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I. Overview of photosynthesis.
A. Plants used green pigment chloropyhyll to capture light
energy.
B. Energy trapped from light used to split water molecules.
C. Hydrogen from split water molecules used to reduce organic
products of carbon fixation.
D. Overall process can be written as: CO2 + H2O --> CH2O + O2.
E. Photosynthesis is the opposite of respiration and is highly
endergonic, because H2O requires a lot of energy to split
into H and O.
F. Importance of photosynthesis.
1. Provides the basis of all life on earth because the
world's food chains are based on plant food.
2. All the oxygen in our atmosphere ultimately comes from
photosynthesis.
II. Chloroplast structure.
A. Physical structure.
1. Outer envelope of chloroplast is composed of two
plasma membranes.
2. Inside chloroplast are another set of membranes
composing flattened sacs called thylakoids.
3. Each individual sac is called a thylakoid and they are
associated is a number of stacks called grana.
4. All of the sacs are believed to be continuous, the
whole complex consisting of essentially one sac.
B. Chemical structure.
1. The interior of the thylakoids and stroma act as an H+
reservoir used for chemiosmotic synthesis of ATP.
2. The solution surrounding the thylakoids and grana
contains the enzymes responsible for carbon fixation
and the chloroplast's own DNA and ribisomes.
3. Imbedded in the thylakoid membranes are:
a. Chlorophyll and other pigments for light
absorption.
b. Chloroplast ETS.
c. Protein complexes with ATP synthetase
C. Cells may contain 1-40 or more chloroplasts.
III. Absorption of light energy.
A. Electromagnetic radiation.
1. Comes from sun in wavelengths from as long as a few
meters to as short as a few nanometers (10-9 m).
2. What we call visible light is in the range from 380-
750 nm.
3. The shorter the wavelength the more energy.
4. What we perceive as color is actually the wavelengths
of light that are reflected back from an object, all
the other wave lengths being absorbed.
B. Photosynthetic pigments.
1. Pigments are compounds that absorb certain wavelengths
of light (and reflect others) and therefore appear
colored.
2. Chlorophyll absorbs specific wavelengths in the blue
and red part of the spectrum while reflecting green.
3. Other pigments in the chloroplast absorb slightly
different wavelengths.
4. The absorption spectrum of a pigment tells us what
wavelengths of light are absorbed by the pigment.
C. Does the absorption spectrum tell what wavelengths are used
in photosynthesis?
1. In 1883 a German named Engelmann studied
photosynthesis in Spirogyra.
2. Put algae on microscope slide.
3. Exposed to light passed through a prism.
4. REMEMBER: Photosynthesis produces oxygen.
5. Part of algae in wavelengths best for photosynthesis
should produce more oxygen.
6. Aerobic bacteria appeared to cluster preferentially in
blue and red portions of spectrum.
D. Pigment molecule structure.
1. Chlorophyll.
a. Complex ring structure at one end with a Mg2+
bound in center (porphyrin ring); this is the
active site for trapping light energy.
b. Other end is a long non polar tail that anchors
it in the membrane.
2. Chlorophyll a is the main photosynthetic pigment and
it occurs in several slightly different forms that
absorb slightly different wavelengths.
3. Pigments that absorb at different wavelengths than
chlorophyll a are called accessory pigments (e.g.
chlorophylls b, c, and d).
4. Carotenoids (carotene).
a. Accessory pigments.
b. Consist of long hydrocarbon chains with ring at
either end.
c. Functions.
i. Pass absorbed energy to chlorophyll.
ii. Protect chlorophyll by binding oxygen
radicals to its double bonds.
d. In autumn chlorophyll gets broken down so the
magnesium and nitrogen can be conserved when
leaves fall. Leaves appear yellow and orange
because carotenoids become visible.
5. Red algae and blue-green bacteria have a different
group of accessory pigments called phycobilins (in
addition to other pigments that green plants have).
E. Photosystems.
1. Pigment molecules are arranged in clusters called
photosystems.
2. Antenna pigments act as the light gatherers and pass
the light energy along as vibrational energy to
adjacent molecules.
3. The vibrational energy is passed on the photosystem's
reaction center , a special chlorophyll a molecule in
the photosystem's reaction center.
4. Photosystem I contains the chlorophyll a reaction
center P700 with an absorption peak at 700 nm (long
red).
5. Photosystem II contains the chlorophyll a reaction
center P680 with an absorption peak at 680 nm (red).
6. Both photosystems need to be present for
photosynthesis to occur.
7. Antenna pigments absorb light of shorter wavelengths
and pass the energy on to a nearby reaction center.
IV. The events of photosynthesis.
A. Photochemical reactions.
1. Electrons in reaction center pigments are excited to a
higher energy level by light.
2. Reactions of this kind are called photochemical
reactions.
3. Also often called thermochemical reactions because
heat increases the rate of the reaction.
4. High energy electrons are psssed to the primary
acceptor molecules.
B. Electron transport.
1. e- passed to ETS.
2. ETS passes e- to outer surface of thylakoid where it
reduces NADP+ to NADPH + H+.
3. Some of the H+ produced is shuttled to inside of
membrane.
