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Photosyntheisis Light Dependent Reactions

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					Photosynthesis I – Light-Dependent Reactions                          Name:
                                                                      Course ID:
Introduction:

       Photosynthesis literally means “synthesis” using light. The net reaction for photosynthesis
involves the synthesis of carbohydrates from carbon dioxide and water in association with the
evolution of oxygen gas (Equation 1)

                                    Light, chlorophyll
       Eq. 1     6 CO2 + 6 H2O                             C6H12O6 + 6 O2

The process of photosynthesis is generally thought to consist of two distinct sets of reactions: a
series of light-dependent reactions, termed the thylakoid reactions, based on their location of the
internal (thylakoid) membranes of chloroplasts and a series of light independent reactions, termed
then the stroma reactions, based on their location in the interior fluid (stroma) of the chloroplasts.
Today’s lab will focus specifically on the light-dependent reactions.
        In the light dependent reactions, light absorption is used to drive the synthesis of ATP and
NADPH (Equation 2) that are used as sources of energy and reducing power, respectively, for
carbon fixation in the light-independent reactions.

                                                Light, chlorophyll
       Eq. 2     H2O + ADP + Pi + NADP+                                ½ O2 + ATP + NADPH + H+

The light reactions originate with the coordinate loss of “excited state” electrons from a pair of
specialized chlorophyll a molecules, termed the “reaction center” chlorophyll molecules (see Part
A, below). Excited-state electrons are electrons that have absorbed energy and moved into a higher
energy orbital (typically an oribital located further from the nucleus of an atom). In the case of the
reaction center chlorophyll a molecules, electrons may be excited through direct light absorption or
via energy transfer from adjacent photosynthetic pigments following their light absorption.
        The electrons lost from chlorophyll a are transferred through a series of electron carriers
located in the thylakoid membranes in association with the generation of a pH gradient across the
membranes that is used to drive ATP synthesis via chemiosmosis. Ultimately, the transported
electrons are used for the reduction of NADP+ and the oxidized chlorophyll a molecules of the
reaction center are reduced via the splitting of water by means of a process termed photolysis.
Aside from generating electrons, photolysis also produces protons that contribute to the
development of a pH gradient across the thylakoids and oxygen gas.

Supplies:

Equipment: Flood lights, ring stands and clamps, forceps, 250 ml beakers, #4 cork borers, 60 cc
syringes, Coupland jars (used chromatography chambers), rulers, pencils, meter sticks, petri plates

Plant Material: Tobacco (Nicotiana tabacum) and English Ivy (Hedera helix) stockplants

Reagents: Boiled distilled water, 0.02% and 0.2% sodium bicarbonate, tapwater with detergent
(Tween-20, 3 drops per 200 ml), chromatography solvent (petroleum ether:acetone:propanol
[84:15:1] ), chloroplast extract in petroleum ether

Consumables: Glass capillaries, disposable gloves, Baker-flex® Silica Gel 1B TLC sheets, foil
                                                                                  Biol 219, Lab 6 -2-

A) Separation and Identification of the Photosynthetic Pigments

       In theory, the only pigment molecule that is essential for photosynthesis in plants is
chlorophyll a, whose photooxidation represents the source of electrons for NADP+ reduction and
ATP synthesis via chemiosmosis. However, the thylakoid membranes of the chloroplasts typically
contain several additional pigments, including chlorophyll b and the carotenoids (carotenes and
xanthophylls), termed accessory pigments (Fig. 1). The photosynthetic pigments and associated
thylakoid proteins are organized so as form larger protein-pigment complexes termed photosystems.
Each photosystem consists of a more or less centrally position pair of reaction center chlorophyll a
molecules surrounded by several hundred accessory pigments including additional chlorophyll a
molecules that are not directly involved in electron transfer. In contrast with chlorophyll pigments
which are green to blue-green in color the carotenoid pigments of chloroplasts are typically yellow,
orange, or pick in color; however, they are normally masked by the green color of the more
abundant chlorophyll molecules.




Figure 1. Structures of several common photosynthetic pigments.


