Photosynthesis

We explain what photosynthesis is, its characteristics, equation and phases. Also, why it is important for global ecosystems.

What is photosynthesis?

Photosynthesis is the biochemical process by which Plants, algae, and photosynthetic bacteria convert inorganic matter (carbon dioxide and water) into organic matter (sugars) taking advantage of the energy from sunlight. This is the main nutrition mechanism of all autotrophic organisms that have chlorophyll, which is the essential pigment for the photosynthetic process.

Photosynthesis constitutes one of the most important biochemical mechanisms on the planet since it involves the manufacture of organic nutrients that store light energy from the Sun in different useful molecules (carbohydrates). In fact, the name of this process comes from the Greek voices photo“light”, and synthesis“composition”.

After photosynthesis, Synthesized organic molecules can be used as a source of chemical energy to sustain vital processes, such as cellular respiration and other reactions that are part of the metabolism of living beings.

To carry out photosynthesis, the presence of chlorophyll is required a pigment sensitive to sunlight, which gives plants and algae their characteristic green coloration. This pigment is found in chloroplasts, cellular organelles of various sizes that are typical of plant cells, especially foliar cells (leaves). Chloroplasts contain a set of proteins and enzymes that allow the development of the complex reactions that are part of the photosynthetic process.

The photosynthesis process It is essential for the ecosystem and for life as we know them, since it allows the creation and circulation of organic matter and the fixation of inorganic matter. Furthermore, during oxygenic photosynthesis, the oxygen that most living beings need for respiration is produced.

See also: Producing organizations

Types of photosynthesis

Two types of photosynthesis can be distinguished, depending on the substances used by the organism to carry out the reaction:

• Oxygenic photosynthesis. It is characterized by the use of water (H₂O) for the reduction of carbon dioxide (CO₂) consumed. In this type of photosynthesis, not only useful sugars are produced for the body, but oxygen is also obtained (O₂) as a product of the reaction. Plants, algae and cyanobacteria carry out oxygenic photosynthesis.
• Anoxygenic photosynthesis. The body does not use water to reduce carbon dioxide (CO₂), but rather takes advantage of sunlight to break down hydrogen sulfide (H) molecules.₂S) or hydrogen gas (H₂). This type of photosynthesis does not produce oxygen (O₂) and, instead, releases sulfur as a product of the reaction. Anoxygenic photosynthesis is carried out by the so-called green and purple sulfur bacteria, which contain photosynthetic pigments grouped under the name bacteriochlorophylls, which are different from the chlorophyll of plants.

Features of photosynthesis

Broadly speaking, photosynthesis is characterized by the following:

• It is a biochemical process of using sunlight to obtain organic compounds, that is, the synthesis of nutrients from inorganic elements such as water (H₂O) and carbon dioxide (CO₂).
• It can be carried out by various autotrophic organisms, as long as they have photosynthetic pigments (the most important is chlorophyll). It is the nutrition process of plants (both terrestrial and aquatic), algae, phytoplankton, photosynthetic bacteria. A few animals are capable of photosynthesis, including the sea slug. Elysia chlorotica and the spotted salamander Ambystoma maculatum (the latter does so thanks to symbiosis with an algae).
• In plants and algae, photosynthesis takes place in specialized organelles called chloroplasts, in which chlorophyll is found. Photosynthetic bacteria also have chlorophyll (or other analogous pigments), but do not have chloroplasts.
• There are two types of photosynthesis, depending on the substance used to fix the carbon from carbon dioxide (CO₂). Oxygenic photosynthesis uses water (H₂O) and produces oxygen (O₂), which is released into the surrounding environment. Anoxygenic photosynthesis uses hydrogen sulfide (H₂S) or hydrogen gas (H₂), and does not produce oxygen but releases sulfur.
• The relationship between sunlight and plants was already postulated since Ancient Greece. However, advances in the study and understanding of photosynthesis began to gain importance thanks to the contributions of a successive set of scientists from the 18th, 19th and 20th centuries. For example, the first to demonstrate the generation of oxygen in plants was the English cleric Joseph Priestley (1732-1804) and the first to formulate the basic equation of photosynthesis was the German botanist Ferdinand Sachs (1832-1897). Later, the American biochemist Melvin Calvin (1911-1997) made another enormous contribution, clarifying the Calvin cycle (one of the phases of photosynthesis), which earned him the Nobel Prize in Chemistry in 1961.

Photosynthesis equation

The general equation for oxygenic photosynthesis is as follows:

The correct way to formulate this equation chemically, that is, the balanced equation of this reaction, is as follows:

Phases of photosynthesis

Photosynthesis as a chemical process occurs in two differentiated stages: the light (or light) stage and the dark stage so called because only the presence of sunlight directly intervenes in the first (which does not mean that the second necessarily occurs in the dark).

• Luminous or photochemical stage. During this phase, light-dependent reactions occur inside the plant, that is, the plant captures solar energy through chlorophyll and uses it to produce ATP and NADPH. It all begins when the chlorophyll molecule comes into contact with solar radiation and the electrons in its outer layers are excited, which generates an electron transport chain (similar to electricity), which is used for the synthesis of ATP (adenosine). triphosphate) and NADPH (nicotin adenine dinucleotide phosphate). The breakdown of a water molecule in a process called “photolysis” allows a chlorophyll molecule to recover the electron it lost when excited (the excitation of several chlorophyll molecules is required to carry out the light phase). As a result of the photolysis of two water molecules, an oxygen molecule is produced that is released into the atmosphere as a byproduct of this phase of photosynthesis.
• Dark or synthetic stage. During this phase, which takes place in the matrix or stroma of the chloroplasts, the plant uses carbon dioxide and takes advantage of the molecules generated during the previous stage (chemical energy) to synthesize organic substances through a circuit of very complex chemical reactions known like the Calvin-Benson Cycle. During this cycle, and through the intervention of different enzymes, the previously formed ATP and NADPH, glucose is synthesized from the carbon dioxide that the plant takes from the atmosphere. The incorporation of carbon dioxide into organic compounds is known as carbon fixation.

Importance of photosynthesis

Photosynthesis is a vital and central process in the biosphere due to multiple reasons. The first and most obvious is that produces oxygen (O₂) a gas essential for breathing in both water and air. Without plants, most living things (including humans) simply could not survive.

On the other hand, by absorbing it from the surrounding environment, Plants fix carbon dioxide (CO₂) converting it into organic matter. This gas, which we exhale when breathing, is potentially toxic if it is not kept within certain limits.

Because plants use carbon dioxide to make their own food, the decline in plant life on the planet increases this gas in the atmosphere, where it functions as an agent of global warming. For example, the CO₂ It acts as a greenhouse gas, preventing excess heat that reaches Earth from radiating out of the atmosphere. It is estimated that each year photosynthetic organisms fix around 100,000 million tons of carbon as organic substances.

Continue with: Calvin Cycle

References

• “Photosynthesis” on Wikipedia.
• “Photosynthesis – biology” (video) in Educatina.
• “Introduction to photosynthesis” at Khan Academy.
• “Photosynthesis: stages” in Hypertexts of the Biology Area.
• “Photosynthesis” in Conicet Mendoza (Argentina).
• “Photosynthesis” by Miguel Ángel de la Rosa at the University of Seville (Spain).
• Biology: Solomon E., Berg L., Martin D. (2013) 9th Edition. Cengage Learning Publishing.