Photosynthesis- conversion of light energy tochemical energy

Where does it occur

What are the pathways

What are the adaptations to hot, dry climates

Autotrophs require only CO2, water and light

(Where does the CO2 come from?)

Terrestrial plantsAlgaeSome protists (e.g., Euglena)CyanobacteriaPurple sulfur bacteria

Many kingdoms represented!

How does photosynthesis occur in plants?

Primarily in leaves, in chloroplasts

Interior tissue of leaf: mesophylldozens of chloroplasts per cell

Stomata let CO2 in and O2 out

Veins provide transport throughout plantwater from roots to leavessugar from leaves to other structures

How does photosynthesis happen?

6CO2 + 12H2O + light C6H12O6 + 6O2 + 6H2O

Glucose is eventually formed from 3-carbonproducts

The oxygen is produced from the splitting ofwater, not carbon dioxide

Confirmed using heavy oxygen

Water is oxidized, CO2 is reduced

Localization of photosynthesis reactions

What does NADPH do? ATP?

How is light energy captured?

Different wavelengths of light contain differentamounts of energy (photons)

Visible light ranges from approx. 380-750 nm

Pigments absorb some wavelengths of lightand reflect others

(refer to pp. 183-4 in anticipation of lab!)

Chlorophyll a directly initiates light reactions

Other pigments absorb light, transfer energyto chlorophyll a

chlorophyll bcarotenoids

May have a “dampening effect” (photoprotection)

When photons are absorbed by pigmentstheir energy is transferred to an electron

Electron falls back to ground state, givingoff heat

Some pigments (such as chlorophyll) alsogive off light (fluorescence; chlorophyllfluoresces red)

Fluorescent “tags” have many applications

Photosystems are specialized to harvestenergy from light

Two types of photosystems

Photosystem I- P700

Photosystem II- P680

Electron flow: cyclic or (predominantly)non-cyclic

Summary: when photosystem II absorbs light itbecomes an oxidizing agent- oxidizes waterand replaces electrons

Electrons fall down the electron transport chainand energy is used to form ATP(photophosphorylation)

Light hits photosystem I and releases electronselectrons from photosystem II “replace themNADPH is generated

Cyclic phosphorylation

When is this pathway used?

When ATP is scarce (Calvin cycle uses moreATP than NADPH)

How are these reactions organized in thethylakoid membrane?