CHAPTER 7 - PHOTOSYNTHESIS
I. OVERVIEW OF PHOTOSYNTHESIS
"Without this flow of energy from the sun, channeled largely through the eukaryotic cells, the pace of life on this
planet would swiftly diminish and then, following the inexorable second law of thermodynamics would virtually
cease altogether." Raven, P. H. et al. 1992. Biology of Plants. 5th Edition. Worth Publ.
A. Overall reaction
CO2 + H2O + energy C6H12O6 (glucose) + O2
Seems simple, but hold on to your hats! This overall reaction , like respiration is carried out in many small
steps, nearly all catalyzed by special enzymes.
1. Base of food chain
The first organisms were heterotrophs which consumed energy-rich organic molecules available in their
environment and, using respiration, used this energy for life’s functions releasing CO2 as a by-product.
This worked OK, but this energy source is basically non-renewable, so as living organisms became more
abundant, competition became keener and extinction of all life (remember, we are talking about single-
celled forms at this point) was inevitable. However, approximately 3 billion years ago, some organisms
stumbled on a very good trick and became photosynthetic autotrophs. This is probably the single most
important event in the history of life on earth because in the current biosphere, photosynthesis is
responsible for almost all of the conversion of energy from forms unusable by living organisms into
chemical energy that living things can use to grow, develop, reproduce and do work.
2. Source of O2 in atmosphere
As a by-product of photosynthesis, autotrophs release oxygen into the atmosphere and it is this reaction
that has produced the oxygenated atmosphere that allowed the evolution of oxygen-using life forms like
us. How do we know this? If you dig up rocks iron-containing older than 3-billion years, the iron is in
them is not oxidized (rusty) indicating that there was no oxygen available for this spontaneous reaction to
proceed. However, if you dig up newer rocks, all the iron in them is oxidized.
3. Greenhouse effect
This change in the atmosphere also changed the overall temperature of the planet. CO 2 is a powerful
greenhouse gas. Because the primitive atmosphere was rich in it, temperatures were much higher. As
humans are releasing much of the ancient carbon stored in fossil fuels like coal, oil, and natural gas, we
are seeing an increase in atmospheric CO2,and we could be pushing the atmosphere back towards its
II. PHOTOSYNTHESIS: A HISTORICAL PERSPECTIVE
Many scientists contributed to the understanding of photosynthesis, each contributing a small piece of the
A. Jan Baptista van Helmont (1644)-Belgian Physician: Pot + 200 lb earth + 5 lb willow shoot. He covered
the pot with perforated plate to keep soil out and watered with rain or distilled water. After 5 years, earth = 200
lbs - 3 oz, tree 169 lbs. Conclusion: Plant growth came from water. (Plants not soil eaters)
B. Joseph Priestley (1771)- English scientist: Discovered that plants can restore air injured by burning of
candles. A candle enclosed in a bell jar will go out. A mouse put into a bell jar will die. If a plant is placed in
the bell jar, the mouse will not die and a candle will burn. (First demonstration that plants produce oxygen). A
medal presented for his work reads “For these discoveries we are assured that no vegetable grows in vain ….
but cleanses and purifies our atmosphere”
C. Jan Ingenhousz (1778)- Dutch Physician: Confirmed Priestley’s work and showed that light is necessary
for the process. He reported that plants in the dark “contaminate the air and make it harmful to animals” but
that in the light it could purify the air. He also reported that only the green parts of a plant could purify the air.
He was alarmed enough by his findings that he recommended removing all plants from the house at night. This
superstition exists today, 200 years later.
D. C. B. Van Neil (1920s): Studied a photosynthetic bacterium (the purple sulfur bacteria) that used sulfur as
an electron source during photosynthesis and deposited sulfur as a byproduct. It does not release oxygen. He
determined that the following reaction takes place.
CO2 + 2 H2S Light CH2O + H2O + 2S
He then extrapolated to a general equation for photosynthesis as -
CO2 + 2H2O Light CH2O + H2O + O2
His conclusion: Oxygen released by plants comes from water, not CO2
E. F. F. Blackman (1905) English Plant Physiologist: Concluded that there were two reactions, one of which
was temperature dependent and required enzymes; one of which was temperature independent (didn’t require
enzymes) but was light dependent. . It opened the door to understanding that photosynthesis takes place in two
separate reactions, a light-dependent reaction and a light-independent reaction called the biochemical reactions
(sometimes called the dark reactions because they don’t require light). These studies were confirmed by
Robin Hill who showed that isolated chloroplasts could produce oxygen in the absence of CO 2. This splitting
of water has become known as the Hill reaction. It is the light dependent reaction.
