Calvin Cycle Also know as Calvin – Benson Cycle Light Independent Reactions (Dark Reactions – not a good term as it implies this only occurs at night)

ELECTRON TRANSPORT CHAIN The production of ATP AND NADPH the light reaction of photosynthesis Thylakoid compartment (high H+) Light Light Thylakoid membrane Antenna molecules Stroma (low H+) ELECTRON TRANSPORT CHAIN PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE Figure 7.9

ATP and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle occurs in the chloroplast’s stroma This is where carbon fixation takes place and sugar is manufactured INPUT CALVIN CYCLE Figure 7.10A OUTPUT:

Details of the Calvin cycle INPUT: 3 In a reaction catalyzed by rubisco, 3 molecules of CO2 are fixed. CO2 Step Carbon fixation. 1 1 3 P P 6 P RuBP 3-PGA 6 ATP 3 ADP Step Energy consumption 2 6 ADP + P 3 ATP CALVIN CYCLE 2 6 4 NADPH 6 NADP+ Step Release of one molecule of G3P. 3 5 P 6 P G3P G3P 3 Step Regeneration of RuBP. 4 Glucose and other compounds OUTPUT: 1 P G3P Figure 7.10B

Step 1 Carbon Fixation CO2 is incorporated (fixed) into a five-carbon sugar named ribulose bisphosphate (RuBP). The enzyme that does this is RuBP carboxylase or rubisco. The most abundant protein on Earth.

Step 2 Energy consumption ATP and NADPH2 (from the light reaction) are used to make the three-carbon precursor that will be used to make glucose and other sugars.

Step 3 Glucose For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3-phosphate (G3P) Used to make Glucose

Step 4 Regeneration of RuBP ATP is used to regenerate RuBP from G3P

Energy cost of Calvin Cycle For each G3P synthesized, the cycle spends: 9 ATP 6 NADPH2. Both are made in the light reaction

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Review: Photosynthesis uses light energy to make food molecules A summary of the chemical processes of photo-synthesis Chloroplast Light Photosystem II Electron transport chains Photosystem I CALVIN CYCLE Stroma Electrons Cellular respiration Cellulose Starch Other organic compounds LIGHT REACTIONS CALVIN CYCLE Figure 7.11

Many plants make more sugar than they need The excess is stored in roots, tubers, and fruits These are a major source of food for heterotrophs

C4 and CAM plants have special adaptations that save water Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle In these types of plants, stomata on the leaf surface close when the weather is hot This causes a drop in CO2 and an increase in O2 in the leaf Photorespiration may then occur No sugar or ATP

Photorespiration in a C3 plant CALVIN CYCLE 2-C compound Figure 7.12A

Some plants have special adaptations that enable them to save water Special cells in C4 plants—corn and sugarcane—incorporate CO2 into a four-carbon molecule This molecule can then donate CO2 to the Calvin cycle 4-C compound CALVIN CYCLE 3-C sugar Figure 7.12B

The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism They open their stomata at night and make a four-carbon compound It is used as a CO2 source by the same cell during the day 4-C compound Night Day CALVIN CYCLE 3-C sugar Figure 7.12C

PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE Due to the increased burning of fossil fuels, atmospheric CO2 is increasing CO2 warms Earth’s surface by trapping heat in the atmosphere This is called the greenhouse effect

Sunlight ATMOSPHERE Radiant heat trapped by CO2 and other gases Figure 7.13A & B

Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect Unfortunately, deforestation may cause a decline in global photosynthesis

Mario Molino received a Nobel Prize in 1995 for his work on the ozone layer His research focuses on how certain pollutants (greenhouse gases) damage that layer Figure 7.14A

Sunlight ATMOSPHERE Radiant heat trapped by CO2 and other gases Figure 7.13A & B

Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect Unfortunately, deforestation may cause a decline in global photosynthesis

7.14 Talking About Science: Mario Molina talks about Earth’s protective ozone layer Mario Molino received a Nobel Prize in 1995 for his work on the ozone layer His research focuses on how certain pollutants (greenhouse gases) damage that layer Figure 7.14A

The O2 in the atmosphere results from photosynthesis Solar radiation converts O2 high in the atmosphere to ozone (O3) Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer International restrictions on these chemicals are allowing recovery Sunlight Southern tip of South America Antarctica Figure 7.14B