In What Does The Conversion Of Solar Energy Into Chemical Energy Occur?

Photosynthesis is the process plants and some bacteria use to convert sunlight into chemical energy in the form of carbohydrate molecules like glucose. It’s arguably the most important biochemical process on Earth, as nearly all life depends on it either directly or indirectly. Photosynthesis by plants and algae provides the energy that supports virtually all food chains and ecosystems. It’s also responsible for producing the oxygen we breathe.


Chloroplasts are organelles found in plant cells and eukaryotic algae that conduct photosynthesis. They are considered the “kitchens” of plant cells where the conversion of light energy from the sun into chemical energy takes place. Chloroplasts are usually about 2-10 micrometers long and 1-2 micrometers thick.

The chloroplasts of higher plants are lens-shaped, while the chloroplasts of green algae are more cup-shaped. Chloroplasts are only found in green tissues since chlorophyll, which gives plants their green color, is found in the chloroplasts. They are commonly concentrated in the cells located in the leaf mesophyll, though they can also be found in stems and other green tissues. Chloroplasts absorb light energy which is used to convert carbon dioxide into glucose through photosynthesis. The number of chloroplasts in a cell can range from one in some algae to 100 in plant cells that contain high concentrations of chloroplasts.


Thylakoids are flattened, disc-shaped sacs inside chloroplasts that contain the components necessary for photosynthesis. They are arranged in stacks called grana that are connected by stroma lamellae. The thylakoid membrane houses chlorophyll pigments as well as integral and peripheral membrane protein complexes that carry out the light-dependent reactions of photosynthesis. This internal membrane system provides a large surface area for light capture and the sequence of reactions that convert light energy into chemical energy.

Light-Dependent Reactions

The light reactions are the first stage of photosynthesis, where sunlight is captured and converted into chemical energy in the form of ATP and NADPH. This process takes place in the thylakoid membranes within chloroplasts. When light strikes chlorophyll, a molecule in the photosynthetic membranes, its energy is absorbed. This excitation of the chlorophyll molecules enables a series of electron transfers, resulting in the separation of charges across the membrane.

This charge separation leads to a proton gradient and membrane potential which drives the creation of ATP. Meanwhile, electrons passing along an electron transport chain are coupled with the production of NADPH. The creation of ATP and NADPH provides the chemical energy that will be used in the second stage of photosynthesis, the Calvin cycle.

The specific steps in the light reactions include photolysis, in which water is split to provide electrons, oxygen release, and the electron transport chain, which pumps protons and yields ATP and NADPH. The light reactions are a prime example of the ability of chloroplasts to capture light energy and convert it into chemical bonds.

The Calvin Cycle

The Calvin cycle is the final stage of photosynthesis and is where the organic molecules are synthesized. This cycle was discovered by Melvin Calvin and takes place in the stroma of the chloroplast. The Calvin cycle utilizes the energy carriers (ATP and NADPH) produced during the light reactions to fix carbon from carbon dioxide into glucose.

The Calvin cycle has three main phases:

  1. Carbon fixation – CO2 is bound to a 5-carbon molecule called ribulose bisphosphate (RuBP) by an enzyme called Rubisco to produce 3-phosphoglycerate.
  2. Reduction – The 3-phosphoglycerate is reduced using electrons from NADPH to produce glyceraldehyde 3-phosphate (G3P), which contains high energy bonds.
  3. Regeneration – Most of the G3P is used to regenerate RuBP so the cycle can continue. Some G3P leaves the cycle to be converted into glucose and other carbohydrates.

For every 3 CO2 molecules that enter the cycle, 1 exits as a 3-carbon G3P molecule. The cycling of carbons continues using the ATP and NADPH until there is enough G3P to make glucose. Oxygen is released as a byproduct of the fixation reactions.


ATP and NADPH are synthesized during the light-dependent reactions of photosynthesis and provide the chemical energy that fuels the Calvin cycle. ATP stands for adenosine triphosphate and contains readily accessible high-energy phosphate bonds that can be used to power reactions. NADPH stands for nicotinamide adenine dinucleotide phosphate and carries energized electrons that help provide the reducing power needed to make sugars.

