Download Plants Do It All— Photosynthesis, Respiration, and Transpiration

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LABORATORY TOPIC
Plants Do It All—
Photosynthesis, Respiration,
and Transpiration
BACKGROUND
Physiology is the branch of biology that focuses on the
function of structures and the variety of dynamic
processes that allow an organism to live. Understanding all the processes, such as photosynthesis or protein
synthesis, can help us understand the organism. This
understanding can also help us adjust conditions
for practical purposes, such as to increase crop yields or
control pests. This laboratory topic introduces three
important processes in the lives of plants—photosynthesis, respiration, and transpiration. Unfortunately, this
is only a glimpse into the fascinating inner workings of
a live plant.
This lab is divided into four exercises. The first
explores the flow of carbon dioxide related to photosynthesis and respiration. The second investigates factors
affecting the production of starch via photosynthesis. The
third and fourth exercises explore the flow of water out
of plants via transpiration; Exercise C examines transpiration for individual leaves and plants, and Exercise D
demonstrates what happens when plants act together in
a community.
LEARNING OBJECTIVES
After completing this laboratory topic, students should be
able to:
1. Better understand the factors that can affect the
physiology of plants.
2. Relate the processes of photosynthesis and respiration
to plant growth.
3. Better understand the process of transpiration
in plants.
4. Explore the factors that affect the rate of transpiration in plants, including anatomical features such
as stomata and environmental conditions such
as temperature.
5. Understand the processes of scientific inquiry by posing questions, designing investigations, collecting
empirical data, testing hypotheses, and communicating the results.
EXERCISE A: The Ins and Outs
of CO 2
The common statement “Animals respire, plants photosynthesize” is only partially correct. Although it is true that
plants are capable of producing their own organic compounds through the process of photosynthesis, plants are
so versatile that they can also respire just like animals.
Photosynthesis is a complex series of reactions
involving the capture of light energy, conversion to chemical energy, and finally the synthesis of carbohydrates. It
is one of the main biosynthetic processes by which energy
and carbon enter the network of living organisms. A summary of the ultimate reactants and products of photosynthesis can be stated as:
EQUATION 1: Light + CO2 + H2O Æ (CH2O)n + O2
carbon
dioxide
Water
Sugar
Oxygen
This equation is deceptive in its simplicity because
photosynthesis involves numerous intermediate steps.
Photosynthesis can be divided into two series of reactions.
The first series, light harvesting, results in the capturing of light energy and the temporary storage of this
energy in ATP and NADPH. The second series of reactions, carbon fixation, uses the energy in the ATP and
NADPH to synthesize sugars from CO2.
Light harvesting starts with the absorption of light
energy by the pigment chlorophyll embedded in the
membranes of the chloroplasts within plant cells. The
energy excites electrons within the chlorophyll molecule
to a higher energy state. At this energy state, the electrons
are easily transferred to other chemicals that accept the
electrons. The electrons are transferred from one chemical to the next much like the way buckets of water are
passed from one person to the next in a bucket brigade.
Eventually, the electrons are transferred to an electron
acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate). To replace the lost electrons of the
chlorophyll molecules, water (H2O) is split to produce
electrons, protons (H+), and oxygen gas (O2). In addition, a proton gradient is generated that is used to produce ATP (adenosine triphosphate) from ADP
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(adenosine diphosphate) and Pi (inorganic phosphate).
ATP is a temporary energy carrier. The NADP+ is converted to NADPH by the adding of two electrons and
one proton (H+). At the end of the light-harvesting phase,
NADPH, ATP, and O2 are produced. A summary of the
light-harvesting phase can be stated as:
EQUATION 2: Light + H2O + ADP + Pi + NADP+ Æ
ATP + NADPH + H+ + 1⁄2O2
In the carbon fixation phase of photosynthesis, the
energy and electrons of ATP and NADPH are used to
form carbon–carbon bonds. Carbon dioxide (CO2) enters
a cyclic series of reactions (the Calvin cycle), and eventually sugars (CH2O)n are produced. The name of the
enzyme that catalyzes the fusion of CO2 to the first chemical in the cycle (ribulose bisphosphate) is rubisco. This
is worth mentioning since rubisco is the most abundant
protein on our globe. The first sugar to exit the cycle is
a three-carbon sugar, G3P (glyceraldehyde 3-phosphate).
G3P can be used to generate other sugars, such as glucose
(C6H12O6). The summary of the carbon-fixation phase of
photosynthesis can be stated as:
EQUATION 3: ATP + NADPH + CO2 Æ ADP + Pi + NADP+
+ (CH2O)n
Take some time to examine the three equations.
Equation 1 is the summary of equations 2 and 3. What
information is lost by only looking at equation 1?
Which phase requires light? Which phase is not directly
dependent on light, yet needs the products of the
light-dependent phase?
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The two phases of photosynthesis actually occur in
two different locations within the chloroplasts. The
light-harvesting phase occurs on the membranes of
the thylakoids, whereas the carbon-fixation phase occurs
in the stroma, the space between the thylakoids.
