Dark ReactionsDark reaction Definition The series of biochemical reactions in photosynthesis that do not require light to proceed, and ultimately produce organic molecules from carbon dioxide. y Supplement The energy from ATP (produced during the light reactions) drives the dark reactions of photosynthesis. The term dark reactions does not mean the reactions happen at night or that they require darkness. It means that the reactions can proceed regardless of the amount of light available. The term is only used to identify the dark reactions with the light reactions, which obviously require light. y These reactions occur in the stroma. C4 carbon fixation. reduction reactions. CAM plants store malic acid in their vacuoles every night and release it by day in order to make this process work. These reactions take the light-dependent reactions and perform further chemical processes on them. and Crassulacean Acid Metabolism). this process occurs only when light is available. There are three phases to the light-independent reactions.5-bisphosphate (RuBP) regeneration. y . y Despite its name. instead. the fluid-filled area of a chloroplast outside of the thylakoid membranes. release sucrose into the phloem from their starch reserves. and ribulose 1. This process happens when light is available independent of the kind of photosynthesis (C3 carbon fixation. They.LightLight-independent Reactions The light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. collectively called the Calvin cycle: carbon fixation. Plants do not carry out the Calvin cycle by night. hot climates have adapted different ways of initially fixing CO2 prior to its entering the Calvin cycle. Carbon dioxide. take place in the cytoplasm of the cell.THE C4 AND CAM PATHWAYS The entry and exit of gasses in plants is through small pores called stomata located on the underside of leaves. is not a very abundant gas in nature. but this also results in a greatly diminished supply of CO2 for the plant. . the gas required for the Calvin cycle. These pathways of carbon fixation. Plants that normally live in dry. know as the C4 and the CAM pathways. Under hot and dry environmental conditions the stomata close to reduce the loss of water vapor. CO2 is fixed to a three-carbon compound called threephosphoenolpyruvate to produce the four-carbon fourcompound oxaloacetate.The C4 pathway The C4 pathway is designed to efficiently fix CO2 at low concentrations and plants that use this pathway are known as C4 plants. much. First. . These plants first fix CO2 into a four carbon compound (C4) called oxaloacetate. PEP carboxylase. fixes CO2 very carboxylase. plants. efficiently so the C4 plants don't need to to have their stomata open as much. this reaction. cells. This occurs in cells called mesophyll cells. oxaloacetate. The enzyme catalyzing oxaloacetate. The CO2 combines with ribulose bisphosphate and goes through the Calvin cycle while the pyruvate re-enters the remesophyll cells. and is converted back to phosphoenolpyruvate. the phosphoenolpyruvate. starting compound of the C4 cycle. NADPH. Here the four-carbon fourmalate is decarboxylated to produce CO2. a threethreecarbon compound called pyruvate. NADPH. NADPH. . reacts with ATP. cells. cycle. and pyruvate.The oxaloacetate is then converted to another four-carbon compound called malate fourin a step requiring the reducing power of NADPH. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. . PEP carboxylase.The C4 pathway is designed to efficiently fix CO2 at low concentrations and plants that use this pathway are known as C4 plants. This occurs in cells called mesophyll cells. fixes CO2 very efficiently so the C4 plants don't need to to have their stomata open as much. The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH. CO2 is fixed to a three-carbon compound called phosphoenolpyruvate to produce the fourcarbon compound oxaloacetate. . These plants fix CO2 into a four carbon compound (C4) called oxaloacetate. The enzyme catalyzing this reaction. 1. The pyruvate re-enters the mesophyll recells. a threethreecarbon compound called pyruvate. The CO2 combines with ribulose bisphosphate and goes through the Calvin cycle. . Here the four-carbon fourmalate is decarboxylated to produce CO2. NADPH. cells. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. cycle.3. 