Light-independent reaction To be able to explain that: Energy from the light-independent reaction is used to fix carbon dioxide.

Write a balanced equation for photosynthesis Give reasons why photosynthesis is important. In which cycle does photosynthesis feature? What is the waste product of photosynthesis? What is the splitting of water called? What is made during the light dependent reaction? 6CO2 + 6H2O  6O2 + C6H12O6 Makes food, oxygen carbon oxygen photolysis ATP, NADPH, oxygen

The Light-Independent Reactions The light-independent, or carbon-fixing reactions, of photosynthesis take place in the stroma of the chloroplasts CO2 readily diffuses into the chloroplast where it is built up into sugars in another cyclic process called the Calvin cyclt Comprises of another cyclic pathway, called the Calvin Cycle, after the American scientist who discovered it.

Glossary Ribulose biphosphate (RuBP) glycerate 3-phosphate (GP) ribulose bisphosphate carboxylase 5 carbon sugar A 3 carbon compound (an acid) An enzyme also known as RuBISCO Rubisco – It is a very slow and inefficient enzyme, so large amounts of it are needed (recall that increasing enzyme concentration increases reaction rate), and it comprises about 50% of the mass of chloroplasts, making the most abundant protein in nature. Rubisco is synthesised in chloroplasts, using chloroplast (not nuclear) DNA. Stroma packed with rubisco

The fixation of Carbon Dioxide is light independent. Carbon Dioxide combines with RuBP = 2 x glycerate 3 phosphate CO2 is covalently bonded to RuBP catalysed by the enzyme RuBISCO The product is two molecules of glycerate 3 phosphate This process is called fixation  CO2  Carbon dioxide binds to the 5-carbon sugar ribulose bisphosphate (RuBP) (carboxylated) to form 2 molecules of the 3-carbon compound glycerate phosphate. This carbon-fixing reaction is catalysed by the enzyme ribulose bisphosphate carboxylase, also known as rubisco. RuBP

Energy and H+ ions are required to turn the Glycerate 3 phosphate into the 3 carbon sugar triose phosphate Energy is provided by ATP and the H+ ion is provided by the NADPH from the light dependant reaction 2NADPH 2NADP 2ADP + Pi 2ATP Glycerate phosphate (GP) is an acid, not a carbohydrate, so it is reduced and activated to form triose phosphate (also referred to as GALP), the same 3-carbon sugar as that found in glycolysis. The ATP and NADPH from the light-dependent reactions provide the energy for this step. The ADP and NADP return to the thylakoid membrane for recycling. = 2 x glycerate 3 phosphate 2 x Triose Phosphate  CO2  RuBP

This uses the rest of the ATP produced in the light dependant reaction Two triose phosphate molecules join together to form one hexose sugar e.g. glucose Glucose may be immediately respired, stored as starch or converted into other products The bulk of triose phosphate is converted into more RuBP to enable the fixation of Carbon to continue This uses the rest of the ATP produced in the light dependant reaction 2NADPH 2NADP 2ADP + Pi 2ATP Triose phosphate is a branching point. Most of the triose phosphate continues through a complex series of reactions to regenerate the RuBP and complete the cycle. 5 triose phosphate molecules (15 carbons) combine to form 3 RuBP molecules (15 carbons). = 2 x glycerate 3 phosphate 2 x Triose Phosphate  CO2  RuBP ADP + Pi Hexose sugar ATP

Plants can’t ingest proteins and fats Calvin cycle is the starting point for all the substances a plant needs 5 out of every 6 molecules of triose phosphate are used to regenerate RuBP Every 3 turns of the Calvin Cycle 3 CO2 molecules are fixed to make 1 new triose phosphate molecule. This leaves the cycle, and two of these triose phosphate molecules combine to form one glucose molecule using the glycolysis enzymes in reverse. The glucose can then be used to make other material that the plant needs.

Calvin cycle in summary CO2 combines with ribulose bisphosphate Glycerate phosphate is made Glycerate phosphate is converted into triose phosphate Triose phosphate is converted into glucose RuBP is regenerated

Questions Which compound is carboxylated? How is carbon dioxide fixed? What does GP stand for? Which compound is regenerated? What is ATP required for? What is NADPH required for? What reaction converts GP into triose phosphate? What kind of sugar is RuBP? Which enzyme is important in the Calvin Cycle? How many carbon atoms in GP? Where does the Calvin Cycle happen? RuBP Joined to RuBP Glycerate-3-phosphate Energy for reduction and regeneration of RuBP Source of hydrogen for reduction reduction pentose Rubisco 3 stroma