E - Bio @ Horton
Unit – Energy Transformation
Notes – Photosynthesis
AP Biology
Transformation of Solar Energy
A. Organisms Depend Upon Photosynthesis
1. Photosynthetic organisms (algae, plants and a few other organisms) serve as ultimate
source of food for most life.
2. Photosynthesis transforms solar energy into chemical bond energy of carbohydrates.
3. Most food chains start with photosynthesizers.
B. 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 is the range of types of solar radiation based on
wavelength.
a. Gamma rays have shortest
wavelength.
b. Radio waves have longest
wavelength.
c. Energy content of photons is
inversely proportional to wavelength
of particular type of radiation.
1. Short-wavelength ultraviolet
radiation has photons of a
higher energy content.
2. Long-wavelength infrared light
has photons of lower energy
content.
3. High-energy photons (e.g., those of ultraviolet radiation) are dangerous
to cells because they can break down organic molecules by breaking
chemical bonds.
4. Low-energy photons (e.g., those of infrared radiation) do not damage
cells because they do not break chemical bonds but merely increase
vibrational energy.
d. White light is made up of many different wavelengths; a prism separates them
into a spectrum.
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 are processes, such
as vision and photosynthesis, are adapted to radiation that is most prevalent in
the environment.
5. Earth’s Energy-Balance sheet
a. 42% of solar energy hitting atmosphere reaches earth surface; rest is reflected or
heats atmosphere
b. Only 2% of 42% is eventually used by plants; rest becomes heat.
c. Of this plant-intercepted energy, only 0.1 to 1.6% is incorporated into plant
tissue.
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d. Of plant tissue, only 20% is eaten by herbivores; most of rest decays or is lost as
heat.
e. Of herbivore tissues, only 30% is eaten by carnivores.
6. Photosynthetic pigments use primarily the visible light portion of the electromagnetic
spectrum.
a. Two major photosynthetic pigments are chlorophyll a and chlorophyll b.
b. Both chlorophylls absorb violet, blue, and red wavelengths best.
c. Very little green light is absorbed; most is reflected back; this is why leaves
appear green.
d. Carotenoids are yellow-orange pigments which absorb light in violet, blue, and
green regions.
e. When chlorophyll in leaves breaks down in fall, the yellow-orange pigments show
through.
7. Absorption and action spectrum
a. A spectrophotometer measures the
amount of light that passes through a
sample of pigments.
1. As different wavelengths are passed
through, some are absorbed.
2. Graph of percent of light absorbed at
each wavelength is absorption
spectrum.
b. Action spectrum
1. Photosynthesis produces oxygen;
production of oxygen is used to
measure rate of photosynthesis.
2. Oxygen production and, therefore,
photosynthetic activity is measured
for plants under each specific
wavelength; plotted on a graph, this
produces an action spectrum.
3. Action spectrum resembles
absorption spectrum; indicates
chlorophylls contribute to
photosynthesis.
Structure and Function of the Chloroplast
A. Key Discoveries of Photosynthetic Process
1. The overall equation for photosynthesis is usually stated as carbon dioxide plus water
forms carbohydrated plus oxygen.
2. In 1930 C.B. van Niel showed that oxygen given off by photosynthesis comes from water
and not from carbon dioxide. The correct equation should then read: carbon dioxide plus
water forms carbohydrate plus water plus oxygen.
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B. Structure of Chloroplasts
1. In chloroplasts, a double membrane
encloses a fluid-filled space called the
stroma; stroma contains enzyme-rich
solution that reduces CO2, converting
it to an organic compound.
2. Even more internal membranes within
stroma form flattened sacs called
thylakoids, which are sometimes
organized into stacks called grana.
3. Spaces within all thylakoids are
connected and form an inner
compartment or thylakoid space.
4. Chlorophylls and other pigments
involved in absorption of solar energy
are embedded within thylakoid
membranes; these pigments absorb
solar energy, energize electrons prior
to reduction of CO2 in stroma.
C. Function of Chloroplasts
1. In 1905, F.F. Blackman proposed two
sets of reactions for photosynthesis.
2. Light-dependent reactions cannot take
place unless light is present.
a. Light-dependent reactions are the energy-capturing reactions.
b. Associated with light-absorbing molecules and electron transport systems of
thylakoids.
c. They involve the splitting of water and the release of O2.
d. Low-energy electrons are removed from H2O; energized when thylakoid
membrane pigments absorb energy.
e. Electrons move from chlorophyll a down electron transport system; produces
ATP from ADP and P.
+
f. Energized electrons are also taken up by NADP , becoming NADPH.
g. NADPH temporarily holds energy in form of energized electrons that will fuel CO2
reduction.
