PHOTOSYNTHESIS

PHOTOSYNTHESIS

A direct source of free energy and reduced C in: ! all higher plants ! many primitive plants and protists (all algae,

large and small) ! some marine animals that take up algae into

their cells ! some prokaryotes (cyanobacteria, green and

purple bacteria) An indirect source of free energy and reduced C in:

! animals ! fungi ! non-photosynthetic protists and bacteria ! anything that eats or parasitizes photosynthetic

organisms

Where did the carbon come from? Where did the energy come from?

Consider light reactions and light-independent reactions separately

Focus on the light reactions

Light capture: conversion of light energy tochemical energy

The main pigment of photosynthesis is chlorophyll

! Planar ring ! Conjugated double bonds ! Mg in center ! Phytol tail

Light

! Different wavelengths ! Different colors ! Different energies per

quantum (photon)

E/photon(kcal/mol)

72

52

41

Atoms/molecules

! Different structures! Different electron orbitals! Different energies per

orbital jump

Atoms/molecules

! Different structures ! Different electron orbitals ! Different energies per

orbital jump

The structure of chlorophyll has two major excited states and thus specifically allows absorption of blue and red photons

Why is chlorophyll green?

(Why is the actionspectrum different from the absorptionspectrum of chlorophyll a?)

Excited-state electrons can:

! fluoresce (release light photon) ! transfer energy to another electron ! move to nearby electron acceptor If the electron moves, light has "pumped" electron from chlorophyll to acceptor

Overview: in plants, there are two connected photosystemsEach has an excitable chlorophyllEach loses an electron

Look first at Photosystem I

Photosystem I: formation of NADPH from NADP+

NADPH carries2 electrons (like NADH)

Overview: in plants, there are two connected photosystemsEach has an excitable chlorophyllEach loses an electron

Look next at Photosystem II

Photosystem II: reduction of PSI, formation of oxygen

H2O provides the electron(s)to replace the one(s) lost by Photosystem II (P680)

Electron transport: formation of ATP

(Called “non-cyclic photophosphorylation” of ADP)

Focus on the light-independent reactions

Reduction of CO2

! Soluble enzymes in stroma add CO2 to sugar

! NADPH adds electrons! Free energy of NADPH oxidation and ATP hydrolysis push the reaction forward

Starch: glucose polymer

C6H12O6+ 6 H2O

6 CO2

12 H2O 6 O2

12 NADPH + H+12 NADP+18 ADP + Pi

18 ATP+ H2O

LightSummary

Free energy:! Light! NADPH and ATP! Glucose (reduced carbon)

Structure:! Organic compounds! C-C bonds

For the convenience of having a stable energy sourcein glucose and structural compounds, energy iswasted as heat.

Summary: energy transformations

Light

Heat

ADPNADP+

NADPHATP

C(H2O)O2

CO2H2O

CO2H2O

C(H2O)O2

ADPNAD+

NADHATP

SynthesisMovementIon pumpsGrowthReproduction

There are several types of bacterial photosynthesisGreen bacteriaPurple bacteria

Rhodopseudomonas viridis: cyclic photophosphorylation

H+

H+

ATP

e-

H+

P870

Was photosynthesis an early source of energy?

See next lecture