What Is Photosynthesis?

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What Is Photosynthesis?

Photosynthesis takes in the carbon dioxide produced by all breathing organisms and reintroduces oxygen into the atmosphere.

Credit: KPG_Payless | Shutterstock

Photosynthesis is the process used by plants, algae and certain bacteria to harness energy from sunlight into chemical energy.

There are two types of photosynthetic processes: oxygenic photosynthesis and anoxygenic photosynthesis. Oxygenic photosynthesis is the most common and is seen in plants, algae and cyanobacteria.

During oxygenic photosynthesis, light energy transfers electrons from water (H2O) to carbon dioxide (CO2), which produces carbohydrates. In this transfer, the CO2 is “reduced,” or receives electrons, and the water becomes “oxidized,” or loses electrons. Ultimately, oxygen is produced along with carbohydrates.

Oxygenic photosynthesis functions as a counterbalance to respiration; it takes in the carbon dioxide produced by all breathing organisms and reintroduces oxygen into the atmosphere. In his 1998 article, “An Introduction to Photosynthesis and Its Applications,” Wim Vermaas, a professor at Arizona State University surmised, “without [oxygenic] photosynthesis, the oxygen in the atmosphere would be depleted within several thousand years.”

On the other hand, anoxygenic photosynthesis uses electron donors other than water. The process typically occurs in bacteria such as purple bacteria and green sulfur bacteria. “Anoxygenic photosynthesis does not produce oxygen — hence the name,” said David Baum, professor of botany at the University of Wisconsin-Madison. “What is produced depends on the electron donor. For example, many bacteria use the bad-eggs-smelling gas hydrogen sulfide, producing solid sulfur as a byproduct.”

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

Oxygenic photosynthesis is written as follows:

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 breathable oxygen and water.

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

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

As explained by Govindjee and John Whitmarsh in “Concepts in Photobiology: Photosynthesis and Photomorphogenesis” (Narosa Publishers and Kluwer Academic, 1999) 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).

The following are cellular components essential to photosynthesis.


Pigments are molecules that bestow color on plants, algae and bacteria, but they are also responsible for effectively trapping sunlight. Pigments of different colors absorb different wavelengths of light. Below are the three main groups.

  • Chlorophylls: These green-colored pigments are capable of trapping blue and red light. Chlorophylls have three sub-types, dubbed chlorophyll a, chlorophyll b and chlorophyll c. According to Eugene Rabinowitch and Govindjee in their book “Photosynthesis” (Wiley, 1969) chlorophyll a is found in all photosynthesizing plants. There is also a bacterial variant aptly named bacteriochlorophyll, which absorbs infrared light. This pigment is mainly seen in purple and green bacteria, which perform anoxygenic photosynthesis.
  • Carotenoids: These red, orange, or yellow-colored pigments absorb bluish-green light. Examples of carotenoids are xanthophyll (yellow) and carotene (orange) from which carrots get their color.
  • Phycobilins: These red or blue pigments absorb wavelengths of light that are not as well absorbed by chlorophylls and carotenoids. They are seen in cyanobacteria and red algae.

Photosynthetic eukaryotic organisms contain organelles called plastids in their cytoplasm. According to Cheong Xin Chan and Debashish Bhattacharya of Rutgers University (Nature Education, 2010), the double-membraned plastids in plants and algae are referred to as primary plastids, while the multiple-membraned variety found in plankton are called secondary plastids. These organelles generally contain pigments or can store nutrients. In “The Cell: A Molecular Approach 2nd Ed” (Sinauer Associates, 2000), Geoffrey Cooper enumerates the various plastids found in plants. Colorless and non-pigmented leucoplasts store fats and starch, while chromoplasts contain carotenoids and chloroplasts contain chlorophyll.

Photosynthesis occurs in the chloroplasts, specifically, in the grana and stroma regions. The grana is the innermost portion of the organelle; a collection of disc-shaped membranes, stacked into columns like plates. The individual discs are called thylakoids. It is here that the transfer of electrons takes place. The empty spaces between columns of grana constitute the stroma (The Cell: A Molecular Approach 2nd Ed, Sinauer Associates, 2000).

Chloroplasts are similar to mitochondria in that they have their own genome, or collection of genes, contained within circular DNA. These genes encode proteins essential to the organelle and to photosynthesis.  Like mitochondria, chloroplasts are also thought to have originated from primitive bacterial cells through the process of endosymbiosis.

