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The Light-Dependent and Light-Independent Reactions

The light reactions, or the light-dependent reactions, are up first. We call them either and both names. The whole process looks a little like this:

Do not freak out or fill your head with all the complicated names in that diagram. No—stop right there. All in all, the process is simpler than it looks. In the light-dependent reactions of photosynthesis, the energy from light propels the electrons from a photosystem into a high-energy state. In plants, there are two photosystems, aptly named Photosystem I and Photosystem II, located in the thylakoid membrane of the chloroplast. The thylakoid membrane absorbs photon energy of different wavelengths of light.

Here again is our friend the chloroplast. All exposed the way he is, he kind of reminds us of a boat with green checkers in it:

Image source

Even though the two photosystems absorb different wavelengths of light, they work similarly. Each photosystem is made of many different pigments. Some of these pigments can be described as absorption pigments, and others are considered action pigments.

The absorption pigments transfer the energy from sunlight to another pigment; at each transfer, the absorption pigments pass the photon energy to another pigment that absorbs a similar or lower wavelength of light. Remember when we said that light is funky and acts like it has both particles and waves? A photon is what we call the particle-like aspect of light. In other words, a photon is the basic unit of light.

Anyway, eventually, the energy makes it to the reaction center, or action pigment. At this point, the photosystem loses a highly charged electron to adjacent oxidizing agents, or electron acceptors, in the electron transport chain. This transfer all occurs mind-bogglingly quickly at an estimated time of 200 × 10-12 seconds!2 Since the photosystem has lost an electron, it no longer has a neutral charge and has instead become a positively charged photosystem.

The positively charged photosystem creates a scenario similar to one that might occur if Twilight stars Robert Pattinson and Kristen Stewart made a surprise appearance at your local high school. You, like the electrons in the photosystem, would be attracted to their presence even if you hated them. (You would. Admit it.) The positively charged photosystem attracts electrons from water (H2O) that can then be excited by light energy. When exactly four electrons are removed from H2O, oxygen (O2) is generated. Why, you ask? If two water molecules have four hydrogens that lose four electrons, exactly four hydrogen ions (H+) and two oxygens are left. Don't believe us? Count it out for yourself.

Side note: since hydrogen normally only has 1 proton and 1 electron, the four hydrogen atoms that have each lost one electron are each referred to as H+. Since each H+ is now without an electron, there is only one proton remaining in the hydrogen atom. At some point, scientists became lazy and started equating H+ with the word proton. If you think about it, they are in fact equivalent.

Back to regularly scheduled programming. The protons are then moved into the thylakoid lumen of the chloroplast using the power of the electron transport chain. This move results in a higher concentration of protons in the lumen than in the stroma of the chloroplast.

With so much positivity around, the protons get a little upset and try to equalize their distribution in the chloroplast by moving from the lumen to the stroma to reach equilibrium (read: equal numbers of protons in both places). The rush of protons moving into the stroma is called a proton gradient. When protons move down the gradient, with down referring to the direction of the area containing fewer protons, the protons are grabbed by enzymes that bring the protons together with the electrons from the electron transport chain. This event ultimately results in the making of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) from the adenosine diphosphate (ADP) and nicotinamide adenine dinucleotide phosphate ion (NADP+) that were hanging around nearby.

Now that everyone is partying it up in the stroma, it becomes the perfect location for the next stage of photosynthesis, the light-independent reactions.

Pictures are worth the thousand words that may or may not have just whizzed by your head as you were reading. Here is a much-simplified version of the earlier picture:

Did you miss something, or do we just suck at drawing these pictures? Nope. Photosystem II is ahead of Photosystem I. You might ask, "What the heck happened, Shmoop?" Well, scientists actually discovered Photosystem II before Photosystem I, and instead of changing the names when they found the other photosystem, they just named them in reverse. We know; it's very annoying.

