For scientists working in this area, global warming offers many challenges that need to be overcome. Plant productivity depends significantly on the speed and efficiency of photosynthesis, specific environmental conditions and the availability of nutrients essential for plant growth. With the planet undergoing environmental changes, such as increased temperature, acidification of oceans due to an increased uptake of carbon dioxide, understanding photosynthesis and how it is affected by these environmental changes is of tremendous importance for our existence.
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Photosynthesis in brief
Photosynthesis occurs in large organelles found in leaf cells named chloroplasts and more particularly on the thylakoid membrane which is the third innermost chloroplast membrane. This membrane is a lipid bilayer, formed into stacks of flat disk shape membranes termed grana and in which are embedded large complexes of proteins and chlorophyll, a pigment molecule similar to heme with a magnesium atom Mg2+ in its centre. These complexes are called photosystems.
Plants and algae utilise two photosystems called PSI and PSII with both having distinct functions. They both contain two chlorophyll a molecules (Chl-a) but due to different protein environments, these chlorophylls in the two reactions centres differ in their light absorption maxima (680 nm for PSII and 700 nm for PSI).
So how does photosynthesis work? Photosynthesis is a combination of different processes that aim to form NADPH and ATP so that plants have the necessary energy and electrons to convert CO2 and water to six carbon sugars.
Photosynthesis can be briefly described as follows.
Light absorbed by chlorophylls attached to proteins in the thylakoid membrane raises the chlorophyll molecules to a higher energy state or excited state which is unstable. In the photosystem PSII, this triggers the transfer of an electron from a P680 Chl-a molecule to an acceptor quinone on the stromal surface. This results in (1) the formation of a positively charged P680 Chl-a molecule, which becomes the strongest oxidising agent able to oxidise water to oxygen (2H2O--> O2 + 4H+ + 4e-), (2) the protons being formed as a result of the oxidation of water to remain in the thylakoid membrane and (3) the electrons on the acceptor quinone to move to the electron-donor site on the luminal surface of the PSI reaction centre.
PSI will then use the energy from an absorbed photon to transfer an electron to ferredoxin where NADP+ will be reduced to NADPH.
It should be noted that the electron transport between the two photosystems PSII and PSI occurs through a chain of carriers in the thylakoid membrane. Cytochrome b6f plays an important role in this process and will simultaneously pump protons from the stroma to the thylakoid space hence creating a pH gradient across the thylakoid membrane. Protons will then move down their concentration gradient from the thylakoid lumen to the stroma via ATP synthase (CF0CF1 complex), hence converting ADP and Pi to ATP.
These reactions are light-dependent hence called “light” reactions and in oxygenic photosynthesis, the overall reaction of these “light” reactions is:
2H2O +2NADP+ + 3ADP + 3Pi -->O2 + 2NADPH + 3ATP
Finally, the last process in photosynthesis is the fixation of CO2 and the conversion of CO2 into glucose utilising previously formed ATP and NADPH. These reactions are independent of light energy and are sometimes described as “dark reactions”. An enzyme of interest, called RuBisCO, is involved in one of the first major steps of carbon fixation and shall be discussed into more details in a subsequent blog post.
Written by Magalie Dale
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