1. Part 2: Photosynthesis

2. 1. Overview of Photosynthesis
A. Think of what farmers need for their crops: water, sunlight, and carbon dioxide. This is due to photosynthesis. The equation for photosynthesis is:
carbon dioxide + water sunlight oxygen + glucose 6CO H2O → O C6H12O6

3. 1. Overview of Photosynthesis
B. There are two main phases of photosynthesis. Phase one is called light dependent reactions, since light is required as the energy source forming molecules such as ATP and NADPH.

4. 1. Overview of Photosynthesis
C. Phase two is called light independent reactions, since light is not needed. The ATP and NADPH from Phase One are used as the energy sources to make glucose molecules. Other needed molecules can be made, too, like proteins, lipids, and nucleic acids, but they are not the main focus of this unit.

5. 2. Phase One: Light Reactions
The prefix “photo” means light. To synthesize means to build or make something. Photosynthesis means using light to build, in this case building glucose.
A. Chloroplasts: Chloroplasts are large organelles found in plant cells with the highest exposure to sunlight, namely leaf cells.

6. 2. Phase One: Light Reactions
1. Within chloroplasts are thylakoids, flattened disc-like membranes. Stacks of thylakoids are called granum (singular) or grana (plural). This is where Phase One light dependent reactions occur.
2. The stroma is the liquid outside the grana but still in chloroplasts. This is where Phase Two light independent reactions occur.

7. 2. Phase One: Light Reactions
B. Pigments are light absorbing molecules found in thylakoid membranes.
1. The most common pigments are chlorophyll a and chlorophyll b. Recall that the colors we see are the colors reflected by objects. Plants appear green because that is the color of the frequency least absorbed by plants, instead reflecting away green light.

8. 2. Phase One: Light Reactions
2. Carotenoids are another pigment. Carotenoids produce less energy but are better at utilizing blue and green light, thus reflecting other colors such yellows, oranges, and reds. In autumn, chlorophyll pigments do not get enough sunlight to function, leaving only the carotenoids to gather energy, changing the colors of leaves from green to red, yellow, and orange.

9. 2. Phase One: Light Reactions
C. Electron Transport
The thylakoids relatively large surface area are ideal for the many molecules and proteins needed for electron transfer in the light dependent reactions. There are two photosystems, again photo meaning light system meaning process. First is photosystem 2, thus named because it was discovered after photosystem 1.

10. 2. Phase One: Light Reactions
1. Photosystem 2: Light excites electrons, which causes water molecules to split. The electron joins the Electron Transport System. The oxygen is waste and leaves the cell. The hydrogen, stripped of its electron, is just a lone proton. H+ ions are often just referred to as protons. A series of molecules with the ability to add or release an electron help carry the electron to photosystem 1.

11. 2. Phase One: Light Reactions
2. Photosystem 1: Light helps energize electrons and moves them to a protein named ferrodoxin. This is the last stop on the electron transport chain. Ferrodoxin helps the electron (e-) bond with NADP+ and H+ to make a new, energy storing molecule called NADPH.
e- + NADP+ + H+ → NADPH

12. 2. Phase One: Light Reactions
3. Chemiosmosis: Another side product of the electron transport chain is chemiosmosis. Water was split in photosystem 2, leaving extra H+ protons. In chemiosmosis, a protein channel helps diffuse the excess H+ protons through the thylakoid membrane to bond with ADP, making the energy storing molecule ATP H+ + P + ADP → ATP

13. 3. Phase Two: The Calvin Cycle (a. k. a
3. Phase Two: The Calvin Cycle (a.k.a. Dark or Light Independent Cycle)
It may seem like creating NADPH and ATP, the energy carriers would be the main goal of photosynthesis. But these molecules are not very stable, so cannot be used for long-term storage.

14. 3. Phase Two: The Calvin Cycle (a. k. a
3. Phase Two: The Calvin Cycle (a.k.a. Dark or Light Independent Cycle)
A. In the photosynthesis equation, water and carbon dioxide are the reactants. Water was used in Phase 1. CO2 is used in Phase 2. Several CO2 molecules bond with other carbon compounds. There are 36 total carbon atoms, 6 from CO2 and 30 from the other carbon compounds.

15. 3. Phase Two: The Calvin Cycle (a. k. a
3. Phase Two: The Calvin Cycle (a.k.a. Dark or Light Independent Cycle)
B. The 36 carbons use energy from ATP, releasing ADP, and energy from NADPH, making NADP+. Energy having been used, NADP+ and ADP return to the light cycles to pick up more energy. This is an endless recycling of these energy carrying molecules.

16. 3. Phase Two: The Calvin Cycle (a. k. a
3. Phase Two: The Calvin Cycle (a.k.a. Dark or Light Independent Cycle)
C. The result of the Calvin Cycle releases 2 3-carbon molecules (6 total carbons) that help make glucose. The other 30 carbons return to the beginning of the Calvin Cycle, another great molecule recycling plan.

18. 4. Alternative Pathways
Some environments don’t have a ready supply of water or carbon dioxide, making photosynthesis difficult. Some plants have evolved special adaptations to handle these shortages.

19. 4. Alternative Pathways
A. C4 Plants have the ability to minimize water loss. They can close their pores called stomata, allowing the intake of CO2 as needed but closing to prevent water loss. They are called C4 plants because they create 4-carbon molecules in the Calvin Cycle instead of the typical 3-carbon molecules.
corn
Sugar canes

20. 4. Alternative Pathways
B. CAM Plants live in environments with little access to water, like deserts or salty environments. They can close their stomata during the day to prevent water loss. Their stomata open at night when it’s cooler outside, allowing CO2 in and minimizing water loss.