Photosynthesis - Macmillan Higher Ed

8T O P I C 8–1

Photosynthesis

General Comment

During photosynthesis, plants capture a small fraction of the sun’s energy and store it in the
chemical bonds of carbohydrates. The carbon source for the organic compounds is the
inorganic atmospheric gas carbon dioxide, which is reduced by the addition of electrons from
H2O to form carbohydrate (C3H6O3) as follows:

3CO2 + 6H2O + 343 kilocalories ———> C3H6O3 + 3O2 + 3H2O
carbon dioxide + water + light energy ———> triose + oxygen + water

Although glucose (C6H12O6) is commonly represented as the carbohydrate product of
photosynthesis in summary equations, in reality, very little glucose is generated in
photosynthesizing cells. The more immediate carbohydrate products are trioses (three-carbon
sugars) which are most often converted into sucrose (C12H22O11) and starches (C6H10O5)n.

Photosynthesis is the source of virtually all energy used by organisms. With the exception
of a few chemosynthetic organisms, photosynthesis is the only method by which chemical
energy is added to the biosphere. As we examine the factors involved in energy capture and
storage, remember that cells involved in photosynthesis are also carrying on cellular
respiration, as do all living cells.

I This topic is designed to acquaint you with the pigments involved in photosynthesis. We
will demonstrate the effect of light intensity and quality on photosynthesis, as well as the
relationship between the absorption spectrum and action spectrum of photosynthetic pigments.

Student Preparation
I Read text Chapter 7, pages 122–149.

Exercise I Light and Its Absorption by Chlorophyll

White light is composed of all the wavelengths (colors) in the spectrum of visible light
(manual Figure 8–1). Typically, these wavelengths are measured in nanometers. We see colors
because objects contain pigments that selectively absorb some wavelengths of visible light
and reflect or transmit others. What we recognize as an object’s color consists of the
wavelengths of light that are transmitted or reflected by the object.

The various pigments found in chloroplasts—including chlorophylls a and b,
xanthophylls, and carotenes—absorb different wavelengths of light, thus making use of a
wide range of light energy for photosynthesis.

8–2 TOPIC 8 Photosynthesis

(Exercise I Continued)

Figure 8–1 The spectrum of white light.

Procedure
a. Observe the spectrum of white light with a spectroscope (manual Figure 8–2). Using the

nanometer (nm) scale superimposed upon the spectrum, estimate the wavelengths
transmitted. In manual Figure 8–1, record the colors of visible light opposite the appropriate
wavelengths. The visible spectrum commonly ranges from 380 to 750 nm.

Figure 8–2 Spectroscope. A small test tube containing pigment extract has been
inserted between the light source and the prism in the spectroscope.

TOPIC 8 Photosynthesis 8–3

(Exercise I Continued)

b. Place various colored filters between the light source and the spectroscope, and observe
any effects the filters have upon the spectrum. Now pour some of the leaf (chloroplast)
pigment extract provided by your instructor into a small, clean test tube and, with the tube
between the light and the spectroscope, observe the spectrum of white light that has passed
through the extract. Record the effects of the filters and pigment extract upon the spectrum
in Table 8–1. Discard the pigment into the waste-solvent container.

Table 8–1 Colors Absorbed Colors Transmitted
Filter
Blue

Green
Red
Leaf extract

1. Explain the absorption spectrum of the various colored filters and of the chloroplast extract.

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2. Why do green plants appear green? ___________________________________________________________________________________

3. Why does a black piece of material appear black? ________________________________________________________________

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c. Observe the demonstration of the tube of chlorophyll extract placed so that a beam of light
from a lamp falls on the front of the tube.

