6.6 Light Independent Reactions: The Sugar Factory

Transcription

1 6.6 Light Independent Reactions: The Sugar Factory Light-independent reactions proceed in the stroma Carbon fixation: Enzyme rubisco attaches carbon from CO 2 to RuBP to start the Calvin Benson cycle

2 Calvin Benson Cycle Cyclic pathway makes phosphorylated glucose Uses energy from ATP, carbon and oxygen from CO 2, and hydrogen and electrons from NADPH Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose) Six turns of Calvin Benson cycle fix six carbons required to build a glucose molecule from CO 2

3 PGA Light-Independent Reactions phosphoglycerate PGAL - phosphoglyceraldehyde

4 6.7 Adaptations: Different Carbon-Fixing Pathways Environments differ Plants have different details of sugar production in light-independent reactions On dry days, plants conserve water by closing their stomata O 2 from photosynthesis cannot escape CO 2 cannot enter the pant

5 Plant Adaptations to Environment C3 plants 85% of plants High O 2 level; Rubisco attaches to O 2 instead of CO 2 to RuBP; Photorespiration reduces efficiency of sugar production (by up to 50%) because respiration produces CO 2 and plant looses C. Do not grow well in dry Climates because lose stomata to conserve water

6 O 2 CO 2 RuBP Calvin- Benson cycle PGA glycolate sugar ATP NADPH a C3 plants. On dry days, stomata close and oxygen accumulates in air spaces inside leaves. The high concentration of oxygen makes rubisco attach oxygen instead of carbon to RuBP. Cells lose carbon and energy as they make sugars. Fig. 6.11a2, p.102

7 Plant Adaptations to Environment C4 plants Plants evolved in tropics Corn and bamboo Close stomata when in dry conditions NO decline in sugar production Carbon fixation occurs twice Use more ATP than C3 plants, but make more sugar on dry days First reactions release CO 2 near rubisco, limit photorespiration when stomata are closed

8 CO 2 from inside plant C4 cycle oxaloacetate CO 2 sugar RuBP Calvin- Benson cycle PGA b C4 plants. Oxygen also builds up in the air spaces inside the leaves when stomata close. An additional pathway in these plants keeps the CO2 concentration high enough to prevent rubisco from using oxygen. Fig. 6.11b2, p.102

9 Plant Adaptations to Environment CAM plants 2 cycles separated by time Open stomata and fix carbon at night During the day the Carbon dioxide production occurs.

10 night day CO 2 from outside plant C4 cycle oxaloacetate CO 2 sugar RuBP Calvin- Benson cycle PGA c CAM plants open stomata and fix carbon with a C4 pathway at night. When stomata are closed during the day, organic compounds made during the night are converted to CO2 that enters the Calvin Benson cycle. Fig. 6.11c2, p.102

11 Key Concepts: MAKING SUGARS Second stage is the synthesis part of photosynthesis Enzymes speed assembly of sugars from carbon and oxygen atoms, both from carbon dioxide Reactions use ATP and NADPH that form in the first stage of photosynthesis

12 Key Concepts: MAKING SUGARS (cont.) ATP delivers energy, and NADPH delivers electrons and hydrogens to the reaction sites Details of the reactions vary among organisms

13 6.8 A Burning Concern Photoautotrophs remove CO 2 from atmosphere; metabolic activity of organisms puts it back Before humans this was a balanced cycle Human activities disrupt the carbon cycle Add more CO 2 to the atmosphere than photoautotrophs can remove Started with burning forest to clear land for agriculture. Imbalance contributes to global warming

14 Fossil Fuel Emissions August 20 is Earth Overshoot Day 2013, marking the date when humanity exhausted nature s budget for the year. We are now operating in overdraft. For the rest of the year, we will maintain our ecological deficit by drawing down local resource stocks and accumulating carbon dioxide in the atmosphere.