What is photosynthesis and what are the mechanisms of it?

Photosynthesis the process in which electromagnetic energy is converted to chemical energy that can be utilised for various biosynthetic processes. Plants require photons, CO2, and H2O to produce sugar and carbohydrates. Below is the equation to demonstrate photosynthesis:

6CO2 + 6H2O → (Photons and Chlorophyll) C6H12O6 + 6O2

Light-dependent reactions occur in the thylakoid membranes (reaction centres) of the chloroplasts and utilise photons to synthesize ATP and NADPH. The process commences with photosystem II (250-400 pigment molecules) when a chlorophyll molecule gains sufficient energy from the adjacent pigment, which enables photolysis of H2O to occur (breaking of hydrogen and oxygen). This process produces O2 which is expelled and electrons and protons (H+) which are required. This occurs in the granum of the chloroplast or the stacks of thylakoids. Chitnis (1996) states that the H+ are then translocated through the cytochrome which energises and excites H+ to the second stage of light-dependent reactions which is shown in the following stages:

Photosystem II (P680) →Plastoquinone → Cytochrome b6 → Cytochrome f → Plastocyanin → Photosystem I (P700)

Photosystem I activates electrons for transfer to the Fd and finally to NADP+, where the protons from water splitting are exhausted to create NADPH+ H+. To demonstrate the ability to liberate oxygen from water Chitnis, (1996) formulated the following:

The positively charged P800 exerts a strong pull on the H20 molecule, splitting it into H+ and OH- ions. It requires CI- and Mn2+ ions to act as a catalysis:

4H20 → (Mn2+, CI-) → 4 (OH-) + 4H+

OH donates its electron to oxidise P680:

4(OH-) → 4e- →4OH

OH radical obtained forms H20 and liberates oxygen:

4(OH) → H20 → 4H+ → 4e- → 02

Donation of H+ ion from the formation of NADPH is utilised for the Calvin-Benson Cycle. Below is a summary of the differences between the photosystems.

Table 1 Photosystem comparison

The end products are transferred to the light-independent reactions, these reactions occur in the stroma found within the chloroplast between the grana and thylakoid. C02 is delivered to the stroma where they are reduced to ATP and combined with H+ to power the Calvin-Benson Cycle. This process converts ATP into the carbohydrates needed to power the photoautotrophs. The key enzyme of the cycle is RuBisCO. This enzyme fixes C02 to ATP by complex redox reactions to gain the carbohydrates, lipids and proteins which are required for development. Koller (1990) expressed the following equation for this process:

3CO2 + 6NADPH + 5H2O + 9ATP → glyceraldehyde-3-phosphates (G3P) + 2H+ + 6NADP+ + 9ADP + 8Pi

Plants convert photons into chemical energy with a photosynthetic efficiency between 3-6%, however photosynthesis varies with the frequency of light and intensity temperature and proportions of C02, which can vary between 0.1-8% in the atmosphere. Photosynthetic systems store 469 kilojoules of energy and in some cases up to 502 kilojoules. This process is more productive when both photosystems are balanced in their uptake of light. By comparison, solar panels convert photons into electrical energy at an efficiency of 6-20%. This table puts the reactions of photosynthesis into a timescale.

Table 2 Time of Photosynthesis

Photorespiration

Photorespiration occurs when the CO2 levels inside a leaf become low. This occurs on hot dry days when a plant is forced to close its stomata to prevent water loss. If the plant continues to fix CO2 when its stomata are closed, the CO2 will get exhausted and the O2 ratio in the leaf will increase relative to CO2 concentrations. When CO2 levels inside the leaf drop to 50ppm, RuBisCO begins to combine O2 with RuBP instead of CO2. Leegood, (1998) suggested that the net outcome of this is that instead of producing 2-3C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called Phosphoglycolate. This transaction becomes inefficient and will result in the plant dying from affixation after 24 hours. C3 fixation is greatly affected by this procedure and reduces efficiency of photosynthesis by 25%. This ends up in the plant producing energy which will be utilised to make more energy, resulting in breakthrough (energy not being stored). Photorespiration is expressed as:

