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 stomataDiffuses into chloroplasts in mesophyll

cellsIn 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 reactions

Occur 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

Photosynthesis

Capturing Light Energy Pigments

Absorb 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 Photosynthesis depends on the

absorption of light by chlorophylls and carotenoids

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

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How Photosystem I and II Work

Light Dependant ReactionsPhotosystem II

Linear Electron Transport and ATP Synthesis

The Role of Light Energy Z scheme

Two photons of light needed for production of NADPH

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 Metabolism

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

CAM Plants Crassulacean Acid Metabolism

Run Calvin Cycle and C₄ at different time of the day○ C₄ - night○ Calvin Cycle – day