Math 3c, page 104+106 and Redox reaction #3

Good day chemists!

Sorry for the delay. Today we will talk about the photosynthesis redox reaction. This electron transfer is light-driven, and absorption of H20 is equally necessary as the shining. In this reaction: 6 CO2 + 12 H2O ---> C6H12O6 + 6 H2O + 6 O2 we can see that CO2 gets reduced. It’s during the Calvin-Benson cycle that happens in plants. It’s the electrons of NADPH that do this process, and simultaneously the reducing agent NADPH gets oxidized and lose the hydrogen. Then the reduction of CO2 is finished, because a reduction is gaining electrons and gaining hydrogen. Now there is sugar. Simultaneously H20 gets oxidized (it’s gaining oxygen, therefore H2O gets oxidized to molecular oxygen, and it’s also losing e^- and losing hydrogen) and now there’s only oxygen as a byproduct. It all happened in the chloroplasts that are cell organelles in plant and algae cells. Plants are autotrophs and they build their own food. Light make them produce O2, ATP and NADPH in the thylakoid membrane, inside the chloroplasts.

A cross section of a leaf will definitely give us a wider image. But first we will talk about colors and wavelengths and why we see green light. Visible light moves forward in shape of waves, and those shapes repeat all the time, but every color has different distance between those points that mathematically repeats. The colors in the rainbow demonstrate this clearly, because the colors settle side by side and not on top of each other. The violet color has wavelengths that repeat rapidly, and those are 380 nanometers short. The red color has long wavelengths on 700 nanometers. Humans possess trichromacy, which is when you have 3 independent channels that translate reflected light to color). Retina (thin film at the back of the eye on the inside) and the cones (photoreceptor cells) in the eyes see most easily the colors blue, green and red. A relevant chemistry reaction about the visible spectrum (wavelengths that are 400-700 nanometers) is about the 11-cis-retinal molecule.

When the molecule in question absorbs light, then thanks to isomerization (when a compound changes its configuration, but keeps the same chemical composition as before) it becomes all-trans-retinal, and then it doesn’t fit as well, like a puzzle, in the protein anymore. That changes the geometry of the compound and with that comes new biochemical reactions that creates charge differences. When those electrical impulses reach the brain we will translate this to sight info. But now more about photosynthesis. A green leaf has deep inside spongy mesophylls and these cells have a lot of chloroplasts, which does that they can do the photosynthesis. Those cells also have some airspace to help the leaf with the diffusion of CO2 and O2. Chloroplasts are the food producers, and they are found in Palisade cells also. These last mentioned cells are more often used for photosynthesis (they are placed closer to the leaves surface). In the middle of these cells it is a vacuole (fluid-filled organelle).

This non spontaneous redox reaction happens in the thylakoid membrane in the chloroplasts, after that the light, water and CO2 passed through the surface. That last mentioned gas goes into the Calvin cycle, which reminds a lot about the citric acid cycle in the mammals. Then some of the CO2 will result in sugars that will end up in another part of the leaf, and other products from the Calvin cycle will be NADP^+ (electron carrier) and ADP (a low energy molecule), that will be transported to the thylakoid membrane organelle. This is the organelle that is light sensitive. After sunshine it will oxidize water to oxygen, and it will also generate ATP (that stores potential energy in the bonds of the nucleotide) and NADPH (reducing agent, hydrogen source, electron carrier) that goes back into the Calvin cycle from a different direction this time, so these two can keep the Calvin cycle going. Photosynthesis redox reaction happens of the reason so the cell and plant can have its energy in the end.