Photosynthetic Process
THE SUN: MAIN SOURCE OF
ENERGY FOR LIFE ON EARTH
FREE ENERGY
(available for work)
vs.
HEAT
(not available for work)
THE BASICS OF PHOTOSYNTHESIS
• Almost all plants are photosynthetic
autotrophs (self producing), as are some
bacteria and prtozoas
– Autotrophs generate their own organic matter
through photosynthesis
– Sunlight energy is transformed to energy
stored in the form of chemical bonds
(a) Mosses, ferns, and
flowering plants
(c) Euglena
(d) Cyanobacteria
Light Energy Harvested by Plants &
Other Photosynthetic Autotrophs
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
WHY ARE PLANTS GREEN?
Plant Cells
have Green
Chloroplasts
The thylakoid
membrane of the
chloroplast is
impregnated with
photosynthetic
pigments (i.e.,
chlorophylls,
carotenoids).
THE COLOR OF LIGHT SEEN IS THE
COLOR NOT ABSORBED
• Chloroplasts
absorb light
energy and
convert it to
chemical energy
Light
Reflected
light
Transmitted
light
Chloroplast
Absorbed
light
Photosynthesis occurs in chloroplasts
• In most plants, photosynthesis occurs
primarily in the leaves, in the chloroplasts
• A chloroplast contains:
– stroma, a fluid
– grana, stacks of thylakoids
• The thylakoids contain chlorophyll
– Chlorophyll is the green pigment that captures
light for photosynthesis
• The location and structure of chloroplasts
Chloroplast
LEAF CROSS SECTION
MESOPHYLL CELL
LEAF
Mesophyll
CHLOROPLAST
Intermembrane space
Outer
membrane
Granum
Grana
Stroma
Inner
membrane
Stroma
Thylakoid
Thylakoid
compartment
Thylakoid
Thylakoid Membrane
Granum
Thylakoid Space
Chloroplast Pigments
• Chloroplasts contain several pigments
– Chlorophyll a
– Chlorophyll b
(Chlorophyll a (alpha) absorbs well at a wavelength
of about 450 nm but its primary absorption is at
675nm in the long red wavelengths.
Chlorophyll b (beat) absorbs most effectively at
blue 470 but also with shorter peaks at 430 and
640nm)
– Carotenoids
– Xanthophyll
Fall Colors
• During the fall, the green chlorophyll pigments
are greatly reduced revealing the other
pigments.
• Carotenoids are pigments that are either red or
yellow.
Chlorophyll Molecules
• Located in the thylakoid membranes.
• Chlorophyll have Mg+ in the center.
• Chlorophyll pigments harvest energy (photons)
by absorbing certain wavelengths (blue-420
nm and red-660 nm are most important).
• Plants are green because the green
wavelength is reflected, not absorbed.
Chlorophyll a & b
•Chl a has a methyl
group
•Chl b has a carbonyl
group
Porphyrin ring
delocalized e-
Phytol tail
Absorption of Chlorophyll
Absorption
violet
blue
green
yellow
wavelength
orange
red
Different pigments absorb light
differently
AN OVERVIEW OF PHOTOSYNTHESIS
• Photosynthesis is the
process by which
autotrophic organisms
use light energy to
make sugar and oxygen
gas from carbon dioxide
and water
Carbon
dioxide
Water
Glucose
PHOTOSYNTHESIS
Oxygen
gas
AN OVERVIEW OF PHOTOSYNTHESIS
• The light reactions
convert solar
energy to chemical
energy
Light
Chloroplast
NADP
ADP
+P
– Produce ATP & NADPH
• The Calvin cycle makes
sugar from carbon
dioxide
– ATP generated by the light
reactions provides the energy
for sugar synthesis
– The NADPH produced by the
light reactions provides the
electrons for the reduction of
carbon dioxide to glucose
Light
reactions
Calvin
cycle
Steps of Photosynthesis
• Light hits reaction centers of chlorophyll,
found in chloroplasts
• Chlorophyll vibrates and causes water
to break apart.
• Oxygen is released into air
• Hydrogen remains in chloroplast
attached to NADPH
• “THE LIGHT REACTION”
Steps of Photosynthesis
• The DARK Reactions= Calvin Cycle
• CO2 from atmosphere is joined to H
from water molecules (NADPH) to form
glucose
• Glucose can be converted into other
molecules with different flavors!
Redox Reaction
• The transfer of one or more electrons from
one reactant to another.
• Two types:
1. Oxidation
2. Reduction
Oxidation Reaction
• The loss of electrons from a substance.
• Or the gain of oxygen.
