PHOTOSYNTHESIS PLANTS. Feeding, clothing, sheltering and medicating the world for millions of years.

Photosynthesis A biochemical pathway where autotrophs use energy from sunlight (light energy) to produce organic compounds (chemical energy) 6CO2 + 6H2O -> C6H12O6 + 6O2 2 stages: 1. Light reactions 2. Calvin cycle

Visible Spectrum White light is composed of a variety of colors – visible spectrum Pigments absorb light Light that is not absorbed is reflected

Pigments Chlorophyll (a and b) Plants are green because green wavelengths of light are REFLECTED Chlorophyll a is responsible for absorbing the energy from sunlight Chlorophyll b is an accessory pigment; also carotenoids (yellow, orange and brown)

Photosynthesis depends upon green pigment CHLOROPHYLL (absorbs light in the blue-violet and orange-red and reflects light in green region Accessory pigments help harvest light energy

CHLOROPLAST STRUCTURE Surrounded by double membrane PLUS an inner membrane system: THYLAKOIDS – flattened sacs GRANA – stacks of thylakoids Light harvesting pigments embedded in thylakoid membrane Surrounding thylakoids, liquid: STROMA

LIGHT DEPENDENT RXNS Visible light (traveling in “photons”, packets of energy) is changed into chemical energy H2O is split into O2 and H PS I and II absorb light energy This light energy is transferred to reaction center, a Chlorophyll a that donates e- to electron carriers

At end of electron flow, electrons combine with NADP+ to form NADPH Lost e- from PSII is replaced by e- from H2O As electrons flow along electron transport chain, protons build up inside thylakoids These built up protons will diffuse down concentration gradient through ATP synthase

TAH-DAH!! ATP is made!!! PRODUCTS: O2, ATP and NADPH!!!! ATP and NADPH area both needed for the Calvin Cycle

Chemiosmosis Electrochemical gradient exists between the stroma and the thylakoid interior As protons diffuse through ATP Synthase, ATP is made Where do these protons come from?

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CALVIN CYCLE Requires ATP and NADPH from light rxns Occurs in stroma 1 molecule of CO2 combines with RuBP (5 carbon compound) = “carbon fixation” Resulting 6C molecule splits into 2- 3 carbon molecules of PGA

Each PGA, using 1 ATP and 1 NADPH makes one PGAL Each PGAL converted into various 4, 5 and 6 carbon sugars RuBP is recycled 3 turns of the cycle -> 6 PGAL, 5 -> RuBP, 1 for other sugars Rubisco is the enzyme which catalyzes rxn that fixes CO2 Calvin cycle AKA C3 cycle

Photosynthesis depends upon green pigment CHLOROPHYLL (absorbs light in the blue-violet and orange-red and reflects light in green region Accessory pigments help harvest light energy

CALVIN CYCLE CO2 converted to G3P; ATP and NADPH are consumed To make one molecule of G3P, three turns of the cycle + 3 CO2 molecules CO2 is fixed to RuBP by rubisco – produces unstable intermediate -> 3-phosphoglycerate

3-phosphoglycerate is phosphorylated -> 1,3-diphosphoglycerate NADPH reduces 1,3-phosphoglycerate to glyceraldehyde-3-phosphate RuBP is regenerated – one per each trip through the cycle Uses 9 ATP and 6 NADPH

C4 FIXATION First carbon compound formed has 4 carbons instead of three Bundle sheath cells – grouped around veins CO2 added to PEP (a 3-C compound) phosphoenolpyruvate to form oxaloacetate (catalyzed by PEP carboxylase) Oxaloacetate exported to bundle sheath cells, which break it down into CO2 molecules CO2 then converted to carbs through Calvin cycle (occurs only in bundle sheath cells)

CAM Photosynthesis Adaptation to hot dry climates Stomates open at night and closed during day to minimize water loss CO2 taken in at night, converted to various organic molecules and stored in vacuoles In morning, stomates close and plants release CO2 for normal photosynthesis

Rate of Photosynthesis 4 limiting factors: Light intensity, temperature, [CO2], [O2] Active site of Rubisco can bind to O2 or CO2: Photorespiration – results in release of previously fixed CO2 that would otherwise remain in organic form

RATES OF PHOTOSYNTHESIS As light intensity increases, so does rate of photosynthesis Levels off at a max rate, when all electrons are excited Same thing for CO2 levels Temperature increase, rate increases to a point; then, enzymes denature and stomates close to prevent water loss, thus decreasing rate at high temperatures