1. Animal, Plant & Soil Science E4-3 Photosynthesis

2. Interest Approach  
At the beginning of class, ask the students to list all they know about photosynthesis, the products of photosynthesis, and the importance of photosynthesis to life. After 10 minutes, ask students to share their thoughts. Allow the discussion to flow into the objectives of the lesson.

3. Objectives
1 Examine the role of chloroplasts and plant pigments in photosynthesis.
2 Analyze the major steps involved in photosynthesis, including the two phases of photosynthesis.
3 Compare and contrast C3, C4, and CAM plants.
4 Identify factors that affect photosynthesis.

4. Terms _ ATP _ bundle sheath cells _ C3 plants _ C4 plants
_ Calvin cycle
_ CAM photosynthesis
_ carbon fixation
_ carotenoids
_ chlorophyll

5. Terms _ chlorophyll a _ chlorophyll b _ chloroplasts _ grana
_ light-dependent reaction
_ light-independent reaction
_ NADPH

6. Terms _ photons _ photophosphorylation _ photosynthesis _ stroma
_ thylakoids
_ xerophytes

7. What is the role of chloroplasts and plant pigments in photosynthesis?
I. Photosynthesis is a chemical process in which plants absorb light and convert solar energy into stored chemical energy.
A. Photosynthesis takes place in organelles known as chloroplasts that are located mainly in the cells of the mesophyll tissue found in leaves. The majority of the chloroplasts are found in the mesophyll cells of leaves. However, photosynthesis does occur in other leaf cells, plant stems, and green flower parts. Even roots exposed to light sometimes have chloroplasts where photosynthesis occurs. Typically, there are 20 to 100 chloroplasts in each mesophyll cell.

8. What is the role of chloroplasts and plant pigments in photosynthesis?
B. Chloroplasts are bound by a membrane.
1. A fluid-filled region within the chloroplast is known as the stroma. The stroma contains most of the enzymes required for photosynthetic reactions.
2. Inside the chloroplast is a system of membranes that form an interconnected set of flat, disk-like sacs referred to as thylakoids. These stacked sacs, similar in appearance to a stack of coins, are called grana.

9. What is the role of chloroplasts and plant pigments in photosynthesis?
C. Pigments within the chloroplast are responsible for capturing light. Chlorophyll and carotenoid (yellow and orange) pigments are the primary collectors of light.
1. Chlorophyll is a complex molecule and is the main pigment of photosynthesis. It has two primary forms.
a. The most important is chlorophyll a. Chlorophyll a is bright green and is responsible for absorbing energy from the violet-blue to red, red-orange light wavelengths. Chlorophyll a initiates the light-dependent reactions in the photosynthetic process.
b. Chlorophyll b is yellow-green and absorbs wavelengths of light slightly different from chlorophyll a.

10. What is the role of chloroplasts and plant pigments in photosynthesis?
2. Carotenoids are yellow and orange accessory pigments that absorb energy from green-yellow-orange wavelengths. They are less efficient than chlorophyll. The energy absorbed by carotenoids can be transferred to chlorophyll a. Xanthophylls are yellow pigments from the carotenoid group.

12. What are the major steps involved in photosynthesis?
II. Photosynthesis is a very complex process.
A. During photosynthesis, light particles (known as photons) strike atoms in chlorophyll. In the process, light energy is transferred to electrons. With this energy, chlorophyll makes the high-energy compounds ATP (adenosine triphosphate) and NADPH. Chlorophyll uses the energy in these compounds to split water molecules. Electrons from the hydrogen are transferred to the chlorophyll and passed through a series of electron acceptors. As the electrons are passed along, their level of excitement is increased by additional light energy. Some of the energy produced from this process is used to unite the hydrogen atoms from the water molecule with carbon dioxide to produce simple sugars. Two oxygen atoms from the water molecules bond to form O2. The O2 escapes through the stomata into the atmosphere.

13. What are the major steps involved in photosynthesis?
B. Photosynthesis has two main reactions.
1. The light-dependent reaction only occurs in the presence of light. Chlorophyll absorbs the light energy, which is converted to electrical energy. Some of the electrical energy is used to make ATP through a process known as photophosphorylation. Some of the light energy trapped by the chlorophyll is used to split water molecules. Oxygen (O2) from the water is released. Hydrogen atoms from the water combine with NADP to form NADPH.

