Practically all activity of living beings is carried out thanks to the energy that reaches us from the sun . All the energy that we use as heterotrophic living beings comes from what we eat, and ultimately from plants, algae and other photosynthetic organisms, which capture energy from sunlight. And so it is in almost the entire biosphere.
Exceptions are some groups of bacteria, archaea and other microorganisms that obtain their energy from chemical reactions, generally at the bottom of the oceans. But for the rest of the living beings, photosynthesis is an essential metabolic process and the mainstay of most life forms on earth.
The general scheme of photosynthesis is easy to understand: the plant obtains carbon dioxide and water from the environment, and thanks to sunlight, it obtains glucose , releases oxygen , and emits half of the water it absorbed. The chemical reaction looks like this:
6CO₂ + 12H₂O + sunlight → C₆H₁₂O₆ (glucose) + 6O₂ + 6H₂O.
But the process is not as simple as it may seem at first; in fact, it occurs in two clearly differentiated phases, both of which are complex.
The bright phase
The process of photosynthesis begins when sunlight hits a leaf, more specifically, on the photosystems , protein structures found in the membranes of chloroplasts , the cellular organelles responsible for photosynthesis in algae and plants.
Photosystems are in turn composed of two parts: the antenna complex and the reaction center. The antenna complex is an area covered with chlorophyll, which captures energy from sunlight and transforms it into chemical energy. Chlorophyll is a pigment that we see as green; this is because it absorbs the red and blue wavelengths, and reflects the green wavelength.
The antenna complex functions as a funnel of light, directing light energy to the reaction center . In it, the photon of light excites the chlorophyll molecule, which releases an electron, which is captured by a primary acceptor. The electron that chlorophyll needs then is obtained by breaking a water molecule, which generates, on the one hand, protons inside the membrane, and on the other, oxygen as a waste product , which will then be released into the atmosphere.
In the process, that excited electron captured by the primary acceptor reduces a coenzyme, called nicotinamide adenine dinucleotide phosphate ( NADP ⁺), to NADPH . On the other hand, the protons that are formed inside the membrane when the water molecule breaks form a gradient with respect to the outside. They are released through a protein called ATP-synthase that forms a tunnel in the membrane, and that, as if it were a turbine, takes advantage of the flow of protons to the outside to reduce an ADP molecule to ATP , the currency of exchange cellular energy par excellence.
Note that although water and sunlight are involved in this phase, carbon dioxide has not yet entered the picture .
The dark or synthesis phase
Both NADPH and ATP are highly reactive molecules, with a very short lifespan, and cannot leave the cell. So that the entire plant can be nourished, down to the last cell of its roots, and so that, ultimately, these plants can nourish the rest of the living beings, it is necessary to store that energy in more stable molecules. This process is called the Calvin cycle .
This is when one of the most important proteins in the biosphere comes into play, and probably the most abundant enzyme in the world. Rubisco (acronym for ribulose 1,5-bisphosphate carboxylase-oxygenase). This protein channels the CO₂ obtained from the atmosphere and, with the participation of water, assembles it into a 5-carbon compound, ribulose 1,5-bisphosphate. As a result of this reaction, two molecules of 3-phosphoglycerate, with three carbons, are obtained.
From there, and using the ATP and NADPH previously produced by the light phase as energy sources —and which remain as ADP and NADP⁺— the Calvin cycle begins. This complex chemical reaction ends up giving as a result glucose and ribulose 1,5-bisphosphate, which regenerates the cycle. For every six CO₂ and six H₂O molecules, one glucose molecule is formed.
Does the dark phase happen at night?
There is some confusion with the term “ dark phase of photosynthesis ”. Although today we speak of the “synthesis phase”, it was originally called the “dark phase” because it was thought to be a process independent of light. Just as it is clear that the light phase only happens during the day, there are those who think that the dark phase can happen both during the day and at night, or even that it only happens at night. Even so, it still appears in some textbooks. However, it is not true.
Carbon dioxide is obtained through the stomata, and in most plants —those with metabolism called C3—, they only open during the day. That is, only while there is light can plants obtain CO 2 from the atmosphere to fuel the Calvin cycle. And although plants with different metabolisms —called C4 and CAM— work differently, they continue to use carbon dioxide only during the day.
The “dark phase” can only happen during the day
The Calvin cycle, the central pillar of the so-called “dark phase” of photosynthesis, requires two very important reagents already mentioned: NADPH and ATP, which are produced during the light phase , by the release of electrons from water. and by the proton gradient that activates ATP synthase. And as indicated, both molecules are very unstable. They cannot be stored; where they are produced they are consumed . No plant can keep its synthesized ATP and NADPH molecules from day to night to perform the Calvin cycle. Just as the light phase of photosynthesis can function on its own, releasing oxygen obtained from the water, the dark phase needs the light phase to be occurring simultaneously. And since the light phase cannot happen at night, neither can the dark phase.
But the problem is even deeper. And it is that, even if there were an alternative source that provided the ATP and NADPH necessary for the Calvin cycle, it would not work either, because the central enzyme of the Calvin cycle, rubisco, is only activated by light . So, in dark conditions, the main engine of the “dark phase” simply stops.
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