        When conditions are favorable for photosynthesis the primary function of the accessory
pigments appears to be as antenna pigments that transfer light energy to the reaction center
chlorophyll molecules by resonance energy transfer. Since the absorption spectra of the accessory
pigments are distinct from that of chlorophyll a, the accessory pigments increase the percentage of
visible light that is able to be used in photosynthesis. However, even if the absorption spectra of the
accessory pigments did not differ from chlorophyll a, they are necessary to maintained more less
continuous electron flow, i.e., the rate of light absorption by the reaction center chlorophyll
molecules is insufficient to maintain a stable level of electron carrier activity.
        In contrast, during periods of excess illumination the accessory pigments protect against
photoinhibition. Photoinhibition is defined as the inhibition of photosynthesis by excess light. The
basis for photoinhibition appears to be the reaction between excited state chlorophyll molecules and
molecular oxygen. This reaction generates active oxygen species, especially single oxygen, which
damages the proteins of the photosystem II reaction center. Reactions between excited chlorophyll
molecules and oxygen are typically only observed when the chlorophyll molecules are delayed from
returning to a “ground state” via oxidation (electron loss) or resonance energy transfer. In such
                                                                                   Biol 219, Lab 6 -3-
instances, carotenoid pigments, which do not contain sufficient energy in their excited state to
produce single oxygen can serve a photoprotective role by removing excited state electrons from
chlorophyll by resonance transfer. In addition, the carotenoid zeaxanthin is also thought to protect
against photoinhibition by binding to the protein component of the photosystems so as to cause
conformational changes that appears to lead to chlorophyll quenching via heat dissipation.
        Aside from accessory pigments, chloroplasts also contain an additional pigment type termed
pheophytin. Pheophytin, is equivalent to chlorophyll, but lacks a magnesium atom. Pheophytin is
not involved in light absorption during photosynthesis but rather functions as an initial component
of the thylakoid electron transport chain.
        In this exercise we will use chromatographic techniques to separate the pigments of a
chloroplast extract. In chromatography, molecules are separated based on their differing affinity for
mobile and stationary phases. The specific type of chromatography that we will employ is termed
thin layer chromatography (TLC). In TLC the stationary phase is adfixed to a sheet of glass or
plastic. Although the composition of the stationary phase may vary, in general, polar silica-based
stationary phases are employed as is this case here (Baker-flex® Silica 1B gel plates). In contrast
the solvent (mobile phase) that we will employ is non-polar consisting of a mix of petroleum ether,
acetone, and propanol (84:15:1, v:v:v) In TLC, movement of the mobile phase is mediated by
capillary action, with the solvent being pulled across the stationary phase. Depending upon their
differing affinities for the mobile and stationary phases the chloroplast pigments in the extract will
travel different distances on the chromatograph with the most non-polar pigments traveling the
farthest. The distance traveled is used to calculate ratio-to-front (Rf) values which are defined as the
quotient of the distance traveled by the compound in question relative to the total distance traveled
by the liquid phase.

Protocol:

1) Working in pairs and wearing gloves, obtain a TLC plate and use a pencil to draw a line
approximately 2 cm from one end of the plate.

2) Using the glass capillaries provided apply two spots of chloroplast extract to the “starting line”.
To ensure that the concentration of pigments is sufficient for a good chromatographic separation,
multiple aliquots of the extract (n=6-10) should be applied to each spot with the spots being allowed
to air dry between applications.

3) Transfer your TLC plate to a chromatography chamber containing solvent. NOTE: To ensure
that the solvent moves through the plate at an equal rate the side of the TLC plate should not contact
the walls of the chromatography chamber.

4) Start the chromatography run, by placing the lid back onto the chromatography chamber.

5) After the solvent has climbed to within 1-2 cm of the far end of the TLC plate the plate should be
removed from the chromatography chamber and the position of the solvent front should be marked
with a pencil. How far did the solvent travel (from the “starting line”)?

6) After the plates have air-dried they can be removed from the exhause hood and be examined for
evidence of multiple pigment types. Since the pigments are susceptible for rapid fading, you should
circle the position of each distinct pigment. The pigments should be numbered sequentially starting
from the base of each plate. Using the table provided below indicate the color and relative yield of
each pigment. The Rf values for each pigment should also be calculated.
                                                                                 Biol 219, Lab 6 -4-
Table 1. TLC chromatography data.

Pigment (#)1    Color                       Relative Yield2    Distance Migrated (cm)       Rf




1
 The pigment spots should be numbered beginning with those closest to the starting line.
2
  Rank the quantity of pigment on a scale from 1-3 (+ - +++) based on the size of the pigment spot.
However, you should beware that chlorophyll spontaneously degrades to pheophytin under acidic
conditions. Since specific steps were not taken to control the pH during extract preparation it is
possible that the proportions of pheophytin to chlorophyll on the plates may not be indicative of the
in vivo proportions.



4) Based on a review of the chemical structures of the chemical structures of the photosynthetic
pigments and your knowledge of factors affecting molecular polarity indicated which pigments
spots represent each of the following pigment types. NOTE: The absence of Mg2+ in pheophytin
serves to make the molecule less polar (more non-polar) the chlorophyll precurser.


       chlorophyll a

       chlorophyll b

       pheophytin

       carotenes

       xanthophylls
                                                                                    Biol 219, Lab 6 -5-
B) Effects of Light and Carbon Dioxide on the Thylakoid Reaction Rate

