What is photosynthesis?

Photosynthesis is the process that releases the oxygen we breathe.

Photosynthesis takes in the carbon dioxide produced by all breathing organisms and reintroduces oxygen into the atmosphere. (Image credit: KPG_Payless | Shutterstock)

Photosynthesis is the process used by plants, algae and certain bacteria to turn sunlight, carbon dioxide (CO2) and water into food (sugars) and oxygen. Here's a look at the general principles of photosynthesis and related research to help develop clean fuels and sources of renewable energy.  

Types of photosynthetic processes

There are two types of photosynthetic processes: oxygenic photosynthesis and anoxygenic photosynthesis. They both follow very similar principles, but oxygenic photosynthesis is the most common and is seen in plants, algae and cyanobacteria. 

During oxygenic photosynthesis, light energy transfers electrons from water (H2O) taken up by plant roots to CO2 to produce carbohydrates. In this transfer, the CO2 is "reduced," or receives electrons, and the water is "oxidized," or loses electrons. Oxygen is produced along with carbohydrates.

Oxygenic photosynthesis functions as a counterbalance to respiration by taking in the CO2 produced by all breathing organisms and reintroducing oxygen to the atmosphere.

Anoxygenic photosynthesis, meanwhile, uses electron donors that are not water and do not produce oxygen, according to "Anoxygenic Photosynthetic Bacteria" by LibreTexts. The process typically occurs in bacteria such as green sulfur bacteria and phototrophic purple bacteria. 

The Photosynthesis equation

Though both types of photosynthesis are complex, multistep affairs, the overall process can be neatly summarized as a chemical equation.

The oxygenic photosynthesis equation is: 

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

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

Similarly, the various anoxygenic photosynthesis reactions can be represented as a single generalized formula:

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

The letter A in the equation is a variable, and H2A represents the potential electron donor. For example, "A" may represent sulfur in the electron donor hydrogen sulfide (H2S), according to medical and life sciences news site News Medical Life Sciences

How is carbon dioxide and oxygen exchanged?

Stomata are the gatekeepers of the leaf, allowing gas exchange between the leaf and surrounding air.  (Image credit: Waldo Nell / 500px via Getty Images)

Plants absorb CO2 from the surrounding air and release water and oxygen via microscopic pores on their leaves called stomata. Stomata are the gatekeepers of gas exchange between the inside of plants and the external environment. 

When stomata open, they let in CO2; however, while open, the stomata release oxygen and let water vapor escape. In a bid to reduce the amount of water lost, stomata close, but that means the plant can no longer gain CO2 for photosynthesis. This tradeoff between CO2 gain and water loss is a particular problem for plants growing in hot, dry environments. 

How do plants absorb sunlight for photosynthesis?

Plants contain special pigments that absorb the light energy needed for photosynthesis.

Chlorophyll is the primary pigment used for photosynthesis and gives plants their green color, according to science education site Nature Education. Chlorophyll absorbs red and blue light to use in photosynthesis and reflects green light. Chlorophyll is a large molecule and takes a lot of resources to make; as such, it breaks down towards the end of the leaf's life, and most of the pigment's nitrogen (one of the building blocks of chlorophyll) is resorbed back into the plant, according to Harvard University's The Harvard Forest. When leaves lose their chlorophyll in the fall, other leaf pigments such as carotenoids and anthocyanins begin to show their true colors. While carotenoids primarily absorb blue light and reflect yellow, anthocyanins absorb blue-green light and reflect red light.

Pigment molecules are associated with proteins, which allow them the flexibility to move toward light and toward one another. A large collection of 100 to 5,000 pigment molecules constitutes an "antenna," according to an article by Wim Vermaas, a professor at Arizona State University. These structures effectively capture light energy from the sun, in the form of photons.

The situation is a little different for bacteria. While cyanobacteria contain chlorophyll, other bacteria, for example, purple bacteria and green sulfur bacteria, contain bacteriochlorophyll to absorb light for anoxygenic photosynthesis, according to "Microbiology for Dummies" (For Dummies, 2019). 

