PHOTOSYNTHESIS. Teacher Packet

Transcription

1 AP * BIOLOGY PHOTOSYNTHESIS Teacher Packet AP* is a trademark of the College Entrance Examination Board. The College Entrance Examination Board was not involved in the production of this material.

2 Objective To review the student on the concepts and processes necessary to successfully answer questions over the process of photosynthesis. Standards Photosynthesis is addressed in the topic outline of the College Board AP Biology Course Description Guide as described below. I. Molecules & Cells C. Cellular Energetics Coupled Reactions Cellular Respiration & Fermentation Photosynthesis AP Biology Exam Connections The principles of photosynthesis are tested every year on the multiple choice and consistently make up portions the free response section of the exam. The concepts of energy flow/conversion via electrons and the process of chemiosmosis seem to be most heavily emphasized. Questions relating to C 4 and CAM pathways are also common. As with many AP Biology free response, these topics are often intertwined with other topics. The list below identifies free response questions that have been previously asked over photosynthesis. It seems that there is often a cellular respiration or photosynthesis related FR most every year. These questions are available from the College Board and can be downloaded free of charge from AP Central Free Response Questions 2008 Practice Exam Questions 4 (a) form b Question Question 3 (c) Question 1 (a), 1 (c) Question 3 (lab based) AP* is a trademark of the College Entrance Examination Board. The College Entrance Examination Board was not involved in the production of this material.

3 Energy is not consistent for all photons (shorter wavelength = higher energy); ROYGBIV Energy from photons may be absorbed by electrons Chlorophyll a absorbs the light energy. Accessory pigments (chlorophyll b, xanthophylls, and carotenoids) absorb other wavelengths of light Absorption Spectrum: Chlorophyll a absorbs mainly violet/blue and red/orange wavelengths of visible light (not green). LIGHT & ENERGY The surface of a Photosystem LIGHT DEPENDENT REACTIONS Photosystem II- producing ATP Where: These reactions on the thylakoid membrane found in stacks (grana) in the chloroplast. The function of the thylakoid membrane is to increase surface area. The area surrounding grana is called stroma (not to be confused with stoma). Photons excite a pair of electrons in the reaction center chlorophyll to a higher energy level. These energized electrons are then passed through the electron transport chain powering a protein carrier (proton pump). This proton pump can use the energy from the electrons to pump H + ions (protons) into the thylakoid space thus creating an electrochemical gradient. The H + ions are only able to leave the thylakoid through the ATP synthase as they flow from high to low concentration. The energy from this movement is utilized to join ADP + P i to create ATP. The ATP will be used in the Calvin cycle. Photosystem I- providing electrons Photosystem I receives electrons and they are reenergized by a slightly different wavelength of light. These electrons are then eventually taken by the electron carrier NADP + and H + to from NADPH. NADPH will supply these electrons to the Calvin cycle Note: NADPH is the electron carrier in Photosynthesis. NADH is the electron carrier in cellular respiration H 2 O- the electron source H 2 O is split releasing 2 e - + H + ions + O 2- The electrons will replace those used in Photosystem II. The H + ions will increase the H + ion concentration in the thylakoid space. The oxygen will combine with another oxygen to make the O 2 that makes aerobic cellular respiration possible.

