Photosynthesis Ch 10 AP Biology Converting Solar Energy to Chemical Energy 6 CO H 2 O + Light energy  C 6 H 12 O O H 2 O

Autotrophs vs. Heterotrophs Autotrophs: photosynthesis (or chemosynthesis) and Cellular Respiration Photoautotrophs: energy source: light Chemoautotrophs: energy source: chemicals Types of Autotrophs: – Plants – Algae – Cyanobacteria – Protists – Bacteria Heterotrophs: – Eat other organisms to obtain energy. – Cellular Respiration (or fermentation) only

Chloroplast: Site of photosynthesis Double Membrane Chlorophyll – Green pigment, absorbs sunlight, reflects back green and yellow light Stroma – fluid in chloroplast Thylakoid – pancake like structure (increases surface area) Grana – stacks of thylakoids Leaf – interior cells: mesophyll – Stomata: pores in leaf for gas exchange Chloroplast Mesophyll 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm

Anatomy of a leaf

Light and Pigments Pigments absorb visible light: – Chlorophyll a: light reactions absorbs violet, blue and red – Accessory pigments Chlorophyll b: absorbs blue & orange Carotenoids: absorbs blue Electromagnetic Spectrum – Light energy – Lower wavelength, higher E – 10_07LightAndPigments_A.swf 10_07LightAndPigments_A.swf Photons are particles of light, light travels as both energy and a particle. E=mc 2 Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7

Spectrophotometer measures wavelength of light absorbed by pigments Shine white light through a prism, separate colors Pick a color to shine through a sample Measure transmission of that color High transmission = low absorption & 1/X Notice the wavelengths for each pigment

Photosynthesis 6 CO H 2 O + Light energy  C 6 H 12 O O 2 Light Reactions – Occur in the thylakoid membranes of the chlorplasts – Absorb light energy & convert it to ATP, NADPH & O 2 – 10_05Photosynthesis.mpg 10_05Photosynthesis.mpg Calvin Cycle (Dark Reactions or light independent Rxns) – Occur in the stroma – Uses Energy from the light reactions to make food!

H2OH2O CO 2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH 2 O] (sugar) NADPH NADP  ADP + P O2O2 Figure 10.5 ATP

Light Reactions Thylakoid Membranes Solar Energy + H 2 O  ATP + NADPH + O 2 – ATP + NADPH will be used in the Calvin Cycle – O 2 is given off by the autotroph as a by product (water is source of Oxygen) – H 2 O is split by photons of light to provide electrons for the electron transport chain NADP + is the equivalent to NAD + in Respiration (it carries electrons (reduced) to be used later in photosynthesis to power an H + pump) Remember: a photon of light that is higher E than an e - can excite the e -,when the e - falls back down E is released when this happens in the chloroplast, the E is captured in the redox reaction.

Light Reactions

Photosystems Linear Electron flow Photosystem II (P680) – Red part of visible light – Absorbs light E – Enzymes splits H 2 O & 2e - are used in photosystems, O 2 from 2 water molecules is given off – Passes the E through electrons, down an ETC (chemiosmosis) – The E is used to make ATP Photosystem I (P700) – Far red part of visible light – Catches the e - from PSII and uses solar energy to excite the e - again. – The exergonic “fall” of e - down the ETC is now used to make NADPH Study Figure on page 197 for an explanation of ETC

Non-cyclic Flow

Cyclic Flow Uses only PSI, no NADPH is made, ATP made is sent to Calvin Cycle Likely an evolutionary leftover, used in some photosynthetic prokaryotes and all eukaryotes tested to date. Cyclic Electron Flow

Calvin Cycle uses the ATP, NADPH & CO 2 to make sugar 1. Carbon Fixation – CO 2 attached to RuBP by the rubisco enzyme – RuBP splits in half to form 2 three carbon molecules 2. Reduction – ATP & NADPH used to make G3P (1 used to make glucose, one used in regeneration) 3. Regeneration of CO 2 acceptor RuBP – RuBP is regenerated so that it is ready to accept more CO 2 9ATP + 6NADPH + 3CO 2  1/2 Glucose (G3P) the cycle turns twice for every glucose made

ProcessWhere?InputOutput 1. Light Reactions Thylakoid Membrane Sunlight + H 2 OATP + NADPH + O 2 2. Dark Reactions StromaATP + NADPH + CO 2 Glucose TOTALChloroplastSunlight + H 2 O + CO 2 Glucose + O 2

Alternative Pathways Photorespiration C 4 plants CAM Plants Plants close Stomata in hot dry climates to conserve water and energy Starves the Calvin Cycle of CO 2

Photorespiration An evolutionary remnant or a protection mechanism? C3 plants – “normal photosynthesis” Rubisco will bind O 2 and CO 2 at its active site If CO 2 is limited O 2 will bind in its place in the Calvin cycle NO ATP or glucose is produced – Energy is used – Decreases photosynthetic production by preventing the formation of 3- phosphoglycerate molecules

C 4 Plants C 4 plants minimize the cost of photorespiration – By incorporating CO 2 into four carbon compounds in mesophyll cells = a physical separation These four carbon compounds are exported to bundle sheath cells, where they release CO 2 used in the Calvin cycle

CAM Plants: CAM-TIME Desert plants – cactus, pineapple etc. Conserve H 2 O by closing the stomata during the day when hot and opening stomata at night, a time separation Fix CO 2 into organic acids at night and store it in vacuoles then use the organic acids in the morning when the light reactions make ATP and NADPH

C4 vs. CAM Plants Both minimize photorespiration & optimize Calvin Cycle C 4 – Partially close stomata to conserve H 2 O – CO 2 fixation w/ PEP carboxylase DAY – Completes Calvin Cycle immediately – Bundle sheath cells CAM – Close stomata completely during day to conserve H 2 O – open at night – CO 2 fixation w/ PEP carboxylase NIGHT – Stores organic acids in vacuole until morning then completes Calvin cycle – CAM-TIME

Concept Maps & Summary Non Cyclic Electron Flow Cyclic Electron Flow Photosystem II Electron Transposrt Photosystem I Light Reactions Carbon Fixation Reduction CO 2 regeneration Calvin Cycle