1. The Reactions of Photosynthesis 8-3

2. Inside a Chloroplast Where photosynthesis takes place Contains thylakoids, which are sac-like photosynthetic membranes Thyalkoids are arranged in stacks called grana

3. Thylakoids contain proteins which organize chlorophyll and other pigments into clusters called photosystems –Photosystems are light-collecting units of chloroplasts The reactions of the photosystems are divided into 2 types: –Light-dependent reactions –Light-independent reactions, or Calvin Cycle

4. Light-dependent reactions take place in the thylakoid membranes Light-independent reactions take place in the stroma, or region outside the thylakoids

5. Electron Carriers When sunlight strikes chlorophyll, it transforms the electrons into a high- energy state These high-energy electrons are like hot coals and need a special electron carrier

6. Carrier molecules are a compound capable of transporting these high- energy electrons, along with most of their energy to other molecules This process is known as electron transport Electron carriers are known as electron transport chain

7. The molecule NADP+ serves as an electron carrier –Its job is to hold 2 high-energy electrons along with a hydrogen ion (H+) This converts NADP+ to NADPH –This conversion is one way in which some energy from sunlight is trapped in chemical form NADPH then transfers the high-energy electrons to chemical reactions elsewhere in the cell

8. Light-Dependent Reactions Require light to happen Converts ADP to ATP and NADP+ to NADPH Produces oxygen gas

10. Step A Photosystem II absorbs light Light energy is absorbed by electrons, increasing energy levels Electrons are passed to electron transport chain (ETC)

11. Enzymes on inner surface of thylakoid membrane break up each water molecule into 2 electrons, 2 H+ ions, and 1 oxygen atom –2 e- replace 2 high-energy e- lost to ETC –Oxygen released to air –H+ ions are released inside the thylakoid membrane This reaction is source of nearly all oxygen in the atmosphere

12. Step B High-energy electrons move through ETC from photosystem II to photosystem I Energy from the e- is used by ETC to transport H+ ions from stroma to inner thylakoid space

13. Step C Pigments in photosystem I use energy from light to reenergize e- NADP+ picks up high-energy electrons and H+ions to become NADPH

14. Step D More H+ ions are pumped across the membrane Inside of the membrane becomes positively charged and the outside of the membrane becomes negatively charged The differences in charges provide energy to make ATP

15. Step E In order for H+ ions to move across the membrane, the protein ATP synthase is needed As the H+ ions pass through ATP synthase, the protein rotates As it rotates, ATP synthase binds ADP and a phosphate together to make ATP

16. Because of this system, light-dependent electron transport produces not only high-energy electrons, but ATP as well In summary, in light-dependent reactions: –Plants use water, ADP, and NADP+ –They produce oxygen, and 2 high-energy compounds: ATP and NADPH