4 LIGHT INDEPENDENT REACTIONS (CALVIN CYCLE) Calvin Cycle needs: 1. source of C,H,O in order to manufacture C 6 H 12 O 6 2. source of electrons to bond the above together 3. source of energy to run Calvin cycle reactions With the aid of the enzyme Rubisco, RuBP (5-C compound) captures CO 2 and surrounding H + to form an unstable 6-C compound. The 6-carbon compound immediately splits into two 3-C PGA molecules Electrons and energy from NADPH and ATP created in the light dependent reactions is used to convert PGA to form G-3-P (PGAL) Two molecules of 3-C G-3-P are used to form a molecule of 6-C glucose Most PGAL molecules are used to form RuBP to start system again Krebs CYCLE The problem of photorespiration and dehydration The problem of photorespiration: The rubisco that fixes carbon in the Calvin cycle is actually Ribulose 1,5- Bisphosphate Carboxylase/Oxygenase [-ase endings after all ]. As the name implies, it can attach CO 2 (good) or O 2 (bad). This is especially problematic since O 2 is being created from the splitting of water nearby in the light dependent reactions on the thylakoid. This can reduce efficiency by up to 50% in some plants!!! The problem of dehydration: Recall that the stomata allow for the diffusion of O 2 and CO 2 into and out of the leaves. Realize that H 2 O can also leave (dehydration) through the stomata. At first glance, one would assume that the answer is to close stomata during the hot summer days to avoid dehydration. However, the light dependent reactions are producing O 2 at a high rate on a hot, sunny day resulting increased photorespiration. PHOTORESPIRATION SOUTION #1- PHYSICAL SEPARATION: C 4 Structure: In C-4 plants, the vascular bundle is surrounded by a ring of bundle sheath cells and an outer ring of functional mesophyll cells. The two layers give the appearance of a wreath resulting in what is called Kranz (German for halo ) anatomy. The bundle sheath cells have large chloroplasts that either lack or have poorly developed grana. Function: C 4 plants use PEP carboxylase which has a high CO 2 affinity and is essentially insensitive to O 2 to fix CO 2 instead of rubisco in the mesophyll cells. A modified 4 Carbon molecule(hence the name C4), malic acid, is then shipped into a CO 2 leak proof chamber (bundle sheaths) where the Calvin cycle will take place. The reaction is now reversed resulting in PEP and CO 2. PEP can now return to the mesophyll cell to retrieve more CO 2. With high [CO 2 ] and low [O 2 ], the odds are now far better for carbon fixation in the bundle sheath cells. Examples: C 4 photosynthesis is common in corn, crabgrass, and many annuals

5 PHOTORESPIRATION SOLUTION #2 TEMPORAL SEPARATION: CAM CAM (Crassalucean Acid Metabolism) plants open stomata at night to bring in CO 2, then they fix it to one of a few organic acids like PEP. These molecules are stored in vacuoles until day time when the stomata close and now the Calvin cycle can begin as the previously made organic acids now decarboxylate and release the CO 2 to keep the relative concentration high for rubisco. C 4 and CAM are similar, but CAM does everything in the mesophyll NO bundle sheaths are involved. Examples: CAM is common in cacti and other succulent plants as well as pineapples and other bromeliads

6 Multiple Choice 1. The color with the shortest wavelength in the visible spectrum is: (A) orange (B) red (C) yellow (D) blue (E) green D Blue has the shortest wavelength of the choices provided. 2. If photosynthesis ceased occurring throughout the world, the ultimate effect would be: (A) a rapid increase in plant reproduction and proliferation (B) plants would become the primary energy producers for all living things (C) immediate decreases in atmospheric carbon dioxide (D) alternating cooling and warming of the earth (E) extinction of plants and animals E Plants are producers. All energy on earth is provided for by autotrophs. 3. Photosynthesis commonly takes place in all of the following EXCEPT: (A) mosses (B) ferns (C) algae (D) fungi (E) leafy plants D Fungi are heterotrophs and do not contain chloroplasts. 4. All of the following are required for photosynthesis to occur EXCEPT (A) oxygen (B) carbon dioxide (C) chlorophyll (D) sunlight (E) NADP + A CO 2 is required for the Calvin cycle. NADP + extracts electrons from the light independent reactions. Sunlight is required to excited electrons in chlorophyll. O 2 is produced in photosynthesis; it is not required for photosynthesis

7 5. All of the following are characteristics of chloroplasts EXCEPT (A) located in mesophyll tissue of leaves (B) bound by a double membrane (C) contain chlorophyll molecules (D) found in the fluids that surround most plant cells (E) somewhat similar in size and structure to mitochondria D Chloroplasts are contained within the plant cell. 6. The oxygen produced in the light dependent reactions of photosynthesis comes from (A) ATP (B) water (C) carbon dioxide (D) NADP + (E) chlorophyll B H 2 O is split to in order to provide electrons to chlorophyll in photosystem II. A byproduct of the splitting of 2 H 2 O molecules is 4H + and an O During photosynthesis the compound that is incorporated into existing organic molecules to build carbohydrates is (A) ATP (B) NADP + (C) chlorophyll (D) water (E) carbon dioxide E CO 2 is a source of C and O during the Calvin cycle. 8. The waxy cuticle found on the leaves of many higher plants functions to (A) transfer gases (B) remove excess water from the plant (C) protect the leaf from water loss (D) store chlorophyll and expose the pigment to the maximum amount of sunlight (E) serve as the site of the electron transport system C The adaptation of the cuticle serves to avoid dehydration. Gas transfer is the function of stomata.

