PHOTOSYNTHESIS
Virtually all energy on earth comes from
sunlight. Plants use energy from the sun to
make the bonds which hold organic molecules
together. When these bonds are broken the
energy is ultimately transferred to ATP, which
is then moved about cells and organisms to
power their needs.
Photosynthesis overview
12H20 + 6CO2 ----- light -----> 6O2+ C6H12O6 +
6H20
Photosynthesis - The Basic Reaction
CO2 +H2O +---Plants (Chloroplasts)
Light Energy---Simple
Sugars+ O2
ROLE OF SUNLIGHT
Sunlight provides the energy
Organisms depend upon photosynthesis
1. Photosynthesis uses sunlight as a source of energy to produce
carbohydrates
2. Photosynthetic organisms (algae, plants and a few other
organisms) serve as ultimate source of food for most life
3. Most food chains start with photo synthesizers
4. The early atmosphere of the earth lacked oxygen and it took
about 3 billion years of photosynthesis to produce the current
21% oxygen atmosphere we now enjoy. This oxygen:
a. Made aerobic respiration possible
b. Formed the ozone layer (O3), which protects us from harmful
solar radiation
SOLAR RADIATION
Solar Radiation
1. Solar radiation is described in terms of its
energy content and its wavelength
2. Photons are discrete packets of radiant
energy that travel in waves
3. The electromagnetic spectrum of solar
radiation is based on wavelength
a. Energy content of photons is inversely
proportional to wavelength
Solar radiation contd.
4. Only 42% of solar radiation that hits earth's
atmosphere reaches surface; most is visible
light
a. Higher energy wavelengths are screened out by
ozone layer in upper atmosphere
b. Lower energy wavelengths are screened out by
water vapor and CO2
c. Consequently, both the organic molecules within
organisms and processes, such as vision and
photosynthesis, are adapted to radiation that is
most prevalent in the environment
PHOTOSYNTHETIC PIGMENTS
Photosynthetic pigments use primarily the
visible light portion of the
electromagnetic spectrum
1.
Pigment is a substance that absorbs
visible light that behave as packets of
energy called photons.
2.
Two major photosynthetic pigments are
chlorophyll a and chlorophyll b
3.
Chlorophylls absorb violet, blue, and red
wavelengths; they reflect green, this is
why leaves appear green. Different
colors/wavelengths of light (ROYGBIV).
CAROTENOIDS
1.
2.
Carotenoids are yellow-orange
pigments which absorb light in
violet, blue, and green regions
When pigments absorb light,
electrons are boosted to a higher
energy level and the energy is
captured in a chemical bond
Photosynthesis occurs in
chloroplasts
Chloroplasts have two parts:
1. A double membrane encloses a fluid-filled space
called the stroma or ground substance
2. Thylakoids = flattened sacs organized into stacks
called grana
3. Chlorophylls and other pigments involved in
absorption of solar energy are embedded within
thylakoid membranes; these pigments absorb solar
energy
Chloroplast Structure
1.
2.
3.
4.
5.
6.
Outer membrane
Inner membrane systems
Thylakoid membranes
Thylakoid space (within the
thylakoids)
Granum(a) (stack(s) of
thylakoidsmembranes)
Stroma (the liquid area outside the
thylakoid membranes)
REACTIONS IN
PHOTOSYNTHESIS
Photosynthesis has two sets of reactions
1. Light-dependent reactions = the
energy from the sun is captured in
energy carrying molecules
2. Light-independent reactions =
energy carrying molecules from the
light-dependent reactions are used to
make carbohydrates
The Photochemical (Light)
Reactions
 Capture
of light energy
 Thylakoid membranes
 Photosystems II and I
 Chlorophyll and accessory
pigments
Electron trasport chain - A specific redox
reaction in which the chlorophyll donates an electron into
a series of molecular intermediates
In Photosystem II, the electron which reduces
P680+ ultimately comes from the oxidation of
water into O2 and H+ through several
intermediates. This reaction is how photosynthetic
organisms like plants produce O2 gas, and is the
source for practically all the O2 in Earth's
atmosphere. Photosystem I typically works in series
with Photosystem II, thus the P700+ of
Photosystem I is usually reduced, via many
intermediates in the thylakoid membrane, by
electrons ultimately from Photosystem
LIGHT ABSORBANCE AND ENERGY
TRANSFORMATIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
Flow of Electrons
Splitting of water molecules
Release of oxygen molecules
Accumulation of H+ in thylakoid Spaces
Reduction of NADP to NADPH
Production of ATP
ATP synthase
Powered by flow of H+
ADP + Phosphate --- ATP
Light-dependent reactions – light energy is
converted to chemical energy (ATP&NADPH)
A. Occur in the thylakoid membranes and require participation of
one or two light-gathering units called photosystems
B. A photosystem is a photosynthetic unit comprised of a
pigment complex and electron acceptor; solar energy is
absorbed and high-energy electrons are generated
C. Each photosystem has a pigment complex composed of green
chlorophyll a and chlorophyll b molecules and orange and
yellow accessory pigments (e.g., carotenoid pigments)
D. Absorbed energy is passed from one pigment molecule to
another until concentrated in reaction-center chlorophyll a
E. Electrons in reaction-center become excited; then move along
an electron transport system, along the way ADP is
converted to ATP and NADP is converted to NADPH
F. The electrons are replaced by the splitting of water molecules
G. The energy in the ATP and NADPH are used to power the lightindependent reactions
Light-dependent reactions – light energy is
converted to chemical energy (ATP&NADPH)
A. Occur in the thylakoid membranes and require
participation of one or two light-gathering units called
photosystems
B. A photosystem is a photosynthetic unit comprised of a
pigment complex and electron acceptor; solar energy is
absorbed and high-energy electrons are generated
C. Each photosystem has a pigment complex composed of
green chlorophyll a and chlorophyll b molecules and orange
and yellow accessory pigments (e.g., carotenoid pigments)
D. Absorbed energy is passed from one pigment molecule to
another until concentrated in reaction-center
chlorophyll a
LIGHT ENERGY CONVERTED TO
CHEMICAL ENERGY
E. Electrons in reaction-center become excited;
then move along an electron transport
system, along the way ADP is converted to
ATP and NADP is converted to NADPH
F. The electrons are replaced by the splitting of
water molecules
G. The energy in the ATP and NADPH are used to
power the light-independent reactions
PIGMENTS IN LIGHT
ABSORPTION