4. What color do you see? ___________________________________

5. This phenomenon, known as fluorescence, occurs only when light energy is absorbed by
isolated chlorophyll molecules in solution. Explain this phenomenon. ___________________________________

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Exercise II Separation of Chloroplast Pigments by Thin-Layer Chromatography*

The process of chromatography, which separates complex mixtures on the basis of mixture
components’ solubility in different kinds of solvents, can be used to help identify some of the
pigments used in photosynthesis. You will utilize thin layer chromatography (TLC) by spotting a

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* Adapted with permission from an exercise by Science and Plants for Schools, http://www.saps.org.uk

8–4 TOPIC 8 Photosynthesis

(Exercise II Continued)

leaf extract onto a TLC strip, a thin layer of absorbent material bound to a plastic sheet, and then
allowing a solvent mix to be drawn up the TLC strip. Different pigments vary in their ability to
dissolve in the solvent mix and will travel up the chromatography strip at different rates. The result
will be a series of pigment stripes separating out along the length of the strip.

CAUTION: The solvents used in this exercise are flammable and toxic. Wear safety
glasses and gloves, and work in a fume hood if possible to minimize your exposure.

Procedure

a. Obtain a TLC strip approximately 1.25 cm × 10 cm, or sized to fit into the test tubes
provided. The oils from your fingers will interfere with the chromatography process, so
handle the strip by holding it on the edges of the top corners only, preferably with forceps.

b. With a pencil and ruler, gently draw a line across the TLC strip about 1 cm above the bottom
on the powdered side. This will mark the location for loading pigment extract onto the strip.

c. Obtain a test tube and stopper (or square of plastic sealing film) to serve as a
chromatography chamber, a mortar and pestle, dropping pipette, watch glass, and fine paint
brush.

d. Tear a small portion of a leaf (about 1 cm square) from the plant material provided into
small pieces and drop them into the mortar. Add a couple milliliters of acetone and grind into
a paste. Transfer the dark green liquid in the mortar to the watch glass. Repeat if needed
until you have transferred about 20 drops of pigment extract. Work quickly to prevent
formation of pigment breakdown products.

e. Use a hair dryer to evaporate the solvent from the extract in the watch glass.

f. When the extract is completely dry, add 3–4 drops of acetone and mix in the dried extract
with a fine paint brush.

g. Use the brush to transfer small amounts of extract to the pencil line on the TLC strip. Try to
create a spot no more than 2 mm in diameter. Dry the spot thoroughly, and re-load. Repeat
several times until the pigment spot is very dark green.

h. Slide the strip into the test tube. Mark the tube below the level of the pigment spot. Remove
the strip.

i. Add chromatography solvent up to the mark. Put the TLC strip back into the test tube,
making sure not to touch the surface of the strip or to allow the strip to touch the sides of
the tube. Seal the tube and watch the chromatogram develop.

j. Note that the pigments move up the strip more slowly than the solvent. When the solvent
front has nearly reached the top of the strip, remove the chromatogram and air-dry it.
Discard the excess solvent into the designated waste bottles.

k. Record the results in a sketch in the space provided on the facing page, showing the colors
and their relative positions on the strip. The pigments will fade within a few hours.
Ordinarily, the pigments can be observed in the following order from the top: orange-yellow
carotenes, blue-green chlorophyll a, yellow-green chlorophyll b, orange-yellow xanthophylls.
You may also observe grayish pheophytin, a breakdown product, below the carotenes.

TOPIC 8 Photosynthesis 8–5

(Exercise II Continued)

l. Measure the distance run by the solvent front and by each of the pigments. All
measurements should be made from the center of the original spot to the front of each
pigment spot.

m. Calculate the Rf value for each pigment: the distance the pigment moved divided by the
distance run by the solvent. Record your results in Table 8–2.

Table 8–2 Color Distance Run (mm) Rf
Pigment Identity

Record the distance (in mm) run of the solvent front:______________

Separation of chloroplast pigments. Sketch your chromatogram in the space above and orient it so
that the pigment-loading end is on the left and the top of the solvent front is on the right.

Your instructor may ask you to use thin-layer chromatography to answer one or more of the
following questions:

• Do different species of plants contain the same photosynthetic pigments?
• Do the differently colored areas of variegated leaves all contain the same photosynthetic

pigments?
• Do non-green leaves contain the same photosynthetic pigments as green leaves?
You may also isolate the different pigments by scraping each of them off the chromatogram
with a razor blade into a small test tube. Redissolve the pigments in a very small volume of the
chromatography solvent. The pure pigment solutions may be viewed with a spectroscope to compare
them with the absorption of the different light wavelengths shown in text Figure 7–5, page 127.

