CAM and C4 Photosynthesis

1. CAM and C4 Photosynthesis • We’ve already learned that plants need to leave their stomata open during the daytime, for the light dependent reactions. • While it’s true, plants don’t need to take in CO2 for the light dependent reactions, one of the by-products of noncyclic photophosphorylation needs to be emitted from the leaf, and it does so through its stomata. (O2 waste from photolysis) • Oxygen build-up in plants is bad! It is a waste material that leads to photorespiration • At the same time, plants lose water through open stomata in the process of transpiration Plants also must take in CO2 in order for the light independent reactions to occur. This CO2 goes on to help create glucose. How can plants in hot, and dry biomes possibly allow their stomata to remain open, if it also promotes desiccation?

2. Photorespiration The most common enzyme on Earth, and indeed protein, is rubisco. It is this enzyme that participates in the C-B cycle and catalyzes the fixation of CO2 (rubisco catalyzes the merging of CO2 and RuBP to produce 12 PGA) • Rubisco isn’t a particularly efficient molecule, because it also wants to “fix” oxygen. When oxygen is present, it becomes “fixed”, more readily than CO2 in a process known as photorespiration. • The products of fixed O2 are not particularly healthy for the plant, and most assuredly don’t lead to the formation of glucose, like the fixation of CO2 does. • Instead, the products of RuBP and O2 are stored in peroxisomes, which are found near the chloroplast. The peroxisome breaks down the products of photorespiration, and gets rid of the by-products. • It is for this reason that scientists know that there was little or no oxygen in Earth’s early atmosphere, around the time of the first plants. If oxygen had been plentiful, rubisco would not have given plants a selective advantage.

3. Over Earth’s vast geologic history, the atmosphere has contained both very high (time of dinosaurs >50%), and very low (when life was first evolving-0%) levels of O2. Now, our planet has approximately 21% O2 in our atmosphere. • Because of the very low O2 levels when life was first evolving, plants using rubisco didn’t struggle as much with the issue of photorespiration. Some special plants evolved an active CO2-concentrating mechanism that increases the CO2 concentration near the site of the photosynthetic reactions, suppressing photorespiration.

4. Open stomata are needed for both light dependent, and light independent reactions. Why? ANS: In light dependent reactions, the stomata are open to allow waste gas, O2 to exit the plant. In light independent reactions, they remain open to allow CO2 into the leaf, in order to create the carbohydrate glucose. Let’s not forget, the water that is moved through the plant, largely because of open stomata, is also needed in non-cyclic photophosphorylation for its donation of electrons, and hydrogen to NADPH formation during the process of photolysis. What does rubisco do when O2 is present in higher concentrations than CO2? ANS: It catalyzes the fixation of O2 instead of CO2, in the process of photorespiration. The products of photorespiration are not particularly good for the plant, and they certainly don’t lead to the production of glucose.

5. C4 Photosynthesis • C4 plants are so named because they form a four-carbon compound as the first product of the dark reactions of photosynthesis known as oxaloacetic acid (OAA), instead of the 3-carbon PGA formed in typical C3 photosynthesis. Several thousand species in at least 19 families use the C4 pathway. • Agriculturally important C4 plants are sugarcane, corn, sorghum, and other members of the grass family.

6. To understand the C4 pathway, it is really important to fully understand the structures of the leaf. Let’s review: • Cuticle • Stomata and guard cells • Palisade and spongy parenchyma (mesophyll) • Xylem and Phloem • Bundle sheath cells • Notice how the bundle sheath cells are far away from the stomata, and spongy parenchyma. This will become important. • Where is most C3 photosynthesis occurring?