As you’ve probably gathered by now, the light-dependent reactions fuel the second stage of photosynthesis called the light-independent reactions. Our good buddy carbon dioxide (CO2) provides an excellent source of carbon for making carbohydrates. However, conversion of one mole (one mole is an amount equal to 6.023 × 1023 molecules) of CO2 to one mole of the carbohydrate CH2O requires a lot of energy. And we mean a lot.

Guess what? The ATP and NADPH generated in the earlier light reactions are strong reducing agents (electron donors) and are able to donate the necessary electrons to make carbohydrates. Altogether, the conversion of one mole of CO2 to one mole of CH2O requires two moles of NADPH and three moles of ATP. If you do the math, that's a heck of a lot of molecules. The cell then uses ATP and NADPH to make carbohydrates in the Calvin cycle. We could use a Calvin and Hobbes cartoon right about now.

A key player in the Calvin cycle is ribulose-1,5-bisphosphate carboxylase oxygenase (affectionately called RuBisCo—thank goodness for nicknames), an enzyme that "fixes" CO2 to a 5-carbon compound called ribulose-1,5-bisphosphate (RuBP). The oxygen in CO2 is released as H2O. Immediately after RuBisCo catalyzes the attachment of the carbon from CO2 to the 5-carbon RuBP, the new 6-carbon compound is broken down into two 3-carbon compounds called phosphoglycerate (PGA, and no, it does not know how to golf). Since these 3-carbon compounds were the first compounds to be identified in the plants, they were named C3 plants. It was originally thought that RuBisCo was catalyzing the attachment of carbon to a 2-carbon molecule to make a 3-carbon molecule. Oopsies. And we thought RuBisCo was a cookie company at first, too.

RuBisCo is actually a poor enzyme. Sorry, RuBie. It is slow at catalyzing the attachment of CO2 to RuBP. To make matters worse, RuBisCo is also capable of catalyzing another less-than-beneficial reaction. This reaction is called photorespiration, and it occurs when the concentration of CO2 drops too low relative to the concentration of O2 in the cell. Photorespiration begins when RuBisCo uses O2 instead of CO2 and adds it to RuBP.

While CO2 is eventually produced in this reaction, and O2 is consumed, the reaction does not seem to produce any useful energy forms. The origination and purpose of photorespiration is controversial and still under active study by scientists. In an attempt to overcome the deficiency of RuBisCo, the plant cell produces a whopping ton of the enzyme. If this sounds slightly masochistic, it kind of is, which is why photorespiration has been labeled an outdated evolutionary relic. However, RuBisCo is thought to be the most abundant protein on Earth.2

Right…this not a moan fest about RuBisCo. We were explaining the Calvin cycle. When RuBisCo catalyzes the attachment of CO2 to the 5-carbon RuBP, the Calvin cycle begins. Reactions are initiated to rebuild RuBP from PGA. In this process, 1 molecule of glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar, is removed from the cycle. Altogether, 1 molecule of G3P is produced using 3 molecules of CO2, 9 molecules of ATP, and 6 molecules of NADPH. This 3-carbon sugar can be exported to the cytoplasm to make sucrose (a sugar and a carbohydrate), which is then moved throughout the plant for energy use. Alternatively, sucrose can be converted into another carbohydrate, starch, and then stored in the chloroplast as a type of energy reserve. Smart, plants…real smart.

What's that? You want a picture? Sure thing; glad to be of service.

Brain Snacks

There are carnivores that undergo photosynthesis. Meat-eating plants do not eat for energy; they eat to obtain nutrients, such as nitrogen and phosphate, to build the proteins needed for photosynthesis.

Living organisms besides plants do photosynthesis. One "biological masterpiece," the sea slug Elysia chlorotica, is able to conduct photosynthesis by extracting DNA and chloroplasts from its plant food source. Sneaky, sneaky.

In seeded plants, chloroplasts do not develop unless the seedlings are exposed to light. This process is called photomorphogenesis.3

Weed killers called herbicides work by targeting enzymes used in the light reactions of photosynthesis. Come here, little chloroplasts.

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