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:
CO2 + 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 photosynthetic apparatus
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.
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.
The photosynthetic process
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.