10.1
Provides 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.
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.
10.2
Structure and Function of Chloroplasts
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.
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.
10.3 Solar
Energy Capture
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).
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 electron-acceptor
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.
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, photolysis,
releasing O2 and H+ ions:
H2O 
2H+ + 2 e- +
1/2 O2.
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 - Chemiosmosis.
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.
n. The
photochemical splitting of water in the light-dependent reactions of
photosynthesis,
catalyzed by a specific enzyme is called Photolysis.
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.
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.
10.4
Carbohydrate Synthesis
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.
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.
also known as Rubisco.
The Rubisco enezyme is
present in the
chloroplasts in the largest amounts of any protein, and it may be the most
abundant proteins in
the biosphere. The Rubisco enezyme
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 (3-phosphoglycerate
[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.
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
use RuBP carboxylase, Rubisco, to fix CO2 to RuBP in mesophyll; first
detected molecule is PGA.
e. C4 plants
use the enzyme PEP carboxylase (PETCase)
to fix CO2 to PEP (phosphoenolpyruvate);
end product to 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 C4 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
carboxylase, 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).
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 CAM plant is minimal, due to limited amount of CO2 fixed at
night; does allow
CAM plants to live under stressful conditions.