6. Relate the results of your chromatographic separation to the absorption spectrum of the
pigment extract. ________________________________________________________________________________________________________________

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8–6 TOPIC 8 Photosynthesis

Exercise III The Role of Light in Chlorophyll Synthesis

Several days ago, a flat of wheat seedlings was placed in the dark, while a second flat of wheat
seedlings continued to be provided with light.

Procedure
Observe the two flats of wheat seedlings on demonstration. Record your observations below.

7. Light grown: _____________________________________________________________________________________________________________________

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8. Dark grown: ___________________________________________________________________________________________________________________

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9. Is light necessary for the synthesis of chlorophyll? ______________

10. Is light necessary for photosynthesis? ______________

11. If chlorophyll is the pigment responsible for capturing the sun’s rays, explain how the dark-
grown plants managed to survive and grow. _______________________________________________________________________

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Exercise IV Studying the Light Reactions

Photosynthesis takes place in two stages. The first stage, the light (energy-capturing) reactions,
sometimes called “light-dependent reactions,” takes place in light; these reactions require light. The
light reactions may be summarized by the equation:

light
H2O + ADP + Pi + NADP+ ———> 1/2O2 + ATP + NADPH + H+

chloroplasts

Light energy is used to split water and energize the electrons of chlorophyll molecules. Some of
this energy is then captured and used to form ATP and NADPH; O2 is released as a by-product. We
can determine the rate of this reaction by measuring the amount of product made, in this case O2.
Both light intensity and light quality (color or wavelength) affect the rate of the energy-capturing
reactions.

PART A. THE RELEASE OF OXYGEN DURING
PHOTOSYNTHESIS

Observe the demonstration that shows the release of O2 by photosynthesizing sprigs of the aquatic
plant Elodea (manual Figure 8–3). The sprigs have been immersed in a container of water, to which
some NaHCO3 (sodium bicarbonate—a ready source of CO2) has been added.

A funnel, with a test tube inverted over its stem, has been placed over the plants and the
container kept under bright light. At the beginning of the experiment the test tube was full of water
but, as O2 was released by the photosynthesizing sprigs, it collected at the top of the tube and
gradually displaced water from the tube.

TOPIC 8 Photosynthesis 8–7

(Exercise IV, Part A Continued)

Figure 8–3 Setup for demonstrating
the release of oxygen during
photosynthesis.

12. What happened when the splinter was inserted into the tube? _______________________________________________

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13. Explain what is responsible for the phenomenon you observed. _____________________________________________

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PART B. RELEASE OF OXYGEN: FLOATING LEAF DISK
ASSAY FOR QUANTITATIVE INVESTIGATION
OF PHOTOSYNTHESIS

You will learn a procedure called a “floating leaf disk assay” to illustrate oxygen production in
photosynthesis, and then employ that technique in an experiment of your own design. First you will
compare the rate of photosynthesis in leaves with a carbon dioxide source present versus the rate in
leaves without carbon dioxide.

Procedure
a. Pour about 50 ml of bicarbonate solution into one labeled beaker, and add 1 drop of dilute

liquid soap to it. The soap will wet the hydrophobic leaf surfaces, allowing the solution to be
drawn into the leaves. Avoid suds. If your solution generates suds as you use it, then dilute it
with more bicarbonate solution. Pour about 50 ml of distilled water into the second labeled
beaker, and add a drop of diluted soap to it.

Figure 8–4 Coarse veins should be
avoided when punching out leaf disks.

8–8 TOPIC 8 Photosynthesis

(Exercise IV, Part B Continued)

b. Cut 24 or more uniform leaf disks from the plant material provided with the hole punch. Avoid
major veins and try to make the cuts as clean and sharp as possible (manual Figure 8–4).

c. Infiltrate the leaf disks with solution.

