyll and other pigments to absorb the sun’s light energy, which is then converted into chemical bonds within glucose molecules. The glucose is stored as chemical energy that the plant can later use for cellular processes. Overall, photosynthesis allows plants to utilize the sun’s readily available light energy and convert it into a form of chemical energy they require.

Absorption by Chlorophyll

Photosynthesis begins when chlorophyll and other pigments absorb light energy. Chlorophyll is a green pigment found in the chloroplasts of plant cells and absorbs visible light primarily from the violet-blue and reddish orange wavelengths (https://www.pc.maricopa.edu/Biology/ppepe/BIO145%20Canvas/labs/lab03_4.html). The absorption spectrum peaks of chlorophyll a and b lie mainly in the blue and red regions, which overlaps with the peak emission wavelengths of the sun. This allows the plant to absorb the maximum solar energy available for photosynthesis.

The chemical structure of chlorophyll absorbs specific wavelengths of visible light most efficiently. Absorption peaks in the red area around 680 nm for chlorophyll a, and in the blue and red areas at 450-475 nm and 630-650 nm for chlorophyll b. This allows the plant cell to capture and utilize the energy from sunlight in the photosynthetic process (https://www.reddit.com/r/askscience/comments/slfhw3/is_it_a_coincidence_that_chlorophyll_absorption/). Chlorophyll gives plants their green color because it reflects green wavelengths while absorbing blue and red wavelengths.

Conversion to Chemical Energy

The light energy absorbed by chlorophyll is used to convert ADP into ATP and NADP+ into NADPH through a series of reactions in the light-dependent stage of photosynthesis. ATP and NADPH will provide the chemical energy that is stored and used in the next stage of photosynthesis.

Specifially, the light energy powers photolysis, which splits water molecules into hydrogen ions, electrons, and oxygen. The electrons then move through an electron transport chain, similar to cellular respiration. As the electrons move through the transport chain, their energy is used to pump hydrogen ions into the thylakoid space, creating a proton gradient. This proton gradient powers ATP synthase, which produces ATP from ADP. The electron transport chain also reduces NADP+ to NADPH by donating electrons.

In this way, light energy from the sun is converted into chemical energy stored in two main molecules – ATP and NADPH (Britannica Kids, 2022). This chemical energy will then fuel the reactions in the next stage of photosynthesis.


Britannica Kids. (2022). Photosynthesis. Retrieved from https://kids.britannica.com/students/article/photosynthesis/276411

Carbon Fixation

In the carbon fixation stage, carbon dioxide from the atmosphere is “fixed” into organic molecules through a series of reactions. The most important carbon fixation step is the enzyme Rubisco attaching CO2 to a 5-carbon sugar called ribulose bisphosphate (RuBP) to form an unstable 6-carbon intermediate which quickly splits into two 3-carbon molecules called 3-phosphoglyceric acid (3-PGA) (Source). This initial carbon fixation by Rubisco accounts for the vast majority of carbon incorporated, essentially capturing atmospheric carbon dioxide and converting it into organic compounds the cell can use.

Glucose Synthesis

After carbon dioxide is fixed into organic molecules during carbon fixation, the resulting 3-phosphoglycerate is used to synthesize sugars like glucose. Glucose is a simple sugar and an important product of photosynthesis. The Calvin cycle, also known as the dark reactions of photosynthesis, converts the carbon in 3-phosphoglycerate into the sugar glucose in a series of reactions. Specifically, 3-phosphoglycerate is phosphorylated and then reduced multiple times to eventually produce glucose. Sugars like glucose store chemical energy from light in their molecular bonds. Glucose is important because it can be used by the plant for energy and to build other organic molecules like cellulose and proteins. The sugar glucose is a major product of photosynthesis and an energy source for plants and other autotrophs.

Source: https://www.visiblebody.com/learn/biology/photosynthesis/reactants-products

Storage and Transport

After glucose is produced during photosynthesis, it needs to be transported around the plant to provide energy and be stored for later use. Glucose is soluble in water, so it can be easily transported in the plant’s vascular system. The main transport mechanisms for glucose are:

Phloem Transport: Glucose is loaded into sieve elements in the phloem and transported to sink tissues like roots, fruits, and storage organs. Phloem transport is driven by osmotic pressure gradients created by active sugar loading and unloading (Selvam, 2019). This allows glucose to be distributed throughout the plant.

