Download PHOTOSYNTHESIS

PHOTOSYNTHESIS
Where did the carbon come from? Where did the energy come from?
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
Consider light reactions and light-independent reactions separately
Focus on the light reactions
Light capture: conversion of light energy to
chemical 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 action
spectrum different
from the absorption
spectrum 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 photosystems
Each has an excitable chlorophyll
Each loses an electron
Look first at Photosystem I
Photosystem I: formation of NADPH from NADP+
NADPH carries
2 electrons
(like NADH)
Overview: in plants, there are two connected photosystems
Each has an excitable chlorophyll
Each 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
Summary
Free energy:
! Light
! NADPH and ATP
! Glucose (reduced carbon)
Structure:
! Organic compounds
! C-C bonds
Light
12 H2O
18 ADP
+ Pi
6 O2
18 ATP 12 NADP+12 NADPH + H+
+ H2O
6 CO2
For the convenience of having a stable energy source
in glucose and structural compounds, energy is
wasted as heat.
C6H12O6
+ 6 H2O
Summary: energy transformations
Light
ADP
NADP+
C(H2O)
O2
Heat
NADPH
ATP
CO2
H2O
C(H2O)
O2
CO2
H2O
ADP
NAD+
NADH
ATP
Synthesis
Movement
Ion pumps
Growth
Reproduction
There are several types of bacterial photosynthesis
Green bacteria
Purple bacteria
Rhodopseudomonas viridis: cyclic photophosphorylation
H+
eH+
ATP
H+
P870
Was photosynthesis an early source of energy?
See next lecture