1. Label one syringe “with CO2” and the other “dH2O.” Remove the plunger from each
syringe, and place 12 leaf disks into each, tapping them down to the bottom. Replace the
plunger, being careful not to crush the leaf disks. Push on the plunger until only a small
volume of air and leaf disk material remain in the barrel (<10% of total volume).

2. Pull 10 ml of sodium bicarbonate solution into the correspondingly labeled syringe. Tap
the syringe to suspend the leaf disks in the solution.

3. Hold a finger over the syringe opening and draw back on the plunger to create a
vacuum. Hold this vacuum for about 10 seconds. While holding the vacuum, swirl the
leaf disks to suspend them in the solution. Release the vacuum by quickly releasing the
plunger. The bicarbonate solution should infiltrate the air spaces in the leaf disks,
causing the disks to sink. You will probably have to repeat this procedure several times
in order to get the disks to sink. If you still have difficulty getting the disks to sink, you
may need more soap in the solution.

4. When the disks have sunk, hold the syringe upright and expel any air bubbles.

5. Repeat the procedure with dH2O–soap solution in the second syringe.

d. Remove the plunger from the syringe barrel and release the infiltrated “with CO2” disks into
the bicarbonate solution beaker. Do not pour the disks through the air or create air bubbles
in the solution. Repeat with the water-infiltrated disks and the dH2O beaker.

e. Place the beakers under the light source and note the time. Every 10 minutes, record
the number of floating disks in each beaker. Swirl the syringes to dislodge any disks that
are stuck against the sides. Continue until all of the disks are floating. Record data in
Table 8–3.

Table 8–3

Time (min.) Number Disks Floating Number Disks Floating
0 (bicarbonate solution) (distilled water; dH20)

10

20
30

40
50
60

The point at which 50% of the leaf disks are floating is the point of reference for this procedure.

TOPIC 8 Photosynthesis 8–9

(Exercise IV, Part B Continued)

14. How many minutes elapsed before 50% of the leaf disks were floating in the sodium
bicarbonate solution? ____________________

15. For the distilled water solution? ____________________

16. Write a conclusion about the results of this brief experiment. ________________________________________________

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17. What causes the leaf disks to float? _____________________________________________________________________________________

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You can use this technique to answer the question: Do plants respire? Continue with the
floating disks from Part A, and think about what would happen if the light reactions of
photosynthesis stopped, but cellular respiration continued.
18. How could you create a condition in which the light reactions stopped? _________________________________

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Use your knowledge of the reactants and products of photosynthesis and cellular respiration to
design and carry out a procedure to determine if cellular respiration is occurring in the leaf disks.
19. Describe the results and conclusions of your investigation: ___________________________________________________

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PART C. INVESTIGATING THE LIGHT REACTIONS
OF PHOTOSYNTHESIS

Using the floating leaf disk assay technique, design and carry out an experiment to test the effect of
a chosen variable on the rate of photosynthesis. You will work with a partner or team as assigned by
your instructor and choose which variable to investigate (other than the differences between having
sodium bicarbonate versus distilled water as the infiltrating solution). Below are lists of possible
variables that provide a starting point for developing questions to investigate the light reactions of
photosynthesis. You may think of additional variables to test. Think about how each of these
variables might affect the rate of photosynthesis.

Environmental Variables
• Light color (wavelength)
• Light intensity (brightness)
• Temperature
• Bicarbonate concentration (CO2 source)
• Solution pH

8–10 TOPIC 8 Photosynthesis

(Exercise IV, Part C Continued)
Plant or Leaf Variables
• Leaf color
• Leaf age
• Cotyledon versus true leaf
• Stomatal density
• Light-starved leaves versus leaves kept in bright light

Note: It is very difficult to adequately infiltrate leaf disks of hairy plants.

