1. Photosynthesis

2. 1. Photosynthesis uses the energy of sunlight to convert carbon dioxide into sugars. This process supplies usable energy for life on Earth. a. The products of photosynthesis provide the carbon building blocks for cells and tissues. b. Photosynthesis also supplies oxygen that most organisms require. Plants, algae, and photosynthetic bacteria are autotrophic and are given the term producers.

3. c. Though producers play a huge role in sustaining life on the planet, they account for only about 17-20 percent of the five million species alive today. d. Autotrophs also store food for future use. Much of this stored food will be used by heterotrophs that cannot make their own food.

4. 2. In plants most photosynthesis occurs in the leaves. Leaf structure is adapted to maximize light absorption and gas exchange. Veins supply water and minerals to the leaf and carry the products of photosynthesis away to other parts of the plant. Gases move in and out of the leaf by diffusion through small openings called stomata.

5. 3. Within the cells of the leaf pigments absorb different wavelengths of light and concentrate it in a molecule called chlorophyll a. Chlorophyll a is contained in the chloroplast, an organelle that performs photosynthesis.

6. a. A photosynthetic cell contains from one to several thousand chloroplasts. b. Inside the chloroplast are grana in which hundreds of chlorophyll molecules and other pigments are organized into units called photosystems. Between the grana is a gel-like material called stroma which contains enzymes, DNA, RNA, and ribosomes.

7. 4. Three major events occur in photosynthesis: absorption of light energy, conversion of light energy into chemical energy, and storage of chemical energy in sugars.

8. a. The absorption of light sets up a flow of electrons, which results in water being separated into oxygen, protons, and electrons. H 2 O → 2 H+ + 2e- + ½ O 2 b. The electrons and protons combine with a hydrogen carrier. c. The hydrogen carrier and the ATP will be needed in subsequent chemical reactions to convert carbon dioxide into sugars.

9. The oxygen produced from the splitting of water is released as O 2 into the atmosphere. Oxygen is needed for the process of cellular respiration that will be carried out in the cytoplasm and mitochondria of all eukaryotic cells.

11. 5. In summary, the energy from light causes electrons from water to flow in the chloroplast, a pigment containing organelle inside the cells of the leaf. The electron flow powers the synthesis of ATP and NADPH, which contain usable chemical energy. The ATP and NADPH, along with oxygen gas are the products of the first stages of photosynthesis, which are called the light-dependent reactions.

12. 6. The next steps of photosynthesis do not require the absorption of light energy, but they do require the products of the light reactions. Light is still needed in order to activate several important enzymes that are essential to the completion of the process.

13. 7. In the Calvin Cycle carbon dioxide is combined with the hydrogen that was previously split from water. Energy for the biochemical reactions is provided by the ATP from the light reactions.

14. 8. The process of photosynthesis is often summed up in the following chemical equation: 6 CO 2 + 12 H 2 O C 6 H 12 O 6 + 6O 2 + 6H 2 O

16. 9. The final products of the photosynthesis process are sugar phosphates, particularly PGAL, which are used in cellular functions. From these sugar phosphates plants can synthesize the other compounds that they need.

18. 10. It is very important to remember that the steps of photosynthesis require the activity of enzymes as do all biochemical reactions. Enzymes are catalysts in living organisms and are highly specific for the reactions that they catalyze. a. Some enzymes cause compounds to break apart.

19. b. Some enzymes cause compounds to be formed. c. Enzymes provide a way to speed up life's essential functions without raising the temperature of the organism's body.