RITGenBio2 Chapter 8

Overview

 * Occurs in chloroplast
 * Produces glucose (carb) from light
 * 6CO2 + 12H2O + light --> C6H12O6 + 6O2 + 6H2O
 * Plants, Algae, Bacteria, Potists can do it (water & land)
 * Carbs make ATP (cellular respiration)

Light Dependent

 * Get the energy out of light

Light Independent

 * Use the energy to build sugar
 * Calvin Cycle

Photons

 * Short wavelength = more energy (indigo, violet)
 * Long wavelength = less energy (red, orange)
 * Light can be reflected, transmitted, or absorbed
 * Photosynthesis absorbs light
 * Pigment = any molecule that can absorb light


 * Each light has its own wavelength that causes pigments to become ?excited?
 * An "excited" pigment means an "excited" electron


 * Nothing can stay at a high energy level very long
 * Possible outcomes of photon absorption
 * Heat
 * Fluorescence (visible light - another photon is emitted)
 * Reduction Photosynthesis

Plants absorb photons of visible light

 * violet and indigo are absorbed and most useful
 * green is reflected
 * red and orange are absorbed but not as useful

Light Dependent Phase

 * "Getting energy from light"
 * Cells contain chloroplasts
 * Chloroplast membranes contain pigments that can absorb different wavelengths of light
 * Chlorophyll gets excited by photon
 * electron is passed down an electron transport chain along carrier proteins through redox reactions

< "Location of photosynthesis" picture in textbook ch 8 >

Thylakoid membrane

 * Pigments
 * Chlorophyll: reflects green light; absorbs violet, blue and red
 * a: most abundant
 * b, c, and d less so
 * Carotenoids: reflect red, yellow, orange; basorb violet and blue
 * Helps eliminate free radicals (eg. Beta carotene)
 * Xanthophylls
 * Oxidized carotenses
 * Reflect red and yellow
 * Phycoerythrin
 * Reflects red
 * red algae
 * Phycocyanin
 * Reflects blue
 * eg. Cyanobacteria aka blue-green algae

Steps

 * 1) Photolysis
 * 2) * Sunlight goes into the Thy. Membrane
 * 3) * Breaks apart a water molecule
 * 4) * Chlorophyll gets its electron back and releases O2
 * 5) * H2O --> 2e- + 2H+ + ½O2
 * 6) Excite Electron and extract energy
 * 7) * Electron reduces
 * 8) ** NaDP+ --> NADPH
 * 9) * Electron flow creates ATP via a [ H+ ] gradient in the thylakoid compartment

Light Independent Phase

 * Using the energy to make sugar via the Calvin-Benson cycle
 * Occurs in chloroplast stroma (big space inside chloroplast)
 * CO2 gas is "fixed" - grabbed and held down to build glucose
 * Hydrogens are added to make carbohydrate (glucose)
 * Energy source is ATP and NADPH from light reactions

3 Phases of the Calvin cycle

 * 1) Fixation of the CO2
 * 2) * Fixated to a 5 carbon molecule called ribulose biphosphate (RuBP) by an enzyme called rubisco
 * 3) * Leads to an unstable 6-carbon compound that breaks down into two 3-carbon molecules called phosphoglycerate (PGA)
 * 4) Reduction of CO2 into carbohydrate
 * 5) Regeneration of the CO2 acceptor RuBP

< Calvin Cycle diagram in the book >

Summary:

 * PGA enters the cycle and changes to PGAL
 * 12 PGA are needed to produce one 6 carbon sugar, the rest is recycled to produce RuBP
 * ADP and NADP+ are cycled back into the light dependent phase

Rubisco is the most abundant protein in the world - 20% of all protein in leaves RuBP-CO2 RuBP-O2

Photorespiration:

 * When Rubisco binds to O2 - Bad because:
 * ATP and NADPH are used up without making sugar
 * CO2 is released instated of fixated
 * Decreases food production

3 Plant Types
< See table 8.1 in text book: Three modes of Photosynthesis >

C4 Plants - Avoiding photorespiration

 * Hot climates
 * grasses, corn, sugarcane
 * C4 plants have an additional enzyme in mesophyll cells near the surface of the leaf. PEP carboxylase
 * CO2 + PEP --> Oxaloacetate (4 carbon)
 * Oxaloacetate diffuses to the interior of the leaf
 * Decarboxylation of oxaloacetate ?drops off? CO2 around Rubisco in bundle sheath cells
 * Why don't all plants do C4 pathway?
 * Costs ATP to shuttle CO2

CAM plants - Another way to avoid photorespiration and conserve water

 * Crassulaceae (Jade) acid metabolism
 * Succulents
 * Hot and dry climates
 * Cactus, pineapple, mint, and orchid
 * Close tomata during the day, open them at night
 * Fix CO2 by PEP carboxylase, but at night
 * Light reactions generate ATP and NADPH. The Calvin cycle runs using the CO2 saved up during the night

C3 plants (regular ones)

 * Rubisco will fix O2 instead of CO2 when conditions are hot and dry
 * First product of carbon fixation is always 3 carbon 3PGA