Before a pigment absorbs light all its
electrons in their ground state (lowest
energy level)
The absorption of one photon of light
excites one electron in a pigment molecule.
The e- is in an excited state
For a photon to be absorbed, the energy of
the photon must match the difference in
energy between the ground state of some
e- and a possible excited state.
What happens to the energy
of an excited state?
It gives off heat
 Gives of light
 Excites an e- in another pigment
 Drives a photochemical reaction
 The excited e- reduces an acceptor.

The Biochemical Reactions:
The Calvin Cycle
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
"Fixing" CO2
Cyclic series of enzyme reactions
Rubisco (the enzyme that fixes CO2)
Stomates and CO2 availability
Addition of CO2 to a 5 carbon compound
Production of Carbohydrate
Energy input from ATP (ATP --- ADP + Phosphate)
Addition of H+ and energy
Production of carbohydrates for storage,
transportation, and biosynthesis
Recycling of 5 carbon compound to fix more CO2
THE DARK REACTION (Lightindependent reactions)
Light-independent reactions (also
called carbon fixation or Calvin Benson cycle)
A. Takes place in the stroma of the
chloroplast
B. Carbon dioxide enters through small
pores in the leaves called stomates
and enzymes extract and bond the
carbon to a 5-carbon intermediate
RuBP(ribulose-1,5-bisphosphate)
DARK REACTION
C. Thorough a series of reactions energy from
the light reactions (ATP, NADPH) is used to
form several molecules of PGAL. PGAL is
used to:
1. Regenerate RuBP so the light-independent
reactions may continue; note this requires
an input of ATP created during light
reactions
2. PGAL that accumulates is also used to
produce glucose, a 6-carbon sugar, and as a
backbone for other organic molecules
PGAL or G3P or
triosephosphate


Glyceraldehyde 3-phosphate (G3P) or triose
phosphate is a 3 carbon metabolic intermediate.
G3P is often referred to as 3phosphoglyceraldehyde (PGAL) with respect to
the product of photosynthetic carbon fixation during
the Calvin cycle.
glyceralaldehyde 3-P
PGAL- end product of
photosynthesis


During plant photosynthesis, two molecules
of glycerate 3-phosphate (GP, but also
known as 3-phosphoglycerate (PGA)) are
produced by the first step of the lightindependent reactions when ribulose 1,5bisphosphate (RuBP) and carbon dioxide are
catalysed by the rubisco enzyme
The GP is converted to PGAL using the
energy in ATP and the reducing power of
NADPH as part of the Calvin cycle
G3P



This returns ADP, Pi, and NADP+ to the
light-dependent reactions of photosynthesis
for their continued functioning
PGAL can then be converted to glucose.
RuBP is regenerated for the Calvin cycle to
continue
PGAL is generally considered the prime endproduct of photosynthesis. PGAL is generally
considered the prime end-product of
photosynthesis