14. What are the major steps involved in photosynthesis?
2. The light-independent reaction does not require light and is sometimes referred to as “the dark reaction.” In this reaction, ATP and NADPH formed during the light-dependent reaction are used to make high-energy carbohydrates. The carbohydrates function as a good long-term energy storage system. The materials used in the process include carbon dioxide (CO2) from the air and hydrogen (H) from the NADPH (originally obtained from water).

15. How do C3, C4, and CAM plants compare?
III. Carbon fixation is the process by which carbon dioxide is converted to organic molecules. Different groups of plants go about fixing carbon in different ways. The three groupings are C3, C4, and CAM plants.

16. How do C3, C4, and CAM plants compare?
A. Most plants use the Calvin cycle (C3) to fix carbon during the light-independent reaction. The Calvin cycle takes place in the stroma within mesophyll cells. Initially, a six-carbon molecule is formed. Instantly, it splits into two three-carbon molecules. The three-carbon molecules join to form simple sugars, glucose, and fructose. It takes six turns of the Calvin cycle, involving the input of six CO2 molecules and hydrogen from NADPH and ATP, to result in one molecule of glucose. In some plants (e.g., sugar beets and maples), glucose and fructose join to produce sucrose. Most plants are called C3 plants because the first product of carbon fixation is a three-carbon compound.

17. How do C3, C4, and CAM plants compare?
B. Many plants with tropical origins have the ability to fix carbon dioxide into four-carbon compounds, sometimes referred to as C4 plants, which evolved in areas of high temperatures, high light intensities, and limited amounts of water. Corn, sugar cane, and crabgrass are C4 plants. These plants are typically fast growing and produce yields two to three times higher than C3 plants.

18. How do C3, C4, and CAM plants compare?
1. The leaves of C4 plants differ physically from C3 plants. C4 plants have specialized cells (bundle sheath cells) that are packed tightly around the veins of a leaf. Mesophyll cells surround them. Reactions of the C4 pathway take place in the mesophyll cells. In the process, the plants fix CO2 into four-carbon compounds.
2. A result of the C4 pathway is an increased concentration of carbon dioxide in bundle sheath cells. Levels of carbon dioxide within the bundle sheath cells reach 10 to 60 times that of cells with only the C3 pathway. During the reaction, compounds produced in the mesophyll cells migrate to the bundle sheath cells and go through the Calvin cycle (the C3 pathway). With a high level of CO2, photosynthesis occurs at a rapid rate.

19. How do C3, C4, and CAM plants compare?
C. Some plants, typically growing in arid conditions, fix carbon through a pathway known as CAM (crassulacean acid metabolism) photosynthesis. Pineapples and cacti are examples of plants that perform CAM photosynthesis.
1. Plants adapted to dry climates are called xerophytes. Characteristics of xerophytes are small and thick leaves, a thick cuticle layer, and sunken stomata. CAM plants close their stomata during the day to reduce water loss and thus prevent the exchange of gases. The stomata open at night when temperatures are cooler and the air is more humid. Carbon dioxide, for use in carbon fixation, enters through the open stomata. At night the plants fix carbon dioxide into malate, which is stored in vacuoles. During the day, the malate is decarboxylated to yield CO2, which is used by the plant to manufacture sugars via the C3 pathway.

20. How do C3, C4, and CAM plants compare?
2. CAM may seem similar to the C4 pathway. However, there are distinct differences. The processes take place in different locations in the plants and at different times. C4 plants capture the CO2 in mesophyll cells and transfer it to bundle sheath cells where it is used in the C3 pathway. CAM plant processes occur within the leaf tissues. As long as there is light available, C4 plants fix carbon. Meanwhile, CAM occurs only at night.

22. What factors affect photosynthesis?
IV. Outside factors influence the rate of photosynthesis.
A. The first requirement for photosynthesis is a healthy, living plant. The plant must also have chlorophyll to absorb light.

23. What factors affect photosynthesis?
B. There must be an ample supply of carbon dioxide in the atmosphere. Higher levels of CO2 result in more rapid growth. In some greenhouses, carbon dioxide is added to the atmosphere to speed photosynthesis and plant growth.

24. What factors affect photosynthesis?
C. Water must be available for plant use. Severe lack of water results in wilting or a limp appearance. Stomata close to conserve water. CO2 levels in the leaf drop, and photosynthesis shuts down.
D. Light or radiant energy drives photosynthesis, which cannot occur without light.

26.  REVIEW
1. What is the role of chloroplasts and plant pigments in photosynthesis?
2. What are the major steps involved in photosynthesis?
3. How do C3, C4, and CAM plants compare?
4. What factors affect photosynthesis?