Transmembrane Transport: Glucose is moved into cells and organelles by transport proteins that span cell membranes. The GLUT/SWEET transporter families mediate facilitated diffusion of glucose across membranes (Ma et al., 2017). Hexose transporters like STP1 are also involved in glucose uptake from the apoplast (Rottmann et al., 2018).

Vacuolar Storage: Glucose is stored in vacuoles as insoluble starch. Starch is a polymeric form of glucose that allows plants to stockpile large amounts of glucose in a compact form. Starch can be broken down to retrieve glucose as needed.

Efficient glucose storage and transport allows plants to distribute this energy source to growing tissues and save excess glucose made during the day for use at night when photosynthesis shuts down. Proper allocation of glucose resources via transport is critical for plant growth and development.

Cellular Respiration

During cellular respiration, glucose is broken down through a series of reactions to produce energy in the form of ATP. This multi-step process begins with glycolysis, where glucose is split into two three-carbon molecules called pyruvate [1]. The pyruvate then enters the mitochondria to be further broken down via the citric acid cycle and oxidative phosphorylation. In the citric acid cycle, pyruvate is converted into acetyl CoA which is fed into a series of reactions that generate NADH and FADH2. These electron carriers then transport electrons to the electron transport chain, which uses the electrons to pump hydrogen ions across the mitochondrial membrane. This creates an electrochemical gradient which drives ATP synthase to produce ATP. Overall, cellular respiration extracts the electrons from glucose and uses their energy to generate ATP, which can be used as energy for cellular processes.

Plant Growth

Glucose produced during photosynthesis is used by plants for energy to support growth and build structure. Plants use glucose and other sugars to synthesize cellulose, the main structural component of plant cell walls and stems (Siddiqui, 2020). Cellulose provides strength and rigidity to plant cells and tissues. The production of cellulose is directly dependent on the availability of glucose synthesized during photosynthesis.

Glucose also supports the production of other structural carbohydrates like starch, which is used for energy storage. Starch can be broken down as needed to provide energy for plant growth and metabolic activities. Having adequate glucose reserves in the form of starch allows plants to continue growing at night or during periods of low light when photosynthesis slows down (Ohto et al., 2001).

In addition to structural roles, glucose is used by plants to produce secondary metabolites that support growth. For example, glucose is a precursor for lignin biosynthesis, which provides mechanical support in plant vasculature. The availability of glucose also affects the levels of plant hormones like ethylene, cytokinin, and abscisic acid that regulate various aspects of plant growth and development (Jeandet et al., 2022).

Energy Transfer

As primary producers, plants absorb the sun’s energy during photosynthesis and convert it into chemical energy stored within organic compounds like glucose. This chemical energy is then transferred up the food chain as herbivores consume plants and carnivores consume herbivores. At each link in the food chain, some energy is used for the organism’s life processes and is lost as heat. Only about 10% of the energy at one trophic level is passed on to organisms at the next trophic level. So as you go up the food chain, the available energy decreases significantly. The efficiency of energy transfer places limits on the length of food chains observed in nature, as there needs to be enough energy remaining to support the higher trophic levels. Ultimately, all of the energy originated from the sun and photosynthetic organisms serve as the doorway through which it enters most ecosystems.

Source: https://education.nationalgeographic.org/resource/energy-transfer-ecosystems/


The journey of the sun’s energy through the ecosystem via photosynthesis is quite remarkable. It starts with the sun, which radiates an enormous amount of energy in the form of electromagnetic radiation. Plants on Earth absorb a portion of this solar energy using light-absorbing pigments like chlorophyll in their leaves. The captured light energy is converted into chemical energy and stored in the bonds of glucose molecules synthesized during the Calvin cycle. The glucose provides energy for plants to grow and maintain cellular processes. Plants can store excess glucose or transport it to other parts of the plant to fuel growth and development. Glucose that is not used immediately can be converted to starch or other compounds for longer-term storage. Through cellular respiration, glucose breaks down and releases energy that allows plants to synthesize ATP, powering biochemical reactions in the plant. Overall, the sun’s energy is harvested by plants through photosynthesis, converted to chemical energy, and passed on through ecosystems as a vital energy source for life.

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