 Section Objectives
 Relate the structure of chloroplasts to the events
in photosynthesis
 Describe light-dependent reactions.
Trapping Energy from Sunlight
 The process that uses the sun’s energy to make
simple sugars is called photosynthesis.
Why do chloroplasts contain
so many membranes?
– When most pigments absorb light, they
eventually lose most of that energy as heat.
Chloroplasts avoid such losses. Membranes are
the key to capturing light energy in the form
of high-energy electrons.
Trapping Energy from Sunlight

Photosynthesis happens in two phases.
1. The light-dependent reactions convert
light
energy into chemical energy.
2. The molecules of ATP produced in the lightdependent reactions are then used to fuel the
light-independent reactions that produce simple
sugars.
 The general equation for
photosynthesis is written as
 6CO2 + 6H2O→C6H12O6 + 6O2
The chloroplast and pigments
 To trap the energy in the sun’s light, the thylakoid
membranes contain pigments, molecules that absorb
specific wavelengths of sunlight.
 Although a photosystem contains several kinds of
pigments, the most common is
chlorophyll.(chloro=greenish yellow, phyllon= leaf)
 Chlorophyll absorbs most wavelengths of light except
green.
Light
Chloroplast
Absorbed
light
Reflected
light
An overview of photosynthesis
Chloroplast
CO2
H2O
NADP+
ADP
+P
LIGHT
REACTIONS
(in grana
ATP
CALVIN
CYCLE
(in stroma
NADPH
O2
O2
Light
Sugar
Light-Dependent Reactions
absorption of light energy by
chlorophyll
 As sunlight strikes the chlorophyll molecules in a
photosystem of the thylakoid membrane, the
energy in the light is transferred to electrons.
Light-Dependent Reactions
absorption of light energy by
chlorophyll
 These highly energized, or excited, electrons are
passed from chlorophyll to an electron transport
chain, a series of proteins embedded in the
thylakoid membrane.
Light-Dependent Reactions
absorption of light energy by
chlorophyll
 At each step along the transport chain, the
electrons lose energy.
Light Dependent Ractions
• Two electron pathways operate in the
thylakoid membrane: the noncyclic pathway
and the cyclic pathway.
• Both pathways produce ATP; only the
noncyclic pathway also produces NADPH.
• ATP production during photosynthesis is
called photophosphorylation; therefore these
pathways are also known as cyclic and
noncyclic photophosphorylation.
Light Dependent Reactions:
Cyclic Electron Pathway
Light Dependent Reactions:
The Cyclic Electron Pathway
• Uses only photosystem I (PS-I)
• Begins when PS I complex absorbs solar energy
Light Dependent Reactions:
The Cyclic Electron Pathway
• Electron ejected from reaction center
– Travels down electron transport chain
– Causes H+ to concentrate in thylakoid chambers
– Which causes ATP production
– Electron returns to PS-I (cyclic)
Light Dependent Reactions:
The Cyclic Electron Pathway
Pathway only results in ATP production
Light Dependent Reactions
Noncyclic Electron Pathway
Photosystem II
• PS II captures light
energy
• Causes an electron to be
ejected from the reaction
center (chlorophyll a)
– Electron travels down
electron transport chain to
PS I
– Replaced with an electron
from water
– causes H+ to concentrate in
thylakoid chambers
– causes ATP production
Photosystem II
• PS I captures light
energy (electrons and
H)
– Transferred
permanently to a
molecule of NADP+
– Causes NADPH
production
Organization of the Thylakoid Membrane
•PS II consists of a pigment complex and electron-acceptor
molecules; it oxidizes H2O and produces O2.
•The electron transport system consists of cytochrome
complexes and transports electrons and pumps H+ ions into
the thylakoid space.
•PS I has a pigment complex and electron-acceptor
molecules; it is associated with an enzyme that reduces
NADP+ to NADPH.
•ATP synthase complex has an H+ channel and ATP synthase;
it produces ATP.
Restoring electrons
2H2O  4H+ + O2 + 2e To replace the lost electrons, molecules of water are
split in the first photosystem. This reaction is called
photolysis.
 The O2 released by photosynthesis is made from the
oxygen in water
H2O
+ +
O2 +
2e-
2e-
2
Chlorophyll
O2 + 2H+
H2O
Restoring electrons
 The oxygen produced by photolysis is released into the air and
supplies the oxygen we breathe.
 The electrons are returned to chlorophyll.
 The hydrogen ions are pumped into the thylakoid, where they
accumulate in high concentration.