Where It Starts – Photosynthesis Chapter 7 Part 2

7.6 Light-Independent Reactions: The Sugar Factory  The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis  Calvin-Benson cycle Enzyme-mediated reactions that build sugars in the stroma of chloroplasts

Carbon Fixation  Carbon fixation Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule Builds glucose from CO 2 Uses bond energy of molecules formed in light- dependent reactions (ATP, NADPH)

The Calvin-Benson Cycle  Enzyme rubisco attaches CO 2 to RuBP Forms two 3-carbon PGA molecules  PGAL is formed PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH Two PGAL combine to form a 6-carbon sugar  Rubisco is regenerated

Inputs and Outputs of the Calvin-Benson Cycle

The Calvin-Benson Cycle

Fig. 7-11, p. 115 A Six CO 2 in air spaces inside of a leaf diffuse into a photosynthetic cell. Rubisco attaches each to a RuBP molecule. The resulting intermediates split, so twelve molecules of PGA form. A 6CO 2 12 ATP 12 PGA 6 RuBP B 6 ADP 12 ADP + 12 P i B Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. Twelve intermediate molecules (PGAL) form. D The remaining ten PGAL get phosphate groups from ATP. The transfer primes them for endergonic reactions that regenerate the 6 RuBP. Calvin–Benson Cycle 6 ATP 12 NADPH 4 P i 12 NADP + D C Two of the PGAL combine and form one molecule of glucose. The glucose may enter reactions that form other carbohydrates, such as sucrose and starch. 12 PGAL10 PGAL C other molecules glucose

Fig. 7-11, p. 115 B 12 ATP 12 PGA 12 ADP + 12 P i B Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. Twelve intermediate molecules (PGAL) form. 12 NADPH 12 NADP + 12 PGAL 6 ADP D The remaining ten PGAL get phosphate groups from ATP. The transfer primes them for endergonic reactions that regenerate the 6 RuBP. 6 ATP 4 P i D 10 PGAL C Two of the PGAL combine and form one molecule of glucose. The glucose may enter reactions that form other carbohydrates, such as sucrose and starch. other molecules glucose C A 6CO 2 6 RuBP A Six CO 2 in air spaces inside of a leaf diffuse into a photosynthetic cell. Rubisco attaches each to a RuBP molecule. The resulting intermediates split, so twelve molecules of PGA form. Calvin–Benson Cycle Stepped Art

Animation: Calvin-Benson cycle

7.7 Adaptations: Different Carbon-Fixing Pathways  Environments differ, and so do details of photosynthesis C3 plants C4 plants CAM plants

Stomata  Stomata Small openings through the waxy cuticle covering epidermal surfaces of leaves and green stems Allow CO 2 in and O 2 out Close on dry days to minimize water loss

C3 Plants  C3 plants Plants that use only the Calvin–Benson cycle to fix carbon Forms 3-carbon PGA in mesophyll cells Used by most plants, but inefficient in dry weather when stomata are closed

Photorespiration  When stomata are closed, CO 2 needed for light- independent reactions can’t enter, O 2 produced by light-dependent reactions can’t leave  Photorespiration At high O 2 levels, rubisco attaches to oxygen instead of carbon CO 2 is produced rather than fixed

C4 Plants  C4 plants Plants that have an additional set of reactions for sugar production on dry days when stomata are closed; compensates for inefficiency of rubisco Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make sugar Examples: Corn, switchgrass, bamboo

C3 and C4 Plant Leaves

Fig. 7-12a, p. 116

palisade mesophyll cell spongy mesophyll cell A C3 plant leaves. Chloroplasts are distributed evenly among two kinds of mesophyll cells in leaves of C3 plants such as basswood (Tilia americana). The light-dependent and light-independent reactions occur in both cell types.

Fig. 7-12b, p. 116

bundle-sheath cell mesophyll cell B C4 plant leaves. In C4 plants such as corn (Zea mays), carbon is fixed the first time in mesophyll cells, which are near the air spaces in the leaf, but have few chloroplasts. Specialized bundle-sheath cells ringing the leaf veins closely associate with mesophyll cells. Carbon fixation occurs for the second time in bundle-sheath cells, which are stuffed with rubisco-containing chloroplasts.