        During photolysis water is split to replace the electrons lost by the oxidation of the reaction
center chlorophyll a molecules with oxygen (O2) being evolved in the process. In this experiment
we will monitor the rate of oxygen production as an indirect measure of the rate of photosynthesis
in leaf discs and will test for the effects of differences in light intensity and carbon dioxide (CO2)
concentration on the rate of photosynthesis. However, since the reducing power and energy (ATP)
generated via photosynthetic electron transport are not used exclusively for the light-independent
reactions of photosynthesis (carbon fixation), the correlation between oxygen evolution rate and the
rate of photosynthesis is not perfect. The process nitrite reduction, for example, occurs in the
chloroplast at the expense of electrons generated via photolysis.
        In this experiment the intercellular spaces of the leaf disks will be infiltrated with water and
oxygen production will be monitored based on changes in leaf disk buoyancy. Light will applied
using flood lamps with the distance between the leaf disks and lamps being used to control the light
intensity. Carbon dioxide will be supplied via sodium bicarbonate addition (NaHCO3 Equation 3).

       Eq. 3       CO2 + H2O         H2CO3       H+ + HCO3-        H+ + H+ CO3-2


Protocol:

1) Working in groups of 3-5 obtain several healthy leaves of tobacco or aspen and use a cork borer
(#4) to isolate 50-60 leaf disks being sure to avoid the large veins of the leaves.

2) Transfer the leaf disks to a 60 cc syringe and draw in 10-15 ml of tap water with detergent (3
drops of Tween-20 per 200 ml). Place the thumb of one of your hands over the tip of the syringe
and pull back the plunger with your other hand to draw a vacuum. Hold the vacuum for 5-10
seconds and repeat as necessary until the leaf discs have been successfully infiltrated and as
evidence by a loss of buoyancy. NOTE: Be careful not to pull the plunger completely out of the
syringe. Also, you may want to wear gloves and or to use paper towel sheets to alleviate the stress
of sealing the syringe tip.

3) After the leaf discs have been successfully infiltrated remove the plunger and transfer the
infiltration solution and leaf discs to a petri plate.

4) Using forceps assign eight leaf disks to each of five beakers representing the following
experimental treatments. Each beaker should have 200 mls of the appropriate incubation solution
and the leaf disks should be placed into the beakers edge first to avoid trapping air bubbles that will
prevent them from sinking to the bottom of the beakers. The high light treatment will use of flood
lamps positioned 55 cm above the surface of the lab bench. The low light treatment will involve the
use of lamps set at a height of 80 cm (50% light intensity).

       1) Distilled Water, High Light
       2) 0.02% Sodium Bicarbonate, High Light
       3) 0.2% Sodium Bicarbonate, High Light
       4) 0.2% Sodium Bicarbonate, Low Light
       5) 0.2% Sodium Bicarbonate, Dark [Wrapped in Foil]
                                                                                  Biol 219, Lab 6 -6-
5) Record the time when the beakers are placed into the appropriate light environments as the start
time for the experiment. Monitor the beakers in the light on a regular basis and record the time at
which each leaf disk floats to the surface in the table provided. For the light disks assigned to the
dark treatment data will be collected on the percentage of buoyant leaf disks at the end of the lab
period.

                                           Experimental Treatment
                 DI Water       0.02% NaHCO3                    0.2% NaHCO3
Leaf Disk       100% Light        100% Light    100% Light      50% Light   Dark




Sum of
times
Reciprocal
of average
time

6) For all treatments where 100% of the leaf disks have floated to the surface of the incubation
solution by the end of the lab period calculate the reciprocal of the average time required for the
leaves to regain buoyancy. The reciprocal calculated is proportion to the rate of oxygen evolution,
with larger reciprocals indicating more rapid rates of oxygen production.
                                                                                 Biol 219, Lab 6 -7-

Review Questions:

1) Would the rate of oxygen evolution be greater for cyclic or non-cyclic electron flow (see
textbook, p. 133). Please explain you answer.




2) Explain how ammonium is able to uncouple the processes of electron transport and ATP
synthesis (chemiosmosis). Please review Fig. 12.2 and explain the mechanism including the role of
pH differences across cell membranes.




3) The carotenoid pigments of plant chloroplast are normally masked by the more abundant
chlorophyll pigments. However, they frequently become apparent in senescent leaves and are
primarily responsible for the fall color of trees with yellow, orange, or pink foliage. Remembering
the plant growth is frequently limited by nitrogen and that chlorophyll contain nitrogen (Fig. 1)
suggest a mechanism that accounts for the color changes observed in the leafs of these species in the
Fall.




4) For several horticultural crops mutants with purple-colored leaves have been isolated. In most
cases the purple color is due to anthocyanins that are localized in the epidermis as we have observed
in previous labs. Anthocyanins, which absorb light from in the UV region of the spectrum, are also
induced by high light intensities. In terms of the visible spectrum, anthocyanins absorb primarily
between 500-580 nm. Would you expect the presence of anthocyanins to adversely affect the
photosynthetic abilities of leaves to a significant degree? (See Fig. 7.7 in your textbook).

				
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