4. NADPH. phosphoenolpyruvate. the starting compound of the C4 cycle. and pyruvate. and is converted back to phosphoenolpyruvate. cycle. reacts with ATP. open their stomata to fix CO2 only at night. other succulents. The oxaloacetate is converted to malate which is stored in cell vacuoles. resulting in stunted growth. CAM plants live in very dry condition and. forming oxaloacetate.The CAM pathway The CAM pathway (Crassulacean Acid Metabolism) is utilized by cacti. The CAM pathway uses up more energy. Like C4 plants. . CO2 is removed from the stored malate and enters the Calvin cycle. but it vital in environments where water loss is the difference between life and death. During the day when the stomata are closed. CAM plants open their stomates at night to take in carbon dioxide and close them (reducing water loss) in the day. the use PEP carboxylase to fix CO2. and members of the crassulaceae. unlike other plants. . 5-bisphosphate. It starts with the enzyme rubisco catalyzing the carboxylation of Ribulose-1. which splits instantaneously into two molecules of the more stable 3-phosphogylycerate.5-bisphosphate by CO2 producing a highly unstable 6-carbon intermediate known as 3-keto-2-carboxyarabinitol 1. the first stable intermediate organic compound containing three carbon atoms. an organic compound containing three carbon atoms (hence the name C3).C3 CARBON FIXATION PATHWAY DEFINITION A metabolic pathway where CO2 is converted to 3phosphogylycerate. . SUPPLEMENT The C3 carbon fixation pathway is in fact the initial phase of Calvin cycle. increases O2 levels. The closure of stomata loss. CO2 . such as the wasteful loss of CO2. concentrations. consequently CO2 losing CO2 CO2 instead of fixing CO2. high temperatures and low nitrogen or CO2 CO2 concentrations. and the enzyme rubisco reacts with O2 instead of CO2. Photorespiration occurs CO2 under conditions of drought.Plants that solely depend to C3 pathway for carbon fixation are faced with the negative effects of photorespiration. These conditions cause the stomata to close in an attempt to prevent excessive water loss. and CAM . C4.Three types of photosynthesis: C3. C3 photosynthesis is the typical CAM. CAM photosynthesis are both adaptations to arid conditions because they result in better water use efficiency. photosynthesis that most plants use. photorespiration. . can "idle." saving precious energy and water during harsh times. C4. In addition. C4 and use. CAM plants efficiency. and CAM.The three types of photosynthesis are C3. and C4 plants can photosynthesize faster under the desert's high heat and light conditions than C3 plants because they use an extra biochemical pathway and special anatomy to reduce photorespiration. 3. .RUBISCO.Called C3 because the CO2 is first incorporated into a 3-carbon compound.C3 Photosynthesis : C3 plants. is also the enzyme involved in the uptake of CO2.Most plants are C3.Photosynthesis takes place throughout the leaf.. . . Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy). . the enzyme involved in photosynthesis.Stomata are open during the day. . This enzyme allows CO2 to be taken into the plant very quickly.Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy) .Stomata are open during the day. . .Called C4 because the CO2 is first incorporated into a 4-carbon compound.Uses PEP Carboxylase for the enzyme involved in the uptake of CO2.C4 Photosynthesis : C4 plants. photsynthesis. and then it "delivers" the CO2 directly to RUBISCO for photsynthesis. . 4. and many of our summer annual plants. not allowing it to grab oxygen and undergo photorespiration. Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis. C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here. corn.Adaptive Value: Value: Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO. . CAM stands for Crassulacean Acid Metabolism . The CO2 is converted to an acid and stored during the night. the acid is broken down and the CO2 is released to RUBISCO for photosynthesis . .CAM Photosynthesis : CAM plants.Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. During the day.CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads .Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis. ). and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells). . lower wind speeds. CAM. When conditions are extremely CAMarid. CAM plants can just leave their stomata closed night and day.Adaptive Value: Value: . but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever.But CAM-idling does allow the plant to survive CAMdry spells. lower temperatures. This is a little like a perpetual energy machine. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis.May CAM-idle. etc.Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight. . the malic acid is decarboxylated to release carbon dioxide which is re-fixed to produce starch inside choloroplast via C3 calving cycle. . During day.CAM pathway showing carbon dioxide uptake through open stomata during night and its utilisation for the formation of malic acid which is stored in the vacuole.