Related: What if humans had photosynthetic skin?

Where in the plant does photosynthesis take place?

Plants need energy from sunlight for photosynthesis to occur. (Image credit: Shutterstock)

Photosynthesis occurs in chloroplasts, a type of plastid (an organelle with a membrane) that contains chlorophyll and is primarily found in plant leaves. Double-membraned plastids in plants and algae are known as primary plastids, while the multiple-membraned variety found in plankton are called secondary plastids, according to a 2010 article in the journal Nature Education by Cheong Xin Chan and Debashish Bhattacharya, researchers at Rutgers University in New Jersey. 

Chloroplasts are similar to mitochondria, the energy centers of cells, in that they have their own genome, or collection of genes, contained within circular DNA. These genes encode proteins that are essential to the organelle and to photosynthesis.

Inside chloroplasts are plate-shaped structures called thylakoids that are responsible for harvesting photons of light for photosynthesis, according to the biology terminology website Biology Online. The thylakoids are stacked on top of each other in columns known as grana. In between the grana is the stroma — a fluid containing enzymes, molecules and ions, where sugar formation takes place. 

Ultimately, light energy must be transferred to a pigment-protein complex that can convert it to chemical energy, in the form of electrons. In plants, light energy is transferred to chlorophyll pigments. The conversion to chemical energy is accomplished when a chlorophyll pigment expels an electron, which can then move on to an appropriate recipient. 

The pigments and proteins that convert light energy to chemical energy and begin the process of electron transfer are known as reaction centers.

The reactions of plant photosynthesis are divided into two major stages: those that require the presence of sunlight (light-dependent reactions) and those that do not (light-independent reactions). Both types of reactions take place in chloroplasts: light-dependent reactions in the thylakoid and light-independent reactions in the stroma.  

Light-dependent reactions

When a plant absorbs solar energy it first needs to convert it into chemical energy. 

When a photon of light hits the reaction center, a pigment molecule such as chlorophyll releases an electron.

The released electron manages to escape by traveling through an electron transport chain, which generates the energy needed to produce ATP (adenosine triphosphate, a source of chemical energy for cells) and NADPH — both of which are required in the next stage of photosynthesis in the Calvin cycle. The "electron hole" in the original chlorophyll pigment is filled by taking an electron from water. This splitting of water molecules releases oxygen into the atmosphere.

Light-independent reactions: The Calvin cycle

Photosynthesis involves a process called the Calvin cycle to use energy stored from the light-dependent reactions to fix CO2 into sugars needed for plant growth.  (Image credit: Nagendra Yadav via Getty Images)

The Calvin cycle uses energy stored from the light-dependent reactions to fix CO2 into sugars needed for plant growth. According to the Khan Academy, these reactions take place in the stroma of the chloroplasts and are not directly driven by light — hence their name "light-independent reactions." However, they are still related to light as the Calvin cycle is fuelled by ATP and NADPH (both from the previously mentioned light-dependent reactions).

Firstly, CO2 combines with ribulose-1,5-bisphosphate (RuBP) which is a five-carbon acceptor, according to the Khan Academy. Next, it splits into two molecules of a three-carbon compound — 3-phosphoglyceric acid (3-PGA). The reaction is catalyzed by an enzyme called RuBP carboxylase/oxygenase, also known as rubisco. 

The second stage of the Calvin cycle involves converting 3-PGA into a three-carbon sugar called glyceraldehyde-3-phosphate (G3P) — the process uses ATP and NADPH. Finally, while some G3P molecules are used to make glucose, others are recycled back to make RuBP, which is used in the first step to accept CO2. For every one molecule of G3P that makes glucose, five molecules are recycled to generate three RuBP acceptor molecules. 

Photorespiration

According to the Khan Academy, rubisco can sometimes fix oxygen instead of CO2 in the Calvin cycle, which wastes energy — a process known as photorespiration. The enzyme evolved during a time when atmospheric CO2 levels were high and oxygen was rare, so it had no reason to differentiate between the two, according to researchers in Canada

Photorespiration is a particularly big problem when plants have their stomata closed to conserve water and are therefore not taking in any more CO2. Rubisco has no other choice but to fix oxygen instead, which in turn lowers the photosynthetic efficiency of the plant. This means that less plant food (sugars) will be produced, which could result in a slowdown of growth and therefore smaller plants. 

