Skip to main content


A new study has discovered a powerful force that is now driving evolution on Earth.

Photosynthesis Explained

Photosynthesis is the process by which plants, algae and certain bacteria use sunlight energy and convert it into chemical energy. Here, we describe the general principles of photosynthesis and highlight how scientists study this natural process to help develop clean fuels and renewable energy.

Types of photosynthesis

There are two types of photosynthesis processes: aerobic photosynthesis and anaerobic photosynthesis. The general principles of anaerobic photosynthesis and aerobic photosynthesis are very similar, but aerobic photosynthesis is the most common, found in plants, algae, and cyanobacteria. 

During oxygen-containing photosynthesis, light energy transfers electrons from water (H2O) to carbon dioxide (CO2) to produce carbohydrates. In this transfer, CO2 is reduced or accepts electrons, while water is oxidized or loses electrons. Ultimately, oxygen is produced along with carbohydrates.

Oxygen-containing photosynthesis counteracts respiration by absorbing carbon dioxide produced by all breathing organisms and reintroducing oxygen into the atmosphere.

On the other hand, anaerobic photosynthesis uses electron donors other than water. This process usually occurs in bacteria, such as purple bacteria and green sulphur bacteria, which mainly exist in various aquatic habitats.

Anaerobic photosynthesis does not produce oxygen-hence the name, what it produces depends on the electron donor. For example, many bacteria use hydrogen sulphide gas that smells like rotten eggs to produce solid sulfur as a by-product.

Although these two kinds of photosynthesis are complex, multi-step transactions, the whole process can be subtly summarized as a chemical equation.

Aerobic photosynthesis is written as follows:

6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O

Here, six carbon dioxide molecules (CO2) use light energy to combine with 12 water molecules (H2O). The end result is the formation of a single carbohydrate molecule (C6H12O6, or glucose) and six breathable oxygen and water molecules.

Similarly, various anaerobic photosynthetic reactions can be expressed as a general formula:

CO2 + 2H2A + Light Energy → [CH2O] + 2A + H2O

The letter A in the equation is a variable, and H2A represents a potential electron donor. For example, A may represent the sulfur in the electron donor hydrogen sulfide (H2S). Plant biologists Govindjee and John Whitmarsh of the University of Illinois at Urbana-Champaign in "Photobiology Concepts: Photosynthesis and Photomorphogenesis" (Narosa Publishers) And Kluwer Academic, 1999).

Photosynthesis device

The following are the essential cellular components for photosynthesis.

Pigments are molecules that impart colour to plants, algae, and bacteria, but they are also responsible for effectively capturing sunlight. Pigments of different colours absorb light of different wavelengths. The following are the three main groups.

Chlorophyll: These green pigments can capture blue and red light. There are three subtypes of chlorophyll, called chlorophyll a, chlorophyll b and chlorophyll c. According to Eugene Rabinowitch and Govindjee in their book "Photosynthesis" (Wiley, 1969), chlorophyll a is present in all plants that undergo photosynthesis. There is also a bacterial variant that is aptly named bacterial chlorophyll, which absorbs infrared light. This pigment is mainly found in purple and green bacteria that perform anaerobic photosynthesis.

Carotenoids: These red, orange or yellow pigments absorb blue-green light. Examples of carotenoids are lutein (yellow) and carotene (orange) from which the colour of carrots is extracted.

Phycobilidin: The wavelength of light absorbed by these red or blue pigments cannot be absorbed by chlorophyll and carotenoids. They are found in cyanobacteria and red algae.


The cytoplasm of photosynthetic eukaryotes contains organelles called plastids. The double-membrane plastids in plants and algae are called primary plastids, while the multi-membrane plastids found in plankton are called secondary plastids.

Plastids usually contain pigments or can store nutrients. Colourless and non-pigmented white matter stores fat and starch, while coloured bodies contain carotenoids, and chloroplasts contain chlorophyll.

Photosynthesis occurs in chloroplasts; especially in the grana and stroma regions. The granule is the innermost part of the organelle; a collection of disc-shaped membranes, stacked like plates into columns. A single disc is called a thylakoid. It is here that the transfer of electrons occurs. The voids between the kernels constitute the matrix.

