photosynthesis
DESCRIPTION
Chapter 5. Photosynthesis. http://www.youtube.com/watch?v=sQK3Yr4Sc_k. Photosynthesis. Converting light energy into chemical energy to assemble organic molecules Two stages Light-dependant reactions Absorption of photons of light PI and PII Light-independent reactions (Calvin Cycle) - PowerPoint PPT PresentationTRANSCRIPT
PHOTOSYNTHESIS Chapter 5
http://www.youtube.com/watch?v=sQK3Yr4Sc_k
Photosynthesis Converting light energy
into chemical energy to assemble organic molecules
Two stagesLight-dependant reactions
○ Absorption of photons of light○ PI and PII
Light-independent reactions (Calvin Cycle)○ Does not require light
6CO2 + 6H2O → C6H12O6 + 6O2
Photosynthesis Photosynthesis takes place in the green
portions of plantsLeaf of flowering plant contains
mesophyll tissueCells containing chloroplastsSpecialized to carry on photosynthesis
CO2 enters leaf through stomata
Diffuses into chloroplasts in mesophyll cells
In stroma, CO2 fixed to C6H12O6 (sugar)
Energy supplied by light
Chloroplasts Photosynthesis takes place
Both stages Consists of
Stroma○ Aqueous environment ○ Houses enzymes used for
reactionsThylakoid membranes
○ Form stacks of flattened disks called grana
○ Contains chlorophyll and other pigments
Photosynthetic Stages
Light-dependant reactionsOccur in the thylakoid membranes capture energy from sunlightmake ATP and reduce NADP+ to
NADPH Carbon fixation reactions
(light-independent reactions)Occurs in the stromause ATP and NADPH to synthesize
organic molecules from CO2
Capturing Light Energy
PigmentsAbsorb photonExcited electron moves to a
high energy stateElectron is transferred to an
electron accepting molecule – primary electron acceptor
Pigments Chlorophyll a
Donates electron to PEA Accessory pigments
Chlorophyll bCarotenoidsKnown as antenna complex
○ Transfers light energy to chlorophyll a
A pigment molecule does not absorb all wavelengths of light
Pigments and Photosystems Chlorophylls and carotenoids do not
float freely within thylakoid Bound by proteins Proteins are organized into
photosystems Two types
Photosystem IPhotosystem II
Photosystem I and II Composed of
Large antenna complex250-400 pigment
molecules surrounding reaction centre
Reaction CentreSmall number of proteins
bound to chlorophyll a moelcules and PEA
PI - Contains p700 PII - Contains p680
How Photosystem I and II Work
Trap photons of light Energy trapped used to energize chlorophyll a
molecule in reaction centre Chlorophyll a is oxidized (loses electrons)
Transfers electrons to PEAWater molecule donates electron chlorophyll a lost
Transported through electron transport chain High energy electrons are used to drive ATP
synthesis and assemble glucose molecules http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Phot
osynthetic%20Electron%20Transport%20and%20ATP%20Synthesis
Oxygen
How many photons of light are needed to produce a single molecule of oxygen?2 H₂O → 4 H⁺ + 4 e⁻ + O₂
Cyclic Electron Transport PI can function independently from PII
Produces additional ATP molecules ○ Reduction of CO₂ requires ATP
Light-Independent Reactions Carbon Fixation Series of 11 enzyme-
catalyzed reactionsNADPH reduces CO₂ into
sugarsOverall process is endergonic ATP is hydrolyzed to supply
energy of reactions Divided into three phases
FixationReductionRegeneration
Calvin Cycle: Fixation: C₃ Metabolism CO2 is attached to 5-carbon RuBP
molecule
Result in a 6-carbon molecule
This splits into two 3-carbon molecules (3PG)
Reaction accelerated by RuBP Carboxylase (Rubisco)
CO2 now “fixed” because it is part of a carbohydrate
Calvin Cycle: Reduction
Two 3PG is phosphorylatedATP is used
Molecule is reduced by NADPH
Two G3P are produced
Calvin Cycle: Regeneration RuBP is regenerated for cycle to
continueTakes 3 cycles Produces 3 RuBP molecules
Process (3 turns of cycle)3CO₂ combine with 3 molecules of RuBP6 molecules of 3PG are formed6 3PG converted to 6 G3P5 G3P used to regenerate 3 RuBP molecules1 G3P left over
G3P Ultimate goal of photosynthesis
Raw material used to synthesize all other organic plant compounds (glucose, sucrose, starch, cellulose)
What is required to make 1 molecule of G3P?9 ATP6 NADPH
What is required to make 1 molecule of glucose?18 ATP12 NADPH2 G3P
Alternate Mechanisms of Carbon Fixation Problems with photosynthesis
Not enough CO₂ - 0.04% of atmosphere
Rubisco ○ can also catalyze O₂○ Slows Calvin Cycle, consumes
ATP, releases carbon – photorespirationDecrease carbon fixation up to 50%
Stomata○ Hot dry climates ○ Low levels of CO₂
C₄ Plants Minimize photorespiration Calvin Cycle
Performed by bundle-sheath cells○ Separates exposure of Rubisco to O₂
C₄ CycleCO₂ combines with PEP (3 carbon molecule)Produces oxaloacetate (4 carbon molecule)Oxaloacetate reduced to malateMalate diffuses into bundle-sheath cells and enters
chloroplastMalate oxidized to pyruvate releasing CO₂
C₄ vs C₃ C₄
Can open stomata lessRequire 1/3 to 1/6 as much
rubiscoLower nitrogen demand Run Calvin cycle and C₄ cycles
simultaneously