1. Photosynthesis

2. Main Idea Light energy is trapped and converted into chemical energy during photosynthesis.

3. Overview Most autotrophs (only 10% of the Earth’s species) make organic compounds, like sugars, by photosynthesis. Photosynthesis occurs in two phases. Overall equation: –6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2

4. Phase One: Light Reactions The absorption of light is the first step in photosynthesis. Two energy storage molecules – NADPH and ATP – are produced to be used in the light independent reaction.

5. Chloroplasts Large organelles that capture light energy in photosynthetic organisms. Found mainly in the cells of leaves Disc-shaped organelles that contain two main compartments essential to photosynthesis

6. Chloroplasts Thylakoids are flattened saclike membranes that are arranged in stacks. The stacks are called grana. Light-dependent reactions occur in the thylakoids. The stroma is a fluid filled space outside the grana where the light-independent reactions take place.

8. Pigments Light-absorbing colored molecules called pigments are found in the thylakoid membranes of chloroplasts. Different pigments absorb specific wavelengths of light. Violet light has the shortest wavelength, while red light has the longest. Chlorophylls are the major light absorbing pigments in plants.

10. Pigments Chlorophyll a and chlorophyll b are the 2 most common types of chlorophyll in autotrophs. These types of chlorophyll absorb most wavelengths except green. This means the green light is reflected; therefore, plants look green.

11. Pigments Other pigments are also found in leaves. In the fall when the chlorophyll starts to break down, the other pigments are able to be seen. Hence, the vibrant fall colors on the trees.

12. Electron Transport The thylakoid membranes have a large surface area, which provides the space needed to hold large numbers of electron-transporting molecules. Photosystem I and photosystem II contain light- absorbing pigments and proteins. Photosystems are composed of photosynthetic pigments clustered together. Each pigment inside the photosystem is capable of capturing photons, which are packets of energy.

13. Electron Transport 1. Light energy excites electrons in photosystem II. 2. Light energy causes a water molecule to split, releasing an electron into the electron transport system, a H+ ion into the thylakoid space, and oxygen as a waste product. THIS SPLITTING IS ESSENTIAL TO PHOTOSYNTHESIS.

14. Electron Transport 3. The excited electrons move from photosystem II to an electron-acceptor molecule in the thylakoid membrane. 4. The electron-acceptor molecule tranfers the electron along a series of electron- carriers to photosystem I.

15. Electron Transport 5. In the presence of light, photosystem I transfers the electron to a protein called ferrodoxin. The electrons lost by photosystem I are replaced by electrons shuffled from photosystem II. 6. Ferrodoxin transfers the electrons to the electron carrier NADP+, forming the energy storage molecule NADPH.

17. Chemiosmosis ATP is produced in conjunction with electron transport by the process of chemiosmosis. The breakdown of water is not only essential for providing the original electron, but for also providing the protons (H+) necessary to drive ATP synthesis during chemiosmosis.

18. Chemiosmosis Eventually H+ will accumulate in the thylakoid interior and diffuse down the concentration gradient to the stroma through ion channels spanning the membrane. These channels are enzymes called ATP synthases. As H+ moves through ATP synthase, ATP is formed in the stroma.

19. What Happens If… Some of the light-energized electrons may return to chlorophyll after they’ve moved around the electron transport chain, but most continue on with NADPH. If these electrons are not replaced, the chlorophyll will be unable to absorb light and the light-dependent will stop and no ATP will be made. To replace lost electrons, water molecules are split by a process call photolysis.

21. Phase Two: The Calvin Cycle Because NADPH and ATP are not stable enough to store chemical energy, there is a second phase of photosynthesis. The Calvin cycle is the phase in which energy is stored in organic molecules such as glucose. AKA the light-independent reactions.

22. Phase Two: The Calvin Cycle 1. Carbon fixation – 6 CO 2 molecules combine with six 5-carbon compounds to form 12 3- carbon molecules called 3-phosphoglycerate (3- PGA). 2. Energy in ATP and NADPH is transferred to the 3-PGA molecules to form high energy molecules called glyceraldehyde 3-phosphates (G3P). ATP supplies the phosphate group, while NADPH supplies the H+ ions and electrons.

23. Phase Two: The Calvin Cycle 3. 2 G3P molecules leave the cycle to be used for the production of glucose and other organic compounds. 4. An enzyme called rubisco converts the remaining 10 G3P molecules into 5-carbon molecules called ribulose 1, 5- biphosphates (RuBP). These molecules combine with new carbon dioxide molecules to continue the cycle.

24. Phase Two: The Calvin Cycle It takes a total of 6 cycles for one glucose to be formed.