III. THE NATURE OF LIGHT
A. The electro-magnetic spectrum [Figure 7.4]. Visible light is only a small part of the electro-magnetic
spectrum. Beyond the visible spectrum on the violet end are short wavelength, high energy forms of radiation
like UV light; X-rays (imaging is possible because they penetrate flesh); Gamma radiation. On the other end
are long-wavelengths with low energy that is insufficient for most biological purposes and is, therefore
converted to heat.
1. Ultraviolet light, X-rays and gamma rays have too much energy for biological systems. They break
bonds, knock electrons out of orbit and generally cause cell damage. We are protected from UV by the
2. Infrared light is heat and has too low an energy level
B. The visible spectrum is from about 390 – 760 nm, and includes the familiar rainbow of colors ranging from
violet to blue. There’s nothing REALLY special about these wavelengths except that their energies are within
the range that biological processes can handle in energy conversions like photosynthesis or vision (conversion
to nervous impulses). That is, biological processes make photons in this range special because they use them,
not the other way around.
C.. Light comes in small packets called photons. Whose wavelength and energy content are functions of how
fast they vibrate. Since a vibrating particle moving in a straight line will produce a wavy trajectory, light and
photons act like both waves and particles which is why we use the terminology for both when talking about
them. One wavelength = The distance moved by a photon during one complete vibration. The shorter the
wavelength, the higher the energy (the faster the vibration).
IV. THE ROLE OF PIGMENTS
A. In order to trap light, light must be absorbed. If light is reflected, it has no value.
B. Pigments are organic molecules that absorb light and capture the energy of the photons. Pigments are
generally specific in terms of which photons they absorb, reflect or transmit, and their colors are determined by
the color of the photons that are NOT absorbed. [Fig 7-5] Black pigments absorb everything; white pigments
absorb nothing. Many biological pigments are built from ring structures that surround an atom of a metal (iron,
magnesium, copper). Photosynthetic pigments are soluble in lipids (they occur in membranes, remember?)
C. The wavelengths at which a pigment absorbs light are called the absorption spectrum. The wavelengths
that activate a biological process such as photosynthesis are called the action spectrum
D. Chlorophyll a absorbs wavelengths of light at 400-500 nm (violet-blue) and 600-700 nm (orange-red) [Fig
7-6]. The structure of chlorophyll is given in Fig 7-9. Note the porphyrin ring.
E. Accessory pigments ( Function: to absorb light at wavelengths not absorbed by chlorophyll a)
1. Chlorophyll b - Maximum absorption at ca. 480 nm and 642 nm (There are also minor chlorophyls c
2. Carotenoids-Absorb maximally between 460 and 550 nm, absorb in the blue range and therefore appear
red, orange and yellow. They are chemically unrelated to chlorophylls. Beta-carotene is the most
common carotenoid. When split in half, it becomes two molecules of Vitamin A.
3. Xanophylls (oxidized carotenoids) (red and yellow pigments in carrots, tomatoes, leaves)
4. A third major class of pigments are the phycobilins, found only in the cyanobacteria and red algae.
V. THE REACTIONS OF PHOTOSYNTHESIS
A. Chloroplasts are the site of photosynthesis. They are thought to be endosymbionts and have their own DNA
and prokaryotic genes. Some essential proteins are imported from outside the chloroplast and are encoded by
the nucleus. There are 40-200 chloroplasts/cell. They have two membranes (remember the endosymbiont
hypothesis). [Fig 7-8]
B. Structure of the chloroplast
1. Stroma: Gelatinous matrix inside the chloroplast containing prokaryotic ribosomes, DNA.
Carbohydrate synthesis occurs in the stroma.
2. Thylakoids: Folded membranes called thylakoids fill the stroma and form into stacks called grana.
Pigment in this membrane is chlorophyll. Much of the lipid in membranes is chlorophyll.
C. The reactions that occur in photosynthesis are divided into two separate processes: light reactions (light-
dependent reactions) and carbon-fixation reactions (also called dark reactions, biochemical reactions or light
independent reactions because they don’t require light). These reactions are summarized in Fig. 7-11.
