CHAPTER 7 Photosynthesis: Using Light to Make Food Modules 7.6 – 7.14

7.6 Visible radiation drives the light reactions THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY 7.6 Visible radiation drives the light reactions Certain wavelengths of visible light drive the light reactions of photosynthesis Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light Wavelength (nm) Figure 7.6A

Reflected light Light Chloroplast Absorbed light Transmitted light Figure 7.6B

7.7 Photosystems capture solar power Each of the many light-harvesting photosystems 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

Fluorescence of isolated chlorophyll in solution Heat Photon (fluorescence) Photon Chlorophyll molecule Figure 7.7A

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

7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O2 Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons The excited electrons are passed from the primary electron acceptor to electron transport chains Their energy ends up in ATP and NADPH

Where do the electrons come from that keep the light reactions running? In photosystem I, electrons from the bottom of the cascade pass into its P700 chlorophyll

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

7.9 Chemiosmosis powers ATP synthesis in the light reactions The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP In the stroma, the H+ ions combine with NADP+ to form NADPH

ELECTRON TRANSPORT CHAIN The production of ATP by chemiosmosis in 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

7.10 ATP and NADPH power sugar synthesis in the Calvin cycle THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS 7.10 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:

The Calvin cycle constructs G3P using carbon from atmospheric CO2 electrons and H+ from NADPH energy from ATP Energy-rich sugar is then converted into glucose

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 and redox. 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

7.11 Review: Photosynthesis uses light energy to make food molecules PHOTOSYNTHESIS REVIEWED AND EXTENDED 7.11 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, tuber, and fruits These are a major source of food for animals

7.12 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

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 7.13 Human activity is causing global warming; photosynthesis moderates it 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

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

International restrictions on these chemicals are allowing recovery 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