Photosynthesis Definition, Process, equation, and examples

Photosynthesis Definition

Photosynthesis is defined as the process, used by green plants and photosynthetic bacteria, where electromagnetic radiation is converted to chemical energy and uses the energy of light to convert carbon dioxide and water to carbohydrates and oxygen.

Photosynthesis Definition, Process, equation and examples
  • Carbohydrates formed from photosynthesis provide not only the energy necessary for energy transfer within ecosystems but also the carbon molecules to form a wide range of biomolecules.
  • Photosynthesis is a light-driven oxidation-reduction reaction where the energy of light is used to oxidize water, releasing oxygen gas and hydrogen ions, followed by the transfer of electrons to carbon dioxide, reducing it to organic molecules.
  • Photosynthetic organisms are called autotrophs because they can synthesize chemical fuels like glucose from carbon dioxide and water using sunlight as an energy source.
  • Other organisms that obtain energy from other organisms also ultimately depend on autotrophs for energy.
  • One of the essential requirements for photosynthesis is the green pigment “chlorophyll” that is present in the chloroplasts of green plants and some bacteria.
  • The pigment is essential for “capturing” sunlight, which then drives the overall process of photosynthesis.

What is photosynthesis?

The word photosynthesis can be separated to form two smaller words:

Photo” which means light

“Synthesis” which means to assemble

Plants need food, but they don’t have to wait for people or animals to maintain them. Most plants can make their own food when they need it. This is done with light and the process is called photosynthesis.

Photosynthesis Definition, Process, equation and examples

Photosynthesis equations|formula

  • The Photosynthesis process differs in green plants and sulfur bacteria.
  • In plants, water is used in conjunction with carbon dioxide to release glucose and oxygen molecules.
  • In the case of sulfur bacteria, hydrogen sulfide is used together with carbon dioxide to release carbohydrates, sulfur, and water molecules.
Photosynthesis Definition, Process, equation and examples

Oxygenic Photosynthesis

The overall reaction of photosynthesis in plants is as follows:

Carbon dioxide + Water + solar energy → Glucose + Oxygen

6CO2 + 6H2O + solar energy → C6H12O6 + 6O2


Carbon dioxide + Water + solar energy → Glucose + Oxygen + Water

6CO2 + 12H2O+ solar energy → C6H12O6 + 6O2 + 6H2O

Anoxygenic Photosynthesis

The overall reaction of photosynthesis in sulfur bacteria is as follows:

CO2 + 2H2S + light energy → (CH2O) + H2O + 2S

Photosynthetic pigments

  • Photosynthetic pigments are the molecules involved in the absorption of electromagnetic radiation, transferring the energy of the absorbed photons to the reaction center, resulting in photochemical reactions in organisms capable of photosynthesis.
  • Photosynthetic pigment molecules are quite ubiquitous and are always made up of chlorophylls and carotenoids.
  • In addition to chlorophyll, photosynthetic systems also contain another pigment, pheophytin (bacteriophilin in bacteria), which plays a crucial role in electron transfer in photosynthetic systems.
  • Furthermore, other pigments can be found in particular photosynthetic systems, such as xanthophylls in plants.


Chlorophyll is the pigment molecule, which is the main photoreceptor in the chloroplasts of most green plants.

Chlorophylls consist of a ring of porphyrin, which is attached to an Mg2 + ion, attached to a phytol chain.

Chlorophylls are very effective photoreceptors because they contain alternating single and double bond networks.

In chlorophyll, the electrons are not located in a particular atomic nucleus and, consequently, they can more easily absorb light energy.

Furthermore, chlorophylls also have solid absorption bands in the visible region of the spectrum.

Chlorophylls are found in the cytoplasmic membranes of photosynthetic bacteria or in thylakoid membranes within plant chloroplasts.


  • Bacteriorhodopsin is another class of photosynthetic pigment that exists only in halobacteria.
  • It is made up of a protein attached to a group of retinal prostheses.
  • This pigment is responsible for the absorption of light photons, leading to a conformational change in the protein, resulting in the expulsion of protons from the cell.


  • Cyanobacteria and red algae use phycobilins such as phycoerythrobilin and phycocybilbilin as their light harvesting pigments.
  • These open-chain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic or central Mg2 + structure.
  • Phycobilins are covalently bound to specific binding proteins, forming phycobiliproteins, which associate in highly ordered complexes called phycobilysomes that constitute the primary light-harvesting structures in these microorganisms.


