Chapter 5: Capturing and Releasing Energy

1. Chapter 5: Capturing and Releasing Energy Part 2

2. The Light-Independent Reactions • The ATP (from Photosystem II) and the NADPH (from Photosystem I) from the light-dependent reactions power the energy-requiring/energy-storing reactions of the second stage of photosynthesis. • In this second stage, carbon dioxide is converted to carbohydrates such as glucose. • The reactions of the second stage of photosynthesis are also called the Calvin-Benson cycle and occur in the stroma of the chloroplast. • It is so called this because it is a cyclic reaction: the end products of one reaction serve as the beginning reactants of the next reaction.

3. The Light-Independent Reactions • During this part of photosynthesis, carbon fixation occurs. • Carbon fixation is a process in which carbon atoms from an inorganic source (such as carbon dioxide) are incorporated into an organic molecule (such as glucose). • The enzyme that performs carbon fixation in the light-independent reactions we will call rubisco.

4. The Light-Independent Reactions • Rubisco attaches a carbon dioxide molecule to a 5-carbon molecule called RuBP. • This forms a 6-carbon molecule that is immediately cleaved into two 3-carbon molecules called PGA. • This process has to occur six times to fix the 6 carbon dioxide molecules necessary to form one 6-carbon molecule of glucose and store the energy from the high-energy products (ATP and NADPH) of the light-dependent reactions.

5. The Light-Independent Reactions (Calvin-Benson Cycle)

6. The Light-Independent Reactions • The glucose produced during these reactions may either be used immediately by the plant or stored for future use in the form of sucrose or starch. • For example, excess glucose may be stored as starch granules in the stroma of the chloroplast.

7. Photosynthesis

8. Plant Adaptations for Photosynthesis • Plants are able to live and continue to perform photosynthesis in regions where water is scarce or only available at certain times due to an adaptation called the cuticle. • The cuticle is a thin, waxy, waterproof coating that prevents water loss by evaporation from plant parts that are above ground. • However, this coating also prevents the exchange of gases (that are important for photosynthesis- CO2 and O2) across the cells on the plant’s surface.

9. Plant Adaptations for Photosynthesis • To overcome this obstacle, plants have evolved tiny, closable “pores” on the surface of their photosynthetic parts called stomata. • When the stomata are open, carbon dioxide can diffuse into the plant’s photosynthetic tissues (so that the light-independent reactions can occur) and oxygen (from the light-dependent reactions) can diffuse out.

10. Plant Adaptations for Photosynthesis • Plants that use the Calvin-Benson cycle are called C3 plants (because the first intermediate in a 3-carbon molecule). • These plants can conserve water on dry days by closing their stomata. • However, when the stomata are closed, oxygen gas from the light-dependent reactions cannot get out and so accumulates in the photosynthetic cells. • The problem with this is that, when oxygen concentrations are high, rubisco binds oxygen to RuBP instead of carbon dioxide and so carbon fixation does not occur. • This is called photorepspirationand is very inefficient and undesirable.

11. Photosynthesis vs. Photorespiration

12. Plant Adaptations for Photosynthesis • However, some plants (such as corn and bamboo)have evolved adaptations to help them minimize photorespiration. • For example, C4 plants (so called because their first intermediate in the light-independent reactions is a 4-carbon molecule) fix carbon twice in two different kinds of cells. • In the first kind of cell, carbon is fixed by a different enzyme that doesn’t use oxygen at all, even at high concentrations. • An intermediate carbon molecule that is produced from this process is then transported to the second kind of cell and converted back to carbon dioxide, where it then enters the Calvin-Benson cycle. • The extra initial step keeps levels of carbon dioxide high in the cells that do the Calvin-Benson cycle, thus minimizing photorespiration.

13. C4 Plants

14. Plant Adaptations for Photosynthesis • In another kind of plant, called CAM plants, the extra reaction of the C4 plants runs at different times of day rather than in different cells. • The C4 reactions run during the day (when oxygen levels are high because the stomata are closed to prevent water loss) and the Calvin-Benson cycle (C3 reactions) run at night (when the oxygen levels are lower because the stomata are open due to lower environmental temperatures and less water loss by evaporation). • CAM stands for Crassulacean Acid Metabolism and plants such as succulents and cacti use this pathway.

15. CAM Plants

16. C4 Plants vs.CAM Plants

17. PHOTOSYNTHESIS HOMEWORK • 1. Using the graph on slide 10 in the Part 1 Powerpoint, list the colors that are absorbed and reflected by each of the following pigments according to the graph. Also, tell what color each pigment would appear to the human eye. • B-carotene • Chlorophyll a • Chlorophyll b • Phycoerythrin • Phycocyanin

18. PHOTOSYNTHESIS HOMEWORK • 2. Use the picture below to explain the names of each of the pigments on the previous question:

19. PHOTOSYNTHESIS HOMEWORK • 3. Explain the problem with rubisco. • 4. Compare and contrast C3, C4, and CAM plants.