PHOTOSYNTHESIS: PROCESS, ORGANISMS, TYPES, FACTORS AND FUNCTIONS - BIOLOGY - 2025

The Photosynthesis is the biological process where sunlight is converted into chemical energy stored in organic molecules. It is a connection between solar energy and life on earth.

Metabolically, plants are classified as autotrophs. This means that they do not need to consume food to survive, being able to generate it themselves through photosynthesis. All plants, algae and even some bacteria are photosynthetic organisms, characterized by the green color of tissues or structures.

Photosynthesis (left) and respiration (right). Image on the right taken from BBC

This process occurs in organelles called chloroplasts: membranous subcellular compartments that contain a series of proteins and enzymes that allow the development of complex reactions. In addition, it is the physical place where chlorophyll is stored, the pigment necessary for photosynthesis to occur.

The path that carbon takes during photosynthesis, beginning with carbon dioxide and ending with a sugar molecule, is known in admirable detail. The pathway has historically been divided into light phase and dark phase, spatially separated in the chloroplast.

The light phase takes place in the membrane of the chloroplast thylakoid and involves the breakdown of the water molecule into oxygen, protons, and electrons. The latter are transferred through the membrane to create an energy reservoir in the form of ATP and NADPH, which are used in the next phase.

The dark phase of photosynthesis takes place in the stroma of the chloroplast. It consists of the conversion of carbon dioxide (CO ₂) into carbohydrates, through the enzymes of the Calvin-Benson cycle.

Photosynthesis is a crucial pathway for all living organisms on the planet, serving as a source of initial energy and oxygen. Hypothetically, if photosynthesis were to stop working, a mass extinction event of all “higher” living things would occur in just 25 years.

Historical perspective

Source: pixabay.com

Previously it was thought that plants obtained their food thanks to the humus present in the soil, in a similar way to animal nutrition. These thoughts came from ancient philosophers like Empedocles and Aristotle. They assumed that the roots behaved like umbilical cords or "mouths" that fed the plant.

This vision changed progressively thanks to the hard work of dozens of researchers between the seventeenth and nineteenth centuries, who revealed the basis of photosynthesis.

Observations of the photosynthetic process began around 200 years ago, when Joseph Priestley concluded that photosynthesis was the reverse of cellular respiration. This researcher discovered that all the oxygen present in the atmosphere is produced by plants, through photosynthesis.

Subsequently, robust evidence began to emerge of the need for water, carbon dioxide and sunlight for this process to occur effectively.

At the beginning of the 19th century, the chlorophyll molecule was isolated for the first time and it was possible to understand how photosynthesis leads to the storage of chemical energy.

The implementation of pioneering approaches, such as gas exchange stoichiometry, managed to identify starch as a product of photosynthesis. Furthermore, photosynthesis was one of the first topics in biology studied through the use of stable isotopes.

Photosynthesis equation

Photosynthesis formula

General equation

Chemically, photosynthesis is a redox reaction where some species are oxidized and give their electrons to other species that are reduced.

The general process of photosynthesis can be summarized in the following equation: H ₂ O + light + CO ₂ → CH ₂ O + O _2. Where the term CH ₂ O (one sixth of a glucose molecule) refers to the organic compounds called sugars that the plant will use later, such as sucrose or starch.

Light and dark phase

We can break down this equation into two more specific equations for each stage of photosynthesis: the light phase and the dark phase.

We represent the light phase as: 2H ₂ O + light → O2 + 4H ⁺ + 4e ^-. Similarly, the dark phase involves the following relationship: CO ₂ + 4H ⁺ + 4e− → CH ₂ O + H ₂ O.

Δ

The free energy (Δ G °) for these reactions are: +479 kJ · mol ^- 1, +317 kJ · mol ^−1, and +162 kJ · mol ⁻¹, respectively. As suggested by thermodynamics, the positive sign of these values ​​translates into an energy requirement and is called the endergonic process.

Where does the photosynthetic organism obtain this energy for the reactions to occur? From sunlight.

