Photosynthesis Presenter APD Cover

1 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: 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. Copyright 2008 Laying the Foundation , Inc., Dallas, TX. All rights reserved. Visit: Photosynthesis Objective To review the student on the concepts and processes necessary to successfully answer questions over the process of Photosynthesis .

2 Standards Photosynthesis is addressed in the topic outline of the College Board AP Biology Course Description Guide as described below. 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.

3 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) I. Molecules & Cells C. cellular Energetics Coupled Reactions cellular respiration & Fermentation Photosynthesis Copyright 2008 Laying the Foundation , Inc.

4 , Dallas, TX. All rights reserved. Visit: Photosynthesis 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).

5 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.

6 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 + O2- 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 O2 that makes aerobic cellular respiration possible. 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.

7 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). Copyright 2008 Laying the Foundation , Inc., Dallas, TX. All rights reserved. Visit: 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.

8 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 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 CO2 (good) or O2 (bad).

9 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.

10 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.