Chloroplasts are similar to mitochondria, the energy centre of cells because they have their own genome or collection of genes contained in circular DNA. These genes code for proteins essential for organelles and photosynthesis. Like mitochondria, chloroplasts are also thought to originate from primitive bacterial cells through an endosymbiosis process.

Plastids originated from phagocytic photosynthetic bacteria obtained by single-celled eukaryotic cells more than one billion years ago. Analysis of chloroplast genes showed that it was once a member of the cyanobacteria, a type of bacteria that can complete aerobic photosynthesis.

The endosymbiosis of cyanobacteria cannot explain the formation of secondary plastids well, and the origin of such plastids is still a controversial issue.


Pigment molecules are associated with proteins, which allows them to move flexibly toward the light and each other. Large collections of 100 to 5,000 pigment molecules make up the antennae. These structures effectively capture light energy from the sun in the form of photons.

Ultimately, light energy must be transferred to the pigment-protein complex, which can convert it into chemical energy in the form of electrons. For example, in plants, light energy is transferred to chlorophyll pigments. When the chlorophyll pigment emits an electron, the conversion of chemical energy is completed, and then the electron can be transferred to a suitable recipient.

Reaction centres

The pigments and proteins that convert light energy into chemical energy and start the electron transfer process are called reactions.

The photosynthetic process

Plant photosynthesis reactions are divided into reactions that require the presence of sunlight and reactions that do not require the presence of sunlight. Two types of reactions occur in the chloroplast: the light-dependent reaction in the thylakoid and the light-independent reaction in the matrix.

Light-dependent reaction (also called photoreaction): When a photon hits the reaction centre, a pigment molecule such as chlorophyll releases an electron.

The trick to doing useful work is to prevent the electron from finding a way back to its original location. This is not easy to avoid because chlorophyll now has an electron-hole that can attract nearby electrons.

The released electrons manage to escape through the electron transport chain, which produces the energy required to produce ATP (adenosine triphosphate, a source of cytochemical energy) and NADPH. The electron-hole in the original chlorophyll pigment is filled by acquiring an electron from the water. As a result, oxygen is released into the atmosphere.

Light-independent reaction (also called dark reaction, called Calvin cycle): Light reaction produces ATP and NADPH, which are rich energy sources that drive dark reactions. Three chemical reaction steps constitute the Calvin cycle: carbon fixation, reduction and regeneration. These reactions use water and catalysts. The carbon atoms from carbon dioxide are fixed when they are built into organic molecules that eventually form three-carbon sugars. These sugars are then used to make glucose or recycled to start the Calvin cycle again.

Photosynthesis of the future

Photosynthetic organisms are a possible means to generate clean-burning fuels such as hydrogen or even methane. Recently, a research group at the University of Turku in Finland tapped into the ability of green algae to produce hydrogen. Green algae can produce hydrogen for a few seconds if they are first exposed to dark, anaerobic (oxygen-free) conditions and then exposed to light The team devised a way to extend green algae's hydrogen production for up to three days.

Scientists have also made progress in the field of artificial photosynthesis. For example, a group of researchers at the University of California, Berkeley developed an artificial system that can capture carbon dioxide using nanowires or wires that are one billionth of a meter in diameter. Wire enters the microbial system, using sunlight energy to reduce carbon dioxide into fuel or polymer.

In 2016, members of this same group published a study in the journal Science that described another artificial photosynthetic system in which specially engineered bacteria were used to create liquid fuels using sunlight, water and carbon dioxide. Generally speaking, plants can only use about one per cent of solar energy and use it to produce organic compounds during photosynthesis. In contrast, the researchers' artificial system can use 10% of the solar energy to produce organic compounds.

Continued research on natural processes such as photosynthesis helps scientists develop new ways to utilize various renewable energy sources. Given that sunlight, plants, and bacteria are everywhere, harnessing the power of photosynthesis is a logical step to create clean-burning and carbon-neutral fuels.

Save money shopping online!

It's easy to spend money online. But getting the most value is another matter. Take shopping as an example. It takes time to find the best deal, prices vary from buyer to buyer, and payment methods are not always in your best interest. We think this is unfair. Honey is here to help you. We provide everyone with the tools they need to find the best savings, benefits, and all-around value. We make them free and easy to use. Therefore, you can always consume with confidence.