This is a big problem for agriculture, as smaller plants mean a smaller harvest. There are mounting pressures on the agricultural industry to increase plant productivity to feed our ever-expanding global population. Scientists are constantly looking for ways to increase photosynthetic efficiency and reduce the occurrence of wasteful photorespiration. 

Types of photosynthesis

There are three main types of photosynthetic pathways: C3, C4 and CAM. They all produce sugars from CO2 using the Calvin cycle, but each pathway is slightly different.

The three main types of photosynthetic pathways are C3, C4 and CAM. Most plants use C3 photosynthesis, including rice and cotton.  (Image credit: Andrew TB Tan via Getty Images)

C3 photosynthesis

Most plants use C3 photosynthesis, according to the photosynthesis research project Realizing Increased Photosynthetic Efficiency (RIPE), including cereals (wheat and rice), cotton, potatoes and soybeans. C3 photosynthesis is named for the three-carbon compound called 3-phosphoglyceric acid (3-PGA) that it uses during the Calvin cycle. 3-PGA is produced when rubisco fixes CO2, forming the three-carbon compound.

C4 photosynthesis

Plants such as maize and sugarcane use C4 photosynthesis. This process uses a four-carbon compound intermediate (called oxaloacetate) which is converted to malate, according to Biology Online. Malate is then transported into the bundle sheath where it breaks down and released CO2, which is then fixed by rubisco and made into sugars in the Calvin cycle (just like C3 photosynthesis). C4 plants are better adapted to hot, dry environments and can continue to fix carbon even when their stomata are closed (as they have a clever storage solution), which reduces their risk of photorespiration, according to Biology Online. 

CAM photosynthesis

Crassulacean acid metabolism (CAM) is found in plants adapted to very hot and dry environments, such as cacti and pineapples, according to the educational website Khan Academy. When stomata open to take in CO2, they risk losing water to the external environment. Because of this, plants in very arid and hot environments have adapted. One adaptation is CAM, whereby plants open stomata at night (when temperatures are lower and water loss is less of a risk). According to the Khan Academy, CO2 enters the plants via the stomata and is fixed into oxaloacetate and converted into malate or another organic acid (like in the C4 pathway). The CO2 is then available for light-dependent reactions in the daytime, and stomata close, reducing the risk of water loss. 

How photosynthesis could combat climate change

Photosynthetic organisms are a possible means to generate clean-burning fuels such as hydrogen. 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 researchers devised a way to extend green algae's hydrogen production for up to three days, as reported in their 2018 study published in the journal Energy & Environmental Science.

Scientists have also made advances in the field of artificial photosynthesis. For instance, a group of researchers from the University of California, Berkeley, developed an artificial system to capture CO2 using nanowires, or wires that are a few billionths of a meter in diameter. The wires feed into a system of microbes that reduce CO2 into fuels or polymers by using energy from sunlight. The team published its design in 2015 in the journal Nano Letters

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 CO2. In general, plants are only able to harness about one percent of solar energy and use it to produce organic compounds during photosynthesis. In contrast, the researchers' artificial system was able to harness 10% of solar energy to produce organic compounds.

In 2019, researchers wrote in the Journal of Biological Chemistry that cyanobacteria could boost the efficiency of the enzyme rubisco. Scientists found that this bacteria is particularly good at concentrating CO2 in its cells, which helps stop rubisco from accidentally binding to oxygen. By understanding how the bacteria achieve this, scientists hope to incorporate the mechanism into plants to help boost photosynthetic efficiency and reduce the risk of photorespiration. 

Continued research of natural processes aids scientists in developing new ways to utilize various sources of renewable energy, and tapping into the power of photosynthesis is a logical step for creating clean-burning and carbon-neutral fuels.

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