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Chapter 6: Photosynthesis

1. What is photosynthesis? What does it synthesize? Where in a cell does photosynthesis occur? Photosynthesis is the process that changes light energy into the energy of chemical bonds. It synthesizes glucose. This process occurs in the chloroplast of plants.

2. Write the overall equation for photosynthesis. How do the reactants get into the leaf? How does oxygen get out of the leaf cells? 
 6CO2 plus 6H2O produces C6H12O6 and 6O2
Carbon gets into the leaf through minute pores in leaves called stomata and water goes into the plant through its roots. Oxygen gets out of the cell through stomata.

3. Which parts of electromagnetic spectrum do plants use in photosynthesis? How does a comparison of the absorption spectrum of chlorophyll and the action spectrum of photosynthesis lead to a conclusion about the molecular basis of photosynthesis? How did such studies lead to the discovery of the role of carotenoids? The plants use blue and red lights in photosynthesis. The action of spectrum of photosynthesis in plants roughly corresponds to the absorption spectrum of the pigment chlorophyll, but not exactly. Light with a wavelength of 500 nm (blue-green light) can stimulate photosynthesis even though blue-green light is not absorbed by chlorophyll. This lead to the discovery of carotenoids, which absorb blue-green light and transfer the energy of their excited electrons to chlorophyll.

4. Describe the reactions that occur in Calvin cycle. What enzyme is responsible for capturing carbon dioxide from the atmosphere? What are the products of this reaction? How is energy from light introduced in the Calvin cycle? Why must the cycle produce more ribulose biphosphate? In the first reaction of the Calvin cycle, three molecules of CO2 combine with three RuBP- five-carbon molecules- with the aid of the enzyme Rubisco (ribulose biphosphate carboxylase). The resulting six-carbon molecules immediately break down into 6 molecules of PGA. PGA molecules are converted into PGAL by using ATP and NADPH from the light dependent reaction. 1 molecule of PGAL is removed from the Calvin cycle and combines with another PGAL to make six-carbon glucose, and others convert into RuBP to repeat the cycle by using ATP.

5. Describe how light energy is used to drive the reaction of glucose synthesis? Light energy from the sun is used to excite electrons of photosystem II, and as electrons flow through electron transport chain, ATP is generated. Light energy also excites electrons of photosystem I and generates NADPH. ATP and NADPH are used in Calvin cycle to convert PGA to PGAL, and two molecules of PGAL combine to make glucose.

6. Why is photosynthesis important to feeding our entire planet? Every organism depends on plants directly or indirectly for food, and plants make food through the process of photosynthesis.

7. How does the light dependent reaction differ from the light-independent reaction? What are the products/reactants in each? Where does each occur within the chloroplast?

Light-dependent Reaction Light-independent Reaction
1. Light-dependent reaction depends on light and converts light energy to chemical energy stored in bonds of 2 molecules, ATP and NADPH. 1. The light-independent reaction used ATP and NADPH from light-dependent reaction to assemble sugars. This reaction can occur in light and dark.
2. Products are ATP, NADPH, and oxygen; and the reactant is water. 2. The products is glucose, and reactants are ATP, NADPH, and CO2.
3. This reaction occurs in thylakoid membranes. 3. This reaction occurs in stroma.


8. How are the lost electrons replaced in photosystem I. How are the lost electrons replaced in chlorophyll of photosystem II?
The lost electrons in photosystem I are replaced from the electrons in photosystem II through the electron transport chain. The lost electrons in photosystm II are replaced by splitting water molecule into oxygen, hydrogen ions, and electrons.

9. What is the role of water in the photosynthetic process? Water splits into oxygen, hydrogen ions, and electrons to replace the lost electrons of photosystem II in light-dependent reaction. Also, as hydrogen ions pass through ATP synthase, ATP is formed to use in the light-independent reaction.

10. What specific wavelengths of light are required for the process of photosynthesis? If you placed a plant into an environment where it only received green light, would you expect a high or low rate of photosynthesis from the plant? Explain your answer. Red (660 nm) and blue (440 nm) lights are required for the process of photosynthesis. Low rate of photosynthesis from the plant is expected if a plant is placed into an environment where only green light is received. Chlorophyll reflects green wavelength (550 nm) and absorbs the blues and the reds most.

11. Why must the light-dependent reaction occur before the light-independent reaction? Light-dependent reaction makes ATP and NADPH that are necessary for light independent reaction.

12. In theory, a plant could photosynthesize in the dark if it were given what three substances? A plant could photosynthesize in the dark, if it were given ATP, NADPH, and CO2.

13. Is photosynthesis endergonic/exergonic, anabolic and/or catobolic? Photosynthesis is endergonic because it requires energy; and glucose- the product- has more energy than its reactants. It is anabolic because it builds big molecule, glucose, from small molecules, CO2 and H2O.

14. Where does the carbon that is used in photosynthesis come from? The carbon comes from carbon dioxide in the air.

15. Where does the energy come from to "link" carbon and other atoms into organic compounds? The energy, ATP and NADPH, come from the light-dependent reaction.

16. How does that energy in these organic compounds become available to do cellular work? Through glycolysis and aerobic respiration, ATP is formed to do cellular work.

 
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