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Photosynthesis Photosynthesis • Process by which plants use light energy to make food molecules from CO2 and H2O – 6 CO2 + 6 H2O C6H12O6 + 6 O2 enzymes and light – Plants are autotrophs- self feeder- and are the basis of all food chains and webs on earth – Require sunlight to produce own food • Photosynthesis occurs in chloroplasts – Chlorophyll absorbs light energy – Concentrated in cells of mesophyl-green tissue in interior of leaf • Each mesophyl cell has numerous chloroplasts – Structure of chlorophyll • Has outer membrane and inner membrane • Inner membrane encloses thick material called stroma – Stroma is site of sugar synthesis from CO2 • Suspended in the stroma are elaborate, disc-like membranous sacs called thylakoids – Thylakoids are concentrated in sacs called grana » Chlorophyll molecules embedded into thylakoid membranes capture light energy » Also in membrane is the machinery for converting light energy to chemical energy • Conversion reactions (fig 7.3c) – Plants split O2 off of CO2 molecules – Photosynthesis is a redox process and happens in stages • First stage- light reaction – Absorption of light energy – e- are excited and pass from reaction-center chlorophylls to primary e- receptors » In turn, each primary acceptor is oxidized as it donates high energy e- to the first e- carrier of etransport chain – Additional redox reactions shuttle e-’s from one carrier molecule to the next, releasing ATP and NADPH – NADPH, ATP and O2 gas are the products of the light reactions » ATP is released through a chemiosmosis process called photophosphorylation • Second stage: Calvin’s cycle (fig 7.10 a, b) – Series of reactions that assemble sugar molecules using CO2, ATP, and NADPH – Steps » Carbon fixation- rubisco (RuBP) enzyme combines 3 molecules of CO2 + 3 molecules of a 5C sugar • End result- 6 3C acids (3-PGA) » Energy consumption and redox reactions: 2 chemical reactions consume energy from 6ATP and oxidize 6 NADPH. 6 molecules of 3-PGA are reduced • End result- 6 molecules of G3P, and energy rich molecule » Release of 1 molecule of G3P leaves 5 G3P’s in cycle. The single molecule of G3P leaving the cycle is the net product of photosynthesis. Plant uses 2 G3P molecules to make 1 glucose. It takes 6 complete turns of the cycle to make 2 G3P to make 1 glucose molecule » Regeneration of rubisco- a series of chemical reactions uses ATP to rearrange 5 G3P to make 3 RuBP to start another turn of the cycle. • Plants use sugars for fuels for cellular respiration and as starting materials for making other organic molecules – Most plants make much more than what they need so they store the excess in roots, tubers, fruits, and chloroplasts • Plants use CO2 directly from the air for the Calvin cycle – Dry weather can reduce rate of photosynthesis • Closing stomata reduces H2O loss, but also cuts off CO2 – O2 levels will build up and RuBP incorporates O2 instead of CO2 – Calvin cycle will produce a 2-C compound instead of a 3C – Plant then breaks down 2C compound into CO2 and H2O – Process is called photorespiration and yields no sugars • C plants have special adaptations to save H2O and prevent photorespiration – When hot and dry, stomata stay shut most of the time to conserve H2O • Continues to make sugars through photosynthesis using an enzyme that fixes C into a 4-C compound – Plant can continue to fix C even when [CO2] is low – 4C compound acts as a carbon shuttle by donating CO2 to calvin cycle in a nearby cell • CAM plants conserve H2O by opening stomata only at night – As CO2 enters leaves, it is fixed into a 4C compound and banked at night – Leaf photosynthesizes during the day without admitting any more CO2