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?

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.

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.

• Stomata in the leaf of a tobacco plant, Nicotiana tabacum.• (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).

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.

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.

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., cactuses and pineapples

“The difference between CAM & C4 Plants”

• http://www.youtube.com/watch?v=VbFHakfg5lk&feature=related

Reflection

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

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.

Reflection

• (2) Explain how environmental factors influence photosynthesis.

Calvin Cycle, Alternative Pathways, & Rate of Photosynthesis Review

1. In what part of the chloroplast does the Calvin cycle take place?

2. Describe what happens to PGAL molecules made in the Calvin cycle.

3. 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?

4. What plant structures control the passage of water out of a plant and carbon dioxide into a plant?

5. What is a C4 plant?6. Why does the rate of photosynthesis increase and then reach

a plateau as the concentration of CO2 around a plant increases?