“Plastids originated from engulfed photosynthetic bacteria that were acquired by a single-celled eukaryotic cell more than a billion years ago,” Baum told LiveScience. Baum explained that the analysis of chloroplast genes shows that it was once a member of the group cyanobacteria, “the one group of bacteria that can accomplish oxygenic photosynthesis.”

However, Chan and Bhattacharya (Nature Education, 2010) make the point that the formation of secondary plasmids cannot be well explained by endosymbiosis of cyanobacteria, and that the origins of this class of plastids are still a matter of debate.


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 “antennae,” according to Vermaas. These structures effectively capture light energy from the sun, in the form of photons. 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, for example, 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.

Reaction centers

The pigments and proteins which convert light energy to chemical energy and begin the process of electron transfer are know as reaction centers, according to Vermaas.

Anoxygenic photosynthetic and oxygenic photosynthetic organisms use different electron donors for photosynthesis. Moreover, anoxygenic photosynthesis takes place in only one type of reaction center, while oxygenic photosynthesis takes place in two, each of which absorbs a different wavelength of light, according to Govindjee and Whitmarsh. However, the general principles of the two processes are similar. Below are the steps of photosynthesis, focusing on the process as it occurs in plants.

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

Light-dependent reactions (also called light reactions): When a photon of light hits the reaction center, a pigment molecule such as chlorophyll releases an electron. “The trick to do useful work, is to prevent that electron from finding its way back to its original home,” Baum told LiveScience. “This is not easily avoided because the chlorophyll now has an “electron hole” that tends to pull on nearby electrons.” 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. The “electron hole” in the original chlorophyll pigment is filled by taking an electron from water. As a result, oxygen is released into the atmosphere.

Light-independent reactions (also called dark reactions): ATP and NADPH are rich energy sources, which drive dark reactions. During this process carbon dioxide and water combine to form carbohydrates like glucose. This is known as carbon fixation.

Photosynthesis in the future

Photosynthesis generates all the breathable oxygen in the atmosphere, and renders plants rich in nutrients. But researchers have been looking at ways to further harness the power of the process.

In his 1998 article, Vermaas mentions the possibility of using photosynthetic organisms to generate clean burning fuels such as hydrogen or even methane. Vermaas notes, “Even though methane upon combustion will form CO2, the overall atmospheric CO2 balance would not be disturbed as an equal amount of CO2 will have been taken out of the atmosphere upon methane production by the photosynthetic organism.”

Advances have also been made in the field of artificial photosynthesis. A group of researchers recently developed an artificial system to capture carbon dioxide using nanotechnology (nanowires). This feeds into a system of microbes that reduce the carbon dioxide into fuels or polymers by using energy from sunlight.

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Watch Strange, Glowing Bacteria Harpoon and Swallow DNA to Evolve

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Watch Strange, Glowing Bacteria Harpoon and Swallow DNA to Evolve

Watch Strange, Glowing Bacteria Harpoon and Swallow DNA to Evolve

A bacterium “harpoons” a bit of stray DNA in this first-of-its-kind recording. On the left, you can see the scene without the fluorescent dyes. On the right, you can see the scene with the fluorescent dyes.

Credit: Ankur Dalia, Indiana University

In an astonishing new video, a bacterium reaches out into space, snatches a piece of DNA and stuffs that DNA into its own body. Its appendage, much longer than its own body, wanders and bends a little but seems to move with intention toward its target. And the whole act is part of the microbe’s effort to evolve.

The video is the first direct observation of bacteria using appendages called pili to “harpoon” loose DNA and incorporate it into the bacteria’s own genetic structures. It shows how the single-celled organisms pull off a neat trick called “horizontal gene transfer” that lets them adapt quickly to new environments. This would be a bit like if a person who’s allergic to pollen needed only to reach out, snatch some loose flesh from a nonallergic friend and swallow it to get through spring without sneezing. [5 Ways Gut Bacteria Affect Your Health]

Researchers already knew that bacteria needed their pili to pull off horizontal gene transfer, but they’d never seen the maneuver in action, in part because the pili are too tiny to easily observe through a microscope. A single pilus, according to the videographers, is less than one-ten-thousandth the width of a human hair. And the hole the bacteria use to haul the loose DNA into their own single-celled “bodies” is “almost the exact width of a DNA helix bent in half,” the researchers said in a statement.

So, to record the video, the researchers dyed the pili of Vibrio cholerae, the bacterium responsible for cholera, with fluorescent dye. The dye also covered the bacteria and the loose DNA. Then, the researchers stuck the bacteria and stray DNA under a regular microscope and waited to see what the now-glowing organism would do.