CAM Plants  CAM plants (Crassulacean Acid Metabolism) Plants with an alternative carbon-fixing pathway that allows them to conserve water in climates where days are hot Forms 4-carbon oxaloacetate at night, which is later broken down to CO 2 for sugar production Example: succulents, cactuses

A CAM Plant  Jade plant (Crassula argentea)

C3, C4, and CAM Reactions

Fig. 7-13a, p. 117

mesophyll cell O2O2 CO 2 RuBP glycolate Calvin– Benson Cycle PGA sugar ATP NADPH A C3 plants. On dry days, stomata close and oxygen accumulates to high concentration inside leaves. The excess causes rubisco to attach oxygen instead of carbon to RuBP. Cells lose carbon and energy as they make sugars.

Fig. 7-13b, p. 117

mesophyll cell CO 2 from inside plant oxaloacetate C4 Cycle CO 2 bundle-sheath cell RuBP Calvin– Benson Cycle PGA sugar B C4 plants. Oxygen also builds up inside leaves when stomata close during photosynthesis. An additional pathway in these plants keeps the CO 2 concentration high enough to prevent rubisco from using oxygen.

Fig. 7-13c, p. 117

mesophyll cell CO 2 from outsid e plant C4 Cycle oxaloacetate night day CO 2 RuBP Calvin– Benson Cycle PGA sugar C CAM plants open stomata and fix carbon using a C4 pathway at night. When stomata are closed during the day, the organic compounds made during the night are converted to CO 2 that enters the Calvin– Benson cycle.

Key Concepts: Making Sugars  The second stage is the “synthesis” part of photosynthesis, in which sugars are assembled from CO 2  The reactions use ATP and NADPH that form in the first stage of photosynthesis  Details of the reactions vary among organisms

7.8 Photosynthesis and the Atmosphere  The evolution of photosynthesis dramatically and permanently changed Earth’s atmosphere

Different Food Sources  Autotrophs Organisms that make their own food using energy from the environment and inorganic carbon  Heterotrophs Organisms that get energy and carbon from organic molecules assembled by other organisms

Two Kinds of Autotrophs  Chemoautotrophs Extract energy and carbon from simple molecules in the environment (hydrogen sulfide, methane) Used before the atmosphere contained oxygen  Photoautotrophs Use photosynthesis to make food from CO 2 and water, releasing O 2 Allowed oxygen to accumulate in the atmosphere

Earth With and Without Oxygen Atmosphere

Fig. 7-15a, p. 118

Fig. 7-15b, p. 118

Effects of Atmospheric Oxygen  Selection pressure on evolution of life Oxygen radicals  Development of ATP-forming reactions Aerobic respiration  Formation of ozone (O 3 ) layer Protection from UV radiation

7.8 Key Concepts: Evolution and Photosynthesis  The evolution of photosynthesis changed the composition of Earth’s atmosphere  New pathways that detoxified the oxygen by- product of photosynthesis evolved

7.9 A Burning Concern  Earth’s natural atmospheric cycle of carbon dioxide is out of balance, mainly as a result of human activity

The Carbon Cycle  Photosynthesis locks CO 2 from the atmosphere in organic molecules; aerobic respiration returns CO 2 to the atmosphere A balanced cycle of the biosphere  Humans burn wood and fossil fuels for energy, releasing locked carbon into the atmosphere Contributes to global warming, disrupting biological systems

Fossil Fuel Emissions

7.9 Key Concepts: Photosynthesis, CO 2 & Global Warming  Photosynthesis by autotrophs removes CO 2 from the atmosphere; metabolism by all organisms puts it back in  Human activities have disrupted this balance, and contribute to global warming

Animation: C3-C4 comparison

Animation: Harvesting photo energy

Animation: Light-dependent reactions

Animation: Photosynthesis overview

Animation: Structure of a chloroplast

Animation: Wavelengths of light

ABC video: Solar Power

Video: Biofuels