7. Instead of being fixed by rubisco to form PGA, CO2 combines with PEP (Phosphoenolpyruvic acid) to form OAA (oxaloacetic acid), using the fixing enzyme PEP carboxylase. • OAA is then converted to malate, and that is shuttled to the bundle sheath cells through plasmodesmata. • Here, malate is converted to pyruvate and CO2. • The pyruvate moves back to the mesophyll cells where one ATP is broken down to form AMP (not ADP) which is required to convert the pyruvate back to PEP (to help continue the cycle) • The CO2 goes on to generate glucose in the Calvin Cycle. The overall effect of this process is to move CO2 from mesophyll cells to the bundle sheath cells, in order to make photosynthesis more efficient. Since bundle sheath cells rarely make contact with intercellular spaces, very little oxygen reaches them, decreasing the likelihood of photorespriation

8. C4 photosynthesis is called C4 because… The first molecule produced is OAA (oxaloacetic acid) which is a 4-carbon moleclue. What happens when the OAA is converted into malate in the mesophyll? It is shuttled to the bundle sheath through through the plasmodesmata, where it goes on to break down again into CO2, and pyruvate. What is the purpose of the plant moving the site of the Calvin Cycle to the chloroplasts within the bundle sheath, instead of the palisade or spongy parenchyma? Bundle sheath cells are farther away from the spaces within the plant cell that may contain oxygen, thereby reducing the potential for photorespiration.

9. As soon as the CO2 is delivered to the bundle sheath cells, rubisco begins the C-B cycle (C3 Photosynthesis). Because so little O2 is present, little photorespiration takes place, and photosynthesis is more efficient. Because it is more efficient, the stomata don’t have to remain open as long, which decreases transpiration of H2O… C4 plants are found in hot, dry places where they possess an advantage over typical C3 plants, and are able to out-compete them. The advantage it gives them more than makes up for the added energy required (1 ATP to AMP…two bonds are broken), for the pyruvate to be converted back into PEP which continues the cycle.

10. In ordinary C-3 plants which form a 3-carbon compound (PGA) during the initial steps of the light independent reactions, photosynthesis in the leaf shuts down without a sufficient supply of CO2. C-4 plants have a competitive advantage during hot summer days because they are able to carry on photosynthesis in the bundle sheaths where CO2 levels are concentrated. Weedy C-4 plants such as Bermuda grass, spurges and purslane grow rapidly during hot summer days, while photosynthesis and growth in C-3 plants shuts down. Close-up view of a purslane leaf showing the prominent green veins. The chloroplasts are concentrated in bundle sheath cells surrounding the veins. • Both C4 and CAM are two evolutionary solutions to the problem of maintaining photosynthesis with stomata partially or completely closed on hot, dry days. • However, it should be noted, that in all plants, the Calvin cycle is used to make sugar from carbon dioxide.

11. How exactly does C4 photosynthesis help a plant conserve water? CO2 is shuttled to an interior part of the plant leaf called the bundle sheath cells in the form of malate (CO2 storage), which is far from any air spaces in spongy parenchyma. Because CO2 is stored in these “malate” molecules, stomata do not have to remain open as much. This conserves water, as transpiration does not occur when stomata are closed. What process produces the carbohydrates in C4 plants? Calvin-Benson Cycle. What else happens, other than water conservation, when stomata remain closed on hot, dry days? Oxygen waste builds up, and photorespiration may occur.

12. Some plants adapted to hot, arid regions have a different photosynthetic mechanism called CAM photosynthesis. CAM (Crassulacean Acid Metabolism) photosynthesis is found in cacti and succulents, including the crassula family. CAM Photosynthesis During the hot daylight hours their stomata are tightly closed; however they still carry on vital photosynthesis as carbon dioxide gas is converted into simple sugars. How do they do it? During the cooler hours of darkness their stomata are open and CO2 enters the leaf cells where it combines with PEP (phosphoenolpyruvate) to form 4-carbon organic acids (malic and isocitric acids). The 4-carbon acids are stored in the vacuoles of photosynthetic cells in the leaf. During the daylight hours the 4-carbon acids break down releasing CO2 for the dark reactions (Calvin cycle) of photosynthesis inside the stroma of chloroplasts.

13. The CO2 is converted into glucose through the Calvin-Benson cycle with the help of ATP and NADPH, which were synthesized during the light reactions of daylight in the grana of chloroplasts. The adaptive advantage of CAM photosynthesis is that plants in arid regions can keep their stomata completely closed during the daytime, thereby reducing water loss from the leaves through transpiration; however, they can still carry on photosynthesis with a reserve supply of CO2 that was trapped during the hours of darkness in the vacuoles, in the form of 4-carbon acids, when the stomata were open. The tropical strangler Clusia rosea exhibits CAM photosynthesis. This unusual tree starts out as an epiphyte on other trees and then completely envelops and shades out its host.