8 9. A major purpose of thylakoids that form the grana in chloroplasts is to (A) provide a fluid environment for the reactions in the light phase (B) control the amount of light that reaches the chlorophyll (C) convert chlorophyll a into various accessory pigments (D) provide increased membrane surface area for the reactions in the light phase (E) accumulate glucose molecules for distribution to other plant cells D Just like cristae in the mitochondria, the thylakoids function to increase surface area. Light dependent reactions take place on the thylakoid surface. 10. Accessory pigments such as chlorophyll b, xanthophyll, and carotenoids are important in photosynthesis in that they are able to (A) absorb wavelengths of light that are not absorbed by chlorophyll a (B) serve as enzymes to create new chlorophyll a molecules (C) directly convert carbon dioxide into glucose and oxygen (D) absorb water in arid conditions (E) control the opening and closing of stomata A Accessory pigments are able to absorb different wavelengths when compared to chlorophyll a the primary photosynthetic pigment. 11. The source of the electrons captured and transferred by NADPH is (A) glucose (B) water (C) NADP + (D) ATP (E) carbon dioxide B H 2 O is responsible for replenishing electrons lost from photosystem II. An electron source is necessary as the electrons are not recycled. The electrons eventually reside in glucose/starch. 12. Ribulose biphosphate (RuBP) is important in the light independent reactions (Calvin cycle) as the molecule that (A) transfers hydrogen ions to carbon dioxide (B) releases oxygen into the atmosphere (C) combines with carbon dioxide (D) begins the electron transport system (E) catalyzes the formation of ATP from ADP C With the aid of the rubisco enzyme RuBP combines with CO 2 at the beginning of the Calvin cycle.

9 13. In the Calvin cycle of photosynthesis, PGA is formed from (A) the combination of water and carbon dioxide (B) the reaction of chlorophyll with carbon dioxide (C) the synthesis of ATP from ADP (D) the synthesis of NADPH and carbon dioxide (E) the breakdown of an unstable six-carbon molecule E RuBP and CO 2 combine to form an unstable molecule that will later be broken down into PGA. The synthesis of ATP and NADPH both occur in the light dependent reactions. 14. Glyceraldehyde-3-Phosphate (PGAL) molecules are used to create: (A) NADP and salts (B) RuBP and glucose (C) proteins and fatty acids (D) ATP and chlorophyll (E) a supply of electrons for the Calvin cycle B Two G-3-P molecules will combine to form a glucose. In addition several G-3-P molecules will continue in the CYCLE.

10 Questions refer to the following terms I. Grana II. Stroma III. Stomata IV. Chlorophyll 15. Contain(s) chlorophyll molecules imbedded in membranes: (A) I only (B) II only (C) I and II (D) II and III (E) III only A Grana (stacks of thylakoids) contain chlorophyll molecules in the phtosystems 16. Region(s) of chloroplasts where light dependent and light independent reactions occur (A) I only (B) I and II (C) II only (D) II and III (E) I and III B The light dependent reactions occur on the thylakoids of grana. The light independent reactions occur in the stroma. 17. Become(s) activated when exposed to certain wavelengths of light (A) I and II (B) II only (C) I and III (D) IV only (E) II and IV D Electrons in chlorophyll molecules are excited when exposed to the appropriate wavelength of light. 18. Control(s) gas exchange through leaf surfaces (A) I and II (B) I and III (C) II only (D) II and III (E) III only E O 2 and CO 2 exchange occurs through the stomata.

11 19. The C-4 photosynthesis pathway found in some plants is characterized by (A) the removal of water from the light dependent reactions (B) elimination of electron transport systems (C) chemical transfer of ribulose biphosphate from one tissue area to another (D) formation of malic acid to transfer carbon dioxide (E) rapid transfer of oxygen to the atmosphere C The reactants for the Calvin cycle occur are transferred to the bundle sheath cells during C-4 photosynthesis. 20. The adaptive advantage of plants with a C-4 pathway include the ability to photosynthesize faster than plants with a typical C-3 pathway under all of the following conditions EXCEPT (A) high light intensity (B) low carbon dioxide concentrations (C) high turgor pressure in guards cells that surround stomata (D) dry conditions with high temperatures (E) area where plants grow tightly clustered together C High turgor pressure in guard cells would imply that stomata are open. When stomata are open, adequate CO 2 is entering and O 2 is not trapped. C-4 plants specialize in situations of low CO 2 concentration.

