a b c Figure: 08-01 Title: Three types of photosynthesizers. Caption: Photosynthesis is carried out not only by familiar plants, such as these sunflowers (a), but by algae, such as this giant kelp (b), and by some bacteria, such as these cyanobacteria (c). (magnified x 1025) a b c

Photosynthesis Importance Where, Who? What is it? Limitations

Photosynthesis (Phosyn) - conversion of light energy to chemical energy Figure 7.1A Figure 7.1B

Autotrophs = “self feeders” Includes plants, algae, some bacteria Figure 7.1C Figure 7.1D

Photosynthesis = light energy is used to make sugar and oxygen from CO2 and water Carbon dioxide Water Glucose Oxygen gas PHOTOSYNTHESIS

Aquatic plant shows production of O2 Aerobic organisms dependent on this O2 Figure 7.3A

The location and structure of chloroplasts LEAF CROSS SECTION MESOPHYLL CELL LEAF Mesophyll CHLOROPLAST Intermembrane space Outer membrane Granum Inner membrane Grana Stroma Thylakoid compartment Stroma Figure 7.2 Thylakoid

Photosynthesis is a redox process Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas These electrons and H+ ions are transferred to CO2, producing sugar Reduction Oxidation Respiration Oxidation Reduction

CO2 sunlight H2O Calvin cycle ADP NADP+ O2 sugar NADPH ATP Figure: 08-08 Title: Summary of photosynthesis. Caption: In the light-dependent reactions, solar energy is converted to chemical energy in the thylakoids, and the chemical energy is stored temporarily in the form of ATP and NADPH. Water is required for this reaction, and oxygen is a by-product. The stored chemical energy is in turn used in the light-independent reactions (the Calvin cycle), taking place in the stroma, in which sugar is made from carbon dioxide and a low-energy sugar, RuBP. The sugar can be used for food, or may become part of the plant’s structure. NADP+ O2 sugar

Certain wavelengths of visible light drive the light reactions of photosynthesis Reflected light Chloroplast Absorbed light Transmitted light Figure 7.6B

How do photosystems capture solar power? Each light-harvesting photosystem consists of: an “antenna” of chlorophyll and other pigment molecules that absorb light a primary electron acceptor that receives excited electrons from the reaction-center chlorophyll

Primary electron acceptor PHOTOSYSTEM Photon Reaction center Pigment molecules of antenna Figure 7.7C

Primary electron acceptor Excitation of chlorophyll in a chloroplast Primary electron acceptor Other compounds Photon Chlorophyll molecule Figure 7.7B

Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product Primary electron acceptor Electron transport Primary electron acceptor Electron transport chain Photons Energy for synthesis of PHOTOSYSTEM I PHOTOSYSTEM II by chemiosmosis Figure 7.8

Chemiosmosis powers ATP synthesis in the light reactions H+ produced by photolysis of H2O electron transport chains pump H+ through the thylakoid membrane The flow of H+ back into the stroma is harnessed by ATP synthase to make ATP In the stroma, the H+ ions combine with NADP+ to form NADPH

thylakoid compartment chloroplast sunlight PHOTOSYSTEM II PHOTOSYSTEM I Figure: 08-06 Title: Light-dependent reactions. Caption: The light-dependent reactions take place in the thylakoid membranes within the chloroplasts. Electrons are donated by water molecules located in the thylakoid compartments. Powered by the Sun’s energy, these electrons are passed along the electron transport chain embedded in the thylakoid membrane and end up stored in NADPH in the stroma. An additional product of the splitting of water molecules is oxygen atoms, which quickly combine into the O2 form. This is the atmospheric oxygen that we breathe in. e- NADPH O2 + 4 H+ 2 H2O thylakoid compartment thylakoid membrane stoma

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

3 molecules 3 molecules of RuBP rubisco 6 molecules of 3-PGA 1. carbon fixation 3 ADP 6 ATP 3 ATP 4. regeneration of RuBP 2. energizing the sugar 6 ADP 3 molecules of G3P Figure: 08-07 Title: Calvin cycle. Caption: Overview: CO2, ATP and electrons (contained in hydrogen atoms) from NADPH are the input into the Calvin cycle, while a sugar (G3P) is its output. 1. Carbon fixation. The enzyme rubisco brings together three molecules of CO2 with three molecules of the five-carbon sugar RuBP; the three resulting six-carbon molecules are immediately split into six three-carbon molecules named 3-PGA (3-phosphoglyceric acid). 2. Energizing the Sugar. In two separate reactions, (a) Six ATP molecules react with six 3-PGA, in each case transferring a phosphate onto the 3-PGA. (b) The six 3-PGA derivatives oxidize (gain electrons from) six NADPH molecules; in so doing, they are transformed into the energy-rich sugar G3P (glyceraldehyde 3-phosphate). 3. Exit of product. One molecule of G3P exits as the output of the Calvin cycle. This molecule, the product of photosynthesis, can be used for energy or transformed into materials that make up the plant. 4. Regeneration of RuBP. In several reactions, five molecules of G3P are transformed into three molecules of RuBP. 3. exit of product 6 molecules of 3-PGA derivative 1 molecule of G3P 6 NADPH 6 NADP+ 6 molecules of G3P glucose and other derivatives

LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma) An overview of photosynthesis H2O CO2 Chloroplast Light NADP+ ADP + P LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma) ATP Electrons NADPH O2 Sugar Figure 7.5

Photorespiration occurs as O2 increases and CO2 decreases Plants close their stomates to conserve water. Result: CO2 cannot reach the mesophyll cells. Photorespiration occurs as O2 increases and CO2 decreases

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

This molecule can then donate CO2 to the Calvin cycle 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

Stomates open at night; plants make a four-carbon compound CAM plants—pineapples, most cacti, and succulents—employ a different mechanism Stomates open at night; plants make a four-carbon compound Then use this as a CO2 source in the same cell during the day 4-C compound Night Day CALVIN CYCLE 3-C sugar Figure 7.12C

Is global warming really a threat to life? Due to the increased burning of fossil fuels, atmospheric CO2 is increasing (+30% since 1900) CO2 warms Earth’s surface by trapping heat in the atmosphere = greenhouse effect

Greenhouse gases trap solar energy in the atmosphere - gases include CO2 and methane Sunlight ATMOSPHERE Radiant heat trapped by CO2 and other gases Figure 7.13A & B

What is the effect on phosyn? Consequences predicted by models: world temp may rise from 1 to 6 degrees C by 2100 Polar ice melts, sea levels rise Drastic weather changes Spread of tropical pests and diseases Extinction of many species What is the effect on phosyn?

Effects of deforestation? How can warming be stopped? 1. Plant more crops - phosyn removes CO2 Effects of deforestation? Stop adding CO2, methane - Change from fossil fuel to other energy - Don’t eat hamburgers.

Solar radiation converts O2 high in the atmosphere to ozone (O3) 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

International restrictions on these chemicals are allowing recovery Industrial chemicals called CFCs speed up 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

Figure: 08-11 Title: Dry-weather photosynthesis. Caption: Plants such as this saguaro cactus in Arizona utilize CAM photosynthesis, thereby preserving precious water.