RuBP + O2 → Phosphoglycolate + 3-phosphoglycerate + 2H+

Cellular respiration is expressed as:

C6H1206 + 602 + H20 → 6C02 + 12H20 + 673 Kcal

C3 carbon fixation is a metabolic pathway for carbon fixation in photosynthesis. This mechanism converts C02 and RuBP (5 carbon sugar) into 3-phosphoglycerate. This reaction happens in most plants as the first step of the Calvin-Benson Cycle. C4 fixation is a complex process of the more common C3 carbon fixation and is known to have evolved recently (5 million years ago). C4 and CAM overcame the tendency of the enzyme RuBisCO to wastefully fixing oxygen rather than C02 in photorespiration. This is achieved by using an efficient enzyme to fix CO2 in mesophyll cells and shuttling this fixed carbon by the bundle-sheath cells, which surround the vascular network. In these cells, RuBisCO is isolated from oxygen and saturated with the CO2 released by decarboxylation of the bundle-sheath. However, additional energy is required to produce ATP.  

This supplementary energy requirement, C4 fixation is able to efficiently fix carbon in arid conditions, with C3 pathways being more efficient in others such as temperate regions. Research conducted by the Horticultural Development Council, (2010) has suggested that photorespiration may be required for assimilating nitrates. A reduction in photorespiration can be achieved by genetic engineering or increasing atmospheric C02 which may not benefit plants that has been proposed in some studies. The overall energy specification of C3 photoautotrophs can be shown as:

6CO2 + 18ATP + 12NADPH + 12H20 → C6H1206 + 18ADP + 18Pi + 12NADPH + 6H+

The overall energy sine qua none of C4 plants can be expressed as:

6CO2 + 30ATP + 12NADPH + 12H20 → C6H1206 + 30ADP + 30Pi + 12NADPH + 18H+

C4 plants have increased C02 assimilation in comparison to C3’s but they consume more energy, they also demonstrate the ability to cope with stress better than C3’s. Refer to the table below for a comparison.

Table 3 Comparison of C3-C4 Pathways

Crassulacean Acid Metabolism is a carbon fixation pathway present in modern plants. These plants fix CO2 during the night, storing it as the four-carbon acid malate. The CO2 is released during the day, where it is concentrated around the enzyme RuBisCO which increases efficiency of photosynthesis. CAM pathways permit stomata to remain closed during the day, reducing evapotranspiration; therefore, it is especially frequent in plants adapted to arid conditions such as Cactaceae.

Table 4 Examples of carbon fixation

Photosynthetically reactive radiation

Photoreceptors are divided into two groups: sensor that absorbs maxmia and trandsue responses to light in the red and blue region. Specialisation of photoreceptors relate to the type of information available during daylight and its application. Absorption of photons by chlorophyll shifts the light transmitted through canopies to lower wavelengths so this section of the spectrum is important for sensing light quality. Blue receptors or cryptochrome is applied by plants as an indicator of sunlight because it is more subject to scattering gradients of light intensity across a short range of tissues.  

In aquatic environments blue light takes on greater importance due to natural absorption of red light by H20. Certain cryptochrome can be induced by UV-A radiation. Red light photoreceptors or phytochrome are employed by plants to sense light quality, presence, intensity and duration, phototropism and to sense the direction of light. Phytochrome is synthesised in the inactive form, which has a maximum absorption rate of 660nm. Carotenoid or the third sensor detects light intensity in specific cells. This photoreceptor has been identified in seed plants by a combination of genetic and biochemical research.

Far-red light is enriched in shade, light shifts the balance between the Pr and Pfr forms significantly towards Pr. This is the process of identifying competition, plants utilise these receptors to obtain advantages over their neighbours. Once a plant detects a competitor it can increase growth to gain the ability to sustain light levels.

