Photosynthesis
Chapter 10

Objectives
 Compare
the overall reaction of photosynthesis with
the overall reaction for respiration
 Describe where the processes of photosynthesis
occur
 Describe the basic processes involved in
photosynthesis: water splitting to obtain electrons,
redox reactions of the electron transport chains,
electron and energy shuttling by means of ATP and
NADPH, and the coupling of the light-dependent
reactions and the Calvin cycle
 Explain
how pigments capture light and excite
electrons
 Describe the structural and functional differences
between photosystem I and photosystem II, and
cyclic and noncyclic photophosphorylation
 Outline the steps in the cyclic fixation of carbon in
the Calvin cycle and where these occur
 Contrast the C4 and CAM photosynthetic systems
with the simpler C3 system, and how they are
adaptions to hot, dry climates
Introduction

Overall equation is reversal of cellular
respiration
 6CO2+12H2O+energy--->C6H12O6+6O2+6H2O

Increasingly probing studies provided
knowledge about how photosynthesis works
 van
Helmont-developed early ideas about
where plants obtain materials for growth
 showed
that soil not sufficient
 concluded that water important
that plants restore “bad” air
 Ingenhousz-plants only restore air when
exposed to light
 Priestly-showed
Autotrophs are Producers

Autotroph-means self-feeding
 applies
to any organism that makes own food
without eating, decomposing or absorbing other
organisms or organic molecules

Photosynthetic autotrophs include plants,
algae and photosynthetic bacteria
Site of Photosynthesis
Photosynthesis occurs in chloroplasts in all
photosynthetic organisms except monerans
 Leaves (specifically, mesophyll cells) are
primary site of photosynthesis
 Light-absorbing pigment is chlorophyll

 located
in protein complexes in internal
membranes of chloroplasts

Sugars assembled in stroma
Underlying Processes

Oxygen produced by splitting water
 demonstrated
using 18O-labeled reactants
given C18O2 does not release 18O2
 plant given H218O does give off 18O2
 plant

Photosynthesis is redox process
oxidized--->1/2O2+2H++2e CO2 reduced to glucose by addition of e-’s and
H+’s
 compare with respiration where glucose
oxidized and O2 reduced
 H2O
In photosynthesis, electrons travel “uphill”
from water to glucose, adding light energy
captured by chlorophyll
 In respiration, electrons travel “downhill”
from glucose to water, releasing energy to
ATP

Overview

Photosynthesis is a two-stage process
 light-dependant
reactions
 convert
light energy to chemical energy, releases O2
as waste product
 occurs in thylakoid membranes and produces energy
shuttles ATP and NADPH
 Calvin
cycle
 cyclic
series of steps that assemble organic
molecules from CO2
 occur
in stroma and use energy and electrons from
ATP and NADPH in carbon fixation
 light not required but usually run during day as
require shuttles from light-dependant reactions
The Light Reactions

Driven by visible light
 light
is electromagnetic radiation
 only small fraction of em radiation perceived
by organisms
 different
wavelengths=different colors
 leaf
absorbs some wavelengths (red-orange and
blue-violet) and reflects others (green)
 in
plants light absorbed by chlorophyll a,
chlorophyll b and carotenoids

only chlorophyll a directly involved in light
reactions; other pigments act as “antenna”
molecules to broaden range of energy
absorbed
The Photosystems
Light behaves like particles-photons
 When pigment absorbs photon, energy level
of one electron is raised to excited, unstable
state

 if
pigment is isolated from molecular
environment, excited electron loses energy as
heat or light and returns to normal level
 chlorophyll
fluoresces red

In chloroplasts, 200-300 chlorophyll
molecules grouped with proteins to form
antenna assembly around two chlorophyll a
molecules-reaction center chlorophylls
 excited
electrons passed from antenna
chlorophylls to reaction center chlorophylls
then to primary electron acceptor
 series
of redox reactions
• final is oxidation of reaction center chlorophyll and
reduction of primary electron acceptor

Two photosystems (antenna
assembly+primary electron acceptor)
identified
 absorb
at different wavelengths
 photosystem
I-absorbs maximally at 700nm (P700)
 photosystem II-absorbs maximally at 680nm (P680)
 function
together to carryout non-cyclic
electron transport
 also
known as non-cyclic photophosphorylation
 photosystem
I can also carryout cyclic electron
transport (cyclic photophosphorylation)
 thought
to be the earliest form of photosynthesis
• present in many primitive photosynthetic bacteria
 synthesizes
only ATP
Chemical Energy Generation

Electron transport chains generate ATP,
NADPH and O2
 kinetic
energy of light absorbed and excites
electrons
 excited electrons passed along electron
transport chain-series of redox reactions
 released energy used to generate ATP, NADPH
and O2
 production
 supplied
of NADPH requires 2 electrons
to PS I by PS II
 replaced in PS II by splitting water
 H2O ---> 1/2O2 + 2H+ + 2e-
Chemiosmosis

Powers ATP synthesis
 H+
ions from splitting water and those pumped
across thylakoid membrane by electron
transport chain form gradient across thylakoid
membrane (inside to outside)
 ATP synthase provides port for H+ to diffuse
back into stroma
 releases
energy and phosphorylates ADP to ATP
 similar process to ATP generation in mitochondria
 known as photophosphorylation
Carbon Fixation

ATP and NADPH from light-dependant
reactions power Calvin cycle
 net
result of Calvin cycle is 3C molecules from
CO2 using energy and electrons in ATP and
NADPH from light-dependant reactions
 CO2 added to 5C intermediate ribulose-1,5bisphosphate (RuBP)
 catalyzed
by RuBP carboxylase/oxygenase (rubisco)
 Number
of rearrangements occur in many steps,
using energy in ATP and oxidation of NADPH
 last
step in cycle regenerates RuBP
 all steps occur simultaneously but ultimately
regenerate starting reactants, hence cycle
Three RuBP enter cycle for each 3C
molecule released from chloroplast
 Calvin cycle occurs in chloroplast stroma
 3C molecules exported to cytoplasm

 used
to synthesize glucose and other organic
molecules

Plants that use only Calvin cycle to fix
carbon called C3 plants
 first
identifiable product of carbon fixation is
3C molecule
Carbon-fixing Variations

C3 plants conserve water by closing stomata
 allows
buildup of O2 in leaves
 Rubisco fixes O2 rather than CO2
 called photorespiration
 uses
ATP and NADPH but makes no sugars
 C4
plants adapted to conserve water and prevent
photorespiration
 CO2
incorporated into 4C molecule in mesophyll
cells
 diffuses into bundle sheath cells and released
 enters Calvin cycle in bundle sheath chloroplasts

CAM (crassulacean acid metabolism) plants
incorporate carbon during night
 stomata
open at night, closed during day
 CO2 incorporated in 4C molecule and stored in
vacuole at night
 during day, 4C molecules exported into
cytoplasm and CO2 released
 CO2 enters Calvin cycle
C4 separate carbon incorporation and
fixation spatially
 CAM plants separate carbon incorporation
and carbon fixation temporally
