Photosynthesis: The Process of Converting Light Energy into Chemical Energy
Photosynthesis
Photosynthesis is a complex process by which living photoautotrophs capture solar light energy and transform it into chemical energy (ATP) and reducing power (NADPH). They then transform water and carbon dioxide into reduced organic compounds (glucose), releasing oxygen.
This process is conducted in the chloroplasts.
Photosynthetic Pigments
Substances that absorb light are the pigments found in chloroplast thylakoid membranes.
Pigments are molecules that contain a chemical group capable of absorbing a particular wavelength of the visible spectrum.
They are chlorophyll a, bacteriochlorophyll, xanthophyll, and carotenoids.
The chlorophyll molecule consists of a ring that contains magnesium and whose function is to absorb light, and a hydrophobic chain that keeps the chlorophyll integrated in the photosynthetic membrane.
The pigments capture photons (energy) and pass to an excited state, meaning they have undergone a change in the distribution of electrons. When the pigments return to their original state, they emit a quantity of energy less than that absorbed to get excited, but are able to excite another molecule.
As there are various pigment molecules, many wavelengths are captured.
Phases of Photosynthesis
Light Stage
A set of reactions, depending on the light, which take place in thylakoid membranes. In it, the electrons released are used to reduce NADP+ to NADPH. Through an electron transport chain, energy is used in the synthesis of ATP.
The pigments present in the thylakoids are organized into photosystems (functional complexes formed by more than 200 molecules of pigments, chlorophyll predominant).
The light captured by the photosystems, acting as antenna chlorophyll, is carried to form the reaction center, where chemical energy is transformed.
There are two photosystems associated with chlorophyll a:
- Photosystem I (PS I or P700) absorbs wavelengths of 700 nm.
- Photosystem II (PS II or P680) absorbs wavelengths of 680nm.
The light is received at PS II by chlorophyll P680, which is oxidized to release an e– that amounts to a higher energy level.
This is picked up by an electron acceptor substance, which is reduced, and since it is passing along an electron conveyor chain until the reaction center of PS I.
On the way down the chain, with oxidation and reduction in each step, energy is released, which is used to pump hydrogen protons from the stroma to the thylakoid interior, creating an electrochemical gradient of electrons.
Protons H+ return to the stroma through bridges formed in the membrane by the ATPase, and ATP molecules originate.
To form a molecule of ATP, it is necessary to step 2 electrons from PS II to PSI.
PS I re-assigns an e– to a new electron transport chain at the end of which is reduced by one molecule of NADP+ to form NADPH.
The electrons released by PS II are replaced by electrons released to split water by photolysis, into protons and oxygen. In this way, you can keep the steady stream of e–.
In PS I, light produces the same effect on chlorophyll a, so that the electron acquires a higher energy level and leaves the molecule, is collected by the electron transport chain until a molecule of NADP+, which is reduced to NADPH. The electrons in this photosystem are also spare parts for water photolysis.
The two photosystems can act in series, a process known as the pattern in z, from PS II to PS I activated without activating, and from PS I turned to NADP+; by two redox chains.
- Noncyclic photophosphorylation – e– make an open run: schema Z. Electrons follow an open path from a primary donor, water, up to a final acceptor, NADP+, molecular oxygen released.
- Cyclic photophosphorylation – chlorophyll reaction center of PS I, excited by light, yields e– to a conveyor chain, and after traveling back again to the reaction center, which acts as electron donor and acceptor.
Noncyclic photophosphorylation produces ATP and reducing power (NADPH).
Cyclic photophosphorylation produces only ATP.
Most prokaryotes have only one photosystem, so do not produce oxygen (anoxygenic).
Dark Stage
The dark phase consists of light-independent reactions that take place in the stroma, which use the energy (ATP) and reducing power (NADPH) of the light phase to reduce and absorb carbon from carbon dioxide and obtain organic molecules.
This process is called CO2 fixation and in most autotrophic organisms takes place through a series of reactions known as the Calvin cycle. It occurs in three phases:
- Carboxylic – ribulose 1,5 diphosphate reflects the CO2 forming an unstable 6-carbon compound that is divided into two three-carbon molecules, the 3-phosphoglyceric acid. This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase, also called Rubisco.
- Reductive – reduction of 3-phosphoglyceric acid to glyceraldehyde in two different reactions: phosphorylation and reduction, respectively using ATP and NADPH.
- Regenerative/synthetic – out of 6 molecules of glyceraldehyde, 5 are used to regenerate ribulose 1,5 bisphosphate, consuming 1 ATP molecule and making the Calvin cycle continue, and will be used to synthesize molecules of glucose, fatty acids, and amino acids.
Calvin Cycle Balance
.- for a molecule of glucose from CO 2, spent 12 molecules of NADPH and 18 ATP