Photosynthesis: Light Absorption, Pigments, and Processes
Photosynthesis
Absorption of Light by Photosynthetic Pigments
Absorption Spectrum and Color
Chlorophylls have two main absorption peaks in the visible light spectrum:
- One in the blue light region (400-500 nm wavelength)
- Another in the red region of the spectrum (600-700 nm)
They reflect the middle of the spectrum, which corresponds to green light (500-600 nm). This is why chlorophylls appear green and give this color to tissues with active chloroplasts in their cells.
Chlorophylls are accompanied by accessory pigments, mainly carotenoids and phycobilins, which have different colors. These pigments contribute to the overall coloration of photosynthetic organisms, such as the brown color of brown algae or the red and purple hues of red algae.
Types of Chlorophyll
There are two main types of chlorophyll:
- Chlorophyll a
- Chlorophyll b
They differ slightly in their porphyrin ring structure, which influences their light absorption properties.
Carotenoids
Carotenoids are red and orange pigments that capture light in the green and violet regions of the spectrum. They contribute to photosynthesis but primarily act as photoprotectants, dissipating excess light energy.
Photosystems
Pigments are organized into photosystems. A photosystem consists of a reaction center surrounded by a light-harvesting complex. When a photon (particle of light) strikes a photosystem, the energy is transferred to the reaction center. This energy excites the photosystem, causing an electron to be expelled. This electron then enters a series of redox reactions.
Types of Photosystems
There are two types of photosystems:
- Photosystem I (PSI): Its reaction center, called P700, absorbs light primarily at a wavelength of 700 nm.
- Photosystem II (PSII): Its reaction center, called P680, absorbs light primarily at a wavelength of 680 nm.
Photolysis of H2O
This reaction is responsible for the presence of oxygen (O2) in the atmosphere. It transformed the primitive, reduced atmosphere into an oxidized one, enabling aerobic life. The reaction is as follows:
H2O + Light Energy → 2H+ + 2e– + ½O2
The electrons released in this reaction are transferred to the coenzyme NADP+.
Electron Transport to NADP+ (NADPH Formation)
The light-dependent or photochemical phase of photosynthesis can occur in two ways:
- Acyclic electron transport
- Cyclic electron transport
Acyclic electron transport requires both photosystems I and II, while cyclic electron transport involves only photosystem I.
Acyclic Light Phase
begins with the arrival of photons to photosystem II. Excites its target pigment P680 losing as many electrons as photons absorbed. After this excitement there is a continuous passage between molecules capable of winning and losing those electrons. But to replace electrons lost P680 pigment is produced by hydrolysis of water (photolysis of water), releasing oxygen. This is done on the inside of the thylakoid membrane. Finally, electrons are introduced into the interior of the thylakoid cytochrome bf and create an electrochemical potential difference (Mitchell’s chemiosmotic hypothesis) on both sides of the membrane. This brings out protons through the ATP synthase with the resulting synthesis of ATP that accumulates in the stroma (phosphorylation of ADP). On the other hand, the photons also affect the PSI, the P700 chlorophyll loses two electrons are captured by successive acceptors. The electrons that chlorophyll loss are parts by the receiving Plastocyanin cytochrome bf. Eventually the electrons move to the enzyme and form NADPH NADPreductasa (photoreduction of NADP). In cyclic light phase involved only the PSI, creating a flow of electrons or cycle, at every turn, results in ATP synthesis. There is no photolysis of water and generates NADPH, or gives off oxygen. Its purpose is to generate more ATP is essential for the subsequent dark phase. Photophosphorylation: The chloroplasts generate ATP by chemiosmosis. the poroceso is porduce in internal menbranes mu thylakoid is similar to that quimosmotico coupling oxidative phosphorylation. the electron transport chain e-transform the E of the reactions of redox in a power gate structure on the same membrane, is a complex enzyme that of the ATP synthase, q is responsible for coupling the diffusion of protons H + afaor gradient cn the fosforilacon of adp to atp to. esxisten similarities with mitochondrial quimosmotico such coupling are similar proteins, complex atp – similar synthetase. but there diferncias as: * oxidative phosphorylation, the mitochondria, and originate from organic molecules. on the contrary, the photophosphorylation e-come from an H2O molecule i been Inpulse to chemiosmotic .* contragradiente inner membrane, the mitochondrial proton bonbeo is done in the mitochondrial matrix space intrmenbranoso (inside? out) but in photosynthesis Occurs as membrane pump Tilaco the stroma (not? inside) q by H + gradient is high q months in oxidative fosforilacon. protons in the stroma intoduced active ATP synthase.