Photosynthesis Presenter APD Cover
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.
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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)
AP* is a trademark of the College Entrance Examination Board. The College Entrance Examination Board was not involved in the production of this material.
Copyright ? 2008 Laying the Foundation?, Inc., Dallas, TX. All rights reserved. Visit:
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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