AP BIOLOGY PHOTOSYNTHESIS Chapter 10 Light Reactions http://vilenski.org/science/safari/cellstructure/chloroplasts.html AP BIOLOGY PHOTOSYNTHESIS Chapter 10 Light Reactions http://www.science.siu.edu/plant-biology/PLB117/JPEGs%20CD/0076.JPG

different wavelengths as different ___________ wavelengths Sunlight is made up of many different _______________ of light Your eyes “see” different wavelengths as different ___________ wavelengths colors http://www.simontucket.com/_Portfolio/PortLarge/L_Il_Prism.jpg

Visible light is part of electromagnetic spectrum Y O R

WHY ARE PLANTS GREEN? We “see” reflected light Light wavelengths that are reflected bounce back to your eyes . . . so leaves “LOOK” green. Image modified from: http://www.visibledreams.net/Web/color/color_3.html

pigments Plants gather the sun’s energy with light absorbing molecules called _______________. pigments Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules *Remember: Heterotrophs obtain their organic material from other organisms By: VanderWal

The main energy absorbing molecule in green plants is http://fig.cox.miami.edu/Faculty/Dana/chlorophyll.jpg The main energy absorbing molecule in green plants is __________________ CHLOROPHYLL a

Leaves are the major locations of photosynthesis Their green color is from chlorophyll within chloroplasts Chloroplasts are found mainly in cells of the interior tissue (mesophyll) of the leaf Each mesophyll cell contains 30–40 chloroplasts CO2 enters and O2 exits the leaf through microscopic pores called stomata

Pigments are found within the chloroplast Chlorophyll & other pigments embedded in thylakoid membrane arranged in a “photosystem” (PS) PS = a group of proteins and lipids that receive energy from the sun

Light: absorption spectra Photosynthesis gets energy by absorbing wavelengths of light chlorophyll a absorbs best in red & blue wavelengths & least in green other pigments with different structures absorb light of different wavelengths

WHY DON’T WE SEE THE OTHER PIGMENTS? Carotenoids are usually hidden by the presence of chlorophyll Other pigments: Caroteniods– appear orange, red, and yellow Xanthophyll- yellow In the fall chlorophyll production shuts down and other pigments “show” A spectrophotometer measures a pigment’s ability to absorb various wavelengths This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

What is photosynthesis such a big deal? Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also most of the living world

PHOTOSYNTHESIS OVERVIEW Pearson Education Inc; Publishing as Prentice Hall

Plant anatomy THYLAKOIDS GRANUM (pl. grana) = sac-like photosynthetic = stack of thylakoids membranes inside chloroplast GRANUM (pl. grana) Image from BIOLOGY by Miller and Levine; Prentice Hall Publishing©2006

SPACES THYLAKOID SPACE (lumen) STROMA cytoplasm Gel-filled space inside chloroplast surrounding thylakoid sac Gel-filled space Inside the thylakoid sac cytoplasm Gel-filled space OUTSIDE chloroplast but inside the cell membrane http://www.science.siu.edu/plant-biology/PLB117/JPEGs%20CD/0076.JPG

Remember redox reactions?! OIL RIG Oxidation is loss Reduction is gain Loss and gain of electrons Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

Photosynthesis Light reactions Calvin cycle light-dependent reactions energy production reactions convert solar energy to chemical energy Make ATP & NADPH Calvin cycle light-independent reactions sugar production reactions use chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6

Photosystems of photosynthesis 2 photosystems in thylakoid membrane Both have a REACTION CENTER • CHLOROPHYLL a molecules • PRIMARY ELECTRON ACCEPTOR Surrounded by light-gathering “ANTENNA COMPLEX” • Accessory pigments (chlorophyll b, carotenoids) Collect light energy and pass it on to chlorophyll a Photosystem II P680 = absorbs 680nm wavelength red light Photosystem I P700 = absorbs 700nm wavelength red light

ELECTRON TRANSPORT CHAIN ETC PROTEINS: Plastoquinone Cytochrome Plastocyanin Ferredoxin Electron Transport Chain membrane-bound proteins (don’t have to memorize names) electron acceptors- @ END NADP+ accepts) Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane Diffusion of H+ (protons) across the membrane drives ATP synthesis

Light Dependent reactions Electron Transport Chain membrane-bound proteins in organelle electron acceptors END = NADPH proton (H+) gradient across inner membrane ATP synthase enzyme Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.

Chloroplasts transform light energy into chemical energy of ATP use electron carrier NADPH ETC of Photosynthesis Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide

LIGHT DEPENDENT REACTIONS ETC produces from light energy ATP & NADPH go to Calvin cycle PS II absorbs light excited electron passes from chlorophyll to “primary electron acceptor” need to replace electron in chlorophyll enzyme extracts electrons from H2O & supplies them to chlorophyll splits H2O O combines with another O to form O2 O2 released to atmosphere and we breathe easier!

Noncyclic Photophosphorylation Light reactions elevate electrons in 2 steps (PS II & PS I) PS II generates energy as ATP PS I generates reducing power as NADPH 1 photosystem is not enough. Have to lift electron in 2 stages to a higher energy level. Does work as it falls. First, produce ATP -- but producing ATP is not enough. Second, need to produce organic molecules for other uses & also need to produce a stable storage molecule for a rainy day (sugars). This is done in Calvin Cycle!