3. Light-independent Reactions
a. These reactions take place in the stroma; can occur in either the light or the dark.
b. The light-dependent reactions are synthesis reactions that use NADPH and ATP
to reduce CO2.
The Capturing of Solar Energy
A. Light-dependent Reactions
1. Occur in the thylakoid membranes and require participation of two light-gathering units:
photosystem I (PS I) and photosystem II (PS II).
2. A photosystem is a photosynthetic unit comprised of a pigment complex and electron
acceptor; solar energy is absorbed and high-energy electrons are generated.
3. Each photosystmem has a pigment complex composed of green chlorophyll a and
chlorophyll b molecules and orange and yellow accessory pigments (e.g., carotenoid
pigments).
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4. Absorbed energy is passed from one pigment molecule to another until concentrated in
reaction-center chlorophyll a.
5. Electrons in reaction-center chlorophyll a become excited; they escape to electronacceptor molecule.
B. Electrons Pathways
1. Cyclic electron pathway generated only ATP; noncyclic pathway results in both NADPH
and ATP.
a. ATP production during photosynthesis is called photophosphorylation since light
is involved.
b. This leads to cyclic photophosphorylation and noncyclic photophosphorylation.
2. Cyclic Electron Pathway
a. The cyclic electron pathway begins after PS I pigment complex absorbs solar
energy.
b. High-energy electrons leave PS I reaction-center chlorophyll a molecule but
eventually return to it.
c. Before they return, the electrons enter and travel down an electron transport
system.
1. Electrons pass from a higher to a lower energy level.
2. Energy released is stored in form of a hydrogen (H+) gradient.
3. When hydrogen ions flow down their electrochemical gradient through
ATP synthase complexes, ATP production occurs.
d. Some photosynthetic bacteria utilize cyclic electron pathway only; pathway
probably evolved early.
e. It is possible that in plants, the cyclic flow of electrons is utilized only when CO2 is
in such limited supply that carbohydrate is not being produced.
f. There is now no need for additional NADPH, which is produced only by the
noncyclic electron pathway.
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3. Noncyclic Electron Pathway
a. During the noncyclic electron pathway, electrons move from H2O through PS II to PSI and
then on to NADP+.
b. The PS II pigment complex absorbs solar energy; high-energy electrons (e’)
leave the reaction-center chlorophyll a molecule.
c. PS II takes replacement electrons from H2O, which splits, releasing O2 and H+
2H+ + 2 e’ + 1/2 O2.
ions: H2O
d. Oxygen evolved from chloroplasts and plant as oxygen gas (O2).
e. The H+ ions temporarily stay within the thylakoid space.
f. High-energy electrons that leave PS II are captured by an electron acceptor,
which sends them to an electron transport system.
g. As electrons pass from one carrier to next, energy to be used to produce ATP
molecules is released and stored as a hydrogen (H+) gradient.
h. As H+ flow down electrochemical gradient through ATP synthase complexes,
chemiosmotic ATP synthesis occurs.
i. Low-energy electrons leaving the electron transport system enter PS I.
j. PS I pigment complex absorbs solar energy; high-energy electrons leave
reaction-center chlorophyll a and are captured by an electron acceptor.
k. The electron acceptor passes them on to NADP+.
l. NADP+ takes on an H+ to become NADPH: NADP+ + 2 e’ + H+
NADPH.
m. NADPH and ATP produced by noncyclic flow electrons in thylakoid membrane
are used by enzymes in stroma during light-independent reactions.
C. ATP Production
1. The thylakoid space acts as a reservoir for H+ ions; each time H2O is split, two H+
remain.
2. Electrons move carrier-to-carrier, giving up energy used to pump H+ from stroma into
thylakoid space.
3. Flow of H+ from high to low concentration across thylakoid membrane provides energy to
produce ATP from ADP + P by using an ATP synthase enzyme.
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4. This is called chemiosmosis because ATP production is tied to an electrochemcial
gradient.
D. The Thylakoid Membrane
1. PS II oxidizes H2O and produces O2.
2. The cytochrome complex transports electrons and pumps H+ ions into the thylakoid
space.
3. PS I is associated with an enzyme that reduces NADP+ to NADPH.
4. ATP synthase complex has an H+ channel and ATP synthase; it produces ATP.
Carbohydrate Formation – The Calvin Cycle
A. Light-independent Reactions
1. The second state of photosynthesis; light is not directly required.
2. Require CO2, which enters through leaf and NADPH and ATP, which have been
produced by light-dependent reactions.
3. PS I initiates regulatory mechanism by which enzymes of the light-independent reactions
are turned on.