20. Formulate and state a hypothesis. ______________________________________________________________________________________

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21. State your null hypothesis. ______________________________________________________________________________________________

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Consult with your instructor to determine the types of materials and conditions available, then
design an experiment to test your hypothesis.
22. Identify the dependent variable in this experiment. ______________________________________________________________

23. Identify the independent variable in this experiment. __________________________________________________________
Conduct your experiment and collect well-labeled data that measure the dependent variable as

it is affected by variations in the independent variable.

24. Do your data support or falsify your null hypothesis? _________________________________________________________

25. Do your data support or falsify your hypothesis? ________________________________________________________________

26. What sources of error might have affected the outcome of this experiment? What improvements
to your design and procedure could you suggest? _________________________________________________________________

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Write a lab report summarizing your investigation if directed to do so by your instructor (see
Appendix A, How to Write a Lab Report).

TOPIC 8 Photosynthesis 8–11

Exercise V The Carbon-Fixation Reactions of Photosynthesis

Earlier, we indicated that photosynthesis takes place in two stages. We noted that in the first stage
(the light reactions), energy from the sun is captured in ATP and NADPH. During the second
stage—the carbon-fixation reactions (sometimes called the “light-independent reactions”)—most
of this energy is stored in energy-rich carbohydrates. The overall equation for the production of a
three-carbon sugar is

3CO2 + 9ATP + 6NADPH + 6H+ ———> C3H6O3 + 9ADP +9Pi + 6NADPH+ + 3H2O

Again, this result is not accomplished in a single step, but requires many separate reactions.
Carbohydrate is packaged as sucrose, a disaccharide, for transport from the leaves to the

nonphotosynthesizing parts of the plants, such as the roots or fruit. For longer-term storage, starch,
consisting of long chains of glucose, is synthesized.

Must the light reactions occur before the carbon-fixation reactions can occur? In some plants
such as Coleus, parts of the leaves do not contain chlorophyll, that is, the leaves are variegated. Do
the non-green parts carry out carbon-fixation reactions even though they cannot carry out the light
reactions without chlorophyll? Perhaps this exercise will help us answer the question.

Procedure

a. Obtain a variegated leaf from a Coleus plant.

b. Draw the leaf (in the space provided at the bottom of this page), indicating the distribution
of chlorophyll. Observe both sides of the leaf, for the distribution of chlorophyll often differs
on the two sides.

c. Using forceps, place the leaf in a 250-ml beaker containing about 100 ml of boiling water for
about 30 seconds. (Add some boiling beads or chips to the beaker before heating the water.)

d. Using forceps, transfer the leaf to hot 95% ethyl alcohol (150 ml alcohol heated in a 400-ml
beaker placed in a water bath). Be very careful heating the alcohol—do not allow it to
boil! Cover the beaker with a watch glass and leave the leaf in the alcohol until all of the
pigment has been leached out.

e. Place the now brittle leaf in a Petri dish and pour Lugol’s solution (IKI) over it. Starch will
stain dark blue-black.

f. When portions of the leaf have darkened, rinse the leaf gently with water, and discard the
alcohol and IKI into the bottles designated by your instructor.

g. Draw the leaf again, indicating the distribution of starch in the leaf.

Coleus leaf Coleus leaf
(distribution of chlorophyll) (distribution of starch)

8–12 TOPIC 8 Photosynthesis

(Exercise V Continued)

27. How does the distribution of starch compare with that of chlorophyll? __________________________________

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28. How does the distribution of starch compare with the parts of the leaf in which the carbon-
fixation reactions take place? _____________________________________________________________________________________________

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29. Do the leaves commonly store starch for long periods of time? ______________

30. Where might one expect to find starch in a plant? ________________________________________________________________

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31. If you placed a plant with leaves that contained starch in the dark and tested those leaves for
starch 24 hours later, what would you expect the results to show? _________________________________________

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32. Is starch an end-product of photosynthesis? ______________

33. Would you conclude that the energy-capturing reactions are a necessary preliminary to the
carbon-fixation reactions? __________________________________________________________________________________________________

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34. How is it possible for a plant to move starch from one cell to another or from one plant part to
another? __________________________________________________________________________________________________________________________

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Exercise VI C3 Photosynthesis versus C4 Photosynthesis

Two main groups of vascular plants can be recognized on the basis of the nature of the first product
of CO2 fixation that can be detected: the C3 plants and the C4 plants.

