A Plant’s dilemma! “When to open stomata and photosynthesize – When to close stomata and conserve water!?” Consider that the limiting factor to photosynthesis is the availability of CO2 in the atmosphere which currently is present in very dilute concentrations of 0.039% whereas O2 is relatively concentrated at 20%. A well hydrated plant would keep stomata open at daytime to allow ready access to atmospheric CO2 while keeping stomata closed at night time in order to conserve the CO2 generated by respiration. The large vacuoles of plant cells could be considered botanical scuba tanks permitting the storage of waste CO2 generated by nighttime respiration. O2 is never in limiting supply; however, those same large vacuoles could overcome the problems of laggard diffusion during gas exchange, by sequestering copious waste O2 generated during photosynthesis. Vacuoles can therefore provide a reserve of CO2 for photosynthesis or a reserve of O2 for respiration as need be. Waste not – want not! So logically, a well hydrated plant’s default setting would be stomata open in daytime and closed at nighttime. And often this is indeed the case. However, there are three unfortunate wrinkles complicating this straightforward story: 1. Desiccation 2. Photorespiration 3. Nutritional (specifically Nitrogen) requirements By definition, photosynthesis happens in light. However when conditions are most “light”, conditions are also most “hot and dry”. Plants are most prone to water loss precisely when they open stomata for gas exchange during the day. Another unanticipated wrinkle is a relic of plant evolution called Photorespiration. Let’s start with what is supposed to happen during Photosynthesis. The start of the Calvin Cycle combines CO2 with the phosphorylated 5-carbon sugar ribulose bisphosphate to produce two molecules of 3-phosphoglycerate. This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase (RuBisCO). From an enzymatic standpoint, the fixation of CO2 by RuBisCO is very inefficient (a high Km – low affinity for substrate); however, whatever RuBisCO lacks in efficiency, it makes up for in quantity! RuBisCO can fairly claim to be the most abundant protein on earth accounting (in some plants) for up to 50% of soluble leaf protein. RuBisCO’s active site can actually bind either CO2 or O2. When photosynthetic algae first arose, the early Earth’s atmosphere contained little, if any, oxygen. RuBisCO would have functioned very well under these conditions. It was only later, when the concentration of oxygen in the atmosphere increased considerably, did the competitive binding of O2 to RuBisCO’s active site pose a problem. What would that problem be? When RuBisCO binds O2, (instead of CO2) only one 3C molecule of PGA is produced (instead of two) and a toxic 2C molecule called Phosphoglycolate is produced. The plant must rid itself of the Phosphoglycolate involving a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria. Since this alternate pathway requires light and produces CO2, it is called “Photorespiration”. However, no useful energy is gained from Photorespiration. Higher temperatures melt the tertiary structure of RuBisCO, rendering it less able to discriminate between CO2 and O2. Meanwhile, warmer temperatures also decrease the solubility of CO2 which poses a problem if CO2 is already in limiting supply. (Remember soda pop is more likely to fizz if it is warm as opposed to cold). CO2 availability is further limited by the constraints of diffusion during gas exchange. Meanwhile relative O2 concentrations accumulate in flagrant excess as a result of Photosystem II located in the very same chloroplast as the Calvin Cycle’s RuBisCO. When the concentration of CO2 drops below 0.01 percent, O2 will out-compete CO2 at RuBisCO’s active site, and no net photosynthesis occurs. To summarize: RuBisCO catalyzes two different reactions: •adding CO2 to ribulose bisphosphate — the carboxylase activity during photosynthesis •adding O2 to ribulose bisphosphate — the oxygenase activity during photorespiration Which of these two reactions predominates would depend on the relative concentrations of O2 and CO2 where: •high CO2, low O2 favors the carboxylase action during photosynthesis, •high O2, low CO2 favors the oxygenase action during photorespiration The light reactions of photosynthesis liberate oxygen and deplete carbon dioxide. Meanwhile, the availability of soluble carbon dioxide is significantly decreased at higher solvent temperatures. Therefore, •high light intensities and •high temperatures (above ~ 30°C) … favor the oxygenase reaction of Photorespiration over the carboxylase activity of regular Photosynthesis. In other words, if a plant is to survive the deleterious effects of photorespiration, it needs to avoid both high light and temperature conditions or find other ways of storing CO2.while isolating O2 from RuBisCO. Let’s start with the simplest scenario. When water is abundant and temperatures are relatively low, a plant’s life is pretty straight forward. No special adaptations are required. Plants that immediately bind CO2 during photosynthesis are referred to as C3 Plants because the molecule 3-phosphoglycerate (see diagram above) has a backbone comprised of three carbon atoms. C3 Plants open their stomata during the daylight hours and as a result cannot survive intense light or heat. “C4 plants” only open their stomata during cooler parts of the day. That means they require a store of CO2 for photosynthesis when stomata are closed. “C4 plants” get their name by storing CO2 as a stable product four-carbon organic compound, usually malate. The details of the C4 cycle (as cut and pasted from John W. Kimball's excellent site) After entering through stomata, CO2 diffuses into a mesophyll cell. o Being close to the leaf surface, these cells are exposed to high levels of O2, but have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle). Instead the CO2 is inserted into a 3-carbon compound (C3) called phosphoenolpyruvic acid (PEP) forming the 4-carbon compound oxaloacetic acid (C4). Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is transported (by plasmodesmata) into a bundle sheath cell. Bundle sheath cells are deep in the leaf so atmospheric oxygen cannot diffuse easily to them; often have thylakoids with reduced photosystem II complexes (the one that produces O2). o Both of these features keep oxygen levels low. Here the 4-carbon compound is broken down into carbon dioxide, which enters the Calvin cycle to form sugars and starch. o pyruvic acid (C3), which is transported back to a mesophyll cell where it is converted back into PEP. In other words, the C4 pathway minimizes photorespiration by separating the so-called light and dark reaction in different locations of the leaf, thereby isolating O2 from RuBisCO. There is a cost to this strategy: every CO2 molecule has to be fixed twice, first by 4-carbon organic acid and second by RuBisCO. As a result, the C4 pathway uses more energy than the C3 pathway. The C3 pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the C4 pathway requires 30 molecules of ATP. For tropical plants, this added energy debt is more than compensated by avoiding the expenditure of over half of photosynthetic carbon to photorespiration. The separation of the so-called light and dark reaction in different locations of C4 leaves explains the separation of Palisade mesophyll cells from the radially oriented bundle sheath cells surrounding the veins. This unique feature of C4 leaves is referred to as “Krantz (i.e. “wreath” in German) Anatomy”. Krantz Anatomy Evolution – Some suggest that C4 plants emerged with a spike in atmospheric O2 levels 25-35 million years ago. http://tinyurl.com/ce8av56 Some authorities make sense of a putative 25-35 million year time limit for C4 emergence by suggesting C4 carbon fixation must have evolved on at least 45 independent occasions in different lineages of plants, making C4 a prime example of convergent evolution. http://tinyurl.com/cegocy7 Such suggestions are reminiscent of Ernst Mayr’s outdated speculation that metazoan eyes may have evolved independently on 40 different occasions! Evo-Devo comes to the rescue by invoking the versatility of metazoan “molecular toolkits”. The metazoan Urbilateran probably had the molecular toolkits for both ciliary and rhabdomeric photoreceptors, either of which was differentially lost in later Eumetazoan lineages. http://tinyurl.com/ykkayn2 A similar story probably happened during plant evolution. Emerging molecular clock analyses will undoubtedly move the emergence of C4 metabolism back to an era much earlier than paleobotanists ever suspected. Evolutionary analysis is further compounded by “horizontal” or “lateral” gene transfer! http://tinyurl.com/d979bqk In other words, Darwin’s “Tree of Life” is beginning to resemble more a “Cobweb of Life”. “Horizontal” or “Lateral” gene transfer is far more likely to occur in plants than in animals by a variety of mechanisms such as “illegitimate pollination”. Photorespiration may serve some purpose? It has been predicted that the increase in ambient CO2 concentrations predicted over the next 100 years may reduce the rate of photorespiration in most plants by around 50%; thereby increasing ecosystem productivity and the sequestration of atmospheric CO2. However, reducing photorespiration may not necessarily result in increased growth rates for plants. Some research has suggested, for example, that photorespiration may be necessary for the assimilation of nitrate from soil. (link) Thus, a reduction in photorespiration either by genetic engineering or by increasing atmospheric CO2 due to fossil fuel combustion may not be as beneficial to plants as some propose. Several physiological processes may be responsible for linking photorespiration and nitrogen assimilation: one is that photorespiration perhaps increases availability of NADH, which is required for the conversion of nitrate to nitrite. Such speculation is confounded by the fact that certain nitrite transporters also transport bicarbonate, and elevated CO2 has been shown to suppress nitrite transport into chloroplasts. That said; Photorespiration also may still be used as a mechanism to dissipate excess energy at high irradiance levels to prevent damage to plant cells. Ecological considerations: Increasing atmospheric CO2 levels favor C3 Plants. However, increasing global temperatures favor C4 Plants. Global or regional changes in climate have the potential to change a predominantly C3 forest into a predominantly C4 grassland or vice versa. When such changes occur quickly, ecological collapse and mass extinction are sure to follow. During the Younger Dryas of the Late Pleistocene; rapid climatic oscillations accompanied planet-wide ecological collapse together with mass extinctions that included Ice Age Mammoths, camels, llamas, and saber-tooth tigers. Biotechnology: Converting plants from C3 to C4 Given the advantages