The use of biofuels (see chapter introduction) avoids many of the problems associated

Autotrophs are the producers of the biosphere. Plants, algae, and some protists and bacteria are photoautotrophs, the producers of food consumed by virtually all heterotrophic organisms.

Photosynthesis occurs in chloroplasts in plant cells. Chloroplasts are surrounded by a double membrane and contain stacks of thylakoids and a thick fluid called stroma.

Scientists traced the process of photosynthesis using isotopes. Experiments using both heavy and radioactive isotopes helped determine the details of the process of photosynthesis.

Photosynthesis is a redox process, as is cellular respiration. In photosynthesis, H2O is oxidized and CO2 is reduced.

Overview: The two stages of photosynthesis are linked by ATP and NADPH. The light reactions occur in the thylakoids, producing ATP and NADPH for the Calvin cycle, which takes place in the stroma.

Visible radiation absorbed by pigments drives the light reactions. Certain wavelengths of visible light are absorbed by chlorophyll and other pigments. Carotenoids also function in photoprotection from excessive light.

Photosystems capture solar energy. Thylakoid membranes contain photosystems, each consisting of light-harvesting complexes and a reaction-center complex. A primary electron acceptor receives photoexcited electrons from chlorophyll.

Two photosystems connected by an electron transport chain generate ATP and NADPH. Electrons shuttle from photosystem II to photosystem I, providing energy to make ATP, and then reduce NADP+ to NADPH. Photosystem II regains electrons as water is split and O2 released.

Chemiosmosis powers ATP synthesis in the light reactions. In photophosphorylation, the electron transport chain pumps H+ into the thylakoid space. The concentration gradient drives H+ back through ATP synthase, driving the synthesis of ATP.

ATP and NADPH power sugar synthesis in the Calvin cycle. The steps of the Calvin cycle include carbon fixation, reduction, release of G3P, and regeneration of RuBP. Using carbon from CO2, electrons from NADPH, and energy from ATP, the cycle constructs G3P, which is used to build glucose and other organic molecules.

Other methods of carbon fixation have evolved in hot, dry climates. In C3 plants, a drop in CO2 and rise in O2 when stomata close divert the Calvin cycle to photorespiration. C4 plants and CAM plants first fix CO2 into a four-carbon compound that provides CO2 to the Calvin cycle even when stomata close on hot, dry days.

Review: Photosynthesis uses light energy, carbon dioxide, and water to make organic molecules

Photosynthesis may moderate global climate change. Reducing deforestation and fossil fuel use and growing biofuel crops, which remove CO2 from the atmosphere, may help moderate the global warming caused by increasing CO2 levels.

Scientific study of Earths ozone layer has global significance. Solar radiation converts O2 high in the atmosphere to ozone (O3), which shields organisms from damaging UV radiation. Industrial chemicals called CFCs have caused dangerous thinning of the ozone layer, but international restrictions on CFC use are allowing a slow recovery.

The following diagram compares the chemiosmotic synthesis of ATP in mitochondria and chloroplasts. Fill in the blanks, which label the molecules shared by both processes and the regions in the chloroplast. Then, for both organelles, indicate which side of the membrane has the higher H+ concentration.

Continue your comparison of chemiosmosis and electron transport in mitochondria and chloroplasts. In each case, a. where do the electrons come from? b. how do the electrons get their high potential energy? c. what picks up the electrons at the end of the chain? d. how is the energy given up by the electrons used?

Complete this summary map of photosynthesis.

In photosynthesis, ______ is oxidized and ______ is reduced. a. glucose . . . oxygen b. carbon dioxide . . . water c. water . . . carbon dioxide d. glucose . . . carbon dioxide e. water . . . oxygen

Which of the following are produced by reactions that take place in the thylakoids and consumed by reactions in the stroma? a. CO2 and H2O b. NADP+ and ADP c. ATP and NADPH d. ATP, NADPH, and CO2 e. O2 and ATP

When light strikes chlorophyll molecules in the reaction-center complex, they lose electrons, which are ultimately replaced by a. splitting water. b. breaking down ATP. c. oxidizing NADPH. d. fixing carbon. e. oxidizing glucose.

The reactions of the Calvin cycle are not directly dependent on light, but they usually do not occur at night. Why? (Explain.) a. It is often too cold at night for these reactions to take place. b. Carbon dioxide concentrations decrease at night. c. The Calvin cycle depends on products of the light reactions. d. Plants usually close their stomata at night. e. Most plants do not make four-carbon compounds, which they would need for the Calvin cycle at night.

How many turns of the Calvin cycle are required to produce one molecule of glucose? (Assume one CO2 is fixed in each turn of the cycle.) a. 1 b. 2 c. 3 d. 6 e. 12

Which of the following does not occur during the Calvin cycle? a. carbon fixation b. oxidation of NADPH c. consumption of ATP d. regeneration of RuBP, the CO2 acceptor e. release of oxygen 1

Why is it difficult for most plants to carry out photosynthesis in very hot, dry environments such as deserts? a. The light is too intense and destroys the pigment molecules. b. The closing of stomata keeps CO2 from entering and O2 from leaving the plant. c. They must rely on photorespiration to make ATP. d. Global warming is intensified in a desert environment. e. CO2 builds up in the leaves, blocking carbon fixation. 1

How is photosynthesis similar in C4 plants and CAM plants? a. In both cases, the light reactions and the Calvin cycle are separated in both time and location. b. Both types of plants make sugar without the Calvin cycle. c. In both cases, rubisco is not used to fix carbon initially. d. Both types of plants make most of their sugar in the dark. e. In both cases, thylakoids are not involved in photosynthesis. 1

What are the major inputs and outputs of the two stages of photosynthesis? 1

Explain why a poison that inhibits an enzyme of the Calvin cycle will also inhibit the light reactions. 1

What do plants do with the sugar they produce in photosynthesis? 1

Most experts agree that global warming is already occurring and will increase rapidly in this century. Many countries have made a commitment to reduce CO2 emissions. Recent international negotiations, however, including a 2009 meeting in Copenhagen, Denmark, have yet to reach a consensus on a global strategy to reduce greenhouse gas emissions. Some countries have resisted taking action because a few scientists and policymakers think that the warming trend may be just a random fluctuation in temperature and/or not related to human activities or that cutting CO2 emissions would sacrifice economic growth. Do you think we need more evidence before taking action? Or is it better to act now to reduce CO2 emissions? What are the possible costs and benefits of each of these two strategies? 1

The use of biofuels (see chapter introduction) avoids many of the problems associated with gathering, refining, transporting, and burning fossil fuels. Yet biofuels are not without their own set of problems. What challenges do you think would arise from a large-scale conversion to biofuels? How do these challenges compare with those encountered with fossil fuels? Do you think any other types of energy sources have more benefits and fewer costs than the others? Which ones, and why?