Light and Pigments
• Pigments – light
absorbing molecules
• Chlorophyll – the
most abundant
pigment in plants,
that absorbs blue
and red light.
High Energy Electron Carriers
(NADPH)
• High energy electrons
produced by chlorophyll
are highly reactive an
require a special “carrier”
• An electron carrier is a
compound that can accept
a pair of high energy
electrons and transfer
them, along with most of
their energy, to another
molecule
• NADPH can carry high
energy electrons that were
produced by light
absorption in chlorophyll
Electron Carrier
• A compound that can accept a pair of high energy
electrons and transfer them, along with most of their
energy, to another molecule
– NADP+ (nicotinamide adenine dinucleotide phosphate)
• Accepts and holds 2 high E electrons and H+ ions
• Converts from NADP+ to NADPH – one way in which some sunlight
energy can be trapped in chemical form
Photosynthesis: An Overview
• Photosynthesis takes place in the
chloroplast
– Thylakoids – saclike photosynthetic
membrane in the chloroplast
– Stroma - the region outside of the
thylakoid membranes
Photosynthesis: An Overview
Photosynthesis is a complex process that can be broken
down into 2 steps:
Electron Carrier
• A compound that can accept a pair of high energy
electrons and transfer them, along with most of their
energy, to another molecule
– NADP+ (nicotinamide adenine dinucleotide phosphate)
• Accepts and holds 2 high E electrons and H+ ions
• Converts from NADP+ to NADPH – one way in which some sunlight
energy can be trapped in chemical form
Light Dependent Reactions:
Generating ATP and NADPH
• The light dependent
reactions use energy from
the sunlight to produce O2
and convert ADP and
NADP+ into the high E
carriers ATP and NADPH
• Occurs in the thylakoids
which contain clusters of
chlorophyll and proteins
known as
PHOTOSYSTEMS
– Photosystems absorb
sunlight and generate high
energy electrons
2 Photosystems connected by an ETC
generate ATP and NADPH
• Electrons removed from water pass from photosystem II to I
and are accepted by NADP+
• The bridge between the photosystems II and I is an electron
transport chain (ETC) that provides energy for the synthesis
of ATP
• NADPH, ATP and O2 are the products of the light reactions
Step 1: Photosystem II
• Photosystem: a cluster of chlorophyll and proteins
found in the thylakoids.
• Light is absorbed by photosystem II which creates high
energy electrons.
• A water molecule is split to provide more electrons and
hydrogen ions and oxygen is released.
Electron Transport Chain (ETC)
Step 2: Electron Transport Chain
• The electrons from photosystem II move to the electron
transport chain.
• Electrons move from molecule to molecule to pump H+
ions from the stroma into the thylakoid space.
• What type of transport is used to move the H+ ions?
Explain
Electron Transport Chain (ETC)
• Series of electron carrier proteins that shuttle high energy
electrons during ATP generating reactions
• High E electrons move down the ETC to Photosystem I
• Energy generated is used to pump H+ ions across the
thylakoid membrane and into the thylakoid space.
Electron Transport Chain (ETC)
• Series of electron carrier proteins that shuttle high energy
electrons during ATP generating reactions
• High E electrons move down the ETC to Photosystem I
• Energy generated is used to pump H+ ions across the
thylakoid membrane and into the thylakoid space.
Step 2: Electron Transport Chain
• The electrons from photosystem II move to the electron
transport chain.
• Electrons move from molecule to molecule to pump H+
ions from the stroma into the thylakoid space.
• What type of transport is used to move the H+ ions?
Explain
Step 3: Photosystem I
• Since the electrons used some energy pumping H+ ions
they need to be renergized.
• Light collected at photosystem I renergizes electrons.
• The electrons along with H+ ion combine with NADP+ to
from NADPH
Photosystem I
• Pigments in photosystem I use E from light to reenergize
the electrons
– E was used to pump H+ ions across thylakoid membrane
– Electrons do not contain as much E
• At the end of a short 2nd ETC, NADP+ molecules in the
stroma pick up the high E electrons along with H+ ions
at the outer surface of the thylakoid membrane to
become NADPH.
Electron Transport Chain (ETC)
Step 4: ATP Formation
• A high concentration of H+ have been pumped into the
thylakoid.
• The H+ “want” to move into the stroma. Why?
• The only way to get to the stroma is to go through a
protein called ATP Synthase.
• When H+ move through ATP Synthase a ADP molecule is
converted into ATP
Hydrogen Ions
and ATP
• H+ ions cannot cross the membrane directly
• ATP synthase spans the membrane and allows H+ ions to pass
through it
• As it rotates, ATP synthase binds ADP and a phosphate group
together to produce ATP
Hydrogen Ions
and ATP
• The buildup of H+ ions makes the stroma negatively
charged relative to the space within the thylakoids.
• This gradient (difference in charge and H+ ion
concentration) across the membrane provides the energy to
make ATP
An Overview of Photosynthesis:
H2O
CO2
O2
Sugars
(C6H12O6)
Sunlight
6 CO2
6 H2O
_____ + _____

6 O2
C6H12O6
_____ + _____
The Dark Reaction
(The light independent Reaction/Calvin Cycle)
• The NADPH and ATP from
the light reaction move to
the stroma.
• The leaf absorbs CO2
from the atmosphere.
• The energy from the
NADPH and ATP is used to
convert CO2 into
carbohydrates.
• What is the main function
of carbohydrates?
Light Independent Reactions
aka Calvin Cycle
• Plants use the E that
ATP and NADPH
contain to build
stable high energy
carbohydrate
compounds that can
be stored for a long
time
• ATP and NADPH
from the light
dependent reactions
are used to produce
high energy sugars
End Result of Photosynthesis
• 2 sets of photosynthetic reactions work together
– Light dependent reactions trap the energy of sunlight in chemical form
– Light independent reactions use the chemical E to produce stable, high energy
sugars from CO2 and water
Photosynthesis: Light reaction, Calvin cycle, Electron Transport (7:26)
End Result of Photosynthesis
• 2 sets of photosynthetic reactions work together
– Light dependent reactions trap the energy of sunlight in chemical form
– Light independent reactions use the chemical E to produce stable, high energy
sugars from CO2 and water
Photosynthesis: Light reaction, Calvin cycle, Electron Transport (7:26)
The Dark Reaction
(The light independent
Reaction)
• Each molecule of glucose
contains enough energy to
produce 36 ATP’s.
• Plants produce starch when
they need to store energy.
• Starch is a long chain of glucose.