In nature there will be various frequencies of photons that are absorbed by the two forms of phytochrome. This will result in individual molecules of phytochrome cycling between them. It has been found that the cycling rates can be influenced by light strength and that these cycling speeds can be converted to a signal regulated by adaptations to different photon intensities. The inactivation leads to the inhibition of seed germination in plants that usually thrive in high light levels. When mature plants respond to shade, the disappearance of Pfr signal which prevents stem elongation and stimulates development, leads to increased stature and limited allocation of resources to leaves until shade has been overcome.

Emerson, (1957) measured the efficiency of photosynthesis using a monochromatic light. He witnessed quantum yield (number of 02 evolved per light quanta) and two quanta are needed to transfer one electron. Therefore, eight quanta are required for the evolution of one 02 molecule. Quantum yield decreases sharply towards the far-red light part of the spectrum (680 nm); this region is described as the red drop. Emerson further observed that if the red light of shorter wavelengths was superimposed with far-red the rate of quantum yield would greatly be enhanced; this is referred to as the Emerson Effect. Peavy and Gibbs, (1975) identified that photosynthesis yield was increased by 25% when two lights were supplied simultaneously. They confirmed this by isolating chloroplasts in Spinacia oleracea. The following equation known as The Hill Reaction demonstrates the energy potential:

2H20 (4 Photons, Mn+ (Water splitting enzyme)) → 4H+ + 4e- + 02

Below is a table outlining the different pigmentation involved in light absorption.

Table 1 Pigments distribution in plants and bacteria

In summary, this chapter has focused on the mechanisms and processes involved in photosynthesis and photorespiration. How they are influenced by blue, red and far-red light; how they are required to work for elongation, flowering, budding, surface area expansion and ageing.  Several equations were present to establish the efficiency of photosynthesis and photorespiration and how it takes one millisecond to convert photons into starch. An explanation was provided to illustrate how these processes can be developed.

The Processes Governing Photosynthesis

Photosynthesis is the process by which carbon dioxide is converted into organic compounds, primarily sugars, using energy from sunlight. This process occurs in plants, algae, and many bacteria, but not in archaea. Organisms that carry out photosynthesis are referred to as photoautotrophs, as they produce their own food. In plants, algae, and cyanobacteria, photosynthesis utilises carbon dioxide and water, releasing oxygen as a by-product. This process is crucial for sustaining life on Earth, maintaining atmospheric oxygen levels and providing energy, either directly or indirectly, for nearly all living organisms. The energy captured through photosynthesis is enormous—around 100 terawatts, approximately six times the total power consumption of human civilisation. In addition to energy, photosynthesis is the source of carbon in all organic compounds within living organisms. Annually, photosynthetic organisms convert around 100 billion tonnes of carbon into biomass.

While photosynthesis varies among species, several key features are universal. The process always starts with light energy being absorbed by photosynthetic reaction centres, which contain chlorophyll. In plants, these centres are located within organelles called chloroplasts, whereas in bacteria, they are embedded in the plasma membrane. Some of the absorbed light energy is stored as adenosine triphosphate (ATP), while the rest is used to remove electrons from a molecule, typically water. These electrons then participate in reactions that convert carbon dioxide into organic compounds. In plants, algae, and cyanobacteria, this occurs via the Calvin cycle, though some bacteria use different pathways, such as the reverse Krebs cycle found in Chlorobium. Many photosynthetic organisms have developed adaptations to concentrate or store carbon dioxide, helping to mitigate photorespiration, a process that can reduce the efficiency of photosynthesis by consuming some of the sugars produced.

Photosynthesis is the primary mechanism by which plants, algae, and certain bacteria produce organic compounds and oxygen from carbon dioxide and water. The cycle between autotrophs (producers) and heterotrophs (consumers) is central to life on Earth.

