Chapter+10+Photosynthesis

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**UNIT 10 PHOTOSYNTHESIS**

**1. Label the diagram below**

**2. Explain the experimental reasoning that Van Niel used to understand photosynthesis**

**3. Label the diagram below**



**4. What is a photon?**

**5. Explain the action spectra vs the absorption spectra below**

An action spectrum is the rate of a physiological activity plotted against wavelength of light. This relationship in plants is shown on the left graph. As we can see, the rate of photosynthesis is greater for wavelengths of light that are blue and red. Thus, the blue and red light is absorbed by plants, while the green (useless in photosynthesis) is refelcted giving the plant a green color. Light spectrums can also be used to identify objects because different elements have distinct absoption spectrums. This idea is explained in this video: []

**6. Explain the excitation of chlorophyll by light**

Chlorophyll is located in the photosystem. Chlorophylls in Photosystem 1 work best with light that has a wavelength of 700nm. In PS2, the chlorophylls work best with light that has a wavelength of 680 nm. [|Excitation of Chlorophyll VIDEO] Light hits the photosystem at the light harvesting complex, causing an electron within the chlorophyll to move to a nearby chlorophyll pigment until the energy reaches the reaction center. -EMILY

**7. Explain the diagram below. How does chloropyll b differ?** The diagram shown is chlorophyll a. Chlorophyll a is found in all plants due to the fact it is essential for photosynthesis. The molecule absorbs energy from the violet/blue and orange/red spectra. Chlorophyll b is an isomer of chlorophyll a. Chlorophyll b acts in complement to chlorophyll a (it is known as an accessory pigment). Chlorophyll b absorbs blue and some green/yellow as seen in the picture below with chlorophyll spelled poorly. These two molecules, chlorophyll a and b, work together to allow for more efficient photosynthesis compared to photosynthesis with only chlorophyll a. -Danny G. a.k.a. DG3

**8. Label the photosystem** -Joanne H.

P680 and P700 are both reaction-center complexes in chlorophyl a that help excite electrons to a high energy state. P680 is located in Photosystem II and is named P680 because it is best at absorbing light with a wavelength of 680 nm. P680 receives an electron from water and then excites this electron to the primary acceptor. P700 is located in Photosystem I and is named after its ability to best absorb light with a wavelength of 700 nm. P700 receives its electrons from the electron transport chain which occurs in between photosystem II and I. After an electron is excited by either P680 or P700, it enters the electron transport chain where the electron can move back down to a normal energy level. - Gordie K and Andrew H
 * 9.What are P700 and P 680? How are the similair? How are they different? **

**10.Explain and label the diagram below** -Monika K

The figure shown here is a depiction of how linear electron flow generates both ATP and NADPH during light reactions

The linear electron flow process begins as a light photon hits the pigment molecule in photosystem two causing a boost of the electrons to a higher level of energy. Soon enough, the electron will fall back to the regular ground state and simultaneously; an electron within the pigment molecule will be altered to its excited state. This becomes an ongoing process as the energy is relayed to many different pigment molecules up until the point where the P680 pair of chlorophyll molecules in the reaction-center complex of photosystem two is reached. Overall, this will excite an electron in this specific pair of chlorophylls to a much higher energy state. Next, the electron will be transferred from the excited state of the P680 into the primary electron accepter. Furthermore, an enzyme will catalyze the splitting of a water molecule into 2 H+ and an O2 atom. These electrons are supplied individually to the P680+ pair as each electron replacing one transferred to the primary electron acceptor. Then, the hydrogen atoms are released into the thylakoid lumen and the oxygen atom will combine with another oxygen atom in order to split another water molecule and form O2. To continue, each photoexcited electron will pass from the primary electron acceptor in photosystem two to photosystem one with the assistance of the ETC. In the next step the exergonic “fall” of electrons into a lower energy level will provide ATP synthesis energy and as electrons pass through the cytochrome complex, hydrogen atoms are pumped into the thylakoid lumen to contribute to chemiosmosis. While this is ongoing, light energy has already been transferred with the help of light harvesting complex pigments to the photosystem one reaction-center complex. This causes an electron of the P700 pair of chlorophyll a molecules to become excited. Next, these photoexcited electrons are transferred to photosystem one’s primary electron acceptor to create an electron “hole” in the P700, allowing the new P700+ to act as an electron acceptor. Soon enough, photoexcited electrons are passed from the primary electron acceptor of photosystem one down a second electron transport chain in a series of redox reactions. Finally, the enzyme NADP+ reductase is able to catalyze the transform of electrons, as two electrons are required for the reduction to NADPH. Now, this molecule is at a higher energy level than water and the electrons are more available for the reactions of the Calvin Cycle then were those of water. H+ are now removed from the stroma Note:
 * 1) The gold arrows shown in the diagram follow the light-driven electron current beginning at water and ending in NADPH
 * 2) P680+ is known as the strongest biological oxidizing agent known today and its electron ‘hole” must be filled in order to assist in transferring electrons from the already split water molecule
 * 3) The ETC between both photosystems consists of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc)

