Light-Dependent Reactions ( Z Scheme ) (Neațu et al., 2014) Light-dependent reactions happen in the thylakoid membrane of the chloroplasts and occur in the presence of sunlight. The sunlight is converted to chemical energy during these reactions.
The electron transport. The electrons then are moved downhill to a molecule of energy-rich NADP+ and the addition of these electrons reduces NADP+ to NADPH + H+. The whole scheme of transfer of electrons is called the z – scheme, due to its characteristic shape.
The electron transport chain of photosynthesis is often put in a diagram called the z – scheme, because the redox diagram from P680 to P700 resembles the letter Z. to NADPH. Activities of the electron transport chain, especially from cytochrome b6f, lead to pumping of protons from the stroma to the lumen.
It involves both PS-I and PS-II. Flow of electrons is unidirectional. Here electrons are not cycled back and are used in the reduction of NADP to NADPH2. This non – cyclic photophosphorylation is also known as Z – scheme (because of shape of path of electron-flow) and this was given by Hill and Bendall (1960).
The “ Z ‐ scheme ” describes the oxidation/reduction changes during the light reactions of photosynthesis. Absorption of a photon excites P680 to P680*, which “jumps” to a more actively reducing species. P680* donates its electron to the quinone‐cytochrome bf chain, with proton pumping.
Robin Hill (biochemist)
|Born||Robert Hill2 April 1899 Leamington Spa, Warwickshire, United Kingdom|
|Died||15 March 1991 (aged 91)|
|Alma mater||University of Cambridge|
The sharp decline in photosynthetic efficiency as the wavelength of exciting light enters the red region (above 680nm) due to the insensitivity of photosystem II to excitation by red light. Return to Search Page.
A photosystem is a protein complex, a group of two or more proteins, that is essential for the photochemistry of photosynthesis. Photosystem II is first in the process of the light-dependent reactions of photosynthesis. The photosystems are the protein structures in plant chloroplasts that absorb light energy.
Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step.
C4 plants —including maize, sugarcane, and sorghum—avoid photorespiration by using another enzyme called PEP during the first step of carbon fixation. This step takes place in the mesophyll cells that are located close to the stomata where carbon dioxide and oxygen enter the plant.
A solution with a higher (more positive) reduction potential than the new species will have a tendency to gain electrons from the new species (i.e. to be reduced by oxidizing the new species) and a solution with a lower (more negative) reduction potential will have a tendency to lose electrons to the new species (i.e.
P680 is composed of chlorophyll a molecule which, after excitation by the absorption of light to form P680 *, gives up an electron to an acceptor, converting it to P680 •+. This radical cation has a redox potential estimated to be ~1.2 V or more, which is required to oxidize water.
In another form of the light reactions, called cyclic photophosphorylation, electrons follow a different, circular path and only ATP (no NADPH) is produced.
Let us have a look at the important differences between cyclic and non-cyclic photophosphorylation. Cyclic vs Non-cyclic Photophosphorylation.
|Cyclic Photophosphorylation||Noncyclic Photophosphorylation|
|Does not occurs||Occurs|
|ATP||Reduced coenzymes and ATP|
In the Z ‐ scheme, electrons are removed from water (to the left) and then donated to the lower (non‐excited) oxidized form of P680. The electron from cytochrome bf is donated to PSI, converting P700 to P700*. This electron, along with others, is transferred to NADP, forming NADPH.