Evolution of Photosynthesis

Photosynthesis evolved early in the history of life, during a time when the atmosphere was rich in carbon dioxide and all life forms were microorganisms. The first photosynthetic organisms likely emerged around 3.5 billion years ago, using hydrogen or hydrogen sulphide instead of water as electron donors. Cyanobacteria appeared later, around 3 billion years ago, and began oxygenating the Earth’s atmosphere roughly 2.4 billion years ago, profoundly altering the planet's environment. This rise in oxygen levels allowed for the evolution of more complex life forms, such as protists. Eventually, one of these protists formed a symbiotic relationship with a cyanobacterium, leading to the development of the ancestor of modern plants and algae. The chloroplasts found in today’s plants are the descendants of these ancient symbiotic cyanobacteria.

The general equation for photosynthesis is as follows:

2n CO2 + 2n H2O + photons → 2(CH2O) n + n O2 + 2n A

Carbon dioxide + electron donor + light energy → carbohydrate + oxygen + oxidized electron donor

Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:

2n CO2 + 2n H2O + photons → 2(CH2O) n + 2n O2

Carbon dioxide + water + light energy → carbohydrate + oxygen

Other processes substitute other compounds (such as arsenite) for water in the electron-supply role; the microbes use sunlight to oxidize arsenite to arsenates; the equation for this reaction is:

(AsO33-) + CO2 + photons → CO + (AsO43-)

Carbon dioxide + arsenite + light energy → arsenate + carbon monoxide (used to build other compounds in subsequent reactions)

Photosynthesis: The Two Stages

Photosynthesis occurs in two main stages:

Light-Dependent Reactions (Light Reactions): In this stage, the energy from light is captured and used to produce energy-storage molecules, namely ATP and NADPH. The process begins when a molecule of the pigment chlorophyll absorbs a photon, causing it to lose an electron. This electron is transferred to a modified form of chlorophyll known as pheophytin, which then passes the electron to a quinone molecule. This initiates a flow of electrons through an electron transport chain, ultimately leading to the reduction of NADP+ to NADPH. Simultaneously, a proton gradient is created across the chloroplast membrane. This gradient drives ATP synthase to produce ATP. Chlorophyll replaces the lost electron through photolysis, a process in which a water molecule is split, releasing an oxygen (O2) molecule.

Light-Independent Reactions (Calvin Cycle): In this stage, the ATP and NADPH generated during the light reactions are used to capture and reduce carbon dioxide, leading to the formation of organic compounds.

While most photosynthetic organisms that produce oxygen use visible light, there are at least three organisms that utilise infrared radiation for this process.

The overall equation for the light-dependent reactions, under non-cyclic electron flow conditions in green plants, is:

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

Wavelengths and photosynthesis

Not all wavelengths of light are effective in supporting photosynthesis. The photosynthetic action spectrum varies depending on the accessory pigments present in the organism. In green plants, the action spectrum closely matches the absorption spectrum of chlorophylls and carotenoids, with notable peaks in violet-blue and red light. In contrast, red algae have an action spectrum that aligns with the absorption spectrum of phycobilins, which are sensitive to blue-green light. This adaptation allows red algae to thrive in deeper waters where longer wavelengths of light, which are absorbed by green plants, are filtered out. The wavelengths of light that are not absorbed are reflected or transmitted and give photosynthetic organisms their characteristic colours, but they are less effective for photosynthesis in these organisms.

The Calvin-Benson Cycle

In the light-independent reactions, also known as the dark reactions, the enzyme RuBisCO captures carbon dioxide (CO₂) from the atmosphere. This process, which requires the newly formed NADPH, is part of the Calvin-Benson Cycle. During this cycle, CO₂ is converted into three-carbon sugars, which are subsequently combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is:

3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2

Carbon fixation 

Carbon fixation begins with the formation of an intermediate product, which is then converted into final carbohydrate products. The carbon skeletons produced through photosynthesis are utilised in various ways: they contribute to the formation of other organic compounds, such as the structural polysaccharide cellulose, serve as precursors for lipid and amino acid synthesis, or are used as fuel in cellular respiration. Cellular respiration occurs in both plants and animals, with energy being transferred through the food chain.