(See pages 194-195 in our textbook) Reece, Jane B., and Neil A. Campbell. //AP* Edition Campbell Biology//. 9th ed. Boston: Benjamin Cummings / Pearson, 2011. Print. -Dan K. **11. Explain and label the diagram below. How is it differ form the LEF diagram above?** Jared and Eli

The Cyclic cylce as seen above occurs in only Photosystem one, unlike the non-cyclic cycle. This cycle creates an end result of only a production of ATP, where as the non-cyclic cycle forms ATP and NADPH. Photons excite electrons and then they release enrgy in order to continue exciting near by electrons to go in the path of the primary acceptor. When the electron is used in photsystem I, it is passed down the electron acceptor molecules in order to return to photosystem I and keep the reaction going. **12. Explain and label the diagram below**

P.O.

The diagram shows the light reactions process in the thylakoid membrane. The yellow arrow shows the flow of electrons through both photosystems toward the calvin cycle. Meanwhile, hydrogen atoms in the thylakoid space are used for the ATP synthase. The synthesized ATP is utilized by the Calvin cycle and later returned in the form of ADP and an inorganic phosphate. Also, NADPH is formed by the photosystem electron transport chains and pushed towards the calvin cycle only to return in the form of NADP.- JLM

**13.** **How does chemiosmosis differ in photosynthesis and cellular respiration.** Chemiosmosis is the final step of cellular respiration and of the light reactions in photosynthesis. It involves the usage of a proton motive force from the proton gradient formed in by ETC in order to produce ATP for the cell. In cellular respiration, chemiosmosis takes place in the mitochondria, and in photosynthesis, it occurs in the thylakoid. ATP synthase carries protons back into the cell, which is used in both photosynthesis and cellular respiration to harness energy as ATP. In photosynthesis, when Photosystem 2 receives light energy, a pair of electrons becomes excited, sending them down the ETC which creates the proton (H+) gradient. The hydrogen ions aid in phosphorylation as ADP become ATP which is used as energy. In cellular respiration, NADH and FADH2 pass their electrons through the ETC. At this point, ATP synthase is used to make ATP from the energy in the ETC. The final electron acceptor in cellular respiration is oxygen, and in photosynthesis is NADP.