  • In addition to chlorophylls, thylakoid membranes contain light-absorbing secondary pigments, or accessory pigments, called carotenoids.
  • Carotenoids can be yellow, red, or purple. The most important are β-carotene, which is an orange-red isoprenoid, and the yellow carotenoid lutein.
  • Carotenoid pigments absorb light at wavelengths not absorbed by chlorophylls and are therefore supplementary light receptors.

Factors affecting photosynthesis

Blackman formulated the Limiting Factors Act while studying the factors that affect the rate of photosynthesis. This law establishes that the rate of a physiological process will be limited by the factor found in the shortest supply. Similarly, the rate of photosynthesis is also affected by a number of factors, namely;

Photosynthesis Definition, Process, equation and examples


  • As light intensity increases, the rate of light-dependent reactions of photosynthesis increases, and in turn, the rate of photosynthesis increases.
  • With increasing light intensity, the number of photons falling on a sheet also increases. As a result, more chlorophyll molecules are ionized and more ATP and NADH are generated.
  • After one point, however, the photosynthesis rate remains constant as the light intensity increases. At this point, photosynthesis is limited by some other factors.
  • Furthermore, the wavelength of light also affects the rate of photosynthesis.
  • Different photosynthetic systems absorb light energy more effectively at different wavelengths.

Carbon dioxide

  • An increase in the concentration of carbon dioxide increases the rate at which carbon is incorporated into carbohydrates in light-independent photosynthesis reactions.
  • Therefore, increasing the concentration of carbon dioxide in the atmosphere rapidly increases the rate of photosynthesis to a point after which it is limited by some other factors.


  • Light-independent photosynthesis reactions are affected by temperature changes since they are catalyzed by enzymes, whereas light-dependent reactions are not.
  • The speed of the reactions increases as the enzymes reach their optimum temperature, after which the speed begins to decrease as the enzymes tend to denature.

Process of Photosynthesis

The overall process of photosynthesis can be objectively divided into four steps/ process:Photosynthesis Definition, Process, equation and examples

Photosynthesis Definition, Process, equation and examples

Process of Photosynthesis

1. Absorption of light

  • The first step in photosynthesis is the absorption of light by chlorophylls that are bound to proteins in the thylakoids of chloroplasts.
  • The absorbed light energy is used to remove electrons from an electron donor such as water, forming oxygen.
  • The electrons are further transferred to a primary electron acceptor, quinine (Q), which is similar to CoQ in the electron transfer chain.

2. Electron Transfer

  • Electrons are now further transferred from the primary electron acceptor through a chain of electron transfer molecules present in the thylakoid membrane to the final electron acceptor, which is generally NADP +.
  • As the electrons are transferred through the membrane, the protons are pumped out of the membrane, resulting in the proton gradient across the membrane.

3. Generation of ATP

  • The movement of protons from the thylakoid light to the stroma through the F0F1 complex results in the generation of ATP from ADP and Pi.
  • This step is identical to the step of ATP generation in the electron transport chain.

4. Carbon Fixation

  • The NADP and ATP generated in steps 2 and 3 provide energy, and the electrons drive the carbon reduction process in six-carbon sugar molecules.
  • The first three steps of photosynthesis are directly dependent on the energy of light and are therefore called light reactions, while the reactions in this step are independent of light and are therefore called dark reactions.

Types/ Stages/ Parts of photosynthesis

Types Stages Parts of photosynthesis

  • Photosynthesis takes place in two stages: light dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use the energy of light to produce ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO2.

Photosynthesis is divided into two stages based on the utilization of light energy

Light-dependent reactions

  • The light-dependent reactions of photosynthesis only take place when the plants/bacteria are illuminated.
  • In light-dependent reactions, chlorophyll and other pigments in photosynthetic cells absorb light energy and conserve it as ATP and NADPH while simultaneously releasing O2 gas.
  • In photosynthesis of light-dependent reactions, chlorophyll absorbs high-energy, short-wavelength light, which excites the electrons present within the thylakoid membrane.
  • The excitation of the electrons now begins the transformation of light energy into chemical energy.
  • Light reactions encompass two photosystems that are present in the thylakoid of chloroplasts.

Photosystem II

  • Photosystem II is a group of proteins and pigments that work together to absorb light energy and transfer electrons through a chain of molecules until it finally reaches an electron receptor.
  • Photosystem II has a pair of chlorophyll molecules, also known as P680 as the molecules that best absorb light of the 680nm wavelength.
  • The P680 donates an electron pair after absorbing the light energy, resulting in a rusty form of P680.
  • Finally, an enzyme catalyzes the division of a water molecule into two electrons, two hydrogen ions, and oxygen molecules.
  • This pair of electrons are then transferred to P680, causing it to return to its initial stage.

