Photosynthesis Outline Date:

Transcription

1 Photosynthesis Outline Date: Energy conversions Autotrophs o Producers o Photoautotrophs Heterotrophs o Consumers Plants Structures o Leaves o Stomates o Roots o Chloroplasts Double membrane Stroma Thylakoids Chlorophyll Pigments o Light o Types Chlorophyll Central atom: magnesium Carotenoids

2 Photosynthesis Reaction Oxygen comes from split water molecules o Provides electrons Two stages o Light reaction o Calvin cycle Redox reactions o Oxidation o Reduction Splitting water releases electrons and H + These bond to CO 2, reducing it to sugar Electrons increase the potential energy Light provides extra free energy o OIL RIG Light reaction o Thylakoids o Solar chemical energy o Net products: NADPH, ATP, oxygen o Main events Light absorbed Electrons transferred from H 2 O to NADP +, forming NADPH Water is split, O 2 released

3 ATP is generated through photophosphorylation o Light Electromagnetic energy made up of photons Pigments absorb light of different wavelengths Absorption spectrum Action spectrum o Photosystems Groups of pigment molecules in thylakoids; absorbs photons Light-harvesting complex: chlorophyll and carotenoid o Photons excite the electrons of chlorophyll Reaction center: chlorophyll only o Conversion of light chemical energy Thylakoids contain two photosystems, named photosystem I and photosystem II Linear (noncyclic) electron flow Predominant route Overview o Sunlight energizes electrons, generating ATP as they are passed PS II to PS I o Excited again in PS I, and transferred from NADP + NADPH o NADPH and ATP are used in the Calvin cycle Specific steps o 1: PS II absorbs light, an electron is excited and lost, and chlorophyll now has an electron hole

4 o 2: Enzyme splits H 2 O into 2 H +, 2 electrons, and O. The O combines with another O atom, forming atmospheric O 2. o 3: Original excited electron passes from PS I to PS II through an electron transport chain (similar to cell respiration). ATP is made through chemiosmosis. o 4: Energy from electron transfer pumps protons into the thylakoid space, creating a buildup (gradient) of H +. H + ions diffuse through ATP synthase, generating ATP to be used in the Calvin cycle. o 5: The electron from PS II ends up in PS I, which has just lost an electron due to light energy. o Excited electrons are passed to another electron transport chain, converting NADP + into NADPH. This will also be used in the Calvin cycle. Cyclic electron flow PS I only uses a short circuit of linear electron flow by cycling the electrons back to their original PS I starting point Creates equal ATP and NADPH, but the Calvin cycle requires more ATP Electrons are rerouted back to the electron transport chain from PS I to produce more ATP Uses chemiosmosis to produce ATP, but does not make NADPH, oxygen, and does not use water Chemiosmosis The way chloroplasts and mitochondria generate ATP Basic steps:

5 o Electron transport chain uses flow of electrons to pump H+ across thylakoid membrane. o A proton-motive force is created within the thylakoid space that is used by ATP synthase to convert ADP ATP (phosphorylation). It is generated in: H+ from water H+ pumped across the membrane Removal of H + from the stroma when NADP+ NADPH Dark reaction (Calvin cycle) o Overview Occurs in stroma Carbon fixation: CO 2 from air is used in organic molecules Uses fixed carbon, NADPH, and ATP from light reactions to form new sugars Actual product is glyceraldehyde 3-phosphate (G3P), which then forms glucose/sugar o Steps Carbon fixation: 3 CO 2 + RuBP (ribulose bisphosphate, a 5-carbon sugar) Catalyzed by rubisco (enzyme RuBP carboxylase) Reduction: 6 ATP and 6 NADPH used to produce 1 net G3P (glyceraldehydes 3-phosphate) One G3P leaves to be used by the plant cell (two are needed to form glucose) Regeneration: 3 ATP regenerate RuBP Remaining 5 G3P become starting molecules, and the cycle continues o Endergonic Reaction The formation of one net G3P requires 9 ATP and 6 NADPH, both replenished by light reactions One of the 6 total G3P that are made is a net gain, and will be used for biosynthesis or energy

6 Photosynthesis Alternatives Problem with photosynthesis and C 3 plants o CO 2 and H 2 O enter and exit via stomates (stomata) o Hot, dry climate close stomates, less sugar because of low CO 2 o Rubisco binds to O 2 in place of CO 2, causing breakdown (oxidation) of RuBP (starting molecule) Low RuBP = low energy and carbon = photorespiration This can drain up to 50% of the carbon fixed by the Calvin cycle Adaptations to arid climates metabolic adaptations reduce photorespiration o C 4 plants o CAM plants C 3 C 4 CAM Description Enzyme