In C3 plants, chloroplasts of similar appearance are distributed throughout the leaf, and the
carbon-fixation reactions (Calvin cycle) occur in each cell, beginning with the fixation of atmospheric
CO2 by carboxylation of ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (PGA).

In C4 plants, the biochemical events of photosynthesis are compartmentalized. The chloroplasts
of certain cells fix atmospheric CO2 by carboxylation of phosphoenolpyruvate (PEP) to yield
oxaloacetate, a four-carbon compound, which is rapidly converted into either malate or aspartate.
These four-carbon compounds are then transported to the chloroplasts of other cells, and there the
CO2 is transferred to the RuBP of the Calvin cycle. Hence, the structure of the leaf of C4 plants
imparts a spatial separation between the C4 pathway and the Calvin cycle.

Typically, the leaves of C4 plants are characterized by an orderly arrangement of the mesophyll
cells around a layer of large bundle-sheath cells: the mesophyll cells and bundle-sheath cells
together form two concentric layers around the vascular bundle, an arrangement termed Kranz
anatomy. The C4 pathway occurs in the mesophyll cells, and the Calvin cycle occurs in the bundle-
sheath cells.

TOPIC 8 Photosynthesis 8–13

(Exercise VI Continued)

Examine manual Figure 8–5, a photomicrograph of a portion of a leaf from a maize, or corn
(Zea mays), plant, one of the better-known C4 plants. 35. Identify and label the mesophyll cells and
bundle-sheath cells, and the many chloroplasts within them.

a

b
c

Figure 8–5 Photomicrograph of a
transverse section of a portion of a
maize (Zea mays) leaf, ×360.

In certain C4 plants, the chloroplasts of the mesophyll cells have well-developed grana,
whereas those of the bundle-sheath cells have poorly developed grana or none at all. Manual
Figure 8–6 is an electron micrograph showing a portion of a chloroplast in a mesophyll cell and a
portion of a chloroplast in a bundle-sheath cell of maize. 36. Identify and label the grana,
stroma, and stroma (intergranal) thylakoids in these chloroplasts. (The relatively dense,
circular bodies in the chloroplasts are oil bodies.) From what you know, you should be able to
determine which chloroplast is the mesophyll cell chloroplast and which one is the bundle-sheath
cell chloroplast.

37. Label the mesophyll cell and bundle-sheath cell in manual Figure 8–6 (on the following page).

38. Where are the chlorophyll and the accessory pigments located within the chloroplasts? __________

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39. What pathway, then, would you expect the intermediates or products of photosynthesis to
follow as they move from the mesophyll cell to the bundle-sheath cell or vice versa? ______________

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8–14 TOPIC 8 Photosynthesis

(Exercise VI Continued)

40. Identify such pathways in the electron micrograph of manual Figure 8–6.

b
c

ad

e CELL f CELL

Figure 8–6 Electron micrograph of a portion of a
plastid with well-developed grana in a mesophyll
cell and a portion of a plastid with poorly
developed grana in a bundle-sheath cell of a maize
(Zea mays) leaf. Note the plasmodesmata in the
wall between these two cells, ×56,500.

TOPIC 8 Photosynthesis 8–15

(Exercise VI Continued)

The plastids of the bundle-sheath cells commonly contain many starch grains while the maize
leaf is photosynthesizing, but the grains gradually disappear when the leaf is not photosynthesizing.

41. Label manual Figure 8–7, which shows a plastid in a bundle-sheath cell in a
photosynthesizing maize leaf.

Figure 8–7 Electron micrograph of a
portion of a bundle-sheath cell of a maize
(Zea mays) leaf, showing starch grains
within a plastid with poorly developed
grana, ×12,800. Courtesy M. A. Walsh.

