Photosynthesis: Light, Dark Phases, and the Calvin Cycle
Light Phase
In the light phase, light absorption occurs in the antenna complex, capturing light and channeling it to the reaction centers. This creates an energy flux that passes through a series of redox molecules. In this process, reducing power (NADPH) and energy (ATP) are formed.
Dark Phase
In the dark phase, the energy formed in the light phase is utilized. Using CO2 and NADPH, and the energy from ATP, sugars are produced.
Chlorophyll a
Chlorophyll a has a methyl group substituent on ring 2. Its absorption spectrum presents two maxima: one in the blue region (420 nm) and one in the red region (660 nm). Chlorophyll is synthesized from alpha-aminolevulinic acid, which is synthesized from the amino acid glutamate.
Chlorophyll b
Chlorophyll b has a formyl substituent (-CHO) instead of a methyl group. The presence of this group makes chlorophyll b more polar. Its absorption spectrum has maxima in the blue region (440 nm) and in the red region (640 nm).
Photosynthesis involves a set of redox reactions in which electrons flow from water through an electron transport chain to form NADPH and ATP. If this transport is interrupted, CO2 assimilation is inhibited, and carbohydrates are not formed. If reducing power is not formed, sugars are not formed. Light is absorbed in the form of light quanta or photons.
P700 and P680
P700 is a component with an absorption maximum at a wavelength of 700 nm, and it is part of photosystem 1. It is formed by a pair of chlorophyll molecules. P680 is similar to P700, but it is part of photosystem 2.
Photosystem 1
Photosystem 1 is also known as plastocyanin-ferredoxin oxidoreductase. Its function is to take electrons from plastocyanin and transfer them to ferredoxin. In this electron transport chain, electrons are transported to the oxidized component, and the reduced component has a Z-scheme configuration.
Photosystem 2
Photosystem 2 is also known as water-plastoquinone oxidoreductase and has two functions: to capture light energy and transform the flow of photons into a flow of energy, and to capture electrons from the hydrolysis of water.
The electron transport starts with the hydrolysis of a water molecule, which, by the action of light, yields an oxygen molecule, two protons, and two electrons. The electrons are captured by PS2, and P680 is excited. The plastocyanin molecule is amphipathic and captures an electron when the molecule is reduced, and it donates it to cytochrome b6f. This complex is integrated into the thylakoid membrane, and its function is to connect the two photosystems. P700 is excited and transfers electrons to ferredoxin, forming NADPH. This entire process takes place in the thylakoid membrane. For this transport to take place, it is essential that electrons receive two light impacts, one in each photosystem. Ferredoxin-NADP+ reductase catalyzes this reaction. It can also donate electrons to plastoquinone, resulting in a cyclic electron flow. The formation of ATP in the chloroplast stroma is directly related to electron transport and light intensity.
Calvin Cycle
The Calvin cycle takes place in the chloroplast stroma and has three phases:
Carboxylation
The CO2 molecule attaches to ribulose-1,5-bisphosphate in a reaction catalyzed by the enzyme RuBisCO, forming two molecules of 3-phosphoglyceric acid. RuBisCO is the most abundant protein in plants, making up 50% of the protein in plant cells. It requires Mg2+ as a cofactor to be activated and uses CO2 as a substrate. The rate of CO2 capture in photosynthesis depends on the amount of RuBisCO and its activity.
Reduction
3-phosphoglycerate, in a series of reactions catalyzed by dehydrogenases and kinases, using ATP and NADPH, is converted into glyceraldehyde-3-phosphate.
Regeneration
Of all the glyceraldehyde-3-phosphate molecules formed, only one of them enters into the synthesis of sugars, and the rest are used to regenerate ribulose-1,5-bisphosphate.
6CO2 + 18 ATP + 12 NADPH -> hexose-P + 17 Pi + 18 ADP + 12 NADP+ (only under light)
Photorespiration
Photorespiration is a non-mitochondrial respiration in which oxygen is consumed and CO2 is produced. It is a process that is stimulated by light and occurs in C3 and C4 plants.
It takes place in three organelles: chloroplasts, peroxisomes, and mitochondria.
In chloroplasts, a molecule of phosphoglycolate is formed. This decreases the photosynthetic efficiency of RuBisCO because it can use both CO2 and O2, although it has a higher affinity for CO2.
Phosphoglycolate is converted into glycolate, and phosphate is lost.
This glycolate is converted into glyoxylate, which transaminates with glutamate to form glycine.
Glycine leaves the peroxisome and enters the mitochondria, where two glycine molecules form one serine molecule, releasing CO2 and ammonium.
The serine molecule leaves the mitochondria and re-enters the peroxisome. A series of reactions using NADPH forms glycerate, which leaves the peroxisome and enters the chloroplast.
Glycerate is regenerated in a reaction that uses ATP, forming the 3-phosphoglycerate needed for the Calvin cycle.
Plants could live without photorespiration, as its function is not beneficial for them. Plants that photorespire dissipate energy under high light and low CO2 concentrations, which prevents the Calvin cycle from occurring. In C4 plants, RuBisCO is only present in the bundle sheath cells, where the Calvin cycle takes place. The CO2 compensation point is the point at which the photosynthetic and respiratory processes are balanced. In C3 plants, this point is between 30 and 70 ppm, and in C4 plants, it is around 10 ppm.