Cyclic photophosphorylation PS I doesn’t pass electron to NADP… it cycles back to ETC & makes more ATP, but no NADPH coordinates light reactions to Calvin cycle Important in maintaining proportion of ATP & NADPH for Calvin Calvin cycle uses more ATP than NADPH X

Photophosphorylation noncyclic photophosphorylation cyclic Cyclic electron flow is thought to have evolved before linear electron flow Cyclic electron flow may protect cells from light-induced damage

Experimental evidence Where did the O2 come from? radioactive tracer = O18 6CO2 6H2O C6H12O6 6O2 light energy  + Experiment 1 6CO2 6H2O C6H12O6 6O2 light energy  + 6CO2 6H2O C6H12O6 6O2 light energy  + Experiment 2 Proved O2 came from H2O not CO2 = plants split H2O

LIGHT REACTIONS summary Where did the energy come from? Where did the electrons come from? Where did the H2O come from? Where did the O2 come from? Where did the O2 go? sunlight From chlorophyll; replaced by H2O In through roots Made when water splits Out through stomata

LIGHT REACTIONS summary Where did the H+ come from? Where did the ATP come from? What will the ATP be used for? Where did the NADPH come from? What will the NADPH be used for? Split off of water Produced by ATP synthase during light rxns Make sugar in Calvin cycle Receives e-’s at end of ETC Make sugar in Calvin cycle …stay tuned for the Calvin cycle

Calvin Cycle Memorization of the steps in the Calvin cycle, the structure of the molecules and the names of enzymes (with the exception of ATP synthase) are beyond the scope of the course and the AP Exam. Click to See Calvin cycle animation  remember that much of this is out of scope for the class but sometimes it helps to see the big picture

CALVIN CYCLE also called light independent reaction - does not require light but usually occurs during the day. It happens in the stroma between thylakoids NADPH donates hydrogen ions and electrons ATP donates energy CO2 donates carbon and oxygen All to make G3P which later can combine into glucose or cellulose or any other sugar a plant may need

CALVIN CYCLE summary Where does the C in G3P come from? Where does the H in G3P come from? Where does the O in G3P come from? Where does the ADP & NADP+ go? Where does the G3P go? CO2 From H2O via NADPH CO2 Back to light reaction to recharge Used to make glucose and other organic molecules

Normal Carbon Fixation C3 plants (Ex: rice, wheat, soybeans) (1st product of carbon fixation has 3 Carbons) On hot, dry days when plant shuts stomata plant switches to photorespiration Rubisco adds O2 to Calvin cycle instead of CO2 This creates a toxic byproduct that must be broken down by mitochondria/peroxisomes Photorespiration is COUNTERPRODUCTIVE: Makes NO ATP, Makes NO sugar, Uses ATP Decreases photosynthesis by siphoning molecules from Calvin cycle

PROBLEMS ON HOT DRY DAYS If stomata are open to receive CO2 results in water loss On hot, dry days if plant shuts stomata to conserve water . . . photosynthesis slows Over 8000 species of angiosperms (plants that flower) have developed adaptations which minimize the losses to photorespiration. Many of these plants are considered C4 plants http://www.ipm.iastate.edu/ipm/icm/files/images/spider-mite-field.jpg

ALTERNATIVE METHODS of CARBON FIXATION SEE ANIMATION Have evolved ways to avoid a decrease in photosynthesis due to photorespriation C4 plants- Ex: corn & sugarcane Such plants are well adapted to habitats with high daytime temperatures and intense sunlight. Another kind of C4 plant called the CAM plants- Ex: succulents, cactus, pineapple Well adapted to conditions of high daytime temperatures, intense sunlight and low soil moisture.

C4 plants C4 plants form a 4-carbon molecule instead of the normal 3-carbon molecules of the Calvin cycle when they take in CO2. 1. CO2 diffuses into a mesophyll cell. 2. CO2 then joins other molecules to form a 4-carbon acid that is stored in the bundle sheath cells – these cells are deep in the leaf where it is hard for O2 to reach – thus preventing photorespiration 3. Then this acid can be broken down in the Calvin Cycle CAM plants — separate their C3 and C4 cycles by time CAM plants take in CO2 through their open stomata (they tend to have reduced numbers of them) CO2 joins other molecules to form a 4-carbon acid which can accumulates during the night in the central vacuole of the cells.

CALVIN CYCLE found in BUNDLE SHEATH CELLS in C4 plants

photophosphorylation Quick Vocab Check- Process of using H+ gradient to generate ATP = _______________(Can refer to ATP made in mitochondria too) Process of creating ATP using a Proton gradient created by the energy gathered from sunlight. = ________________________ Process that consumes oxygen, releases CO2, generates no ATP, and decreases photosynthetic output; generally occurs on hot, dry, bright days, when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide chemiosmosis photophosphorylation photorespiration

Hank Green on Photosynthesis 3 amazing photosynthetic ANIMALS is also a good video to watch