Ella P **14. Explain the steps of calvin cycle**

The Calvin cycle is an enzymatic pathway associated with photosynthesis that is responsible for converting carbon dioxide into G3P, a carbohydrate. There are three primary steps within the Calvin cycle: carboxylation, reduction, and regeneration. Within the first step, carboxylation, the enzyme rubisco catalyzes a reaction that adds carbon dioxide to ribulose 1, 5-biphosphate, a five-carbon sugar phosphate, creating a six-carbon molecule that promptly splits to form two three-carbon phosphoglycerate molecules (3-PGA). This molecule, 3-PGA, is the product of carbon fixation within the vast majority of plants, hence the designation C3. Carboxylation is complete after carbon binds to the five-carbon sugar phosphate and the six-carbon molecule splits, allowing the next phase, reduction, to occur. Within reduction, 3-PGA is energized and reduced by ATP and NADPH, products of the light reactions of photosynthesis. The enzyme phosphoglycerate kinase catalyzes the phosphorylation of 3-PGA with ATP, producting 1,3-biphosphoglycerate and ADP. Then, the enzyme G3P dehydrogenase catalyzes the reduction of 1,3-biphosphoglycerate by NADPH, producting G3P, and NADP+. Thus, the ATP and NADPH involved in the reduction stage come out at ADP and NADP+ and enter the light reactions once more, where each respective molecules is energized. Following reduction, regeneration occurs; more ATP is expended for the purpose of converting some of the G3P back into RuBP, the acceptor for carbon dioxide. Various enzymes catalyze reactions involved in regeneration, including triose phosphate isomerase, aldolase, and transketolase. Through this complex series of reactions, RuBP is regenerated, after the carbon skeleton is rearranged from 5 glyceraldehyde-3-phosphate molecules into 3 RuBP molecules. This reaction series will require 3 ATP, and in the end RuBP is regenerated, and the cycle begins again.

-Kellen and Jason

**15. What is a C4 plant? Why is it different than a C3 plant?** A C4 plant most simply refers to a plant that undergoes the C4 photosynthesis pathway rather than C3 or CAM photosynthesis. Comparison of C4 plants and C3 plants
 * C4 photosynthesis is efficient in high daytime temperatures and intense sunlight while C3 photosynthesis is more efficient in moist, cool conditions and moderate sunlight.
 * The most prominent difference between C4 and C3 plants is the first product of CO2 fixation; C4 plants incorporate CO2 into a 4 carbon compound called Oxaloacetate while C3 plants incorporate CO2 into a 3 carbon compound called 3-phosphoglyceric acid (3-PGA).
 * C4 photosynthesis is a 2 stage pathway while C3 only has 1 stage. C3 plants only use the Calvin Cycle for carbon fixation. The primary CO2 acceptor in the mesophyll cells of C3 plants is RuBP which uses the enzyme Rubisco to produce 3-PGA. In C4 plants, PEP is the primary CO2 acceptor in the mesophyll cells. PEP, along with the enzyme PEP carboxylase, assists in the formation of a 4-carbon intermediate. The 4-carbon compound (oxaloacetic acid) is pumped into a bundle sheath cell and is there split into CO2 and a 3 carbon compound. This CO2 is then fixed by Rubisco in the Calvin Cycle.
 * C4 photosynthesis uses more ATP than C3 photosynthesis because in C4 photosynthesis, there are 2 stages of fixation for every CO2 molecule. There is less loss of carbon and therefore, minimal photorespiration takes place in C4 photosynthesis in comparison to the extensive photorespiration in C3 plants. Supritha

**16. Explain the C4 photorespriation pathway**

http://www.youtube.com/watch?v=dIepJV_obW4

Photorespiration is the process in which RuBP, a sugar, has oxygen added to it by the enzyme rubisco, rather than carbon dioxide during normal photosynthesis. This process is rather inefficient, so some plants--C4 and CAM plants--have adapted mechanisms to reduce the uptake of oxygen by rubisco. To do so, they increase the concentration of CO2 in the leaves so that rubisco is less likely to react with O2. More specifically, C4 plants capture carbon dioxide in their mesophyll cells and oxaloacetate is formed. This oxaloacetate is then converted to malate and released into the bundle sheath cells (the site of carbon dioxide fixation by rubisco) where oxygen concentrations are low to avoid photorespiration. Here, CO2 is removed from the malate and combined with RuBP in the usual, more efficient way, and the Calvin cycle continues as normal. --Sabrina Zionts

**17. Label and explain the following diagrams**