The researchers said they hope the findings, which were published June 11 in the journal Nature Microbiology, might be helpful for research into antibiotic-resistant bacteria.

Originally published on Live Science.

Corpse of Mysterious Sea Creature Washes Ashore in Namibia

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Corpse of Mysterious Sea Creature Washes Ashore in Namibia

Corpse of Mysterious Sea Creature Washes Ashore in Namibia

Scientists found the decomposed body of a sea creature on Nambian shores. They think it is a Cuvier’s beaked whale (Ziphius cavirostris).

Credit: Nambian Dolphin Project/Caters News

A bizarre-looking, nearly 20-foot-long (6 meters) sea creature washed ashore at Dorob National Park in Namibia last week. When scientists found the body, it was so decomposed that they didn’t really know what they were looking at — it could’ve been a dolphin or a whale, or something else, according to the Daily Mail.

After measuring the carcass and analyzing the shape of its head, the scientists are now almost certain that the mysterious creature is a Cuvier’s beaked whale (Ziphius cavirostris) — a creature that hasn’t been sighted in Namibia since 2000, according to Simon Elwen, a principal investigator of the nonprofit Namibian Dolphin project and one of the researchers who found the creature, as reported by the Daily Mail.

“I was quite surprised,” Elwen told the Daily Mail. “These animals are rarely seen in the water, so to see them on land is very unique.” [Marine Marvels: Spectacular Photos of Sea Creatures]

Cuvier’s beaked whales can be found across the world and tend live in temperate, subtropical and tropical waters. They can weigh up to 6,800 lbs. (3,090 kilograms) and can grow up to 23 feet (7 m) long, according to the National Oceanic and Atmospheric Administration (NOAA). They have a “goose-like” head with an upward-slanted jawline that makes them look as if they are smiling, according to NOAA.

Because the body was so decomposed, the scientists couldn’t figure out the cause of death, according to the Daily Mail. Though the jawbone was cracked and broken, the scientists think that happened after death, since the creature didn’t have any other visible injuries, according to the Daily Mail.

On the IUCN Red List of Threatened Species, the Cuvier’s beaked whale is listed as “least concern.” Although global trends and population numbers for this elusive creature don’t exist, there are at least 100,000 of them in the world, according to the IUCN. Possible threats to this species include entanglement in fishing gear, collisions with ships and human-caused noise, such as from ships.

The Cuvier’s beaked whale is one of the deepest divers — plunging to a depth of about 3,300 feet (1,000 m). In addition, the species uses sound to find food, communicate with each other and navigate.

The team collected parts of the animal, including its skull, to investigate further, Elwen said.

Originally published on Live Science.

Did These Children Have Their Hearts Ripped Out as a Sacrifice to an Ancient Rain God?

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Did These Children Have Their Hearts Ripped Out as a Sacrifice to an Ancient Rain God?

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Did These Children Have Their Hearts Ripped Out as a Sacrifice to an Ancient Rain God?

An archaeologist excavates a 1,500-year-old site in northern Peru on March 21. Researchers have found 77 burials belonging to a mixture of pre-Inca Chimú, Salinar and Viru cultures at this site.

Credit: Peru excavation

Construction workers in northern Peru recently uncovered a grisly discovery: The skeletal remains of 47 ancient people, including those of at least 12 children who were likely sacrificed by the ancient Chimú culture about 1,500 years ago.

The children’s chest bones had cut marks on them, likely a sign of an attempt to break their ribs so that their hearts could be removed, archaeologist Víctor Campaña León, director of the Las Lomas Archaeological Rescue Project, told La República, a Peruvian newspaper.

After finding the bones, the workers — who were laying down drinking-water pipes in the beach town of Huanchaco — notified archaeologists. In the following excavation, the researchers unearthed 77 tombs and burials, as well as camelid bones (likely from vicuña or alpaca) and 115 vessels from the Chimú, Salinar and Virú cultures, La República reported. [25 Cultures That Practiced Human Sacrifice]

In addition to the 12 children, “We have also found a neonate, a newborn, who has also been sacrificed,” Campaña León said in Spanish. The excavations began on Oct. 23, 2017, and are projected to end on June 23, 2018, according to Andina, a Peruvian news outlet.

This is hardly the first evidence of human sacrifice in pre-Columbian societies. Archaeologists have also uncovered the remains of sacrifice victims associated with the Inca, Maya and Aztec cultures. Meanwhile, human sacrifice was also practiced in ancient Rome, China and Japan, as well as at Cahokia, an early Native American city located by modern-day St. Louis, Live Science previously reported.