of C4 metabolism at higher temperatures, a group of scientists from institutions around the world are working on the C4 Rice Project to turn rice, a C3 plant, into a C4 plant. Rice is for more than half the world’s population its most important staple food. Rice that is more efficient at converting sunlight into carbohydrate could significantly augment global food security. Some suggest C4 rice could produce up to 50% more food energy - and be able to do it with less water and nutrients. (link) (link) Questions: How did C3 Plants get their name? _______________________________________________________ C4 Plants? _________________________________________________________________________ What is/are the overall function(s) of photosystem I? Are the compounds listed here used or produced in: Glucose O2 H2O CO2 ATP ADP + Pi NADPH NADP+ What is/are the overall function(s) of photosystem II? Photosystem I? Photosystem II? What is/are the overall function(s) of the Calvin cycle? The Calvin cycle? C3 C4 Draw simplified diagrams of the cross sections of a leaf from a C3 and C4 plant. C3 Plants have one kind of chloroplast whereas C4 Plants have two. Explain: ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ Present-day C4 plants are generally concentrated in the tropics and subtropics (below latitudes of 45°). Why would that be? ___________________________________________________________________ ____________________________________________________________________________________ Some call photorespiration a “mistake” in the functioning of the plant cell? Explain the Pros & Cons to that statement: _______________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ Rubisco is thought to have evolved when Earth had a reducing atmosphere. Does this help to explain the so-called “mistake”? __________________________________________________________________ ____________________________________________________________________________________ What makes C4 photosynthesis more efficient than C3 photosynthesis in tropical climates? ____________________________________________________________________________________ ____________________________________________________________________________________ What makes C3 photosynthesis more efficient than C4 photosynthesis in temperate climates? ____________________________________________________________________________________ ____________________________________________________________________________________ The project to turn rice, a C3 plant, into a C4 plant is a bit of a gamble. Explain why this experiment is being attempted: ______________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ … and what could go wrong? ___________________________________________________________ ____________________________________________________________________________________ Define convergent evolution: ___________________________________________________________ ____________________________________________________________________________________ The suggestion that C4 emerged ONLY 25-35 million years ago is pretty contentious! Why is there disagreement? _______________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ For the sake of argument, let’s say that that C4 did indeed emerge ONLY 25-35 million years ago. Without invoking repeated incidents of convergent evolution; provide another explanation how so many different lineages of plants could have all acquired C4. _______________________________________ ____________________________________________________________________________________ If you grow bean plants (C3) and corn plants (C4) in a sealed terrarium, one quickly dies even as the other thrives. Explain these results: ______________________________________________________ ____________________________________________________________________________________ ______________________________________________________ (Hint: as a plant grows, carbohydrates are made) Marine planktonic diatoms are responsible for up to 20% of primary production on earth, fixing more than 10 billion tons of inorganic carbon each year. The existence of both the C3 and C4 pathways were recently discovered in a marine diatom. In this unicellular organism, the two paths are kept separate by having the C4 path in the cytosol, and the C3 path confined to the chloroplast. What are the evolutionary implications of this discovery? _________________________________________________________ ___________________________________________________________________________________ Diatoms are by definition aquatic organisms. Does that make their C4 pathway surprising? Explain ____________________________________________________________________________________ ____________________________________________________________________________________ If the Greenhouse Effect continues unabated, will C4 diatoms thrive or perish? Explain ____________________________________________________________________________________ ____________________________________________________________________________________ Essay Question / Research The C4 Rice Project represents the acme of Genetically Modified Foods (GM foods) aka Biotech Foods. GM foods are extremely controversial and are even sometimes referred to as “Frankenfood”. Write an essay citing the Pros and Cons of GM foods and be sure to explain why you come down on one side or the other on this contentious issue. Please use proper citations and do not copy and paste from any source.
