Chapter 8

1. Chapter 8 Photosynthesis

2. 8-1 Energy and Life • Autotrophs and Heterotrophs • Autotrophs, such as plants and some other types of organisms, can use light energy from the sun to make their own food. • Heterotrophs, such as animal, fungi, and a few others, obtain energy directly from other organisms as they consume food.

3. ATP and ADP • Adenosine triphosphate (ATP) is one of the major chemical compounds that the cell uses to fuel its activities. • Contains an adenine, a 5 carbon sugar, and 3 phosphate groups. • Adenosine diphosphate (ADP) contains only two phosphate groups. • Cells store energy by adding a phosphate to ADP.

4. Releasing Energy from ATP • Energy stored in ATP is released when ATP is converted to ADP + phosphate. • ATP can store just enough energy to power a variety of cell activities, making it a very useful molecule that is used by all types of cells as an energy resource.

5. Using Biochemical Energy • Cells can use the energy from ATP in a number of ways, such as active transport and movement within the cell. • ATP and Glucose • Most cells have only a few seconds worth of ATP, because it is not very good for long-term storage. • Instead, energy is stored as glucose, which has 90 times the chemical energy of ATP per molecule. • Cells regenerate ATP from ADP from glucose as needed.

6. ATP-Glucose-Protein Cycle LinkIf link does not work, go to http://www.indiana.edu/~oso/animations/ATPprotcycle.html

7. 8-2 Photosynthesis: An Overview • The process of photosynthesis is when plants use the energy from sunlight to convert water and carbon dioxide to oxygen and high-energy carbohydrates such as sugars and starches.

8. Investigating Photosynthesis • Van Helmont’s Experiment • Wanted to know if plants grew by taking material out of the soil. • Determined the mass of dry soil and the mass of a small seedling. • Watered regularly and remeasured after five years. • Mass of soil was almost unchanged, so he concluded that most of the mass gained by the plant was due to water. • This accounts for the “hydrate” portion of carbohydrates, but he had no way to determine that the “carbo” comes from CO2 in the air.

9. Priestley’s Experiment • In 1771, Joseph Priestley noticed that a candle put under a jar would go out. He reasoned that something in the air was necessary for the candle to keep burning. • He found that if he placed a mint sprig under the jar for a few days, the candle would burn longer. • He reasoned that the plant was producing the substance the plant needed. • Later on, scientists determined that plants give off oxygen gas.

10. Jan Ingenhousz • Determined that the effect Priestley observed happened only when the plant was exposed to light. These experiments revealed that plants transform CO2 and water into carbohydrates and starches and release O2, but only in the presence of light.

11. The Photosynthesis Equation • Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into oxygen and high-energy sugars. • The plant then uses the sugars to produce complex carbohydrates such as starches.

12. Light and Pigments • In addition to water and CO2, photosynthesis requires light and chlorophyll. • “White” light is actually a mixture of different wavelengths of light. • Plants gather this energy with pigments (energy-absorbing molecules.) • The main pigment is chlorophyll, which has two types, a and b. • Chlorophyll, does not absorb green light well, so plants appear green. All other wavelengths are absorbed, not reflected.

13. 8-3 The Reactions of Photosynthesis • Inside a Chloroplast • Contain saclike photosynthetic membranes called thylakoids. • Stacks of thylakoids are called grana. • Thylakoids contain clusters of chlorophyll and other pigments and protein (photosystems) to capture the energy of sunlight.

14. Photosynthesis can be broken down into two stages, light-dependent reactions and light-independent reactions (Calvin cycle). • In the light reactions, the energy from light is transferred directly to electrons in the chlorophyll molecule, raising the energy level of these molecules. • The energy for photosynthesis comes from these high-energy molecules. • The Calvin cycle takes place in the stroma (region outside the thylakoids) and does not directly require light energy.

16. NADPH • Carrier molecule for high-energy electrons. • A carrier molecule is a compound that can accept a pair of high-energy electrons and transfer them along with most of their energy to another molecule. • One carrier molecule is NADP+. (nicotinamide adenine dinucleotide phosphate) • It can accept and hold two high-energy electrons along with a hydrogen, changing it to NADPH. • This is one way that the energy from sunlight can be trapped in chemical form. • NADPH then carries these high-energy electrons produced from light absorption in chlorophyll to chemical reactions elsewhere in the cell. • Used to help build molecules the cell needs.

17. Light-Dependent Reactions • Produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH. A- The first step in photosynthesis is when pigments in Photosystem II absorb a photon (packet) of light. Energy from the light is absorbed by electrons, increasing their energy.

18. A- These high energy electrons are passed on to the electron transport chain. New electrons are gained from splitting water molecules using enzymes from the thylakoid membrane. -Water is split into two H+ ions and one oxygen atom. The oxygen is eventually released as O2, and the hydrogens are released inside the thylakoid membrane.

19. B- High-energy electrons move through the electron transport chain from photosystem II to photosystem I. Energy from these electrons is used by the molecules in the electron transport chain to move H+ ions from the stroma to the inner thylakoid.

20. C- Pigments in photosystem I use light energy to reenergize the electrons. NADP+ picks up these high-energy electrons at the outer surface of the thylakoid membrane, plus an H+ ion, and becomes NADPH.

21. D- The inside of the thylakoid membrane is now positively charged because of the H+ ions released during water-splitting and electron transport. The outside of the membrane is negatively charged due to a shortage of H+. This difference in charges on either side of the membrane provides the energy to make ATP.

22. E- H+ ions cannot cross the membrane directly, but they can pass through a protein channel called ATP synthase. As H+ ions flow through the protein, it rotates like a turbine, binding ADP and a phosphate group to produce ATP.

23. A different look at the light-dependent reactions. What happens at each step?

24. The Calvin Cycle • During the light reactions, lots of chemical energy is formed as ATP and NADPH, but it is not stable enough to store for more than a few minutes. • The energy in ATP and NADPH is used in the Calvin cycle to build high-energy molecules, such as sugars, that can be stored for a long time. • The Calvin cycle is also called the light-independent reactions, because light is not necessary for it to occur.

25. A- Six CO2 molecules enter the cycle from the atmosphere. The 6 CO2 molecules combine with six 5 carbon molecules to form 12 3-carbon molecules. B- The 12 3-carbon molecules are converted to higher energy forms by using ATP and high-energy electrons from NADPH.

26. C- Two of the 12 3-carbon molecules are converted into similar 3-carbon molecules. These can now be used to form various 6-carbon sugars (such as glucose) and other compounds. D- The remaining 10 3-carbon molecules are converted back into six 5-carbon molecules. These 5C molecules are used to begin the next cycle.

27. The Calvin cycle uses 6 CO2 molecules from the atmosphere to produce one 6-carbon sugar molecule. • The plant can use the sugars for energy, or convert them into more complex carbohydrates such as starches or cellulose. • Other organisms can use the energy stored in carbohydrates when they eat plants.

28. Factors Affecting Photosynthesis • Water • Shortage can slow or stop photosynthesis • Plants in dry conditions have adaptations such as waxy coatings to reduce water loss. • Temperature • The enzymes necessary for photosynthesis function best between 0°C and 35°C. Photosynthesis slows or stops above or below these temperatures. • Light intensity • Increased light intensity increases the rate of photosynthesis, but only to a point. When the maximum rate is reached, increasing light intensity does not affect photosynthesis.