The fixation or reduction of carbon dioxide involves the reaction of CO₂ with the five-carbon sugar ribulose 1,5-bisphosphate (RuBP) to produce two molecules of a three-carbon compound, glycerate 3-phosphate (GP), also known as 3-phosphoglycerate (PGA). GP is then reduced to glyceraldehyde 3-phosphate (G3P) with the aid of ATP and NADPH from the light-dependent reactions. G3P is also known as 3-phosphoglyceraldehyde (PGAL) or triose phosphate, a three-carbon sugar. Most G3P is used to regenerate RuBP, allowing the cycle to continue. Of the six molecules of triose phosphate produced, one is typically used to form hexose phosphates, which are eventually converted into sucrose, starch, and cellulose. The resulting sugars provide carbon skeletons for other metabolic processes, such as the synthesis of amino acids and lipids.

C4, C3, and CAM Plants

In hot and dry conditions, plants close their stomata to minimise water loss. This reduction in CO₂ and the accumulation of oxygen, a by-product of photosynthesis, increase photorespiration and decrease carbon fixation. Some plants have evolved mechanisms to enhance CO₂ concentration in the leaves under these conditions.

C4 Plants: These plants fix carbon dioxide in the mesophyll cells by attaching it to the three-carbon molecule phosphoenolpyruvate (PEP), a reaction catalysed by PEP carboxylase. This produces the four-carbon organic acid oxaloacetic acid. The oxaloacetic acid, or malate, is then transported to specialised bundle sheath cells, where CO₂ is released through decarboxylation and fixed by the enzyme RuBisCO into the three-carbon sugar 3-phosphoglyceric acid. This separation of RuBisCO from the oxygen-generating light reactions reduces photorespiration and increases CO₂ fixation, enhancing the photosynthetic capacity of the leaf. C4 plants, such as maize, sorghum, sugarcane, and millet, can produce more sugar than C3 plants under high light and temperature conditions.

C3 Plants: These plants rely on the enzyme RuBisCO for carbon fixation, producing the three-carbon sugar 3-phosphoglyceric acid directly in the Calvin-Benson cycle. They do not use PEP carboxylase and are less efficient than C4 plants under hot and dry conditions.

CAM Plants: Xerophytes, such as cacti and many succulents, utilise Crassulacean acid metabolism (CAM). Unlike C4 metabolism, which physically separates CO₂ fixation from the Calvin cycle, CAM plants temporally separate these processes. CAM plants fix CO₂ at night when their stomata are open, storing it mainly as malic acid. During the day, the stored malic acid is decarboxylated to release CO₂, which is then fixed by RuBisCO into 3-phosphoglycerate.

Plants typically convert light into chemical energy with a photosynthetic efficiency of 3-6%. This efficiency can vary based on light frequency, intensity, temperature, and atmospheric CO₂ levels, ranging from 0.1% to 8%. In comparison, solar panels convert light into electrical energy with an efficiency of approximately 6-20% for mass-produced panels, and up to 41% in research settings.

Origin of chloroplasts

Various animal groups have developed symbiotic relationships with photosynthetic algae. This is particularly common among corals, sponges, and sea anemones, which often have relatively simple body plans and large surface areas relative to their volumes. Additionally, some marine mollusks, such as Elysia viridis and Elysia chlorotica, also form symbiotic relationships with chloroplasts. These mollusks capture chloroplasts from their algal diet and incorporate them into their own bodies, allowing them to survive through photosynthesis for several months. Remarkably, some plant genes have even been transferred to these mollusks, enabling the chloroplasts to receive the necessary proteins for survival.