Oxidation
6CO2 + 6H2O 
C6H12O6 + 6O2
glucose
Reduction Reaction
• The gain of electrons to a substance.
• Or the loss of oxygen.
Reduction
6CO2 + 6H2O  C6H12O6 + 6O2
glucose
• Two types of
photosystems
cooperate in the
light reactions
ATP
mill
Water-splitting
photosystem
NADPH-producing
photosystem
1. Light Reaction (Electron Flow)
• Occurs in the Thylakoid membranes
• During the light reaction, there are two
possible routes for electron flow.
A. Cyclic Electron Flow
B. Noncyclic Electron Flow
A. Cyclic Electron Flow
•
•
•
•
•
Occurs in the thylakoid membrane.
Uses Photosystem II only
P700 reaction center- chlorophyll a
Uses Electron Transport Chain (ETC)
Generates ATP only
ADP +
P
ATP
Cyclic Photophosphorylation (addition of
phosphate to ADP to make ATP.)
• Process for ATP generation associated with some
Photosynthetic Bacteria
• Reaction Center => 700 nm
Plants produce O2 gas by splitting H2O
• The O2 liberated by photosynthesis is made
from the oxygen in water (H+ and e-)
In the light reactions, electron transport
chains generate ATP, NADPH, & O2
• Two connected photosystems collect
photons of light and transfer the energy to
chlorophyll electrons
• The excited electrons are passed from the
primary electron acceptor to electron
transport chains
– Their energy ends up in ATP and NADPH
Chemiosmosis powers ATP
synthesis in the light reactions
• The electron transport chains are arranged
with the photosystems in the thylakoid
membranes and pump H+ through that
membrane
– The flow of H+ back through the membrane is
harnessed by ATP synthase to make ATP
– In the stroma, the H+ ions combine with NADP+
to form NADPH
Chemiosmosis
SUN
H+ H+ (Proton Pumping)
Thylakoid
E
T
PS II
PS I
C
H+
H+ H+
H+ H+
H+
ADP + P
H+
H+
high H+
concentration
ATP Synthase
ATP
Thylakoid
Space
low H+
concentration
B. Noncyclic Electron Flow
• Occurs in the thylakoid membrane
• Uses PS II and PS I
• P680 rxn center (PSII) - chlorophyll a
• P700 rxn center (PS I) - chlorophyll a
• Uses Electron Transport Chain (ETC)
• Generates O2, ATP and NADPH
Noncyclic Photophosphorylation
• Photosystem II regains electrons by splitting
water, leaving O2 gas as a by-product
Primary
electron acceptor
Primary
electron acceptor
Photons
Energy for
synthesis of
PHOTOSYSTEM I
PHOTOSYSTEM II
by chemiosmosis
B. Noncyclic Electron Flow

• ADP +
ATP
P
(Reduced
)
+
• NADP + H

NADPH
(Reduced)
• Oxygen comes from the splitting of
H2O, not CO2
H 2O 
(Oxidized)
1/2 O2 + 2H+
How the Light Reactions Generate ATP and NADPH
Primary
electron
acceptor
Primary
electron
acceptor
Energy
to make
NADP
3
2
Light
Light
Primary
electron
acceptor
1
Reactioncenter
chlorophyll
Water-splitting
photosystem
2 H + 1/2
NADPH-producing
photosystem
Summary—Light Dependent
Reactions
a. Overall input
light energy, H2O.
b. Overall output
ATP, NADPH, O2.
Light Independent Reactions
aka Calvin Cycle
Carbon from CO2 is
converted to glucose
(ATP and NADPH
drive the reduction
of CO2 to C6H12O6.)
Light Independent Reactions
aka Calvin Cycle
CO2 is added to the 5-C sugar RuBP by the
enzyme rubisco.
This unstable 6-C compound splits to two
molecules of PGA or 3-phosphoglyceric acid.
PGA is converted to Glyceraldehyde 3-phosphate
(G3P), two of which bond to form glucose.
G3P is the 3-C sugar formed by three turns of the
cycle.
Summary—Light Independent
Reactions
a. Overall input
CO2, ATP, NADPH.
b. Overall output
glucose.
Review: Photosynthesis uses light
energy to make food molecules
• A summary of
the chemical
processes of
photosynthesis
Chloroplast
Light
Photosystem II
Electron
transport
chains
Photosystem I
CALVIN
CYCLE
Stroma
Cellular
respiration
Cellulose
Starch
LIGHT REACTIONS
CALVIN CYCLE
Other
organic
compounds
Photorespiration (Competing Reactions)
• Occurs under the following conditions:
•
•
•
•
•
– Intense Light (high O2 concentrations, hot, dry,
bright days)
– High heat (Stomatas close)
Rubisco grabs CO2, “fixing” it into a carbohydrate in
the light independent reactions.
