Electron transport, Chemiosmosis and the Photosystems

Last time, i talked about photosynthesis, and pigments. I'm just admitting it now, I forgot to mention/explain one thing, so I am explaining it here instead. Pigments are compounds that absorb light, and in the thylakoid membrane there are the chlorophylls a and b. However, there are more compounds, including carotenoids. Carotenoids don't absorb yellow, orange or brown. In fall, many plants lose their chlorophylls, but keep their carotenoids, making the leaves take on many different colors. Why is this important? Well, the chlorophylls and carotenoids are grouped together in clusters within the thylakoid membrane. Each cluster is referred to as a photo system. there are types of Photosystem, Photosystem 1 and Photosystem 2. Although similar in terms of the pigments each contains, they have very different roles in the light reaction process. The light reactions begin with absorbing light. By absorbing light, the pigment molecules aquire some of the energy carried by light waves. Within each photosystem, the aquired energy is quickly passed to other pigment molecules until it reaches a specific pair if chlorophyll a molecules. Now, I can explain this in five steps. Step 1: The light energy forces electrons to have a higher energy level in the two chlorophyll a molecules in photosystem 2. These electrons are said to be 'excited' Step 2: The excited now have enough energy to leave the chlorophyll a molecules. Since they have lost electrons, the chlorophyll a molecules have now undergone an oxidation reaction. Step 3: The electrons are now passed along a series of molecules called an electron transport chain. However, along the way, they lose most of the energy gained from sunlight. This energy is used to move protons into the thykaloid. Step 4:As light excites the electrons in photosystem 2, it does the same in photosystem 1. It specifically excites the electrons in the chlorophyll a molecules. The electrons move away from that pair, and continue being passed along until they reach a primary electron acceptor. The electrons from photosystem 2 replace the ones lost in photosystem 1 Step 5: Electrons from photosystem 1 travel along an electron transport chain, and at the end of the chain, they combine with NADP+ and H+ to make NADPH. Finally, we are at chemiosmosis. This is derived from greek, chemia meaning alchemy and osmosis meaning pushing. Why bring this up? Well, alchemy was meant to be able to turn lead to gold, which is a little unrelated, or is it? The driving chemical energy of photosynthesis is ATP. ATP is produced by glucose. Comparison; "lead" glucose is turned into the "Gold" or ATP. Chemiosmosis relies on the concentration gradient of protons across the thylakoid membrane. I cant remember if I included this or not last time, but when water is broken down inside the thylakoid, some protons are produced. This makes up for some of the protons used, but not all of them. The others are brought in from the stroma. The energy required to due so is gained from the excited electrons from photosystem 2. See? It all ties into itself. Oh, wait there's more to chemiosmosis. The protons brought in from the stroma are used to build a higher and higher concentration gradient. Now, the concentration gradient of protons represents potential energy. ATP harnesses this potential energy into chemical energy. NADPH uses some of the protons too, and together, ATP and NADPH provide energy for the second set of reactions in photosynthesis. So, this is all a very detailed explanation of the first set of photosynthesis. It is called the light cycle too sometimes, because it harnesses energy from light. The second cycle is called the dark cycle, or, more commonly, the Calvin cycle. This will be explained further in the next post. Sorry for so many posts on photosynthesis, but the process is very long and very complicated. Also, it is probably important for the survival of every species of the planet, but let's not get ahead of ourselves