4. Other H+ produced in side thylakoids by splitting of
water as well as O2.
C. Chemiosmosis.
D. Carbon fixation.
1. CO2 is attached to existing organic molecules.
2. Energy for carbon fixation comes from NADPH and ATP.
3. Final products are sugars.
V. Cyclic electron flow.
A. Called cyclic because P700 is the donor and final acceptor
of the electrons passing through this cycle.
B. Electrons are passed through what is called the
plastoquinone shuttle.
C. Electron stransport is used to make ATP by oxidative
phosphorylation.
VI. Non-cyclic electron flow.
A. Overview
1. Water inside the thylakoids is split.
2 e- from water are passed to Photosystem II in the
thylakoid membrane.
3. Light striking the pigments of photosystem II excite
the 2 e- to a higher energy state and they are passed
down a series of carriers to photosystem I.
4. Light striking photosystem I excites e- to another
carrier that passes them finally to reduce NADP+ in
the stroma.
B. Water to photosystem II.
1. The electrons needed by photosystem II are provided by
the splitting of water.
2. Not much is known about the molecule that removes the
electrons from water except that it is a protein with
an associated Mn2+.
3. H+ is left behind in the interior of the thylakoid.
4. Left over O atom joins with another to form O2 which
diffuses out of cell.
5. The release of H+ into the thylakoid acts to decrease
the H+ potential.
C. Photosystem II to photosystem I.
1. Light striking photosystem II excites an electron.
2. Electron is passed to a carrier and then to
plastoquinone.
3. Plastoquinone picks up 2 electrons along with two H+
from dissociated water in the stroma.
4. On the inner surface of the thylakoid plastoquinone
reduces a cytochrome which only carries electrons.
5. Plastoquinone releases two H+ into thylakoid further
decreasing the H+ potential.
6. The electrons move down the transport chain to P700.
D. Photosystem I to NADP+.
1. Light striking photosystem I excites electrons in
P700.
2. After being energized the electrons are passed down
another series of carriers.
3. Two electrons from P700 eventually reduce NADP+ in the
stroma.
4. When accepting two electrons NADP+ also takes up two
H+ from the stroma to make NADPH + H+.
VII. Chemiosmotic synthesis revisited.
A. Potential energy.
1. RECALL: in mitochondria the pH difference was ca. 1.4
and membrane potential was ca. -140 mV.
2. Energy in chloroplasts almost entirely due to H+
gradient.
a. pH difference is 3.5.
b. Membrane is freely permeable to Cl- so that
essentially no membrane potential exists.
c. Chemical gradient supplies ca. 4.8 kcal per mole
of H+.
B. Contrast with respiration.
photosynthesis respiration
e- pass away from O. e- pass away from H
carriers.
e- pass to NADP+ e- pass to O.
H+ accumulates inside. H+ accumulates outside.
ATP synthetase outside. ATP synthetase inside.
ATP released into stroma. ATP released inside matrix.
VIII. The Calvin Cycle.
A. Carbon fixation occurs in the stroma and uses NADPH and ATP
to fuel the reactions involved.
1. Enzymes in stroma attach CO2 to a preexisting organic
molecule.
2. Molecules is then processed through a variety of
different pathways.
B. Discovery of Calvin Cycle.
1. Melvin Calvin and his fellow scientists elucidated the
steps in the Calvin cycle or C3 cycle.
2. Used the radioactive isotope carbon-14 to trace the
series of reactants.
3. After CO2 deprivation, photosynthetic cells were
exposed to 14CO2 as their only CO2 source.
4. At very short intervals cells were removed and killed
by putting in boiling methanol.
5. The organic molecules of the cells were then separated
by paper chromatography.
6. Because only the reactions that incorporated 14CO2
would be radioactive on the paper chromatographs,
Calvin was able to determine the order that the
different compounds appeared in the cycle.
7. When CO2 is eliminated from a suspension of
photosynthetic cells ribulose bisphosphate (RuBP)
accumulates.
8. When 14CO2 is added the RuBP disappears and 14C-
containing molecules of phosphoglycerate (PGA) appear.
C. Details of Calvin cycle.
1. CO2 adds to 5-C RuBP making an unstable 6-C
intermediate that breaks down to two 3-C PGA
molecules.
2. PGA's are reduced to PGAL by phosphorylation from ATP
and then hydrolysis of the phosphate group. The H for
the reduction comes from NADPH.
3. Some of the PGAL is sent into pathway for formation of
6-C sugars (2 3-C PGAL's joined to form a hexose
sugar).
4. Most PGAL goes to make more RuBP. Out of each 12 made
2 go to hexose synthesis, 10 go to remake 6 5-C
RuBPs.
5. Some PGAL may also enter cycle for production of amino
acids.
6. RuBP carboxylase makes up about 25% of the protein in
chloroplasts.
7. Overall reaction:
RuBP + CO2 + 2 NADPH + 2 H+ + 3 ATP -----> RuBP +
CH2O + 2 NADP+ + 3 ADP + 3 Pi.
8. 6 turns of the cycle are needed to fix the equivalent
of a 6-C sugar.