Obtain a strip of maize leaf, 2 cm long, taken from a plant that has been kept continuously
under light for two days. (The strip, which should be immersed in water when you obtain it, should
first have been aspirated in tap water to remove the air from its intercellular spaces.) Place the leaf
segment in water on a microslide and, with a sharp razor blade, cut sections at right angles to the
veins (cross sections of the leaf). Mount the sections in water on another slide, and cover with a
coverslip. With your compound microscope, find a vein and identify the bundle-sheath cells. Note the
distribution of chloroplasts in these cells. Compare the sheath plastids with those of the mesophyll
cells. Now replace the water with IKI by placing a piece of folded paper towel along one edge of the
coverslip while adding the IKI to the other side (see manual Figure 3–3), and examine the leaf for
the presence of starch.

42. Where is the starch located? _______________________________________________________________________________________________

If time permits, similarly examine the leaves of a selection of plants (i.e., prepare transverse
sections of the leaves and examine them before and after treating them with IKI).

8–16 TOPIC 8 Photosynthesis

(Exercise VI Continued)

43. On the basis of leaf structure and the presence or absence of starch in the bundle-sheath cells,
identify which plants are C3 and which are C4.

Plant Name C3 or C4 Plant

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Exercise VII CAM Photosynthesis

In addition to the C3 and C4 pathways of CO2 fixation, a variant of the C4 pathway has evolved
independently in many succulent plants, such as cacti and stonecrops. In these CAM (meaning
Crassulacean acid metabolism) plants, CO2 fixation occurs at night, when their stomata are
open, and decarboxylation to provide CO2 to the Calvin cycle takes pace in the light, when their
stomata are closed. In CAM plants, the two events, which are temporally separated, take place
within the same cell.

Examine the CAM plants on demonstration.

44. What are the adaptive advantages of the stomatal behavior and the method of carbon
metabolism employed by CAM plants? _______________________________________________________________________________

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Laboratory Review Questions and Problems
1. In order for photosynthesis to occur in green plants, the following must be present:

a. ______________________________________________________ as the energy source.
b. ______________________________________________________ as the carbon source.
c. ______________________________________________________ for the absorption of light energy.
d. ______________________________________________________ as the electron donor.

2. a. Explain the difference between an action spectrum and an absorption spectrum.

TOPIC 8 Photosynthesis 8–17

(Laboratory Review Questions and Problems Continued)

b. For chlorophylls, carotenes, and xanthophylls, what is the relationship of the action
spectrum to the absorption spectrum?

c. Why do plants contain so many pigments?

3. When using the IKI test, why did you extract the pigments from the leaf before adding
IKI?

4. Many plants contain water-soluble red pigments called anthocyanins. Why would these not
be visible in a chromatogram of chlorophyll extract?

RELATIVE RATE OF PROCESS5. The processes of photosynthesis and respiration were studied in separate Topics, but any cell
that is carrying on photosynthesis is also carrying on respiration. On average, if a plant is to
grow, the rate of photosynthesis must exceed the rate of respiration by a factor of at least
three.
The following graph shows the effect of temperature on the rates of photosynthesis and
respiration of one plant; the temperature at which the two rates are equal is referred to as the
compensation point and is not the same for all plants.

PHOTOSYNTHESIS
RESPIRATION

TEMPERATURE (°C)

a. At what temperature is the compensation point reached in this example?

b. At what temperature(s) would you expect growth to be most rapid? Explain.

8–18 TOPIC 8 Photosynthesis

(Laboratory Review Questions and Problems Continued)
c. As temperature rises, what happens to the rate of photosynthesis? Of respiration? Why?

d. Certain fruits such as apples are frequently stored under refrigeration in a carbon-
dioxide-rich atmosphere. Explain the reason for this.

6. Explain how each of the following could limit the rate of photosynthesis:
CO2 concentration
Light quantity

Light quality

Temperature

Water
7. Compare C4 and CAM plants with regard to each of the following:

Time of day during which the stomata are open

Initial products of carbon fixation

Location of the C3 pathway (Calvin cycle)
Location of the C4 pathway