Chapter 10: Photosynthesis
Photosynthesis transforms solar light energy
into chemical bond energy stored as sugar
Autotrophs vs heterotrophs
Autotrophic organisms synthesize their own
organic food from inorganic raw materials
* photoautotrophs
* chemoautotrophs
Heterotrophic -consumers
* Animalsand decomposers
Structure of a chloroplast
Chloroplasts are primarily in cells of mesophyll, green tissue in
the leaf’s interior.
Parts to know:
Intermembrane space – between the inner and outer
membranes
Thylakoids – flattened membranous sacs inside the chloroplast
thylakoid membranes- contain chlorophyl /light reactions/
chemiosmosis
thylakoid space- inside the thylakoid.
grana – stacks of thylakoids
Stroma – fluid outside the thylakoids
Properties of Light
 electromagnetic energy
- composed of discrete particles called
“photons” with energy dependent on
wavelength
 Visible light is only a small portion of the
electromagnetic spectrum
-blue and red are the two wavelengths
most effectively absorbed by chlorophyll
-green is least effective (reflected)
Photosynthetic Pigments
 Pigments are substances that absorb
visible light
 Every pigment has a characteristic
absorption spectrum
 Chlorophyll a is the blue/green lightabsorbing pigment that participates directly
in the light reaction
 Accessory Pigments include:
- Chlorophyll b (a yellow-green
pigment similar to chlorophyll a)
- Carotenoids (yellow and orange)
- The absorbed photon boosts one of the
pigment molecule’s electrons in its lowest
energy state (ground state) to an orbital of
higher potential energy (excited state)
Photosystems
 Photosystems are the light-harvesting
complexes of the thylakoid membrane
 Photosystems contain chlorophyll a,
chlorophyll b, and carotenoids
Photosystem components
1) Antenna Complex
- several hundred chlorophyll a,
chlorophyll b, and carotenoid molecules to
absorb photons and pass energy from
molecule to molecule
- different pigments have different
absorption spectra
2) Reaction-center Chlorophyll
- only one of the many chlorophyll a
molecules in each complex transfers an
excited electron to initiate the light reactions.
Location is the reaction center
3) Primary electron acceptor
located near the reaction center, a primary
electron acceptor molecule traps excited
electrons released from the reaction center
chlorophyll
Photosystems
 The reaction center of photosystem I has a
chlorophyll a molecule known as P700
(absorbs best at 700 nm)
 The reaction center of photosystem II has a
chlorophyll a molecule known as P680
(absorbs best at 680 nm)
Noncyclic Electron Flow
 Noncyclic electron flow involves both
photosystem I and photosystem II.
* occurs in the thylakoid membrane to split
water into H+, electrons, and oxygen
produces ATP, NADPH, and O2
1) Photons excite photosystem I and
electrons are transferred from P700 to the
primary electron acceptor
2) These electrons are passed on to
ferredoxin (an iron containing protein)
and are passed on to NADP+, producing
NADPH (reducing power for Calvin cycle)
3)Photosystem II supplies electrons to fill the
electron “holes” in photosystem I
 Electrons ejected from P680 are trapped by
photosystem II primary electron acceptor
 These electrons are passed down an ETC
embedded in the thylakoid membrane.
 As excited electrons move to P700, protons
are picked up on one side (stroma) and
deposited on the other side (thylakoid
space)
•  An ATP synthase enzyme in the thylakoid
membrane uses the proton-motive force to
make ATP.
noncyclic photophosphorylation
How do we replace these missing electrons from
photosystem II?
 We get them from water!
 A water-splitting enzyme extracts electrons
from water and passes them to the P680
reaction center
II. Cyclic Electron Flow
 Cyclic electron flow only involves
Photosystem I. This process:
- produces ATP
- does not produce NADPH or O2
1) Photons excite photosystem I and electrons
are transferred from P700 to the primary
electron acceptor
2) The electrons are not passed on to NADP+
Reductase (which would produce NADPH).
Instead the electrons are passed down the
ETC back to P700. [Fig. 10.14]
 This process is called cyclic
photophosphorylation
Why do plants need this cyclic pathway?
 To create more ATP for the Calvin cycle.
 NADPH concentration might influence
whether electrons flow through cyclic or
noncyclic pathways?
Comparison of Chemiosmosis in
Chloro. & Mitoch.
Similar:
 electrons passed on to more electronegative
carriers
 ATP synthase complexes
 electron carriers (quinones and cytochromes)
similar
Different:
 Chloroplast use light energy, mitochondria
use chemical energy to produce ATP
 Chloroplasts pump protons from stroma to
thylakoid compartment… mitochondria from
matrix to intermembrane space
Calvin Cycle
 The Calvin cycle uses the ATP and NADPH
produced from the light reaction to reduce
CO2 to sugar
Phase I: Carbon Fixation
 Each CO2 attaches to a five-carbon sugar
(RuBP) – ribulose bisphosphate
- This reaction is catalyzed by rubisco
ribulose bisphosphate carboxylase
- The product of this rxn is an unstable 6carbon intermediate that immediately splits
into two 3-carbon molecules (3phosphoglycerate)
Phase II: Reduction
ATP phosphorylates and NADPH reduces
3 phosphoglycerate to Glyceraldehyde 3
Phosphate (G3P)
 G3P stores more potential energy than
3-phosphoglycerate
 For every 3 CO2 that enter the Calvin cycle,
six G3P are produced, only one of which can
be counted as a net gain.
 The cycle begins with three 5-carbon
molecules - a total of 15 carbons
 The six G3P molecules produced contain 18
carbons (a net gain of 3 carbons from CO2)
 One G3P molecule exits the cycle… what
happens to the other five?
Phase III: Regeneration of Starting
Material (RuBP)
 The other five G3P are recycled through a
complex series of rxns to regenerate three
molecules of the original 5-carbon sugar
(RuBP)
 These rxns require 3 more ATP (why we
needed cyclic electron flow )
Summary of Calvin Cycle:
 The net synthesis of one G3P molecule
requires: ATP? 9 NADPH molecules? 6
CO2? 3
 G3P is used to construct glucose and other
carbohydrates
 The Calvin cycle uses 18 ATP and 12 NADPH
and 6 CO2 to produce one glucose 
1) Carbon enters the Calvin cycle as CO2 and
leaves as sugar glyceraldehyde 3- phosphate
(G3P)
2) ATP is the energy source, while NADPH is
the reducing agent
3) Calvin cycle is similar to the Kreb cycle in that
the starting material is regenerated
Photorespiration
Rubisco can also accept Oxygen – results in the
loss of 2-carbon fragments which peroxisomes
and mitochondria convert to carbon dioxide
Leaches carbon away from Calvin cycle!!!
Alternate photosynthesis
C 4 Plants
1) fix incoming CO2 to an organic acid in
mesophyll cells(via *PEP carboxylase*)
2) deliver this acid directly to bundle sheath
cells, in which the Calvin cycle occurs
Cost: extra cells (bundle sheath) and enzymes
Gain: reduced photorespiration
C A M Plants (crassulacean acid metabolism)
1) fix incoming CO2 to an organic acid only
during night (when stomata are open)
2) close stomata during day and feed CO2 to
Calvin cycle as light reactions occur
Cost: extra enzymes and CO2 availability
Gain: reduced photorespiration AND
transpiration