It should be mentioned that, in contrast to photosynthesis, aerobic respiration is an exergonic process - in this case the value of ΔG ° is accompanied by a negative sign - where the energy released is used by the organism. Therefore, the equation is: CH ₂ O + O ₂ → CO ₂ + H ₂ O.

Where it happens?

In most plants, the main organ where the process occurs is in the leaf. In these tissues we find small globose structures, called stomata, that control the entry and exit of gases.

The cells that make up a green tissue can have up to 100 chloroplasts inside them. These compartments are structured by two outer membranes and an aqueous phase called the stroma where a third membrane system is located: the thylakoid.

Process (phases)

Light phase

Photosynthesis begins with the capture of light by the most abundant pigment on planet earth: chlorophyll. The absorption of light results in the excitation of electrons to a higher energy state - thus converting the energy from the sun into potential chemical energy.

In the thylakoid membrane, photosynthetic pigments are organized into photocenters that contain hundreds of pigment molecules that act as an antenna that absorbs light and transfers energy to the chlorophyll molecule, called the "reaction center."

The reaction center is composed of transmembrane proteins bound to a cytochrome. This transfers electrons to other molecules in an electron transport chain through a series of membrane proteins. This phenomenon is coupled with the synthesis of ATP and NADPH.

Proteins involved

Proteins are organized into various complexes. Two of them are photosystems I and II, responsible for absorbing light and transferring it to the reaction center. The third group consists of the cytochrome bf complex.

The energy produced by the proton gradient is used by the fourth complex, ATP synthase, which couples the flow of protons with ATP synthesis. Note that one of the most relevant differences with respect to respiration is that energy is not only converted into ATP, but also into NADPH.

Photosystems

Photosystem I consists of a chlorophyll molecule with an absorption peak of 700 nanometers, therefore it is called P ₇₀₀. Similarly, the absorption peak of photosystem II is 680, abbreviated P ₆₈₀.

The task of photosystem I is the production of NADPH and that of photosystem II is the synthesis of ATP. The energy used by photosystem II comes from the breakdown of the water molecule, releasing protons and creating a new gradient across the thylakoid membrane.

The electrons derived from the break are transferred to a fat-soluble compound: plastoquinone, which carries the electrons from the photosystem II to the cytochrome bf complex, generating an additional pumping of protons.

From photosystem II, the electrons move to plastocyanin and photosystem I, which uses the high-energy electrons to reduce NADP ⁺ to NADPH. The electrons eventually reach the ferrodoxin and generate the NADPH.

Cyclic flow of electrons

There is an alternate pathway where ATP synthesis does not involve NADPH synthesis, generally to supply energy to needed metabolic processes. Therefore the decision of whether to generate ATP or NADPH depends on the momentary needs of the cell.

This phenomenon involves the synthesis of ATP by photosystem I. The electrons are not transferred to the NADP ⁺, but to the cytochrome bf complex, creating an electron gradient.

Plastocyanin returns the electrons to photosystem I, completing the transport cycle and pumping the protons into the cytochrome bf complex.

Other pigments

Chlorophyll is not the only pigment that plants have, there are also so-called "accessory pigments", including carotenoids.

In the light phase of photosynthesis, the production of elements potentially harmful to the cell, such as "singlet oxygen", occurs. Carotenoids are responsible for preventing the formation of compound or preventing it from damaging tissues.

These pigments are what we observe in autumn, when the leaves lose their green color and turn yellow or orange, since the plants are breaking down chlorophyll to obtain nitrogen.

Dark phase

The goal of this initial process is to use the sun's energy for the production of NADPH (Nicotinamide-Adenine-Dinucleotide-Phosphate or "reducing power") and ATP (adenosine triphosphate, or "the energy currency of the cell"). These elements will be used in the dark phase.

Before describing the biochemical steps involved in this phase, it is necessary to clarify that, although its name is "dark phase", it does not necessarily occur in total darkness. Historically, the term tried to refer to the independence of light. In other words, the phase can occur in the presence or absence of light.