In the case of the Chimú discovery, it’s possible these children were sacrificed with the hope of encouraging the gods to bring rain to the arid region,  Campaña León said, according to Newsweek.

The Chimú civilization lasted from about A.D. 900 to 1470, when the Inca conquered them, according to the Encyclopedia Britannica. The Chimú people are known for their pottery, textiles, irrigation and metalwork with gold, silver and copper. In fact, the Chimú capital, called Chan Chan, is recognized by the United Nations Educational, Scientific and Cultural Organization(UNESCO) for its “absolute masterpiece of town planning.”

Original article on Live Science.

The Incas Mastered the Grisly Practice of Drilling Holes in People’s Skulls

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The Incas Mastered the Grisly Practice of Drilling Holes in People’s Skulls

The Incas Mastered the Grisly Practice of Drilling Holes in People's Skulls

Ancient skulls from Peru show signs of trepanation. The chances of surviving trepanation were better in ancient Peru than during the American Civil War.

Credit: University of Miami

If you had a hole drilled through your skull in historical times, the odds of surviving the ordeal were far better in the ancient Inca Empire of South America than they were in North America during the American Civil War, a new study finds.

Researchers made the finding by studying more than 800 Inca skulls found in Peru that had undergone trepanation — a practice in which a surgeon cuts, scrapes or drills a hole in a person’s head. Between 17 and 25 percent of these Inca patients died before their skulls healed, the researchers found.

In comparison, during the American Civil War (1861 to 1865), more than twice that percentage — between 46 and 56 percent of soldiers — died so soon after trepanation that their skulls had no time to heal, the researchers discovered. [25 Grisly Archaeological Discoveries]

“That’s a big difference,” study researcher Dr. David Kushner, a clinical professor of physical medicine and rehabilitation at the University of Miami Miller School of Medicine, said in a statement. “The question is: How did the ancient Peruvian surgeons have outcomes that far surpassed those of surgeons during the American Civil War?”

Trepanation is thousands of years old and, historically, was done to suppress headaches, seizures and mental illness, as well as to oust perceived demons. Given that the Inca Empire existed a good 300 years before the American Civil War, it’s impressive that Inca trepanation patients had twice the survival rate of Civil War patients, Kushner said.

That difference likely comes down to hygiene, as sanitation was notoriously horrible on Civil War battlefields, the researchers said. For instance, Civil War surgeons regularly used unsterilized medical tools, and even their bare fingers, to dig inside head wounds or break up blood clots, said study co-researcher John Verano, a world authority on Peruvian trepanation at Tulane University in New Orleans.

Nearly every Civil War soldier wounded by gunfire later suffered from an infection, but the Inca appear to have experienced a much lower infection rate, the researchers said.

“We do not know how the ancient Peruvians prevented infection, but it seems that they did a good job of it,” Kushner said. “Neither do we know what they used as anesthesia, but since there were so many [cranial surgeries], they must have used something — possibly coca leaves. Maybe there was something else, maybe a fermented beverage. There are no written records, so we just don’t know.”

The Inca skulls the researchers studied — some with as many as seven holes in them — date back to 400 B.C. These skulls indicate that the Inca refined their trepanation skills over the centuries. For example, the Inca learned not to perforate the dura, or the protective membrane covering the brain — a guideline that Hippocrates codified in ancient Greece at about the same time, in the fifth century B.C.

However, early Inca trepanation patients — who lived from about 400 B.C. to 200 B.C. — fared slightly worse than Civil War patients, as about half of these ancient Inca patients died. It was much better to be a trepanation patient from A.D. 1000 to A.D. 1400, when up to 91 percent of patients survived.

“Over time, from the earliest to the latest, they learned which techniques were better and less likely to perforate the dura,” Kushner said. “They seemed to understand head anatomy and purposefully avoided the areas where there would be more bleeding. They also realized that larger-sized trepanations were less likely to be as successful as smaller ones. Physical evidence definitely shows that these ancient surgeons refined the procedure over time. Their success is truly remarkable.”

Doctors still practice trepanation today, although now when they remove a piece of someone’s skull, it’s usually called a craniotomy. This operation and other types of modern brain surgery have “very, very low” mortality rates compared to historical times, Kushner said.

“And, just like in ancient Peru, we continue to advance our neurosurgical techniques, our skills, our tools and our knowledge,” he said.

The study was published in the June issue of the journal World Neurosurgery.

Original article on Live Science.