14. What is the basic difference between CAM and C4 photosynthesis? C4 stores it’s CO2 molecules in the form of malate in the bundle sheath cells. CAM stores CO2 in the form of malic and isocitric acid within vacuoles. In CAM photosynthesis, all CO2 can be taken in through the stomata during the night time hours, so they can remain closed all day long.

15. Adaptive Advantage? Typical, or C3 photosynthesis is carried out by most plants growing in areas with sufficient water.  In this type of photosynthesis, an enzyme called RuBP carboxylase grabs CO2 in the light independent reactions of photosynthesis.  This works fine as long as there is plenty of carbon dioxide and relatively little oxygen.  If there is too much oxygen, RuBP carboxylase will grab that instead of the CO2, and a process called photorespiration will occur. Photorespiration does not help build up any sugars, so if photorespiration occurs, growth stops.  C3 plants will wither. Normally, oxygen (produced in photosynthesis) exits the plant through the stomata; however, if there isn't enough water available (as would happen under bright, hot, sunny conditions), excess oxygen may build up and trigger photorespiration, because the stomata close to conserve water.  If water is present, however, C3 photosynthesis is very efficient because both the light reactions and Calvin Cycle can occur simultaneously in the same cell, and almost all of the cells in the leaf will be producing sugars.

16. C4 photosynthesis differs from C3 in 2 key ways.  First, instead of RuBP carboxylase, a different enzyme, PEP carboxylase, is used to grab CO2.  The PEP carboxylase is less likely to bind to oxygen, thus photorespiration is less likely to occur, a decided advantage under hot, dry conditions where water may be scarce and the stomata remain closed for long periods, trapping oxygen in the plant.  This process is relatively inefficient, but if water is in short supply the inefficient C4 route is still better than the C3 route with photorespiration.  Also, since there are fewer cells involved in making sugars (bundle-sheath), fewer sugars can be made. Thus desert plants can survive the dry conditions, but at the cost of rapid growth.  Desert plants are often very slow to grow, and this is one of the reasons they invest so much energy in defensive structures (spines) and bitter chemicals C3  is best under moist conditions, C4 under warm, sunny, dry conditions, CAMunder desert conditions

17. Characteristics of Photosynthesis in C3, C4 and CAM plants Characteristic C3 Plant C4 Plant CAMPlant Photorespiration Yes Little none Lower temp limit for photorespiration 15-25o C 30-40o C -------------------- Rubisco present Yes Yes Yes PEP Carboxylase present No Yes Yes Initial CO2 fixation directly into Calvin Cycle via Rubisco into OAA via PEP carboxylase, then to malic acid which moves from mesophyll cell to bundle sheath cell and then releases CO2. into OAA via PEP carboxylase, then to malic acid which moves into vacuole (during night). CO2 released from malate during day. Secondary CO2 fixation ---------------- In bundle sheath cell using Rubisco In “mesophyll”* cell using Rubisco – in morning Site of Calvin cycle mesophyll cells bundle sheath cells “mesophyll” cells Site of Light Reactions mesophyll cells mostly in mesophyll cells “mesophyll” cells

18. Why is C3 photosynthesis selected for on the planet today, over CAM and C4 photosynthesis? Because in the presence of water, light dependent and independent reactions can occur simultaneously, and nearly all the cells in the leaf can be producing sugars. In optimum circumstances, it is by far, the most efficient producer of glucose. Which enzyme fixes CO2 in CAM and C4 photosynthesis, instead of Rubisco, and why is this an advantage? PEP Carboxylase; because it won’t fix oxygen, and photorespiration will not occur as a result. Where in the leaf do each of the following first fix carbon dioxide? C3; C4; CAM C3: mesophyll; palisade parenchyma primarily C4: bundle sheath cells CAM: mesophyll, CO2 stored in vacuoles as acid