Light + 6CO2 + 6 H2O C6H12O6 + 6 O2 + energy
Photochemical reactions: 6 H2O 6 O2
Carbon fixation reactions: 6CO2 C6H12O6
1. Light dependent stage (takes place in the thylakoid membranes)
a. Requires light
b. Independent of temperature (an indication that enzymes are not involved)
c. Light is used to make ATP and to reduce electron carrier molecules
d. During the light reaction, light energy generates NADPH and ATP and splits water to produce
2. Dark dependent stage (takes place in the chloroplast stroma)
a. Occurs in the light but does not require light
b. Temperature dependent or sensitive reactions. Reactions increase up to 30C then decrease
c. This temperature profile is an indication that enzymes are involved
d. Energy products of the light reaction are used to reduce carbon from CO 2 to a sugar (carbon
VI. TWO PHOTOSYSTEMS ARE INVOLVED IN THE LIGHT REACTION
A. There are two kinds of light-trapping centers located in the chloroplast thylakoid membranes. They
are photosystem I and photosystem II. In most photosynthetic organisms, these work together.
Photosystem I and photosystem II are connected by an electron transport chain. Photosystem I: Absorbs
maximally at 700 nm. Photosystem II: Absorbs maximally at 680 nm.
B. Each photosystem is composed of pigments arranged in an aggregate of pigment molecules consisting
of a protein matrix, ~300 molecules of chlorophyll a, ~50 molecules of carotenoids and other accessory
pigments (to expand the wavelengths of light energy that can be absorbed).
C. The pigment aggregates have been given the name antenna complex because it acts as an antenna to
trap light and transfer the absorbed energy.
1. Carotenoids: Outermost pigments that first capture the energy from photons and transfer them to
2. Chlorophyll b accepts energy from carotenoids and transfers them to chlorophyll a.
3. Chlorophyll a in antennae complexes receive energy from chlorophyll b and transfers it to special
chlorophyll a molecules called the reaction center.
D. Reaction center: Within the antenna complex, there is a special pair of chlorophyll a molecules and
associated proteins called a reaction center. Function of the reaction center: The only pigment complex
which can accept electrons transferred from the antenna complex and boost them into higher orbit where
they can be transferred to another molecule.
1. The three fates of this energy are: a) heat, b) fluorescence, c) energy is passed to a neighboring
E. Photophosphorylation and Chemiosomosis in chloroplasts. There are an additional set of protein
complexes in chloroplast thylakoid membranes which transfer electrons (this is similar to electron transport in
the mitochondrial membranes). Electrons are passed along a series of electron carriers. When electrons reach
transmembrane pumps, protons are pumped into the lumen. This proton gradient drives synthesis of ATP.
VII. IN THE LIGHT REACTIONS, ELECTRONS FLOW FROM WATER TO PHOTOSYSTEM II TO
PHOTOSYSTEM I TO NADPH+
A. Where do electrons come from? Electrons are removed from water. Four electrons are removed from 2
water molecules, splitting water into H+ and oxygen (so now you know why photosynthesis generates oxygen).
B. Noncyclic Photophosphorylation and chemiosmosis: So how is this energy actually used? Basically, the
energy of a photon is used to boost an electron in the pigment molecule into a higher energy orbital around the
atom’s nucleus. Much like chemiosmosis in respiration, this high energy electron gets passed through a series
of carrier molecules called an electron-transport chain, participating in redox reactions. Some of these
molecules act as proton pumps which move protons into the thylakoids and generate a gradient between the
thylakoid space and the stroma (chemiosmosis). The potential energy of this gradient is then used to
synthesize ATP by special protein complexes called coupling factors which include an ATP synthase
C. The process [Figs 7-14, 7-15]
a. 2 Photons absorbed in PSII embedded in thylakoid membrane and their energy are transferred to the
P680 reaction center, where they excite 2 electrons in the chlorophyll a of P680. This chlorophyll
transfers these electrons to the “primary acceptor” pheophytin.
b. This loss of electrons by P680 results in their replacement with electrons stripped from water by the
“Z-protein” also using light energy. This releases four electrons, and these are fed one at a time via the
manganese in the Z-protein. This step uses water and releases oxygen.
