Photosynthesis

Photosynthesis is a process by which solar energy is converted into chemical energy to carry out the synthesis of organic compounds. Photosynthesis is primarily responsible for energy input into the biosphere and is carried out by organisms called photosynthetic, such as plants and algae .

Next, we will delve a little deeper into this topic, showing how this process occurs, its steps and importance, and making a comparison with another important process for obtaining energy , chemosynthesis .

More about Photosynthesis

It is a process carried out by autotrophic photosynthetic organisms such as plants, algae and some prokaryotes. These organisms capture sunlight, convert it into chemical energy, which will be used for the production of organic compounds , based on water and carbon dioxide. One of the end products of this process is oxygen, which is released into the environment.

Photosynthesis takes place in two steps (described later) in chloroplasts , organelles present in the cells of photosynthetic eukaryotic organisms . These organelles store photosynthetic pigments , which are responsible for absorbing light. Among these various pigments, such as chlorophylls, carotenoids and phycobilins, chlorophyll-a stands out as the main one, being found in all photosynthetic organisms.

stages of photosynthesis

Photosynthesis occurs in two stages called: light or photochemical phase or reaction and carbon fixation phase or reaction :

• Light or photochemical phase or reaction

In this phase two photosystems are involved, photosystem I and photosystem II. In the first, pigments absorb wavelengths of 700 nm or greater ; in the second, they absorb wavelengths 680 nm or smaller. The components of the two photosystems are the antenna Each type of memory has its own operation, although all of them cooperate to carry out a complete memorization process. This is complex and the reaction center.

In photosystem II, pigment molecules of the antenna complex absorb light energy, and energized electrons are transferred from one molecule to another until they reach the reaction center. At that location, one of the chlorophyll-a molecules of the pair present there absorbs the energy, and one of its electrons is transferred to an electron acceptor . These electrons are replaced by others from the photolysis of water.

The photolysis of water occurs in photosystem II , through the action of an enzyme, and presents as the final product of the process: two electrons, two hydrogen ions and an oxygen atom. H ⁺ are released into the thylakoid space, from which they will be removed in later reactions.

The oxygen atom released in this process will be responsible, along with another oxygen atom released from another water molecule, for the formation of O _2 . The photoexcited electrons will pass to photosystem I via an electron transport chain. The water photolysis process will also release protons that will be bombarded into the thylakoid lumen, stimulating ATP synthesis .

While these reactions occur in photosystem II, in photosystem I light energy is transferred by pigment molecules to the reaction center, energizing an electron from one of the pairs of chlorophyll-a molecules. This electron will be transferred to the primary electron acceptor.

The electron from photosystem II is received at the end of the electron transport chain. These will be transferred to a second transport chain through the ferredoxin protein, the final electron acceptor. The transfer of electrons to NADP+ then occurs , reducing them to NADPH, a process catalyzed by the enzyme NADP+ reductase.

Photosystem I can act independently of photosystem II in a process called cyclic electron flow. This is performed, for example, by some bacteria and produces ATP, however, it does not produce NADPH or O ₂ .

• Carbon fixation phase or reaction

It occurs through reactions, carried out in three stages, called the Calvin Cycle , in the chloroplast stroma. The NADPH and ATP molecules, produced in the light phase for the production of sugars from the reduction of fixed carbon, will be used in this phase.

The first step consists of attaching the carbon to a sugar consisting of five carbons with two phosphate groups, known as ribulose 1,5-bisphosphate, usually forming two molecules of 3-phosphoglycerate or 3-phosphoglyceric acid (PGA).

In the second step , 3-phosphoglycerate is reduced to glyceraldehyde 3-phosphate or 3-phosphoglyceraldehyde (PGAL). In the third step, five of the six molecules of glyceraldehyde 3-phosphate formed in the second are Chinese . In addition, it was used to regenerate three molecules of ribulose 1,5-bisphosphate.

Importance of photosynthesis

As said, photosynthesis is a process by which organic molecules are produced, based on water and carbon dioxide, and which also presents oxygen as a final product, which is released into the environment.

Thus, it is an essential process for the existence of life on Earth as we find it today, as it is through photosynthesis that the oxygen existing on the planet, essential for the survival of most organisms, is produced.

In addition, photosynthesis is also responsible for producing energy for virtually all living beings. Autotrophic photosynthetic organisms are the basis of both terrestrial and aquatic food chains . The energy present in the organic matter produced by them is transmitted to heterotrophic beings through the food chain .

Photosynthesis and chemosynthesis

Photosynthesis is one of the main processes for producing organic matter , however, it is not the only one. Another, performed only by some organisms, such as some natural resources  by man. In fact, in these areas, not only species of bacteria , is chemosynthesis.

Chemosynthesis uses the energy obtained from the oxidation of inorganic molecules — such as methane, ammonia, nitrites or hydrogen sulfides — to carry out a set of reactions that will give rise to organic matter (glucose) used as a nutrient source for autotrophic beings .