In the thylakoid membranes of chloroplasts, photons of light hit the chlorophyll pigments and excite their electrons to higher energy levels. This energy is used to generate a proton gradient across the membrane, creating an electrochemical potential. Protons then flow down this gradient through ATP synthase, which uses the energy to phosphorylate ADP and make ATP. The energized electrons from photosystem I are also used to reduce NADP+ to NADPH.

The ATP and NADPH generated by the light reactions are then transported to the stroma and used in the Calvin cycle reactions. The energy from ATP is required to power the fixation of carbon dioxide by the enzyme Rubisco and other steps of sugar synthesis. Meanwhile, the electrons carried by NADPH provide the reducing power needed to convert 3-phosphoglycerate into the 3-carbon sugar glyceraldehyde 3-phosphate. Without the continued production of these energy carriers, the Calvin cycle could not operate.


The enzyme Rubisco plays a key role in the light-independent reactions of photosynthesis. Rubisco, which stands for ribulose-1,5-bisphosphate carboxylase/oxygenase, catalyzes the first major step of carbon fixation, by adding carbon dioxide to the five-carbon sugar ribulose-1,5-bisphosphate (RuBP). This initial carboxylation reaction produces two molecules of 3-phosphoglycerate.

Rubisco is an extremely abundant enzyme and accounts for most of the soluble protein in chloroplasts. It binds both carbon dioxide and oxygen, but has a preference for carbon dioxide. The dual carboxylase and oxygenase functions of Rubisco are important in both the Calvin cycle for carbon fixation and photorespiration, which helps protect the plant from excess light and heat. The relative rate of carboxylation versus oxygenation helps determine the efficiency of photosynthesis. Thus, the properties and kinetic performance of Rubisco play a central role in the conversion of light energy to chemical energy during photosynthesis.

Glucose Synthesis

The Calvin cycle, which takes place in the stroma, uses the ATP and NADPH produced by the light-dependent reactions to produce glucose from CO2. This process is also called the dark reactions or light-independent reactions since it does not directly require light to occur.

In the Calvin cycle, CO2 from the atmosphere is fixed onto a 5-carbon sugar called ribulose bisphosphate (RuBP) by an enzyme called Rubisco, forming a 6-carbon intermediate. This intermediate is then reduced to form two 3-carbon glyceraldehyde 3-phosphate (G3P) molecules, using the ATP and NADPH from the light reactions.

Most of the G3P produced is used to regenerate RuBP so the cycle can continue. However, one out of every six G3P molecules is removed from the cycle and combined to form glucose, a 6-carbon sugar which the plant uses as an energy source or building block.

In summary, the Calvin cycle harnesses the ATP and NADPH from the light reactions to fix carbon from CO2 into glucose, which can be used by plants for energy and growth. The cycle regenerates RuBP so that the process can continue as long as light is available.

Oxygen Release

One of the byproducts of photosynthesis is the release of oxygen into the atmosphere. As water is split during the light-dependent reactions, oxygen atoms are freed. These oxygen atoms accumulate in the chloroplast and are eventually released into the surrounding air through the stomata. This release of oxygen is vital for many living organisms that require it for cellular respiration. Over millennia, the oxygen produced through photosynthesis gradually accumulated in Earth’s atmosphere, helping oxygenate the environment. Oxygen makes up around 20% of the air we breathe today thanks to the accumulated contributions from plant photosynthesis over eons. So every breath we take, we have photosynthesis to thank for the oxygen we inhale.


In summary, the conversion of solar energy into chemical energy occurs through the process of photosynthesis in chloroplasts within plant cells. This complex process takes place in several steps within the thylakoids. First, light energy is captured and converted into chemical energy in the form of ATP and NADPH through the light-dependent reactions. Next, these energy carriers fuel the Calvin cycle, where carbon dioxide is fixed and synthesized into glucose. The entire process produces oxygen as a byproduct and stores energy from sunlight in the glucose molecules. Through this elegant mechanism, plants are able to harness the power of the sun to build energy-rich carbohydrates that sustain virtually all life on Earth.

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