Problem of the Day: What does the graph below say about non- germinating corn seed vs. germinating corn seed? Explain this result.

Chapter 10 Photosynthesis You Must Know: The summary equation of photosynthesis including the source and fate of the reactants and products How leaf and chloroplast anatomy relates to photosynthesis How photosystems convert solar energy to chemical energy How linear electron flow in the light reactions result in the formation of ATP, NADPH and O 2 How chemiosmosis generates ATP in the light reactions. How the Calvin cycle uses the energy molecules of the light reactions to produce G3P.

Essential Knowledge 2.A.2 Organisms capture and store free energy to use in biological processes. Photosynthesis first evolved in prokaryotic organisms Scientific evidence supports that prokaryotic (bacterial) photosynthesis was responsible for the production of an oxygenated atmosphere Prokaryotic pathways were the foundation of eukaryotic photosynthesis

Transfer of Energy Autotrophs: biotic producers; capture free energy from sunlight (photoautotrophs) or small inorganic molecules in the environment (chemoautotrophs) Heterotrophs: biotic consumers; obtains organic food by eating other organisms or their by- products (includes decomposers)

Plant structures and photosynthesis Chloroplasts are the sites of photosynthesis. Chlorophyll A is the main pigment involved. Photosynthesis takes place in plant tissue called mesophyll Plants obtain CO 2 and release O 2 through openings called stomata Chloroplasts have a double membrane, thylakoids, grana and stroma.

The Nature of Sunlight Electromagnetic energy- wave & particle (photon) Chlorophylls absorb violet, blue and red wavelengths; reflect green Carotenoids absorb violet, blue, and green regions of the spectrum When pigments absorb light, electrons are boosted ot a higher energy level and the energy is captured in a chemical bond

Absorption Spectrum of Chlorophyll Leaves appear green because they reflect green light. Chlorophyll absorbs blue the best, then red. These are the wavelengths used in photosynthesis. More photosynthesis happens when more light is absorbed.

An overview of Photosynthesis

Photosystems: light-harvesting complexes of the thylakiod membrane Antenna complex- contains chlorophyll A and B and carotenoids Reaction center chlorophyll-A Primary electron acceptor-traps e- released from the reaction center

The Pathway of Photosynthesis Light dependent reactions –Capture free energy present in light –Makes ATP and NADPH Calvin Cycle (light independent reactions) –Uses the ATP and NADPH to power the production of organic molecules (sugar)

Light Dependent Reactions Thylakoid membranes contain the photosystems Energy from light is used to boost electrons to a higher energy level in Photosystems I and II. Photosystem II is first.

Light Dependent Reactions The photosystems are connected by the transfer of higher free energy electrons through an electron transport chain (ETC)

Light Dependent Reactions When electrons are transferred between molecules in a sequence of reactions as they pass through the ETC, an electrochemical gradient of protons across the thylakoid membrane is established Chemiosmosis is used to make ATP from ADP and inorganic phosphate

Linear vs. Cyclic Electron Flow Linear Electron Flow-happens during the light reactions; uses both photosystems and produces NADPH, ATP and oxygen; Moves electrons from water to NADP Cyclic Electron Flow employs only photosystem I, producing ATP but not NADPH or oxygen Some species of bacteria only have cyclic flow; probably an evolutionary “left-over” May be photoprotective for some species that do not grow well in bright light

Photolysis 2H 2 O  4H + + 4e- + O 2 Source of electrons for the ETC Oxygen is a byproduct Happens during the light reactions because electrons bounced off of chlorophyll need to be replaced.

Calvin Cycle: makes sugar Occurs in the stroma of the chloroplast Three phases Carbon enters in the form of CO 2 and leaves in the form of sugar (G3P) 2 G3P molecules combine to form glucose Terminal electron acceptor is NADP + Uses energy from ATP and NADPH made in light reactions.

Photosynthesis vs. Cellular Respiration (chemiosmosis) Photosynthesis Source of H + and e- is water Light provides the energy ATP synthase ETC present Proton motive force Cellular Respiration Source of H + and e- is food Chemical bonds from food provide the energy ATP synthase ETC present Proton motive force

Alternative to photosynthesis: Photorespiraton This is a process in which, instead of adding CO2 to the growing sugar, Rubisco adds oxygen. In situations in which photosynthesis is taking place fast (at high temperature, high levels of light or both), there is so much O2 available that this reaction becomes a significant problem.

Alternatives to photosynthesis: C 4 photosynthesis C 4 : separate 2 steps of carbon fixation anatomically = 2 different cells PEP carboxylase in outer ring of mesophyll cells, 4C “storage” compounds (oxaloacetate, malate) Passes carbon by regenerating carbon dioxide in inner bundle sheath cells to rubisco and Calvin cycle Grasses, corn, rice, sugar cane

Alternatives to Photosynthesis: CAM Photosynthesis crassulacean acid metabolism Separates 2 steps of carbon fixation temporally =2 different times Fix carbon at night (when stomates open), put it in “storage compounds (organic acids), then in day (when stomates close), release carbon dioxide from storage compounds to Calvin cycle Cactic succulents, pineapple

A review of photosynthesis Light Reactions Thylakoid membranes Convert light energy to ATP and NADPH Split H 2 O and release O 2 to the atmosphere Calvin Cycle Stroma Use ATP and NADPH to covert CO 2 to the sugar G3P Return ADP, inorganic phosphate, and NADP + to the light reactions