Photosynthesis

1. Photosynthesis 6CO2 + 6H2O -> C6H12O6 + 6O2 Capturing the Energy in Light

2. I. Energy for Life Processes • Plants are autotrophs (make their own food) • Photosynthetic organisms: plants, protists, algae, some bacteria. • Photosynthesis is a complex series of chemical reactions known as a biochemical pathway.

3. II. Light Absorption in Chloroplast • The first reactions in photosynthesis are called light reactions • A chloroplast contains thylakoids, grana or granum (stacks of thylakoids), and stroma (solution).

4. Light and Pigments • Light travels through space as waves of energy • White light from the sun is composed of an array of colors called the visible spectrum (ROY G. BIV) • Different colors have different wavelengths (nm) • Pigments absorb certain colors of light and reflect or transmit the colors of others

5. B. Chloroplast Pigments 1. Chlorophylls: the type of pigment present in the thylakoids (chlorophyll a and chlorophyll b). 2. Only chlorophyll a is directly involved in the light reactions of photosynthesis\ 3. Chlorophyll b is called an accessory pigment (it assist chlorophyll a in capturing light energy). 4. Carotenoids also function as accessory pigments

6. 5. In leaves (photosynthetic parts) chlorophylls are more abundant and they hide the colors of the other pigments 6. Fruits and flowers: colors of other pigments might show

7. III. Electron Transport • Chlorophylls and carotenoids are grouped in clusters referred to as a photosystem (photosystem I and photosystem II) • Steps involved in the light reaction (inside the thylakoid membrane) • Light excites electrons in chlorophyll a molecules of photosystem II. • These electrons move to the primary electron acceptor • The electrons are transferred along a series of molecules called the electron transport chain (ETC)

8. 4. Light excites electrons in chlorophyll a (photosystem I) as the electrons move to the primary electron acceptor, they are replaced by electrons from photosystem II. 5. The electrons from photosystem I are transferred along a second ETC. At the end of the chain they combine with NADP+ and a proton (H+) to make NADPH.

9. Restoring Photosystem II • The replacement electrons (Step 4) are provided by the splitting of water molecules. • 2H2O 4H+ + 4e- + O2 • Protons (H+) produced are left inside the thylakoid, oxygen diffuses out of the chloroplast and can leave the plant. For this reason, oxygen is called a byproduct of the light reaction.

10. IV. Chemiosmosis A. This is a process in chloroplasts and mitochondria where protons move across a membrane (from high to low concentrations) which also synthesizes ATP.

11. B. There is a build up of protons inside the thylakoid (from the splitting of water) C. ADP + P ATP (by the enzyme ATP synthase) D. The energy that drives the above reaction is from the movement of the protons inside the thylakoid to the stroma.

12. Describe the structure and function of the thylakoids of a chloroplast. • What role do accessory pigments (hog breath fresheners) play in photosynthesis. • What happens to the electrons that are lost by photosystem II ? What happens to the electrons that are lost by photosystem I. • Name the substances that are produced when water molecules are broken down during the light reaction. • How is ATP made in the light reaction? • Explain how the light reactions would be affected if there were no concentration gradient of protons across the thylakoid membrane.