9. In darkness the supply of ATP and NADPH quickly
deplete and carbon fixation stops.
IX. Factors affecting rate and efficiency of photosynthesis.
A. Environmental factors.
1. At low light ATP and NADPH are used up for carbon
fixation as quickly as they are produced.
2. At high light more ATP and NADPH are produced than can
be used up by carbon fixation and the rate of
photosynthesis levels off.
3. From low to medium temperatures photosynthesis
increases with rising temperature.
4. At high temperatures photosynthesis is inhibited.
5. Lack of water can be an indirect cause for reduced
photosynthesis.
a. Plants lose water through their stomata.
b. Stomata must be open so plant can obtain CO2.
c. If plant loses too much water stomata close and
no CO2 can enter leaf.
6. O2 buildup can also reduce photosynthesis.
a. Interferes with electron transport by binding to
electron transport molecules.
b. At very high levels it can destroy photosynthetic
pigments by oxidation.
c. More profound effect of O2 is photorespiration.
B. Photorespiration.
1. Because molecular structures of CO2 and O2 are similar
O2 competes for RuBP carboxylase.
2. When O2 is bound RuBP carboxylase oxidizes RuBP
forming one PGA that remains in the Calvin cycle and a
2-C molecule that must be processed elsewhere.
a. 2-C molecule is processed in a peroxisome and a
mitochondrion.
b. Some carbon released as CO2 and the rest is
salvaged and returned eventually to chloroplast.
3. Bright light, high temperatures, high O2, and low CO2
lead to increased photorespiration.
4. Up to 50% of the fixed CO2 may be lost to
photorespiration on some days.
X. Special adaptations.
A. Leaf anatomy.
1. Usually flat and broad for maximum exposure to
sunlight.
2. Epidermis composed of tightly packed cells covers
surface.
3. Surface cells secrete waxy substances that form a
cuticle that prevents water loss.
4. Pores in epidermis called stomata allow gas to enter
and leave air spaces in leaf and are generally open
during the day.
5. Between the upper and lower epidermal cells are
photosynthetic cells called mesophyll cells.
a. Columnar cells called palisade mesophyll lies
under the upper epidermis.
b. Below palisade layer is spongy mesophyll in which
the cells are more loosely arranged with air
spaces.
6. Running through the mesophyll are vascular bundles.
B. C4 or Hatch-Slack pathway.
1. Pathway named after Hatch and Slack (Australians)
(Kortschalk also did some of the work but his name was
left out).
2. Most plants possess PEP carboxylase in at least small
quantities; it adds CO2 to PEP to form oxaloacetate.
3. Plants that possess large quantities of this enzyme
usually have structurally different leaves than C3
photosynthesizers.
a. Leaves have Kranz (= wreath) anatomy in which
photosynthetic bundle sheath cells form a tight
circle around vascular bundles.
b. Mesophyll cells fill the rest of the leaf's
interior with very little air space.
c. Carbon dioxide is fixed in the mesophyll cells.
d. Resultant oxaloacetate is converted to other
compounds and moved via plasmodesmata to bundle
sheath cells.
e. In bundle sheath cells CO2 is removed and
shuttled into the normal C3 cycle.
4. C4 pathway increases photosynthesis and decreases
photorespiration.
a. Enzyme that fixes CO2 in mesophyll cells uses
bicarbonate ion (HCO3-) rather than CO2 so no
competition problem.
b. Transfer of C4 molecule to bundle sheath and
decarboxylation increases CO2 in bundle sheath so
CO2 has a better chance of binding to RuBP
carboxylase.
c. Transfer C-4 molecule usually malate which also
yields NADPH upon decarboxylation.
d. NADPH produced by decarboxylation reduces the
bundle sheath's need for NADPH in C3 cycle.
e. Bundle sheath cells contain reduced amounts of
photosystem II and carry on a lot of cyclic
electron flow.
f. Photosynthetic rate is faster even though 5 ATP
used per carbon fixed rather than 3 ATP as in C3
synthesis.
5. Especially common in dry tropical plants.
a. Good adaptation for water stress; stomates may be
partly closed but because of the C4 pathway the
plant can still capture enough CO2.
b. examples include crabgrass and other weeds; crop
plants like sugarcane and corn.
C. Crassulacian acid metabolism.
1. Most CAM plants are desert succulents.
2. To reduce water loss stomata are kept closed during
the day.
3. Stomata open at night when CO2 is fixed into organic
acids.
4. During day CO2 is removed from acids and is refixed
into the C3 pathway using light energy.
5. Because they must very carefully conserve water they
photosynthesize very slowly.
6. Both CAM and C4 pathways store CO2 for later use; CAM
does it temporally whereas C2 does it spatially.
D. Sun vs. Shade plants.
Sun Shade
Soybeans, cotton, tomato. many ferns, African Violets.
< photosynthesis with < light. Low photosynthesis.
More palisade cells. Fewer palisade cells.
More RuBP carboxylase. Less RuBP carboxylase.
More stomata/surface area. Fewer stomata/surface area.
Thicker leaves. Thinner leaves.
Lower chlorophyll:carboxylase. Higher chlorophyll:carboxyl.
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