4. NADPH and ATP are used to reduce CO2; CO2 becomes CH2O within a carbohydrate
molecule.
5. The reduction of CO2 occurs in the stroma of a chloroplast by series of reactions called
the Calvin cycle.
B. The Importance of PGAL
1. PGAL (glyceraldehyde-3-phosphate) is product of Calvin cycle; is converted to many
organic molecules.
2. Glucose phosphate is product of PGAL metabolism; important source of sucrose, starch,
cellulose.
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3. Hydrocarbon skeleton of PGAL is used to form fatty acids and glycerol of plant oils, and
amino acids.
C. The Calvin Cycle
1. Fixation of Carbon Dioxide
a. CO2 fixation is the attachment of CO2 to an organic compound.
b. RuBP (ribulose bisphosphate) is a five-carbon molecule that combines with
carbon dioxide.
c. Enzyme RuBP carboxylase speeds reaction; is 20-50% of the protein in
chloroplasts.
2. Reduction of Carbon Dioxide
a. Six-carbon molecule immediately breaks down, forms two PGA (3phosphoglycerate [C3]) molecules.
b. Each of two PGA molecules undergoes reduction to PGAL in two steps.
c. Light-dependent reactions provide NADPH (electrons) and ATP (energy) to
reduce PGA to PGAL.
3. Regeneration of RuBP
a. Every three turns of Calvin cycle, five molecules of PGAL are used to re-form
three molecules of RuBP.
b. Every three turns of Calvin cycle, there is net gain of one PGAL molecule; five
PGAL regenerate RuBP.
c. First molecule identified by Calvin was PGA [C3], a three-carbon product; Calvin
cycle is also known as C3 cycle.
d. More recent research shows that sometimes the first molecule formed is not a
C3.
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D. Modes of Photosynthesis
1. In C3, plants Calvin cycle fixes CO2 directly; first molecule following CO2 fixation is PGA,
a C3 molecule.
2. C4 leaves fix CO2 by forming a C4 molecule prior to the involvement of the Calvin cycle.
3. CAM plants fix CO2 by forming C4 molecule at night when stomates can open without
loss of water.
4. C4 Photosynthesis
a. In a C4 plant, mesophyll cells contain well-formed chloroplasts arranged in
parallel layers.
b. In C4 plants, bundle sheath cells as well as the mesophyll cells contain
chloroplasts.
c. In C4 leaf, mesophyll cells are arranged concentrically around the bundle sheath
cells.
d. C3 plants fix CO2 to RuBP in mesophyll; first detected molecule is PGA.
e. C4 plants use the enzyme PEP to fix CO2 to PEP end produce oxaloacetate (a
C4 molecule).
f. In C4 plants, CO2 is taken up in mesophyll cells and malate, a reduced form of
oxaloacetate, is pumped into the bundle-sheath cells; here CO2 enters Calvin
cycle.
g. In hot, dry climates, net photosynthetic rate of C4 plants (e.g., corn) is 2-3 times
that of C3 plants.
5. Photorespiration
a. In hot weather, stomates close to save water; CO2 concentration decrease in
leaves; O2 increases.
b. In C3 plants, O2 competes with CO2 for the active site of RuBP resulting in
production of only one molecule of PGA.
c. Called "photorespiration" since oxygen is taken up and CO2 is produced;
produces only one PGA.
d. Photorespiration does not occur in C4 leaves even when stomates are closed
because CO2 is delivered to Calvin cycle in bundle sheath cells.
e. C4 plants have advantage over C3 plants: in hot and dry weather,
photorespiration does not occur (e.g., bluegrass dominates lawns in early
summer, crabgrass takes over in hot midsummer).
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6. CAM Photosynthesis
a. CAM (crassulacean-acid metabolism) plants form a C4 molecule at night when
stomates can open without loss of water; found in succulent desert plants in
family Crassulacaeae and other.
b. CAM plants use PEPCase to fix CO2 by forming C4 molecule stored in large
vacuoles in mesophyll.
c. C4 formed at night is broken down to CO2 during the day and enters the Calvin
cycle within the same cell, which now has NADPH and ATP available to it from
the light-dependent reactions.
d. CAM plants open stomates only at night, allowing CO2 to enter photosynthesizing
tissues; during the day, stomates are closed to conserve water and CO2 cannot
enter photosynthesizing tissues.
e. Photosynthesis in a CM plant is minimal, due to limited amount of CO2 fixed at
night; does allow CAM plants to live under stressful conditions.
Images: Campbell, Neil and Reece, Jane. Biology (6th ed.) San Francisco: Benjamin Cummings
Modified Notes: Mader, Sylvia. Biology (7th ed) New York: McGraw Hill Publishing
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