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Many believe that, early in the Earth's history, microorganisms consumed organic molecules in much the same way that most animals and microorganisms do today. As you might imagine, these ancient organisms ran into a little problem: they were eating away all of the available food but not producing any of their own. (Sounds a little bit familiar, doesn't it? Looks like humans still have a lot in common with microorganisms.) Even though ancient organisms hadn’t even seen a cake yet, they were still trying to have it and eat it, too.
Estimates suggest that photosynthetic organisms appeared on Earth about 3.5 billion years ago.4 The original photosynthetic organisms may have actually used hydrogen sulfide (H2S) as their electron source. Scientists then speculated that cyanobacteria (named for their color) evolved the ability to split a water molecule (H2O), which made the super-strong reducing agents needed for fixing carbon dioxide (CO2) and producing the carbohydrates required for life as we know it.
The evolution to water-driven photosynthesis must have required a lot of changes in how organisms at the time conducted photosynthesis. The main reason for the needed changes is that H2O holds onto its electrons a lot better than H2S does. Once organisms figured out how to pull H2O into the reaction, a lot of oxygen (O2) was pumped into the atmosphere, and organic materials began to accumulate on the young Earth.
While we might currently believe that the production of O2 was nothing but a fantastically wonderful step forward for life on Earth, O2 in the atmosphere actually created a problem for early life forms. O2 is a great oxidizing agent, meaning that it can pull electrons away from other biological compounds in the cell. The gradual increase in O2 in the atmosphere provided a means of selection. In other words, organisms had to evolve protective methods to prevent oxidative damage to their cells…or they would die. Many organisms eventually acquired the ability to live in the presence of O2. Other organisms shielded themselves from O2 in the environment by settling into environments where they were not exposed to O2. Need an example? The microbes that live in the human gut are one. Yummy.
When we talk about photosynthesis in today's world, we often think about plants and the chloroplast. You might be surprised to find out that the chloroplast was a rather late addition in evolutionary history. Scientists hypothesize that, more than a billion years ago, a eukaryotic cell engulfed a photosynthetic bacterium through a process called endosymbiosis. This "little" event may have resulted in the chloroplast organelle that we know and love so much. While present-day chloroplasts are the result of many, many years of evolution, some artifacts of the chloroplast's previous life as a separate and independent entity remain. Spooky.
Scientists think that both mitochondria and chloroplasts organelles were the results of endosymbiotic events. What led scientists to hypothesize the endosymbiotic theory? Mitochondria and chloroplasts both contain their own genomes (read: sets of encoded DNA), although many of the genes needed for photosynthesis and respiration have been incorporated into the bigger cell’s nuclear genome. Nonetheless, the genome that stayed behind in these organelles tells us a lot about their evolutionary history. Scientists found that many of these genes more closely resembled bacterial genes than equivalent genes in the bigger cell’s nuclear genome. Mitochondria and chloroplasts also encode for their own tRNAs (transfer ribonucleic acids), and in some cases, they even have a different genetic code! Therefore, occasionally, a codon, or a set of three adjacent nucleotides from the RNA molecule that usually represent one amino acid, might code for one amino acid, while the same codon derived from the chloroplast genome might encode for an entirely different amino acid. Crazy stuff. Mitochondria and chloroplasts also segregate their deoxyribonucleic acid (DNA) in a manner completely independent from the bigger cell's nucleus.
All in all, the photosynthetic organisms that we see today are the product of millions, or even billions, of years of evolution. Over time, organisms have made small changes to their existing photosynthetic machinery. If these changes happened to give an organism an advantage over other organisms, the changed organism would be more likely to survive. Therefore, these changed features were "selected" through survival of the "fitter" organisms. However, there is no "best" way to do photosynthesis. Photosynthetic organisms closely evolve and adapt to their own special environments, and the result is a diverse array of existing photosynthetic organisms. In fact, a trait that may have benefited one organism's survival in one environment may actually be detrimental to another organism's survival in a different environment. A very serious take on "whatever floats your boat."
While there are many similarities in how cells conduct photosynthesis, there are also many differences. In order to find their own niche in the ecosystem, plants have continued to evolve in the hopes of maximizing their ability to perform photosynthesis. The result? A wide variety of "biological masterpieces."
If you look at the photosystems in different organisms today, you will see that they absorb different wavelengths of light. These differences are attributed to changes in the pigments in the different photosystems. Nonetheless, the similarities in the overall architecture and structure of the photosystems also speak to their common evolutionary roots.
The specialization of photosynthesis in different organisms that use particular wavelengths of light is responsible for the diversity in colored organisms we see on our planet. Plants appear green, for instance, because they use primarily red and blue light for photosynthesis. Yellow and green light is not absorbed as well, and is therefore reflected, which results in the pretty green colors of forests and other shrubbery. Yes, we really wanted to use the word shrubbery.