c. The 2 electrons are passed from pheophyton to plastiquinones which are on the outer surface of the
d. Plastiquinone Qb passes the 2 electrons to the cytochrome complex,
The cytochrome complex passes the electrons to plastocyanin, pumping a proton from the stroma to the
thylakoid space in the process. This stores energy in the proton gradient for ATP synthesis.
e. The electrons then enter PS I where both they are re-excited using light energy by the PS I antennae
complex go the P700 reaction center.
f. These electrons are passed through a series of carriers to ferredoxin (an iron containing protein on the
outer surface of the thylakoid) and NADP+ reductase generating an NADPH from NADP+ H+ and the 2
electrons in the process. This NADPH is released into the stroma and used in the so-called “dark”
g. So far, what these reactions have produced is just stored energy in two forms: 1) a proton gradient
across the thylakoid membrane 2) high energy bonds in ATP and NADPH. NO CARBON HAS BEEN
D. Cyclic photophosphorylation generates only ATP
1. Cyclic electron flow (prokaryotes): Involves only photosystem I and is found only in prokaryotic
organisms (photosynthesizing bacteria). Electrons are cycled through a series of carriers and return to the
same reaction center with the same starting energy. Hydrogen is released to the thylakoid interior,
creating an electrical/chemical gradient (see pH). The hydrogen gradient (chemiosmosis) drives ATP
phosphorylation via ATP synthase imbedded in the membrane). The added energy from photon
absorption is used for ATP production via chemiosmosis or lost as heat. No NADP is made. [Fig 7-16].
Plants and algae can also carry out this reaction in addition to non-cyclic light reactions.
2. Cyclic phosphorylation alone may have existed for a billion years in bacteria which used sulfur as an
electron donor but modern plants use water as a source of electrons and that required a major change
because the energy in P700 isn’t high enough to remove an electron from water. Green plants, algae and
some photosynthetic bacteria overcame this problem by grafting on a second, more powerful photosystem
which could harvest shorter, higher energy wavelengths of light (The Z scheme of showing electron flow
shows energy levels of electrons in the pathway rather than showing molecules imbedded in membranes).
VIII. THE CARBON-FIXATION REACTIONS
A. In a second series of reactions, ATP and NADPH generated in the “light” reactions is used to fix carbon
dioxide into simple sugars. For most plants, CO2 reaches the photosynthetic cells through stomates.[Fig 7-17 ].
For water plants, CO2 is dissolved in the surrounding water.
B. The Calvin Cycle: The Three-Carbon Pathway (C3): (Sometimes called the C3 pathway)
Overall reaction: 6 CO2 + 18 ATP + 12 NADPH + 12 H2O C6H12O6 + 18 ADP + 18 Pi + 12 NADP + + 6H2O
+ 6O2 (note that this is double the reaction given in your book – this is the reaction required to produce 1
C. Process: Plants that use the Calvin cycle to fix carbon into sugar are called C3 plants: cereals, peanuts,
cotton, sugar beets, tobacco, most trees and lawn grasses. Energy that drives the reaction comes from ATP and
NADPH. Glucose is produced after 6 revolutions of the cycle. Three ATPs enter per cycle (18 ATPs per
molecule of glucose) and two NADPH molecules enter per cycle (12 per molecule of glucose). Glucose-3-
phosphate (G3P) reforms ribulose bisphosphate. The enzyme involved is RuBP carboxylase/oxygenase.
Does it take as many ATPs and NADPHs to make a 6-carbon sugar as is released in ATPs and NADPHs
of aerobic respiration? In theory, 36 ATPs are produced by aerobic respiration and 18 ATPs + 12 NADPHs are
used to make a molecule of glucose but energy is lost all along the way. You can’t get something for nothing.
D. Most fixed carbon is converted to sucrose or starch: Glucose from the Carbon-fixation reactions is
usually converted to starch and stored in the chloroplast as starch grains. At night, the starch is converted to
sucrose and transported to the leaves via the vascular system. In the leaves and other tissues, sucrose is
converted to glucose and enters respiration.