12 Free Response 1. Photosynthesis is often described as two series of reactions, the light-dependent and the lightindependent or Calvin cycle reactions. A. Describe all steps that occur in the light-dependent reactions, including raw materials, energy transfer, and products. (8 pt maximum) description of a photosystem OR that photosystems contain chlorophyll molecules chlorophyll absorbs specific wavelengths of light electrons are excited to higher energy level electrons passed down the ETC carriers pump H + ions into the interior of the thylakoid H + exit down the electrochemical gradient OR due to proton motive force H+ exit through ATP synthase results in production of ATP electrons are picked up at the end of the ETC by electron carriers (or NADP + ) H 2 O split to replenish electrons in photosystem II O 2 released as a result of the splitting of H 2 O B. Explain how the products of the light-dependent reactions are necessary to drive the reactions of the Calvin cycle. (4 pt maximum) CO 2 from atmosphere is incorporated into cycle Rubsico enzyme responsible for carbon fixing step ATP used to power Calvin cycle reactions NADPH will supply electrons to the Calvin cycle G-3-P (PGAL) will be produced and combined to create C 6 H 12 O 6 cyclic nature: much G-3-P will stay in the Calvin cycle

13 Free Response 2. Describe and discuss the C-4 pathway that can occur in some photosynthetic plants. Include in your discussion: A. a description of the compounds that are necessary for the process CO 2 as carbon and oxygen source for PGAL or glucose molecule PEP or PEP carboxylase a mechanism for capturing the CO 2 OAA or malate or malic acid as a means of transporting the CO 2 B. the tissue regions where the reactions occur meosophyll cells are the location for the pick up of CO 2 CO 2 will be dropped off in bundle sheath cells bundle sheath cells are advantageous because they are essentially O 2 and CO 2 leak proof arrangement of bundle sheath cells around the mesophyll cells C. the cyclic nature of the process Krebs cycle (citric acid cycle, TCA) is itself cyclical due to PGAL cyclical nature of CO 2 transport: Malic acid PEP + CO 2, repeat cyclical nature of electron transport between light dependent reactions and Krebs: NADPH/NADP + D. examples of plants where the C-4 pathway can be found accept legitimate options (sugar cane, crab grass, many annuals, etc.) E. the selective advantages to plants that use this photosynthetic pathway function more efficiently than C 3 at higher temperatures function more efficiently than C 3 when stomata are closed OR better at avoiding photorespiration function more efficiently than C 3 when CO 2 levels are low

14 Light Reaction Details Location: Chloroplast (thylakoid membranes of the grana) Function: Produces ATP and NADPH to be used in the Calvin Cycle P680 photon Photosystem II Photosystem I e - now has little energy P680 antenna molecule e - passed to P700 antenna molecule chlorophyll a absorbs P700 photon; re-energizes electron; passes the e- to an acceptor loses electron (e - ) H 2 O e - + H + + O 2 electron acceptor molecule passes e - transferred to carrier molecules the e - (and energy) to NADP+ in Electron Transport System (loss of energy with each transfer) NADP+ 2e - + 2H + NADPH the energy from each transfer is used to pump H + into thylakoid creating a H + concentration gradient H+ flowing out of the thylakoid through ATP synthase results in: ADP + Pi ATP (photophosphorylation) NOTE: Chlorophyll a is the main photosynthetic pigment in all photosynthesizing organisms except bacteria. Other accessory pigments (chlorophyll b, carotenoids, etc.) are able to absorb different wavelength of light and can pass the energy to chlorophyll a. Photosystem I was discovered first and evolved very early; Photosynthesis II evolved later and occurs in plants most plants.

15 Diagram of photosynthesis. What I absolutely have to know to survive the AP Exam (A) (C) (D) (G) (B) (pigment) LIGHT REACTION (E) CALVIN CYCLE (H) (F) (I) (J) ADP + Pi ATP carbon dioxide chlorophyll glucose light NADP + NADPH oxygen water

16 How to write it: 6 CO 2 (1) Each 5-carbon RuBP molecule combines with carbon dioxide in the presence of the rubisco enzyme to form an unstable 6-carbon compound; rubisco enzyme 6 6-Carbon (unstable) (2) the 6-carbon compound immediately breaks into two molecules of 3-carbon PGA 6 ADP + Pi 6 ATP 6 RuBP 5-Carbon Calvin Cycle 12 PGA 3-Carbon 12 ATP 12 ADP + Pi 12 NADPH (3) reactions that provide energy from ATP and hydrogen ions from NADPH convert the PGA into 3-carbon PGAL (4) some of the PGAL is converted to glucose (5) most of the PGAL (with energy from ATP) is converted to the 5-carbon RuBP. 10 PGAL 3-Carbon 12 NADP + 12 PGAL 3-Carbon 1Glucose 6-Carbon