O2 can also react with rubisco, inhibiting its active site
– not good for glucose output
– wastes time and energy (occupies Rubisco)
So Fixation of O2 instead of CO2.
Produces no sugar molecules or no ATP.
Photorespiration is estimated to reduce
photosynthetic efficiency by 25%
Types of Photosynthesis
C3
C4
CAM
Rubisco: the world’s busiest enzyme!
Types of Photosynthesis
• Certain plants have developed ways to limit
the amount of photorespiration
– C3 Pathway
– C4 Pathway*
– CAM (Crassulacean Acid Metabolism)
Pathway*
* Both convert CO2 into a 4 carbon intermediate
 C4 Photosynthesis
C3 Photosynthesis : C3 plants
• Called C3 because the CO2 is first incorporated
into a 3-carbon compound.
• Stomata are open during the day.
• RUBISCO, the enzyme involved in
photosynthesis, is also the enzyme involved in
the uptake of CO2.
• Photosynthesis takes place throughout the leaf.
• Adaptive Value: more efficient than C4 and
CAM plants under cool and moist conditions and
under normal light because requires less
machinery (fewer enzymes and no specialized
anatomy)..
• Most plants are C3.
C4 Photosynthesis : C4 plants
• Called C4 because the CO2 is first incorporated into a
4-carbon compound.
• Stomata are open during the day.
• Uses PEP Carboxylase for the enzyme involved in the
uptake of CO2. This enzyme allows CO2 to be taken
into the plant very quickly, and then it "delivers" the
CO2 directly to RUBISCO for photsynthesis.
• Photosynthesis takes place in inner cells (requires
special anatomy called Kranz Anatomy)
C4 Photosynthesis : C4 plants
• Adaptive Value:
– Photosynthesizes faster than C3 plants under high
light intensity and high temperatures because the
CO2 is delivered directly to RUBISCO, not allowing it
to grab oxygen and undergo photorespiration.
– Has better Water Use Efficiency because PEP
Carboxylase brings in CO2 faster and so does not
need to keep stomata open as much (less water lost
by transpiration) for the same amount of CO2 gain for
photosynthesis.
• C4 plants include several thousand species in at least 19
plant families. Example: fourwing saltbush pictured here,
corn, and many of our summer annual plants.
CAM Photosynthesis : CAM plants. CAM
stands for Crassulacean Acid Metabolism
• Called CAM after the plant family in which it was first
found (Crassulaceae) and because the CO2 is stored in
the form of an acid before use in photosynthesis.
• Stomata open at night (when evaporation rates are
usually lower) and are usually closed during the day.
The CO2 is converted to an acid and stored during the
night. During the day, the acid is broken down and
the CO2 is released to RUBISCO for photosynthesis
CAM Photosynthesis : CAM plants.
• Adaptive Value:
– Better Water Use Efficiency than C3 plants under arid conditions
due to opening stomata at night when transpiration rates are lower
(no sunlight, lower temperatures, lower wind speeds, etc.).
– May CAM-idle. When conditions are extremely arid, CAM plants
can just leave their stomata closed night and day. Oxygen given
off in photosynthesis is used for respiration and CO2 given off in
respiration is used for photosynthesis. This is a little like a
perpetual energy machine, but there are costs associated with
running the machinery for respiration and photosynthesis so the
plant cannot CAM-idle forever. But CAM-idling does allow the
plant to survive dry spells, and it allows the plant to recover very
quickly when water is available again (unlike plants that drop their
leaves and twigs and go dormant during dry spells).
• CAM plants include many succulents such as cactuses and agaves
and also some orchids and bromeliads
Leaf Anatomy
• In C3 plants (those that do C3
photosynthesis), all processes occur in the
mesophyll cells.
Mesophyll cells
Bundle sheath
cells
C4 Pathway
• In C4 plants
photosynthesis occurs
in both the mesophyll
and the bundle sheath
cells.
C4 Pathway
• CO2 is fixed into a 4carbon intermediate
• Has an extra enzyme–
PEP Carboxylase
(Phosphoenolpyruvat
e carboxylase) that
initially traps CO2
instead of Rubisco–
makes a 4 carbon
intermediate
C4 Pathway
• The 4 carbon intermediate
is “smuggled” into the
bundle sheath cell
• The bundle sheath cell is
not very permeable to CO2
• CO2 is released from the
4C malate  goes through
the Calvin Cycle
C3 Pathway
How does the C4 Pathway
limit photorespiration?