Photosystem I

  • Photosystem I is a complex similar to photosystem II, except that photosystem I have a pair of chlorophyll molecules known as P700, as they better absorb the 700nm wavelength.
  • Because the photosystem absorbs energy from light, it also excites and transfers electrons.
  • The now oxidized form of P700 accepts an electron from Photosystem II, restricted back to its initial stage.
  • The photosystem I electrons then pass in a series of redox reactions through the ferredoxin protein.
  • The electrons eventually reach NADP +, reducing them to NADPH.


2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2

2.Light independent reactions (Calvin cycle)

Light-independent photosynthesis reactions are anabolic reactions that lead to the formation of a sexual carbon compound, glucose in plants. Reactions at this stage are also called dark reactions since they do not depend directly on light energy, but require the products formed from light reactions.

This stage consists of 3 additional steps leading to carbon fixation/assimilation.

Step 1: Fixation of CO2 into 3-phosphoglycerate

  • In this step, a CO2 molecule binds covalently to the five-carbon compound 1,5-bisphosphate ribulose catalyzed by the enzyme 1,5-bisphosphate carboxylase, also called rubisco.
  • The binding results in the formation of an unstable six-carbon compound which is then cleaved to form two 3-phosphoglycerate molecules.

Step 2: Conversion of 3-phosphoglycerate to glyceraldehydes 3-phosphate

  • The 3-phosphoglycerate formed in step 1 is converted to glyceraldehyde 3-phosphate by two separate reactions.
  • Initially, the 3-phosphoglycerate kinase enzyme present in the stroma catalyzes the transfer of a phosphoryl group from ATP to 3-phosphoglycerate, producing 1,3-bisphosphoglycerate.
  • NADPH then donates electrons in a reaction catalyzed by the chloroplast-specific isoenzyme of glyceraldehyde 3-phosphate dehydrogenase, producing glyceraldehyde 3-phosphate and phosphate (Pi).
  • Most of the glyceraldehyde 3-phosphate produced in this way is used to regenerate ribulose 1,5-bisphosphate.
  • The rest of the glyceraldehyde is converted to starch in the chloroplast and stored for later use or exported to the cytosol and converted to sucrose for transport to the growing regions of the plant.

Step 3: Regeneration of ribulose 1,5-biphosphate from triose phosphates

  • The three-carbon compounds formed in the previous steps are then converted to the five-carbon compound, ribulose 1,5-bisphosphate through a series of transformations with intermediates of three, four, five, six and seven. carbon sugar
  • Like the first molecules in the process, if they regenerate, this stage of photosynthesis results in one cycle (Calvin cycle).


3 CO2 + 9 ATP + 6 NADPH + 6 H+ → glyceraldehyde-3-phosphate (G3P) + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. It would take six turns of the cycle to produce a glucose molecule.

Photosynthesis examples

Photosynthesis in green plants or oxygenic bacteria

  • In oxygenated plants and bacteria such as cyanobacteria, photosynthesis is carried out in the presence of the green pigment, chlorophyll.
  • It takes place in the thylakoids of chloroplasts, resulting in products such as gaseous oxygen, glucose, and water molecules.
  • Most of the glucose units in plants are bound together to form starch or fructose or even sucrose.

Photosynthesis in sulfur bacteria

In purple sulfur bacteria, photosynthesis takes place in the presence of hydrogen sulfur instead of water.

Some of these bacteria, like green sulfur bacteria, have chlorophyll, while other purple sulfur bacteria have carotenoids as photosynthetic pigments.

The result of photosynthesis in these bacteria is carbohydrates (not necessarily glucose), sulfur gases, and water molecules.

Photosynthesis Definition, Process, equation and examples

Importance of photosynthesis

  • Photosynthesis is the main source of energy in autotrophs where they produce their food through the use of carbon dioxide, sunlight, and photosynthetic pigments.
  • Photosynthesis is equally essential for heterotrophs, as they derive their energy from autotrophs.
  • Photosynthesis in plants is necessary to maintain oxygen levels in the atmosphere.
  • In addition, the products of photosynthesis contribute to the carbon cycle that occurs in the oceans, land, plants, and animals.
  • Similarly, it also helps maintain a symbiotic relationship between plants, animals, and humans.
  • Sunlight or solar energy is the main source of all other forms of energy on earth, which is used through the photosynthesis process.

Photosynthesis Definition, Process, equation, and examples

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Photosynthesis Definition, Process, equation, and examples

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