A. RuBP carboxylase/oxygenase has can bind both oxygen and CO2.
B. If CO2 concentration gets low, then the enzyme rubisco starts binding oxygen instead. This can happen
because plants let carbon dioxide into leaves via stomata and they lose water at the same time. When it is hot
and dry, they can lose too much water, and to prevent this, they close their stomata. The reactions of the Calvin
cycle can then deplete the carbon dioxide available. This forces another pathway - O2 + RuBP PGA +
phosphoglycolate. The reactions called photorespiration because they occur in the light, use oxygen and
release CO2. Unlike normal respiration, it produces no ATP. It is a wasteful salvage pathway
X. THE FOUR-CARBON PATHWAY IS A SOLUTION TO RESPIRATION
A. Some plants have developed mechanisms to bypass the RuBP carboxylase/oxygenase problem
B. C4 plants [Figs 7-24, 7-25]: These plants are called C4 plants because they add another cycle in front of
the Calvin cycle that involves synthesis of a 4-carbon sugar. The Calvin cycle is based on 3-carbon sugars, and,
therefore, is also called C3 photosynthesis. C4 photosynthesis fixes carbon initially as a 4-carbon molecule and
is one way that plants can minimize photorespiration. Basically, these plants store carbon dioxide temporarily
as oxaloacetic acid in mesophylll cells in the light and then release it to the Calvin cycle in a controlled manner.
1. Special anatomy: C4 plants first fix carbon in mesophyll cells and then transfer it to bundle sheath cells
which surround the vascular tissues.
2. Special biochemistry: Mesophyll cells lack the enzyme rubisco and contain PEP carboxylase instead.
They use this enzyme to fix carbon into oxaloacetic acid which is then converted to malic acid or aspartic
acid. These compounds cross into the bundle sheath cells via ATP-dependent active transport. Bundle
sheath cells split the CO2 from malic acid and aspartic acid and feed it into the normal Calvin cycle
reactions with rubisco.
3. Consequences: Bundle sheath cells can maintain 20 –120 times the concentration of CO2 as normal
cells. Rubisco does not fix oxygen instead of CO2, even in very high light under dry conditions.
Therefore, rubisco is more efficient because it is “insulated” from high concentrations of oxygen. This
provides increased water efficiency because stomata can be closed longer. Plants out compete C3 plants
in hot dry weather only, BUT this advantage is only under hot/dry conditions because they have to use
ATP to do it, and ATP isn’t free.
4. Current experimental evidence suggests that this evolved several times is many different evolutionary
lines, both monocot and dicot. Important C4 plants are corn, sugar cane and sorghum.
XI. PLANTS HAVING CRASSULACEAN ACID METABOLISM (CAM) CAN FIX CO 2 IN THE
DARK (ANOTHER SOLUTION TO THE PROBLEM): CAM plants open their stomata and fix C02 using
PEP carboxylase to form malic acid at night when water losses are minimal, and store it in vacuoles. During
the day, they keep their stomates closed to save water, but liberate the stored CO 2 and use the energy of the sun
and the normal Calvin cycle reactions to form 3-carbon sugars. Some of these are polymerized into starch, a
precursor to PEP. Most of these plants are succulents living in hot/dry habitats and are very efficient with
water. Cam is more widespread than C4 plants and is found in at least 23 families of flowering plants.
Pineapple and Spanish moss are CAM plants. Generally CAM plants grow more slowly and compete poorly
with C4 and C3 plants except in very arid environments.
XII. THE CARBON CYCLE AND GLOBAL WARMING
A. CO2 is one of several greenhouse gasses that act to hold heat by absorbing infrared radiation.
B. Living plants incorporate and store CO2 and release O2 to the atmosphere. Fossil fuels account for
4000 billion tons of carbon that has been stored for millions of years.
C. The deep ocean is also a CO2 “sink”
D. Northern (boreal) forests are an important sink for CO2
E. The level of CO2 in the atmosphere is increasing, in major part due to human activities (deforestation,
burning of fossil fuels, pollution). This increased warming has been predicted since the early 1900’s (100
years ago) but has been largely ignored.
F. Global warming is now in progress. The earth’s temperature has risen, the ice caps and glaciers are
melting at unprecedented rates, tropical elements are moving north. Weather is increasingly violent and
G. There are some dire and not so dire predictions for the effects of global warming depending on the
point of view of the predictor.
H. Virtually all of the scientific community and many world leaders agree that global warming is a fact.
The Kyoto treaty was designed to reduce greenhouse emissions but the US, one of the worst polluters, has
now refused to sign the treaty.