Photosynthesis • Conversion of

light energy from the sun into stored chemical energy in the form of glucose and other organic molecules

Site of Photosynthesis• Photosynthesis takes place

in mesophyll tissue• Cells containing chloroplasts

– Specialized to carry out photosynthesis

• CO2 enters leaf through stomata (pore)– Exchange of gases occurs

here– Controlled by guard cells

(opening/closing)

• CO2 diffuses into chloroplasts

• CO2 fixed to C6H12O6 (sugar)

• Energy supplied by light

Chloroplasts• Site of Photosynthesis• Consists of • Stroma– Aqueous environment– Houses enzymes used

for reactions• Thylakoid membranes– Form stacks of flattened

disks called grana– Contains chlorophyll

and other pigments

Photosynthesis • 2 stages1. Light-dependant reactions

– Photosystem II and I– Occurs in the thylakoid

membrane of chloroplasts– capture energy from sunlight– make ATP and reduce NADP+

to NADPH

2. Calvin Cycle (light-independent reactions)– Occurs in stroma of

chloroplast– use ATP and NADPH to

synthesize organic molecules from CO2

Capturing Light Energy • Pigments– Absorb photon (wave

of light)– Excited electron moves

to a high energy state– Electron is transferred

to an electron accepting molecule (primary electron acceptor)

• Chloryphyll a – donates electrons to

PEA

Accessory Pigments• Chlorophyll b and carotenoids

– Known as antenna complex– Transfers light energy to chlorophyll a– Chlorophyll donates electrons to PEA

• 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 I– Photosystem II

Photosystem I and II• Composed of

– Large antenna complex

– 250-400 pigment molecules surrounding reaction centre

• Reaction Centre– Small number of

proteins bound to chlorophyll a molecules and PEA

• PI - Contains p700• PII - Contains

p680

Photosystem II 1. Oxidation of p680– Photon absorbed excites p680– Transfers e⁻ to PEA– e⁻ supplied by splitting of a

water molecule inside lumen

2. Oxidation-reduction of plastiquinone– PEA transfers e⁻ to

plastiquinone • Plastiquinone

– shuttles electrons between PII and cytochrome complex

– responsible for increase proton concentration in thylakoid lumen

3. Electron transfer to PI– Cytochrome complex transfers

e⁻ to plastocyanin• Plastocyanin

– Shuttles electrons from cytochrome complex to PI

Photosystem I1. Oxidation-reduction of p700– Photon absorbed excites

p700– p700 transfers electron to

PEA– P700⁺ forms ready to

accept another e⁻ from plastocyanin

2. Electron transfer to NADP⁺ by ferredoxin– PEA transfer e⁻ to

ferredoxin• Ferredoxin

– Iron-sulfur protein– Oxidation of ferredoxin

reduces NADP⁺ to NADP

3. Formation of NADPH– Ferredoxin transfers

second e⁻ and H⁺– NADP⁺ reductase

reduces NADP to NADPH

Linear Electron Transport and ATP Synthesis

The Role of Light Energy • Z scheme– Two photons of light needed for production of NADPH– p700 molecule too electronegative to give up e⁻– Second photon needed to move e⁻ further away from nucleus of

p700 so it can transfer to NADP⁺

Oxygen• How many photons of light are needed to

produce a single molecule of oxygen?– 2 H₂O → 4 H⁺ + 4 e⁻ + O₂

Chemiosmosis and ATP Synthesis• Proton gradient inside lumen increases

– e⁻ transfer by plastoquinone between PII and cytochrome complex– Water molecule splitting inside lumen – Removal of H⁺ from stroma for each NADPH molecule produced

• Proton-motive force created inside thylakoid lumen • ATP synthase uses proton-motive force to synthesize ATP

molecule

Cyclic Electron Transport• PI can function independently from PII• Ferredoxin does not transfer e⁻ to NADP⁺• Ferredoxin transfers e⁻ back to plastoquinone• Plastoquinone continually moves protons into thylakoid lumen• Splitting of water molecule not needed • Produces additional ATP molecules (photophosphorylation)

– Reduction of CO₂ requires ATP – Occur during drought (no water) or abundance of NADPH

Light-Independent Reactions• Carbon Fixation

– Series of 11 enzyme-catalyzed reactions

– NADPH reduces CO₂ into sugars

– Overall process is endergonic

– ATP is hydrolyzed to supply energy of reactions

• Divided into three phases– Fixation– Reduction– Regeneration

Calvin Cycle: Fixation• CO₂ is attached to 5C

RuBP molecule• 6C molecule is produced– 6C splits into 2 3C

molecules (3PG)• RuBisco– RuBP carboxylase– Most abundant protein

on earth– Involvd in first major step

of carbon fixation • CO₂ is now fixed– Becomes part of

carbohydrate

Calvin Cycle: Reduction• Two 3PG is

phosphorylated– ATP is used

• Molecule is reduced by NADPH

• Two G3P are produced

Calvin Cycle: Regeneration• RuBP is regenerated for cycle to continue

– Takes 3 cycles – 3 molecules of CO₂– Produces 3 RuBP molecules

• Process (3 turns of cycle)– 3CO₂ combine with 3 molecules of RuBP– 6 molecules of 3PG are formed– 6 3PG converted to 6 G3P– 5 G3P used to regenerate 3 RuBP molecules– 1 G3P left over (This process occurs 2x – 6CO₂ found in reactants)

Glyceraldehyde-3-phosphate (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 ATP– 6 NADPH

• What is required to make 1 molecule of glucose?– 18 ATP– 12 NADPH– 2 G3P

Alternate Mechanisms of Carbon Fixation • Problems with photosynthesis in C₃

plants • Not enough CO₂ - 0.04% of atmosphere• Rubisco

– can also catalyze O₂– Slows Calvin Cycle, consumes ATP,

releases carbon (photorespiration)• Decrease carbon fixation up to 50%

– Wasteful to cell – Costs 1 ATP and 1 NADPH

• Stomata– Hot dry climates – closes to prevent

water loss – Low levels of CO₂

• Instead of plant producing 2 G3P molecules

• Plant produces 1 G3P molecule and 1 phosphoglycolate (toxic)

C₄ Cycle• Minimize photorespiration• Calvin Cycle

– Performed by bundle-sheath cells• Separates exposure of Rubisco to O₂

• C₄ Cycle– CO₂ combines with PEP (3 carbon molecule)– Produces oxaloacetate (4 carbon molecule)– Oxaloacetate reduced to malate– Malate diffuses into bundle-sheath cells and enters chloroplast– Malate oxidized to pyruvate releasing CO₂

Benefits of C4 Plants • Can open

stomata less• Require 1/3 to

1/6 as much rubisco

• Lower nitrogen demand

• Run C3 and C4 cycles simultaneously

• Corn

CAM Plants• Crassulacean

Acid Metabolism– Run Calvin

Cycle and C4 at different time of the day

– C4 - night– Calvin Cycle

– day • Cactus