• In addition, a proton gradient is generated that is used to produce ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and Pi (inorganic phosphate). • ATP is a temporary energy carrier. The NADP+ is converted to NADPH by the adding of two electrons and one proton (H+). • At the end of the light-harvesting phase, NADPH, ATP, and O2 are produced. • In the carbon fixation phase of photosynthesis, the energy and electrons of ATP and NADPH are used to form carbon–carbon bonds. Carbon dioxide (CO2) enters a cyclic series of reactions (the Calvin cycle), and eventually sugars (CH2O)n are produced. The name of the enzyme that catalyzes the fusion of CO2 to the first chemical in the cycle (ribulose bisphosphate) is rubisco. • since rubisco is the most abundant protein on our globe; The first sugar to exit the cycle is a three- carbon sugar, G3P (glyceraldehyde 3-phosphate).G3P can be used to generate other sugars, such as glucose (C6H12O6). • The summary of the carbon- fixation phase of photosynthesis can be stated as: EQUATION 3: ATP + NADPH + CO2 Æ ADP + Pi + NADP+ (CH2O)n Summary—Light Independent Reactions a. Overall input CO 2 , ATP, NADPH. b. Overall output glucose. Calvin Cycle • Biochemical pathway in photosynthesis that produces organic compounds using ATP & NADPH
• Carbon fixation – carbon atoms from CO 2 are bonded or ‘fixed’ into carbohydrates • occurs in stroma • The Calvin cycle begins when CO 2 enters the cycle and is joined to RuBP this forms a 6 carbon compound which immediately splits into two 3 carbon compounds (the 6 carbon intermediate has never been isolated) - the 3 carbon compound is 3-phosphoglycerate (PGA) • Because each PGA has three carbons, this is sometimes also called the C3 pathway • Each full turn of the Calvin cycle begins with entry of a CO 2 molecule and ends when RuBP is regenerated - it takes 6 full turns of the Calvin cycle to generate a 6 carbon sugar such as glucose • Although we usually report glucose as the product of photosynthesis, the cell usually produces either sucrose or starch as its storage products • At night, sucrose is produced from the starch and it is transported from the chloroplast to the rest of the cell Light-independent reactions AKA Calvin Cycle Happens in the stroma of the chloroplast Is fueled by the energy made from the light-dependent reactions (does not need sunlight) This is the “synthesis” part of photosynthesis Calvin cycle 1. Carbon dioxide is added to a 5 carbon chain molecule, forms Six- carbon molecules 2. Energy from light-dependent reactions, ATP AND NADPH, split the six-carbon molecules to three-carbon molecules 3. Three-carbon molecules exit. For every 2 three-carbon molecules that exit, one sugar molecule is formed 4. Energy from ATP is used to change the remaining three-carbon molecules back into 5-carbon molecules that stay in the Calvin cycle • Carbon Fixation • C 3 plants (80% of plants on earth) • Occurs in the stroma • Uses ATP and NADPH from light reaction as energy • Uses CO 2 • To produce glucose: it takes 6 turns and uses 18 ATP and 12 NADPH.
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