However, as the phase depends on the reactions that occur in the light phase - which requires light - it is correct to refer to these series of steps as carbon reactions.

Calvin cycle

In this phase, the Calvin cycle or three-carbon pathway occurs, a biochemical pathway described in 1940 by the American researcher Melvin Calvin. The discovery of the cycle was awarded the Nobel Prize in 1961.

In general, three fundamental stages of the cycle are described: the carboxylation of the CO ₂ acceptor, the reduction of the 3-phosphoglycerate and the regeneration of the CO ₂ acceptor.

The cycle begins with the incorporation or "fixation" of carbon dioxide. It reduces carbon to carbohydrates, through the addition of electrons, and uses NADPH as a reducing power.

In each turn, the cycle requires the incorporation of a carbon dioxide molecule, which reacts with ribulose bisphosphate, generating two three-carbon compounds that will be reduced and regenerate a ribulose molecule. Three turns of the cycle results in a glyceralhyde phosphate molecule.

Therefore, to generate a six-carbon sugar like glucose, six cycles are necessary.

Photosynthetic organisms

The photosynthetic capacity of organisms appears in two of the domains, made up of bacteria and eukaryotes. Based on this evidence, individuals that comprise the archaea domain are devoid of this biochemical pathway.

Photosynthetic organisms appeared approximately 3.2 to 3.5 billion years ago as structured stromatolites similar to modern cyanobacteria.

Logically, a photosynthetic organism cannot be recognized as such in the fossil record. However, inferences can be made taking into account its morphology or geological context.

In relation to bacteria, the ability to take sunlight and transform it into sugars seems to be widely distributed in various Phyla, although there does not seem to be an apparent evolution pattern.

The most primitive photosynthetic cells are found in bacteria. They have the pigment bacteriochlorophyll, and not the well-known green plant chlorophyll.

Photosynthetic bacterial groups include cyanobacteria, protobacteria, sulfur green bacteria, firmicutes, filamentous anoxic phototrophs, and acidobacteria.

As for plants, they all have the ability to photosynthesize. In fact, it is the most distinguishable feature of this group.

Types of photosynthesis

Oxygenic and anoxygenic photosynthesis

Photosynthesis can be classified in different ways. A first classification takes into account if the organism uses water for the reduction of carbon dioxide. Thus, we have oxygenic photosynthetic organisms, which includes plants, algae, and cyanobacteria.

In contrast, when the body does not use water, they are called anoxygenic photosynthetic organisms. This group includes green and purple bacteria, for example the genera Chlorobium and Chromatium, which use sulfur or hydrogen gas to reduce carbon dioxide.

These bacteria are not capable of resorting to photosynthesis in the presence of oxygen, they need an anaerobic environment. Therefore, photosynthesis does not lead to the generation of oxygen - hence the name “anoxygenic”.

Types of metabolisms C

Photosynthesis can also be classified based on the physiological adaptations of plants.

In photosynthetic eukaryotes, the reduction of CO ₂ coming from the atmosphere to carbohydrates occurs in the Calvin cycle. This process begins with the enzyme rubisco (ribulose-1,5-bisphosphate carboxylase / oxygenase) and the first stable compound formed is 3-phosphoglyceric acid, with three carbons.

Under conditions of heat stress, called high radiation or drought, the rubisco enzyme cannot differentiate between O ₂ and CO ₂. This phenomenon markedly decreases the efficiency of photosynthesis and is called photorespiration.

For these reasons there are plants with special photosynthetic metabolisms that allow them to avoid this inconvenience.

C4 metabolism

Type C ₄ metabolism aims to concentrate carbon dioxide. Before rubisco acts, C ₄ plants carry out a first carboxylation by PEPC.

Note that there is a spatial separation between the two carboxylations. C ₄ plants are distinguished by having “kranz” or crown anatomy, formed by mesophyll cells and are photosynthetic, unlike these cells in normal or C ₃ photosynthesis.