• Bundle sheath cells are far from the
surface– less O2 access
• PEP Carboxylase doesn’t have an
affinity for O2  allows plant to collect a
lot of CO2 and concentrate it in the
bundle sheath cells (where Rubisco is)
CAM Pathway
• Fix CO2 at night and
store as a 4 carbon
molecule
• Keep stomates
closed during day to
prevent water loss
• Same general
process as C4
Pathway
How does the CAM Pathway
limit photorespiration?
• Collects CO2 at night so that it can be
more concentrated during the day
• Plant can still do the calvin cycle during
the day without losing water
CAM Plants
Night (Stomates Open)
Day (Stomates Closed)
Vacuole
CO2
C-C-C-C
C-C-C-C
C-C-C-C
Malate
Malate
Malate
CO2
C3
C-C-C
PEP
ATP
C-C-C
Pyruvic acid
glucose
Summary of C4
Photosynthesis
• C4 Pathway
– Separates by
space (different
locations)
• CAM Pathway
– Separates
reactions by
time (night
versus day)
Bio cell uses photosynthesis to generate
electricity
• Bio cell inserted into a cactus and a graph showing the
intensity of the electric current generated as a function
of light that fell on the plant (in black, glucose, and red,
O2).Scientists at the research institute CNRS, France,
changed the chemical energy generated by
photosynthesis of a plant into electrical energy.
• The research demonstrates a new route for artificial
photosynthesis , a promising area of research that aims
to develop a strategy for conversion of sunlight into
electricity even more efficient and more environmentally
friendly than solar cells .
Artificial photosynthesis
• Artificial photosynthesis is a research field
that attempts to replicate the natural
process of photosynthesis,
converting sunlight, water, andcarbon
dioxide into carbohydrates and oxygen.
Sometimes, splitting
water into hydrogen and oxygen by using
sunlight energy is also referred to
as artificial photosynthesis.
Photoelectrochemical cell
• Research is being done into finding catalysts that can convert
water, carbon dioxide, and sunlight to carbohydrates. For the
first type of catalysts, nature usually uses the oxygen evolving
complex. Having studied this complex, researchers have made
catalysts such as blue dimer to mimic its function, but these
catalysts were very inefficient. Another catalyst was engineered
by Paul Kögerler, which uses four ruthenium atoms.
• The carbohydrate-converting catalysts used in nature are
the hydrogenases. Catalysts invented by engineers to mimic
the hydrogenasesinclude a catalyst by Cédric Tard,[3] the
rhodium atom catalyst from MIT,[4] and the cobalt catalyst from
MIT. Dr. Nocera of MIT is receiving funding from the Air Force
Office of Scientific Research to help conduct the necessary
experiments to push forward in catalyst research.
Dye-sensitized solar cell
Possibly the most exciting technological development in
nanotechnology is a photovoltaic cell that uses photosynthesis to
generate electricity. The first solar photovoltaic chip was made
using ground-up spinach tissue by scientist Shuguang Zhang at
MIT. He was building on work by a group of researchers who had
earlier figured out how to harness energy from a plant. That
group was able to extract electrical current using a plant’s
photosynthesis for a period of three weeks. Zhang’s chip
converted approximately 12% of the light energy absorbed to
electrical current. This compares to the 24% efficiency of silicon
power cells. In the future, it is hoped that by adding layers of
chips, efficiency will be increased to 20%. size of this
photosynthetic solar chip is Ten to twenty nanometers or, small
enough to fit about a hundred of them in the width of a human
hair. Would result in lightweight computers and other electronic
devices, not to mention more environmentally friendly.
Electricity Generation by
Photosynthetic Biomass
Advantages
• Dye-sensitized cells can be made at one-fifth of the price
of silicon cells.
• The solar energy can be immediately converted and
stored, unlike in PV cells, for example, which need to
convert the energy and then store it into a battery (both
operations implying energy losses). Furthermore,
hydrogen as well as carbon-based storage options are
quite environmentally friendly.
• Renewable, carbon-neutral source of energy, which can
be used for transportation or homes. Also the
CO2 emissions that have been distributed from fossil
fuels will begin to diminish because of the photosynthetic
properties of the reactions.
Disadvantages
• Artificial photosynthesis cells (currently)
last no longer than a few years[ (unlike PV
and passive solar panels, for example,
which last twenty years or longer).
• The cost for alteration right now is not
advantageous enough to compete
with fossil fuels and natural gas as a viable
source of mainstream energy.