Photosynthesis: Process, Phases, and Pigments
Autotrophic and Heterotrophic Anabolism
Autotrophic anabolism is the synthesis of complex molecules from simple inorganic molecules such as glucose or glycerol. Heterotrophic anabolism is the transformation of simple organic molecules into more complex ones, like starch. There are two types of autotrophic anabolism based on the energy source: photosynthetic anabolism, which uses light energy (e.g., photosynthesis in plants, cyanobacteria, and photosynthetic bacteria), and chemosynthetic anabolism, which uses energy from oxidation reactions of inorganic compounds (only in some bacteria, called chemosynthetic bacteria).
Photosynthesis
Photosynthesis is the process of converting light energy from the sun into chemical energy, stored in organic molecules. This is made possible by photosynthetic pigments, special molecules that capture light energy and transfer it to other molecules. There are two main types of photosynthesis: oxygenic photosynthesis, where electrons are obtained from water molecules, releasing oxygen into the environment (plants and algae), and anoxygenic photosynthesis, where sulfur precipitates are formed (purple and green sulfur bacteria in sulfide-rich waters). Anoxygenic photosynthesis is the simpler and older form.
Photosynthetic Structures
In plant cells and algae, photosynthesis occurs in chloroplasts, membranous organelles. Within the stroma are thylakoids, sac-like structures containing photosynthetic pigments. Cyanobacteria lack chloroplasts; their thylakoids are located in the cytoplasm. Some bacteria have neither chloroplasts nor thylakoids but use protein-walled organelles called chlorosomes.
Photosynthetic Pigments
Photosynthetic pigments are lipid molecules bound to proteins in the thylakoid membranes. In plants, these include chlorophyll and carotenoids; in bacteria, bacteriochlorophyll. Chlorophyll consists of a porphyrin ring with a magnesium atom at the center and a long chain alcohol called phytol. There are two types: chlorophyll a and chlorophyll b. Carotenoids are isoprenoids that absorb light at 440nm; they can be red carotenes or yellowish xanthophylls.
Photosystems
A photosystem is a complex of proteins containing photosynthetic pigments, forming functional subunits in the thylakoid membranes. The light-harvesting complex captures light energy, exciting pigment molecules, and transmitting the excitation energy to other molecules until it reaches the reaction center. The reaction center contains two subunit molecules of a special chlorophyll a pigment. Upon receiving energy, this pigment transfers electrons to an acceptor molecule. Photosynthesis involves two photosystems:
Photosystem I
Photosystem I (PSI) has a target pigment (P700) that absorbs light at wavelengths less than or equal to 700 nm. It is abundant in the stroma thylakoids and cannot split water to release electrons.
Photosystem II
Photosystem II (PSII) has a target pigment (P680) that absorbs light at wavelengths of 680 nm or less. It is more abundant in the thylakoid grana stacks and can split water molecules to release electrons.
Light-Dependent Reactions
The light-dependent reactions occur in the thylakoids. Light energy is captured and used to generate ATP and reduced nucleotides (NADPH + H+). There are two modes: acyclic and cyclic electron transport. Acyclic transport involves both photosystems I and II, while cyclic transport involves only photosystem I. Other elements involved are the electron transport chain and ATP synthase.
Acyclic Light-Dependent Reactions
The acyclic light-dependent reactions involve three processes: water photolysis, ADP photophosphorylation, and photo-NADP reduction. Light excites PSII’s chlorophyll P680, transferring electrons to an acceptor. To replace the lost electrons, water is hydrolyzed, releasing protons into the thylakoid lumen. This creates an electrochemical gradient, driving protons through ATP synthase to produce ATP. For every three protons, one ATP molecule is synthesized.
Cyclic Light-Dependent Reactions
In the cyclic light-dependent reactions, only photophosphorylation of ADP occurs. Photosystem I is involved, creating a cyclic electron flow that pumps protons into the thylakoid lumen. The electrochemical gradient is used for ATP synthesis. Since PSII is not involved, there is no photolysis of water or oxygen release, and no reduction of NADP+.
Balance of Light-Dependent Reactions
For each NADP+ reduced during the acyclic light-dependent reactions, two electrons and two protons are required, coming from the photolysis of a water molecule. In the cyclic phase, only ATP is produced.
Light-Independent Reactions (Calvin Cycle)
The light-independent reactions (Calvin Cycle) take place in the stroma of chloroplasts. ATP and NADPH produced in the light-dependent reactions are used to synthesize organic molecules. The overall equation is: 6CO2 + 12H2O + light energy → C6H12O6 + 6O2 + 6H2O. In this phase, energy from ATP and NADPH is used to convert inorganic substances into organic matter.