Essay 1: Answer

Essay 1: 1) Compare and Contrast the C3, C4 and CAM pathway. Include the following: Chemical form and function, evolutionary form and function, and ecological form and function. 2) In addition, thoroughly explain each pathway and how the functions of stomatal opening/closure affects 3) plant growth, geographic location and photorespiration.

1) The C4 and CAM pathways are designed to avoid the problem of photorespiration (when O2 out-competes CO2 binding to RuBP, which happens when O2 levels rise and CO2 levels fall, such as when stomata are closed on a hot, arid day). This results in less glucose being formed or no glucose formed at all. Each of the three biochemical pathways (C3, C4 and CAM) is designed (evolved) to produce the maximum amount of glucose (C6H12O6) for X plants habitat as efficiently as possible. C3 plants (standard Calvin Cycle) have not particular shortage of CO2 and therefore have not developed any specific way to deal with the problem of photorespiration. C4 plants avoid photorespiration by using a spatial solution (the biochemical processes occur in two different cells). CAM plants avoid photorespiration by using a temporal solution (the biochemical processes occur at different times of the day). Both processes store CO2 as an organic acid and release the CO2 directly to the C3 pathway, avoiding photorespiration. The big difference is that the C4 plants can open their stomata during the day until it gets too dry and hot, while the CAM plants live in regions (desert) that do not allow the stoma to be open during the day at all (too hot and arid, too much H2O loss) and therefore can only open their stomata at night when the conditions are not as harsh. This all falls into the basic form and function concept: if the environment is not too arid then the plant can open it stomata most of the time, if it is too arid part of the time C4 developed because the stomata can open during the day part of the time, if it is always to arid during the day, then CAM developed where the stoma only open at night and store the CO2 as an organic acid.

2) The CO2 acceptor for the C3 pathway (and indirectly the other pathways as both the C4 and CAM feed CO2 to the C3) is RuBP [a 5 carbon compound] which forms Rubisco [a 6 carbon compound]. Rice, wheat and soybeans are examples of C3 plants. The CO2 acceptor for the C4 pathway is phosphoenolpyruvate (PEP) with the help of PEP carboxylase. PEP carboxylase has a much greater affinity for CO2 than RuBP and NO affinity for O2, thus bypassing photorespiration. (in addition this highlights once more the importance of enzymes!) This occurs in the mesophyll cells and then the product (malate) is sent to the bundle-sheath cells thus the C4 pathway is separated spatially. The organic molecule that forms from the PEP and CO2 is oxaloacetate a 4-carbon compound and thus the name C4 pathway. Another 4-carbon compound called malate is converted from the oxaloacetate and releases their bound CO2 to the C3 cycle in the bundle-sheath cells. Corn and sugarcane are examples of C4 plants. CAM (crassulacean acid metabolism) plants which have temporally separated biochemical pathways. The acid is made at night with the CO2 being brought in through the open stomata when the environment is not as hot or arid as in the day. The organic acid is stored in vacuoles in the mesophyll cells until the stomta close in the daytime and the light reaction starts. When the stomata close during the day, the acid releases its CO2 to the C3 pathway as the light reaction is occurring. Catcti, and succulents (pineapple, aloe) are good examples of CAM plants. C3 plants live in temperate or hot and humid climates where photorespiration is not a significant factor. C4

plants live in environments that are often hot and arid, but not always hot and arid. CAM plants (mostly succulents) live in regions that are so hot and arid that the stomata are not ever opened during the day, only at night.

3) Plants growth is the best in regions where the stomata can be open at any time and do not have to incorporate extra biochemical pathways (C4 and CAM) to avoid photorespiration/wilting, such as in a tropical rainforest.

When going over a new concept, such as the C4 and CAM pathways, always think about form and function, how the concept evolved, how the concept affects homeostasis in the organism in question,

and how this applies to the real world the organism lives in.

C3, C4, and CAM plants all carry out the same photosynthetic functions. They all have light-dependent reactions and the Calvin-Benson cycle. The major difference in C4 and CAM plants is when and where the

carbon fixation initially occurs.

In C3 plants, the light reactions occur in the palisade mesophyll cells. Carbon fixation also occurs in these cells during the Calvin-Benson cycle. An enzyme called ribulose bisphosphate carboxylase oxygenase

(rubisco for short) fixes incoming carbon dioxide molecules into three carbon molecules called glyceraldehyde-3-phosphate (G3P). The G3P goes thorough a series of reductions to form carbohydrate

molecules. Because these biochemical pathways are virtually identical in all three types of plants, I won't elaborate on them.

C4 plants typically live in warmer, drier climates than normal C3 plants can withstand. When the outside air is hot and dry, C3 plants must close their stomata or they risk losing too much water via transpiration. But closing the stomata also cuts off the supply of CO2. As the influx of sunlight drives photosynthesis, CO2

levels fall and O2 levels rise. Because rubisco can fix oxygen as well as carbon dioxide, some of the molecules needed for regular photosynthesis become oxidized and useless if O2 levels get too high.

C4 plants solve this problem by not having carbon-fixation occur in the palisade mesophyll cells, where oxygen concentrations are high due to the splitting of water molecules (photolysis). Photolysis is necessary

to replenish the electrons "lost" in reducing carbon compounds to carbohydrates. Instead, when carbon dioxide enters the leaf of a C4 plant, it is bonded to a three-carbon compound called phosphoenolpyruvate

(PEP) by an enzyme called PEP carboxylase. PEP is much more specific for carbon dioxide than rubisco is, so there is less risk of photorespiration.

The resulting four-carbon molecule is called oxaloacetic acid (OAA). It is converted into another four-carbon molecule (malic acid), and actively transported to the bundle sheath cells. It's from this four-carbon "taxi"

molecule that C4 photosynthesis gets its name. Because light reactions are not occurring in the bundle sheath cells, the O2 levels aren't high there. Therefore, carbon can be safely fixed by rubisco in the oxygen-poor

bundle sheath cells.

Once malic acid reaches the bundle sheath cells, it is relieved of its fourth carbon, which converts it back into a three-carbon molecule. It returns to the mesophyll cells to gather more carbon dioxide. Meanwhile, the left-

behind carbon molecule is fixed into G3P by rubisco and the rest is clockwork.

So the only real difference between C3 and C4 photosynthesis is that between the light-dependent and Calvin-Benson cycle, there is an additional step where carbon is transported to the interior of the leaf.

CAM stands for Crassulacean Acid Metabolism (due to its first having been identified in plants of the family Crassulaceae). CAM plants live in extremely dry environments, where desiccation is an even bigger threat than in environments favored by C4 plants. CAM plants open their stomata at night to collect the CO2 they need. Since it's cooler at night, transpiration is not much of a problem. As in C4 photosynthesis, the plant fixes the carbon molecules into malic acid. Instead of shipping the malic acid to the bundle sheath cells,

however, CAM plants store their malic acid in the mesophyll cells' vacuoles until dawn. When the sun (and the temperature) rises, the plants close their stomata to conserve water and begin using the carbon stored in

their vacuoles in the Calvin-Benson cycle.

So...C3 plants carry out their light-dependent and Calvin-Benson reactions in the same place at the same time: in the mesophyll cells during the day.

C4 plants carry out their light-dependent and Calvin-Benson reactions at the same time, but in different places: the light reactions take place in the mesophyll cells while carbon fixation occurs in the bundle sheath

cells.

CAM, like C3 plants, perform their light-dependent and Calvin-Benson reactions in the same place and at the same time, but they get their carbon from a store they build up overnight.