Photoautotrophic Anabolism: Photosynthesis

Photosynthesis is the process by which photoautotrophs or photolithotrophs capture energy from sunlight and transform it into chemical energy (ATP) that they use to synthesize organic matter from other simpler inorganic compounds.

These photosynthetic organisms (bacteria, algae, plants), from sunlight, produce chemical energy (ATP) and reducing power (NADPH), transforming water and carbon dioxide in the atmosphere into carbohydrates. Oxygen is also produced, which will be used in the respiration of plants and animals. In addition, ATP and NADPH is used to reduce and absorb other bioelements, such as N and S.

The energy of sunlight causes the passage of electrons from some molecules to others. Water is the first electron donor, and CO₂ is the one that will receive these electrons. CO₂ combines with other molecules and forms a three-carbon carbohydrate (glyceraldehyde-3-phosphate), from which other organic molecules that plants need as a source of energy and for their development will originate.

Photosynthesis is carried out thanks to photosynthetic pigments that can capture light energy. When a photon hits an electron in a photosynthetic pigment, the electron is excited to a higher energy level, in a position further from the nucleus, and that atom can be lost and oxidized. The molecule that will give another electron to that atom is called the first electron donor, and the electrons that have been lost, charged with the energy of the photon, go to another molecule called the first electron acceptor, then passing through a series of molecules that they capture (reduce) and yield (oxidize) successively, releasing energy that is used by the enzyme ATP-synthase to form ATP, and be stored in its ester-phosphoric bonds.

Two types of photosynthetic processes are distinguished: oxygenic photosynthesis and anoxygenic photosynthesis. 

• The oxygenic photosynthesis (is what we will study in this issue) is characteristic of higher plants, algae and cyanobacteria, in which the electron donor is water (photolysis of water), and consequently follows oxygen.
• The anoxygenic photosynthesis is characteristic of the purple and green bacteria of sulfur, in which the electron donor is not water, but, generally, the hydrogen sulfide, H₂S, so it is not apparent oxygen.

By DARTORIUS (Own work) [GFDL or CC BY 3.0], via Wikimedia Commons

Through photosynthesis, cells can use the sun's energy to transform it into chemical energy that is stored and later used to synthesize immediate principles. Photosynthesis takes place in chloroplasts.

Photosynthesis consists of two stages:

• The light phase (better to call it photochemical) develops in the thylakoid membranes, and consists of a series of chemical reactions that depend on light. When photons of light reach the photosystems, they cause the release of electrons that serve to reduce NADP⁺ to NADPH. In addition, oxygen has been released from the water molecules. With the energy of these electrons, obtained in the electron transport chain, ATP is synthesizedfrom ADP, in a process called photophosphorylation.

In the photochemical phase, from the light energy of the sun, chemical energy is generated in the form of ATP and reducing power (in the form of NADPH).

• The dark phase (or biosynthetic phase) takes place in the stroma. Several reactions, independent of light, take advantage of the energy (ATP) and the reducing power (NADPH) of the photochemical phase to reduce carbon from CO₂, obtaining organic molecules in a carbon fixation process.

In the biosynthetic phase, from simple inorganic molecules (CO₂, H₂O) and using the reducing power (NADPH) and energy (ATP) of the light phase, more complex organic molecules (glucose) are synthesized through a series of reactions called the Calvin cycle.

Game: Photosynthesis interactive.