Photosynthesis Presenter APD Cover

[Pages:17]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 .

Free Response Questions

2008 Practice Exam Question 3

2006- Questions 4 (a) form b

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).

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 + 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) 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

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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