Cyanobacteria and the evolution of photosynthesis

The ability to use water as a source of electrons in photosynthesis evolved just once, in a common ancestor of modern cyanobacteria. Geological evidence suggests this significant event occurred early in Earth’s history, between 2450 and 2320 million years ago (Ma), though it may have happened much earlier. Evidence from geobiological studies of Archean (>2500 Ma) sedimentary rocks indicates that life existed around 3500 Ma, but the exact timing of oxygenic photosynthesis's evolution remains uncertain. By around 2000 Ma, the fossil record reveals a diverse array of cyanobacteria. These microorganisms were the primary producers throughout the Proterozoic Eon (2500-543 Ma), largely because the ocean’s redox state favoured photoautotrophs capable of nitrogen fixation. Green algae began to complement cyanobacteria as major primary producers on continental shelves towards the end of the Proterozoic. However, it was not until the Mesozoic Era (251-65 Ma) that primary production in marine shelf waters took on its modern form, with the radiation of dinoflagellates, coccolithophorids, and diatoms. Cyanobacteria continue to be crucial in marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified forms, as plastids in marine algae.

Carbon dioxide levels and photorespiration (C2)

As carbon dioxide concentrations increase, the rate of sugar production in the light-independent reactions also rises, up to a point where it is limited by other factors. RuBisCO, the enzyme responsible for capturing carbon dioxide in these reactions, has an affinity for both carbon dioxide and oxygen. At high carbon dioxide levels, RuBisCO fixes carbon dioxide effectively. However, at low carbon dioxide levels, RuBisCO binds oxygen instead, a process known as photorespiration. This process consumes energy but does not result in sugar production.

Photorespiration poses several disadvantages for plants

One outcome of the oxygenase activity of RuBisCO is the production of phosphoglycolate (a two-carbon compound) instead of 3-phosphoglycerate (a three-carbon compound). Phosphoglycolate cannot be metabolised by the Calvin-Benson cycle, leading to a loss of carbon from the cycle. High oxygenase activity thus depletes the sugars needed to recycle ribulose 5-bisphosphate and sustain the Calvin-Benson cycle.

Phosphoglycolate is rapidly converted to glycolate, which is toxic at high concentrations and inhibits photosynthesis.

The salvage of glycolate is an energy-intensive process, involving the glycolate pathway. Only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate, and this process also produces ammonia (NH₃), which can diffuse out of the plant, leading to a loss of nitrogen.

A highly simplified summary is:

2 glycolate + ATP → 3-phosphoglycerate + carbon dioxide + ADP +NH3

 Transpirational pull

Transpirational pull results from the evaporation of water from the surfaces of cells within the leaves. As water evaporates, it creates a negative pressure in the pores of the cell walls, forming a concave meniscus. The high surface tension of water causes this meniscus to pull outward, generating a force strong enough to lift water up to 100 meters from the ground to the highest branches of a tree. This process relies on the small diameter of the water-conducting vessels, as larger diameters would lead to cavitation, which disrupts the water column. As water evaporates from the leaves, more water is drawn up through the plant to replace it. If the water pressure in the xylem becomes too low due to insufficient water from the roots, gases may come out of solution and form bubbles, creating embolisms. These bubbles can quickly spread to adjacent cells unless bordered pits are present to prevent their movement.

Cohesion tension theory

The Cohesion Tension Theory, proposed by John Joly and Henry Horatio Dixon, explains how water travels upward through the xylem of plants against gravity. Water molecules are polar due to the high electronegativity of the oxygen atom, which has two lone pairs of electrons. This polarity causes water molecules to form hydrogen bonds with one another, where the negatively charged oxygen atom of one molecule attracts the positively charged hydrogen atom of another. These hydrogen bonds contribute to several properties of water: they keep it in liquid form at room temperature, create surface tension, and enable cohesion and adhesion. Through these forces, water is drawn from the roots, transported through the xylem, and reaches the leaves, where it participates in photosynthesis to produce glucose from water and carbon dioxide.

Water is continually lost from the leaf through transpiration. As one water molecule evaporates, another is pulled along due to cohesion and adhesion. Transpirational pull, which relies on capillary action and the surface tension of water, is the primary mechanism for water movement in plants. However, it is complemented by other processes, as any use of water in the leaves also contributes to its movement.

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