The Calvin Cycle

Fig 7.22 In the light, acidification of the lumen creates a pH gradient across thylakoid membranes.

ATP-synthase is a protein motor Driving force is chemiosmotic gradient (Mitchell 1960s) Fig 7.33

Products and substrates of Light and Dark reactions Substrate Energy source ProductsLocation Light reactions H2OH2OH2OH2OlightNADPHATPThylakoids Dark reactions CO 2 NADPH ATP Carbo- hydrates Stroma Relating the Light and Dark Reactions

Carbon Fixation by the Calvin Cycle Second set of reaction in photosynthesis involves a biochemical pathway known as the Calvin Cycle Second set of reaction in photosynthesis involves a biochemical pathway known as the Calvin Cycle Pathway produces organic compounds, using energy stored in ATP and NADPH during the light reactions Pathway produces organic compounds, using energy stored in ATP and NADPH during the light reactions Named after Melvin Calvin Named after Melvin Calvin

Carbon Fixation by the Calvin Cycle Atoms from CO 2 are bonded or “fixed” into organic compounds Atoms from CO 2 are bonded or “fixed” into organic compounds Incorporation of CO 2 into organic compounds is known as carbon fixation Incorporation of CO 2 into organic compounds is known as carbon fixation

Fig. 8.2 The Calvin Cycle (reductive pentose phosphate cycle) 3 Stages Carboxylation Reduction Regeneration A 3 carbon molecule An outline of C3 photosynthesis

3 major steps to Calvin Cycle Occurs within the stroma of the chloroplast Occurs within the stroma of the chloroplast STEP 1: CO 2 diffuses into the stroma from the surrounding cytosol CO 2 diffuses into the stroma from the surrounding cytosol Enzyme combines a CO 2 molecule with a five- carbon carbohydrate called RuBP Enzyme combines a CO 2 molecule with a five- carbon carbohydrate called RuBP Product is a six-carbon molecule that splits immediately into a pair of three-carbon molecules known as PGA Product is a six-carbon molecule that splits immediately into a pair of three-carbon molecules known as PGA

STEP 2: PGA is converted into another three-carbon molecule called PGAL in a 2-part process: PGA is converted into another three-carbon molecule called PGAL in a 2-part process: 1.) each PGA molecule receives a phosphate group from a molecule of ATP. 1.) each PGA molecule receives a phosphate group from a molecule of ATP. 2.) The resulting compound then receives a proton from NADPH and releases a phosphate group producing PGAL 2.) The resulting compound then receives a proton from NADPH and releases a phosphate group producing PGAL In addition to PGAL, these reactions produce ADP, NADP + and phosphate In addition to PGAL, these reactions produce ADP, NADP + and phosphate

STEP 3: Most of the PGAL is converted back into RuBP in a complicated series of reactions. Most of the PGAL is converted back into RuBP in a complicated series of reactions. Require a phosphate group from another molecule of ATP, which is changed into ADP. Require a phosphate group from another molecule of ATP, which is changed into ADP. By regenerating the RuBP that was consumed in Step 1, the reactions of Step 3 allow the Calvin Cycle to continue By regenerating the RuBP that was consumed in Step 1, the reactions of Step 3 allow the Calvin Cycle to continue Some PGAL molecules are not converted into RuBP – they leave the Calvin Cycle and can be used by the plant to make other organic compounds Some PGAL molecules are not converted into RuBP – they leave the Calvin Cycle and can be used by the plant to make other organic compounds

Balance Sheet for Photosynthesis Each turn of the Calvin Cycle fixes one CO 2 molecule…since PGAL is a three- carbon compound, it takes three turns of the cycle to produce each molecule of PGAL. Each turn of the Calvin Cycle fixes one CO 2 molecule…since PGAL is a three- carbon compound, it takes three turns of the cycle to produce each molecule of PGAL. For each turn of the cycle 2 ATP, and 2 NADPH molecules are used in Step 2, and 1 ATP molecule used in Step 3 For each turn of the cycle 2 ATP, and 2 NADPH molecules are used in Step 2, and 1 ATP molecule used in Step 3 Therefore 3 turns of the Calvin cycle uses 9 molecules of ATP and 6 molecules of NADPH Therefore 3 turns of the Calvin cycle uses 9 molecules of ATP and 6 molecules of NADPH

Simplest overall equation for photosynthesis, including both the light reactions and the Calvin Cycle can be written as: Simplest overall equation for photosynthesis, including both the light reactions and the Calvin Cycle can be written as: CO 2 + H 2 O + light energy  (CH 2 O) n + O 2 The (CH 2 O) represents the general formula for a carbohydrate.

