APBIO- Chpt 10B Supplement Alternative Pathways & Rate of Photosynthesis

1. APBIO- Chpt 10B SupplementAlternative Pathways & Rate of Photosynthesis Warm-Up: Many plants have stomata that take in CO2 at night and release it during the day. Describe why this form of photosynthesis is an advantage for plants living in a hot, dry climate?

2. Objectives • (1) Students will be able to distinguish between C3, C4, and CAM plants. • (2) Students will be able to explain how environmental factors influence photosynthesis.

3. Alternative Pathways • The Calvin cycle is the most common pathway for carbon fixation. Plant species that fix carbon exclusively through the Calvin cycle are known as C3 plants. • Other plant species fix carbon through alternate pathways and then release it to enter the Calvin cycle. • Usually found in plants that evolved in hot dry climates.

4. Stomata in the leaf of a tobacco plant, Nicotianatabacum. • (a) When stoma are open water, carbon dioxide, and other gases can pass through it to enter or leave a plant (814x SEM). • (b) When stoma close passage is greatly restricted (878x SEM).

5. Alternative Pathways • When a plant’s stomata are partially closed the level of CO2 in plants falls as CO2 is consumed in the Calvin cycle. • At the same time, the level of O2 rises as the light reactions split water and generate O2. • Both of these conditions- low CO2 level and high O2 level- inhibit carbon fixation by the Calvin cycle. • Plants with alternative pathways for carbon fixation have evolved ways of dealing with problem.

6. The C4 Pathway • Alternative pathway that enables certain plants (C4 plants) to fix CO2 into four carbon compounds. • During the hottest part of the day the stomata are partially closed. They lose only half as water as C3 plants when producing the same amount of carbohydrate. • i.e., corn, sugar cane, and crabgrass.

7. The CAM Pathway • Open their stomata at night and close during the day- the opposite of what other plants do. • At night, CAM plants take in CO2 and fix it into a variety of compounds. During the day, CO2 is released from these compounds and enters the Calvin cycle. • Because CAM plants have their stomata open at night, when the temperature is lower, they grow slowly. But, the lose less water than either C3 or C4 plants. • i.e., cactusesand pineapples

8. “The difference between CAM & C4 Plants” • http://www.youtube.com/watch?v=VbFHakfg5lk&feature=related

9. Reflection • (1) Distinguish between C3, C4, and CAM plants.

10. Rate of Photosynthesis • The environment affects the rate of photosynthesis. • (1) Light intensity • When light intensity increases and then levels off to a plateau (which represents the maximum rate of photosynthesis). • (2) CO2 levels • Increasing CO2 levels around a plant stimulates photosynthesis until the rate of photosynthesis reaches plateau. • (3) Temperature • Raising the temperature accelerates various chemical reactions involved in photosynthesis. The rate of photosynthesis increases as temperature increases, over a certain range.

11. Reflection • (2) Explain how environmental factors influence photosynthesis.

12. Calvin Cycle, Alternative Pathways, & Rate of Photosynthesis Review • In what part of the chloroplast does the Calvin cycle take place? • Describe what happens to PGAL molecules made in the Calvin cycle. • How many turns of the Calvin cycle are needed to produce a molecule of PGAL? How many molecules of ATP and NADPH are used in the process? • What plant structures control the passage of water out of a plant and carbon dioxide into a plant? • What is a C4 plant? • Why does the rate of photosynthesis increase and then reach a plateau as the concentration of CO2 around a plant increases?