Download LEAVES AND PHOTOSYNTHESIS

LEAVES AND PHOTOSYNTHESIS
Leaf
Shapes
dicots:
reticulate
venation
Leaf
Shapes
monocots: parallel venation
(protection)
(photosynthesis)
(gas exchange)
Photosynthesis
--inputs: light, CO2, (H2O)
--outputs: O2, carbohydrate (sucrose, starch)
6CO2 + 6H2O ---red, blue light---> C6H12O6 + 6O2
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
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)
Summary (so far): production of O2 and synthesis of NADPH
Electron transport: formation of ATP
H+ transport into the thylakoid lumen
H+ return through ATP synthase
(Called “non-cyclic photophosphorylation” of ADP)
Electron transport: formation of ATP:
works even if no O2 or NADPH is produced
(Called “cyclic photophosphorylation” of ADP)
Reduction of CO2
! Soluble enzyme in stroma (“Rubisco”) adds CO2 to
RuBP (a sugar)
! NADPH adds electrons
! Free energy of NADPH oxidation and ATP
hydrolysis push the reaction forward
Why is cyclic photophosphorylation needed?
• Non-cyclic photophosphorylation makes one
ATP per NADPH
• Calvin cycle uses 18 ATP per 12 NADPH
•
C yclic photophosphorylation makes the rest
of the ATP
Photosynthesis
--inputs: light, CO2, (H2O)
--outputs: O2, carbohydrate (sucrose, starch)
6CO2 + 12H2O ---light---> C6H12O6 + 6O2 + 6H2O
PGAL
Rubisco