Besides specializing their photosystems, organisms have had to balance the amount of water (H2O) needed for photosynthesis while at the same time letting carbon dioxide (CO2) into the system. Some plants, such as cacti, are called CAM plants, or Crassulacean acid metabolism plants. These plants only open their stomata to collect CO2 at night because it helps to minimize the amount of H2O lost in the hot and dry daytime weather. They only fix CO2 at night. CAM plants primarily store their CO2 in the form of malate, which can be broken down to release CO2 inside the leaf during the day, when it can be acted on by RuBisCo.
Although ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) prefers to use CO2, it can use oxygen (O2) if the concentration of CO2 becomes too low. This process, called photorespiration, is a way for the plant to use extra oxygen while at the same time producing CO2. However, it does not seem to result in any useful energy forms, and the reason for its existence is still debated today. Photorespiration proves to be a real problem for plants living in hot and dry areas. In these climate conditions, plants are often forced to close their stomata to prevent the loss of H2O. The result, however, is that CO2 cannot enter the cells. With CO2 concentrations getting lower and lower, photorespiration becomes a favored process. Some plants, such as corn, a type of C4 plant, have evolved a special procedure to try to decrease the amount of photorespiration that happens inside the plant. In these cases, photosynthesis is actually spatially separated within the leaf.
In C4 plants, RuBisCo is only present in the bundle sheath cells, and carbon fixation only occurs there. These cells are not exposed to the air, which effectively decreases the availability of O2 to RuBisCo. CO2 is pumped into these cells by the mesophyll cells that perform the light reactions. This pumping is an advantage because the cells can conduct photosynthesis with a much lower concentration of CO2. Furthermore, it allows cells to close their stomata during hot and dry conditions because CO2 can be stored and used later. The mesophyll cells capture CO2 by attaching each one to a 3-carbon molecule, making it a 4-carbon molecule. This attachment is also why they are called C4 plants. In C4 plants, not only does photosynthesis occur in different cells, but the chloroplasts within those cells have also become specialized to deal with their specific roles in the photosynthesis. And you thought that the assembly line was invented in the 1900s. Psh. Plants have had it down pat for a few million years before that.
Why don’t all plants use the C4 system? Because, dear Shmooper, it is only advantageous under certain conditions, like those in hot and dry climates, where CO2 is limiting. Unfortunately, it takes a lot of energy to pump CO2 into those bundle sheath cells. C4 plants comprise about 4% of plant species, but represent a higher percentage of our food crops.
Photosynthesis is a process that shows how structure and function unite as one in biology. We can first think about the structure at the level of the plant itself, and how the location of photosynthesis is important for its function. The plant optimizes where it conducts photosynthesis; that is, photosynthesis does not occur in every cell in every plant. Chloroplasts are mostly localized in the leaves, primarily in a special type of cell called a mesophyll cell. In a special type of plant called C4 plants, the light-dependent and light-independent cells are separated, with the light-dependent reactions occurring in the mesophyll cells, and the light-independent reactions occurring in the bundle sheath cells. This spatial separation of photosynthesis decreases the amount of photorespiration that occurs because it limits the exposure of these cells to O2.
On a deeper level, we can also think about where photosynthesis is localized within the cell: the chloroplast. During the light-dependent reactions, the pigments in Photosystems I and II absorb photons and transfer that energy through the electron transport chain. The localization of these reaction centers in the thylakoid membrane creates an environment well suited for easy energy transfer. Each carrier molecule is nearby, which makes energy transfer more efficient. Structure and function working together. Isn't that sweet?
However, separation is also critical for the light reactions to occur. Protons move from the lumen of thylakoid in the chloroplast to the stroma in an attempt to reach a proton concentration equilibrium. This gradient is critical for bringing the protons together with the electrons from the electron transport chain to then make ATP and NADPH from ADP and NADP+.
ATP and NADPH are then conveniently located in the stroma of the chloroplast, which is where these energy powerhouses are needed to generate carbohydrates during the light-independent reactions. The localization of the light-independent reactions in the chloroplast is important for another reason as well. Many of the enzymes used in the Calvin cycle are also used in other cellular biochemical reactions, and the localization of these enzymes in the chloroplast facilitates their use in the Calvin cycle.
In all of this, the ATP and NADPH generated during photosynthesis rarely make it out of the chloroplast. That has got to be a pretty boring existence for them. Reaction after reaction? Give them a break! But no, these energy reserves remain almost exclusively in the chloroplast, fueling the production of carbohydrates. The plant then sustains life by breaking down these carbohydrates and generating ATP and NADPH again outside of the chloroplast.