Chapter 10 – Photosynthesis

Photosynthesis Conversion process of captured light energy to chemical energy stored in sugar and other organic materials. Autotrophs – organisms which can sustain themselves without eating anything derived from other organisms Heterotrophs – organisms which obtain their organic material from compounds produced by other organisms Photoautotrophs use light

Chloroplasts The site of photosynthesis Primary pigment: chlorophyll Located in: mesophyll, 30-40 in each cell Gas exchange: stomata Double membrane Stroma: dense fluid Thylakoid: membranous sac, contains pigment Grana: stacks of thylakoids ..\ppt lectures cd\animations\10_07LightAndPigments_A.swf

Overview 6CO2 + 6H2O + light  C6H12O6 + 6O2 A REDOX reaction Actually produce 3C compounds that are linked together to make glucose Actually have 12 water on left and 6 water on right, but equation is simplified Occurs in 2 stages: Light Reactions – NADP+ reduction and photophosphorylation Calvin Cycle – carbon fixation

Photosystems A reaction center surrounded by a number of light-harvesting complexes These complexes consist of a variety of pigment molecules When absorbs a photon, energy is transferred to reaction center – a protein complex that contains 2 special chlorophyll a molecules and a primary electron acceptor High energy electrons transferred from these chlorophyll a molecules to primary electron acceptor Step 1 of light reactions

Photosystems There are 2 types of photosystems: photosystem I (PS I) and photosystem II (PS II) Each have a primary electron acceptor next to a special chlorophyll a molecules. The reaction center of PS II is known as P680 (best at absorbing 680nm wavelength light) PS I is called P700 (absorbs 700nm best) Named in the order of their discovery but PS II functions first

Light Reactions Light driving the synthesis of NADPH and ATP animation Two different pathways: Noncyclic electron flow Cyclic electron flow Both are required for the Calvin cycle to function properly

Noncyclic Electron Flow

Noncyclic Electron Flow Photon of light strikes pigment in harvesting complex of PS II. Excites an electron in P680. Excited electron captured by primary electron acceptor Enzyme splits water into 2 electrons, 2 hydrogen ions and an oxygen atom. Electrons supply P680. Photoexcited electron passes from P680 to P700 through an electron transport chain. Energetic fall of electron drives ATP synthesis Meanwhile, photon of light strikes pigment in harvesting complex of PS I, excites an electron in P700. Captured by primary electron acceptor and electron replaced by electron from P680. Photoexcited electron passes from P700 down a second electron transport chain Enzyme NADP+ reductase transfers electrons from the final protein in the chain to reduce NADP+ to NADPH (takes 2 electrons) movie

Cyclic Electron Flow

Cyclic Electron Flow On occasion, the excited electrons from the P700 will not fall to NADP+, but will instead fall back through the 1st electron transport chain. This generates additional ATP and recycles the excited electrons back to P700 Why? Calvin cycle consumes more ATP than NADPH and the noncyclic pathway produces an equal amount of ATP to NADPH. Could be driven by the concentration of NADPH in the chloroplast.

Chemiosmosis Essentially the same process in both photosynthesis and cellular respiration Different electron donors Different electron acceptors But chemiosmosis is the same.

Light Reactions Summary ..\ppt lectures cd\animations\10_17LightReactions_A.swf

Calvin Cycle ..\ppt lectures cd\animations\10_18CalvinCycle_A.swf Animation - experiment animation2

Calvin Cycle An anabolic pathway, building sugar from smaller molecules and consuming energy Occurs in 3 phases: Phase 1 – carbon fixation Attaches CO2 one at a time to ribulose bisphosphate (RuBP) with rubisco Creates 6 carbon compound that is unstable Splits into 2, 3 carbon 3-phosphoglycerate molecules Phase 2 – reduction Phosphate from ATP added to 3-phosphoglycerate to make 1,3-phosphoglycerate Electrons from NADPH reduces 1,3-phosphoglycerate to G3P 1 G3P molecule generated for every 3 CO2 molecules used Phase 3 – regeneration of CO2 acceptor With use of energy from ATP, convert 5 G3P molecules into 3 RuBP molecules

Review

Alternative Forms of Photosynthesis When it is hot/dry, stomata close, CO2 scarce, O2 abundant, binds to rubisco, produces a 2 carbon compound instead of a 3 carbon compound, consumes ATP So what do plants do that live in hot/dry climates? Use an alternative form of the Calvin cycle

C4 Plants Common in members of the grass family Have different anatomy – bundle-sheath cells and mesophyll cells CO2 fixed into a 4 carbon intermediate before entering the Calvin cycle Forces CO2 into the bundle-sheath causing rubisco to bind CO2 instead of O2

CAM plants CAM (crassulacean acid metabolim) Occurs in water storing plants like cacti and pineapples. Close stomata during day, open at night (opposite of C3 plants) Helps prevent water loss Store organic acids during the night, Calvin cycle occurs during the day

Question Using your knowledge of photosynthesis, explain what would happen to the photosynthetic process in the following situations: 1. There is no available water 2. NADP+ is not efficiently regenerated 3. Something prevents cyclic-electron flow