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

• Photosynthesis takes place in mesophyll tissue
• Cells containing chloroplasts
– Specialized to carry out photosynthesis
• CO
2 enters leaf through stomata (pore)
– Exchange of gases occurs here
– Controlled by guard cells
(opening/closing)
• CO
2 diffuses into chloroplasts
• CO
2 fixed to C
6
H
12
O
6
(sugar)
• Energy supplied by light

• 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

• 2 stages
1. 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 (lightindependent reactions)
– Occurs in stroma of chloroplast
– use ATP and NADPH to synthesize organic molecules from CO
2

• 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

• 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

• Photosynthesis depends on the absorption of light by chlorophylls and carotenoids

• Chlorophylls and carotenoids do not float freely within thylakoid
• Bound by proteins
• Proteins are organized into photosystems
• Two types
– Photosystem I
– Photosystem 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

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

1.
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

• Z scheme
– Four photons of light needed for production of NADPH
– Single e⁻ moves through Z scheme at a time
– 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⁺

• How many photons of light are needed to produce a single molecule of oxygen?
– 2 H₂O → 4 H⁺ + 4 e⁻ + O₂

• 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

• 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

• Carbon Fixation
– Series of 11 enzymecatalyzed 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

• 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

• Two 3PG is phosphorylated
– ATP is used
• Molecule is reduced by
NADPH
• Two G3P are produced

• 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)

• 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

• 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)

• 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

• Can open stomata less
• Require 1/3 to
1/6 as much rubisco
• Lower nitrogen demand
• Run C3 and C4 cycles simultaneously
• Corn

• Crassulacean
Acid
Metabolism
– Run Calvin
Cycle and
C4 at different time of the day
– C4 - night
– Calvin Cycle
– day
• Cactus