In these cells, the first carboxylation occurs by PEPC, giving as product oxaloacetate, which is reduced to malate. This diffuses to the sheath cell, where a decarboxylation process occurs, generating CO ₂. Carbon dioxide is used in the second carboxylation directed by rubisco.

CAM photosynthesis

CAM photosynthesis or acid metabolism of crassulaceae is an adaptation of plants that live in extremely dry climates and is typical of plants such as pineapple, orchids, carnations, among others.

The assimilation of carbon dioxide in CAM plants occurs at night, since the loss of water due to the opening of the stomata will be less than in the daytime.

CO ₂ combines with PEP, a reaction catalyzed by PEPC, forming malic acid. This product is stored in the vacuoles that release its content in the morning hours, then it is decarboxylated and the CO ₂ manages to enter the Calvin cycle.

Factors involved in photosynthesis

Among the environmental factors that intervene in the efficiency of photosynthesis, the following stand out: the present amount of CO ₂ and light, temperature, the accumulation of photosynthetic products, the amount of oxygen and the availability of water.

Plant-specific factors also play a fundamental role, such as age and growth status.

The concentration of CO ₂ in the environment is low (it does not exceed 0.03% of the volume), therefore any minimal variation has significant consequences on photosynthesis. In addition, plants are only capable of 70 to 80% of the carbon dioxide present.

If there are no limitations by the other variables mentioned, we find that photosynthesis will depend on the amount of CO ₂ available.

Similarly, light intensity is crucial. In low intensity environments, the respiration process will outperform photosynthesis. For this reason, photosynthesis is much more active in the hours when the solar intensity is high, such as the first hours of the morning.

Some plants may be affected more than others. For example, forage grasses are very insensitive to temperature.

Features

Photosynthesis is a vital process for all organisms on planet earth. This pathway is responsible for supporting all forms of life, being the source of oxygen and the basis of all existing trophic chains, since it facilitates the conversion of solar energy into chemical energy.

In other words, photosynthesis produces the oxygen that we breathe - as mentioned above, that element is a by-product of the process - and the food that we consume on a daily basis. Almost all living organisms use organic compounds derived from photosynthesis as a source of energy.

Note that aerobic organisms are capable of extracting energy from organic compounds produced by photosynthesis only in the presence of oxygen - which is also a product of the process.

In fact, photosynthesis is capable of converting an exacerbated number (200 billion tons) of carbon dioxide into organic compounds. As for oxygen, production is estimated to be in the range of 140 billion tons.

In addition, photosynthesis provides us with most of the energy (approximately 87% of this) that humanity uses to survive, in the form of fossilized photosynthetic fuels.

Evolution

First photosynthetic life forms

In the light of evolution, photosynthesis appears to be an extremely ancient process. There is a large amount of evidence that places the origin of this path near the appearance of the first forms of life.

Regarding the origin in eukaryotes, there is overwhelming evidence that proposes endosymbiosis as the most plausible explanation for the process.

Thus, organisms reminiscent of cyanobacteria could become chloroplasts, thanks to endosymbiotic relationships with larger prokaryotes. For this reason, the evolutionary origin of photosynthesis is born in the bacterial domain and could be distributed thanks to massive and repetitive events of horizontal gene transfer.

Role of oxygen in evolution

There is no doubt that the energy conversion of light through photosynthesis has shaped the current environment of planet earth. Photosynthesis, seen as an innovation, enriched the atmosphere with oxygen and revolutionized the energetics of life forms.

When the release of O _{2 began} by the first photosynthetic organisms, it probably dissolved in the water of the oceans, until it was saturated. In addition, oxygen was able to react with iron, precipitating in the form of iron oxide, which is currently an invaluable source of minerals.

The excess oxygen advanced to the atmosphere, to finally concentrate there. This massive increase in the concentration of O ₂ has important consequences: damage to biological structures and enzymes, condemning many groups of prokaryotes.

In contrast, other groups exhibited adaptations to live in the new oxygen-rich environment, shaped by photosynthetic organisms, probably ancient cyanobacteria.

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