CO 2 + H 2 O + light energy  (CH 2 O) + O 2 The (CH 2 O) represents the general formula for a carbohydrate. Often replaced in this equation by the carbohydrate glucose, C 6 H 12 O 6 giving this equation: The (CH 2 O) represents the general formula for a carbohydrate. Often replaced in this equation by the carbohydrate glucose, C 6 H 12 O 6 giving this equation: 6CO 2 + 6H 2 O + light energy  C 6 H 12 O 6 + 6O 2

Alternative Pathways Calvin Cycle is the most common pathway for the carbon fixation Calvin Cycle is the most common pathway for the carbon fixation Plant species that fix carbon exclusively through the Calvin Cycle are known as C3 plants because of the three-carbon compound PGA that is initially formed Plant species that fix carbon exclusively through the Calvin Cycle are known as C3 plants because of the three-carbon compound PGA that is initially formed Other plant species fix carbon through alternative pathways and then release it to enter the Calvin Cycle Other plant species fix carbon through alternative pathways and then release it to enter the Calvin Cycle

Alternative pathways are found in plants that are in hot, dry climates Alternative pathways are found in plants that are in hot, dry climates Can rapidly lose water to the air Can rapidly lose water to the air Most of the water loss from a plant occurs through small pores called stomata which are usually located on the undersurface of leaves Most of the water loss from a plant occurs through small pores called stomata which are usually located on the undersurface of leaves

Types of Photosynthesis C3 Photosynthesis C3 Photosynthesis C4 Photosynthesis C4 Photosynthesis CAM Photosynthesis CAM Photosynthesis

Stomata are the major passageways through which CO 2 enters and O 2 leaves a plant Stomata are the major passageways through which CO 2 enters and O 2 leaves a plant When stomata are partly closed, the level of CO 2 in the plant falls as CO 2 is consumed in the Calvin cycle When stomata are partly closed, the level of CO 2 in the plant falls as CO 2 is consumed in the Calvin cycle At the same time, the level of O 2 in the plant rises as the light reactions split water and generate O 2 At the same time, the level of O 2 in the plant rises as the light reactions split water and generate O 2

C3 Photosynthesis : C3 plants. Called C3 because the CO2 is first incorporated into a 3-carbon compound. Called C3 because the CO2 is first incorporated into a 3-carbon compound. Stomata are open during the day. Stomata are open during the day. RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2. RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2. Photosynthesis takes place throughout the leaf. 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).. 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).. Most plants are C3. Most plants are C3.

C 4 Pathway Allows certain plants to fix CO 2 into FOUR- Carbon Compounds. Allows certain plants to fix CO 2 into FOUR- Carbon Compounds. During the hottest part of the day, C4 plants have their stomata partially closed. During the hottest part of the day, C4 plants have their stomata partially closed. C 4 plants include corn, sugar cane and crabgrass. C 4 plants include corn, sugar cane and crabgrass. Such plants lose only about half as much water as C 3 plants when producing the same amount of carbohydrates Such plants lose only about half as much water as C 3 plants when producing the same amount of carbohydrates

C4 Photosynthesis : C4 plants. Called C4 because the CO2 is first incorporated into a 4-carbon compound. Called C4 because the CO2 is first incorporated into a 4-carbon compound. Stomata are open during the day. Stomata are open during the day. Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis. Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis. Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy) Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy) Adaptive Value: Adaptive Value: Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration. Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, 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. 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, and many of our summer annual plants. C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants.

The CAM Pathway Cactus, pineapples and certain other plants have different adaptations to hot, dry Climates. Cactus, pineapples and certain other plants have different adaptations to hot, dry Climates. They fix carbon through a pathway called CAM. They fix carbon through a pathway called CAM. Plants that use the CAM Pathway by opening their Stomata at night and Closing it during the day, the opposite of what other plants do. Plants that use the CAM Pathway by opening their Stomata at night and Closing it during the day, the opposite of what other plants do. At night, CAM plants take in CO 2 and fix it into organic compounds. At night, CAM plants take in CO 2 and fix it into organic compounds. During the day, CO 2 is released from these compounds and enters the Calvin Cycle. During the day, CO 2 is released from these compounds and enters the Calvin Cycle. Because CAM Plants have their Stomata open at night, they grow very slowly, But they lose less water than C 3 or C 4 plants. Because CAM Plants have their Stomata open at night, they grow very slowly, But they lose less water than C 3 or C 4 plants.

CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism 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. 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. Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis Adaptive Value: Adaptive Value: Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.). Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.). May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, 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 dry spells, 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). May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, 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 dry spells, 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). CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliadscactuses

Rate of Photosynthesis The rate at which a plant can carry out photosynthesis is affected by the plant’s environment. The rate at which a plant can carry out photosynthesis is affected by the plant’s environment. Three things in the plant’s environment affect the rate of photosynthesis: Three things in the plant’s environment affect the rate of photosynthesis: 1. LIGHT INTENSITY 2. CO 2 LEVELS 2. CO 2 LEVELS 3. TEMPERATURE

3 things affecting Photosynthesis 1. LIGHT INTENSITY - One of the most important. As light intensity INCREASES, the rate of photosynthesis initially INCREASES and then levels off to a plateau

3 things affecting Photosynthesis 2. CO2 LEVELS AROUND THE PLANT - Increasing the level of CO 2 stimulates photosynthesis until the rate reaches a plateau

3 things affecting Photosynthesis 3. TEMPERATURE - Raising the temperature ACCELERATES the Chemical Reactions involved in Photosynthesis. 3. TEMPERATURE - Raising the temperature ACCELERATES the Chemical Reactions involved in Photosynthesis. The rate of Photosynthesis increases as temperature increases. T The rate of Photosynthesis generally PEAKS at a certain temperatures, and photosynthesis begins to decrease when the temperature is further increased