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AP BIOLOGY PHOTOSYNTHESIS

Teacher Packet

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Photosynthesis

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 C4 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 http://apcentral.collegeboard.com. Free Response Questions 2008 Practice Exam 2006- Questions 4 (a) form b Question 3 2007- Question 3 (c) 2006- Question 1 (a), 1 (c) 2004- Question 3 (lab based)

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Photosynthesis

LIGHT & ENERGY 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 DEPENDENT REACTIONS The surface of a Photosystem

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 + Pi 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 H2O- the electron source H2O is split releasing 2 e- + H+ ions + O2The 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 O2 that makes aerobic cellular respiration possible. ®

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Photosynthesis

LIGHT INDEPENDENT REACTIONS (CALVIN CYCLE) Calvin Cycle needs: 1. source of C,H,O in order to manufacture C6H12O6 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 CO2 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 1,5Bisphosphate Carboxylase/Oxygenase [-ase endings after all ☺]. As the name implies, it can attach CO2 (good) or O2 (bad). This is especially problematic since O2 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 O2 and CO2 into and out of the leaves. Realize that H2O 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 O2 at a high rate on a hot, sunny day resulting increased photorespiration.

PHOTORESPIRATION SOUTION #1- PHYSICAL SEPARATION: C4 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: C4 plants use PEP carboxylase which has a high CO2 affinity and is essentially “insensitive” to O2 to fix CO2 instead of rubisco in the mesophyll cells. A modified 4 Carbon molecule(hence the name C4), malic acid, is then shipped into a CO2 leak proof chamber (bundle sheaths) where the Calvin cycle will take place. The reaction is now “reversed” resulting in PEP and CO2. PEP can now return to the mesophyll cell to retrieve more CO2. With high [CO2] and low [O2], the odds are now far better for carbon fixation in the bundle sheath cells. Examples: C4 photosynthesis is common in corn, crabgrass, and many annuals ®

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Photosynthesis

PHOTORESPIRATION SOLUTION #2 TEMPORAL SEPARATION: CAM CAM (Crassalucean Acid Metabolism) plants open stomata at night to bring in CO2, 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 CO2 to keep the relative concentration high for rubisco. C4 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

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Photosynthesis

Multiple Choice 1. The color with the shortest wavelength in the visible spectrum is: (A) (B) (C) (D) (E) D

orange red yellow blue green

Blue has the shortest wavelength of the choices provided.

2. If photosynthesis ceased occurring throughout the world, the ultimate effect would be: (A) (B) (C) (D) (E) E

a rapid increase in plant reproduction and proliferation plants would become the primary energy producers for all living things immediate decreases in atmospheric carbon dioxide alternating cooling and warming of the earth extinction of plants and animals

Plants are producers. All energy on earth is provided for by autotrophs.

3. Photosynthesis commonly takes place in all of the following EXCEPT: (A) (B) (C) (D) (E) D

mosses ferns algae fungi leafy plants

Fungi are heterotrophs and do not contain chloroplasts.

4. All of the following are required for photosynthesis to occur EXCEPT (A) (B) (C) (D) (E) A

oxygen carbon dioxide chlorophyll sunlight NADP+

CO2 is required for the Calvin cycle. NADP+ extracts electrons from the light independent reactions. Sunlight is required to excited electrons in chlorophyll. O2 is produced in photosynthesis; it is not required for photosynthesis

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Photosynthesis

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

H2O is split to in order to provide electrons to chlorophyll in photosystem II. A byproduct of the splitting of 2 H2O molecules is 4H+ and an O2.

7. 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

CO2 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.

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Photosynthesis

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

H2O 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 CO2 at the beginning of the Calvin cycle.

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Photosynthesis

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 CO2 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.

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Photosynthesis

Questions 15 -18 refer to the following terms I. II. III. IV.

Grana Stroma Stomata 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

O2 and CO2 exchange occurs through the stomata.

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Photosynthesis

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 CO2 is entering and O2 is not trapped. C-4 plants specialize in situations of low CO2

concentration.

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Photosynthesis

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+) __H2O split to replenish electrons in photosystem II __O2 released as a result of the splitting of H2O B. Explain how the products of the light-dependent reactions are necessary to drive the reactions of the Calvin cycle. (4 pt maximum) __CO2 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 C6H12O6 __cyclic nature: much G-3-P will stay in the Calvin cycle

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Photosynthesis

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 __CO2 as carbon and oxygen source for PGAL or glucose molecule __PEP or PEP carboxylase a mechanism for “capturing” the CO2 __OAA or malate or malic acid as a means of transporting the CO2 B. the tissue regions where the reactions occur __meosophyll cells are the location for the “pick up” of CO2 __CO2 will be dropped off in bundle sheath cells __bundle sheath cells are advantageous because they are essentially O2and CO2 “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 CO2 transport: Malic acid PEP + CO2, 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 C3at higher temperatures __function more efficiently than C3 when stomata are closed OR better at avoiding photorespiration __function more efficiently than C3 when CO2 levels are low

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Photosynthesis

Light Reaction Details Location: Chloroplast (thylakoid membranes of the grana) Function: Produces ATP and NADPH to be used in the Calvin Cycle

Photosystem II

Photosystem I

P680 photon

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 -

e- + H+ + O2

H2O

loses electron (e ) electron acceptor molecule passes the e- (and energy) to NADP+

-

e transferred to carrier molecules 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.

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Photosynthesis

Diagram of photosynthesis. What I absolutely have to know to survive the AP Exam…

(A) ____________

(C) ____________ (D) ____________

(G) ____________

(B) ___________ (pigment) (E) ____________

LIGHT REACTION

CALVIN CYCLE

(H) ____________

(F) ____________

(I) ____________

(J) ___________

ADP + Pi

ATP

carbon dioxide

chlorophyll

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glucose

light

NADP+

NADPH

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oxygen

water

Photosynthesis

How to write it: (1) Each 5-carbon RuBP molecule combines with carbon dioxide in the presence of the rubisco enzyme to form an unstable 6-carbon compound;

6 CO2

rubisco enzyme

6 6-Carbon (unstable)

(2) the 6-carbon compound immediately breaks into two molecules of 3-carbon PGA (3) reactions that provide energy from ATP and hydrogen ions from NADPH convert the PGA into 3-carbon PGAL

6 RuBP 5-Carbon 12 PGA 3-Carbon

Calvin Cycle

6 ADP + Pi 6 ATP

(4) some of the PGAL is converted to glucose 12 ATP 12 ADP + Pi

(5) most of the PGAL (with energy from ATP) is converted to the 5-carbon RuBP.

12 NADPH 12 NADP+ 10 PGAL 3-Carbon

12 PGAL 3-Carbon 1Glucose 6-Carbon ®

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Copyright © 2008 Laying the Foundation®, Inc., Dallas, TX. All rights reserved. Visit: www.layingthefoundation.org