Given this, what compounds cycle back and forth
between the two phases? Does carbon enter the system
in both phases?
All animals, including humans, ultimately depend on
plants to produce the oxygen gas (O2) they need. O2 is
actually a by-product of photosynthesis. We are lucky that
plants produce oxygen. For example, an average hectare
of corn produces enough oxygen per day to support 325
people. Where do the atoms that make up the O2 come
from? If you remove a plant from the light, what happens
to its oxygen production?
What is the fate of the products of photosynthesis?
G3P can easily be converted into various six-carbon sugars, such as glucose or fructose, or stored as starch, a polysaccharide formed as a chain of glucose molecules. When
these basic sugars are combined with other elements, such
as nitrogen or phosphorus, all the other organic compounds in a plant, such as proteins, nucleic acids, lipids,
or alkaloids, can be formed. In this way, plants make all
their basic building blocks. And since animals are incapable of carbon fixation themselves, plants make the basic
building blocks for animals as well. Animal food has to
come from other organisms, and ultimately most of it
comes from plants. In Laboratory Topic 10, you will
relate your own needs for energy to production of chemical forms of energy by plants. Of course, the plants could
probably care less about the welfare of animals.
Respiration is another fundamental process of living organisms. Before we proceed, it is important to compare two definitions of respiration. Many of you may
consider respiration the process of breathing air in and
out of your body. Mammals like yourself pull air into their
lungs by contraction of the diaphragm and exhale the air
as the diaphragm relaxes. The inhalation and exhalation
of air is one valid definition of respiration. Humans even
use breathing as a sign of life itself.
Biologists often use another definition of respiration to
describe what happens to some of the components of the
air, particularly CO2 and O2, at the cellular level. Cellular
respiration is defined as the process by which cells release
energy from organic compounds to generate ATP through
a series of chemical reactions involving the transfer of electrons. In aerobic respiration, oxygen (O2) is the final
electron acceptor. In anaerobic respiration, or fermentation, some other chemical is the final electron acceptor. The
main results of cellular respiration are organic compounds
broken down to simpler compounds, with some energy
becoming available for use in other metabolic steps.
The overall process of aerobic cellular respiration can
be stated as:
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EQUATION 4: (CH2O)n + O2 Æ CO2 + H2O +
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Compare equation 4 with equation 1. On the surface,
respiration appears to be merely the reverse of photosynthesis. But in reality, aerobic respiration is another complex
series of reactions that can be divided into three phases: glycolysis, the Krebs cycle, and the electron transport chain.
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energy (ATP and heat)
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In glycolysis, a molecule of the six-carbon sugar glucose is
oxidized to two molecules of the three-carbon pyruvate, and
some of the energy is recaptured in the production of ATP.
The Krebs cycle completes the oxidation of pyruvate to produce carbon dioxide (CO2) and reduced electron carriers. In
the electron transport chain, a proton (H+) gradient drives
the production of even more ATP and is coupled with the
transfer of electrons to oxygen (O2), producing water
(H2O). After the entire process of respiration is complete,
much of the energy released from the glucose is recaptured
in the production of ATP. Since no conversion of energy is
100% efficient, some of the energy is lost as entropy and is
no longer available to the organism. The ATP, however,
can be used for all the other normal processes of life, such
as synthesis of new tissue, response to external stimuli, or
movement of materials throughout the body.
Just like animals, plants use aerobic respiration to
recapture the energy held in the sugar molecules produced during photosynthesis. Just like animals, plants use
glycolysis, the Krebs cycle, and the electron transport
chain to produce ATP. Plants, however, do not need to
consume preformed organic compounds as animals must.
They produce their own organic compounds via photosynthesis. For this reason, some people say, “Plants make
their own food.” Again we emphasize, plants photosynthesize and respire. Animals only respire.
As long as the net production of new material via photosynthesis exceeds the breakdown of molecules to produce
ATP via respiration (P > R), a plant will grow and increase
in biomass. But sometimes, plants need to rely heavily on
the energy stored in sugars, and respiration can exceed net
gain from photosynthesis (R > P). This is common when
photosynthetic tissues are not yet available, such as in a
germinating seed or regrowth of buds in the tips of
branches each spring. What do you suppose happens when
a plant is kept in the dark for a long time or at night?
Take a look at equations 1 and 4 again. Notice how on
the surface, carbon dioxide is consumed by photosynthesis, yet produced by respiration (fig. 5.1). If you had an
experimental system with only plants, the balance between
photosynthesis and respiration could be determined by
monitoring the levels of carbon dioxide around a plant.
In this activity, we will use a CO2 sensor to measure the
concentration of CO2 in the air surrounding plants in various conditions and see what happens. Relate what you
observe to what could be happening biologically.
Materials Needed for Exercise A
Bell jar
Cellulose acetate or diluted clear nail polish
(see Instructors’ Notes at the end of this manual
for preparation)
Coleus (coleus) plants
Cotton balls
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TRANSPIRATION
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Light energy
Carbon
dioxide
Oxygen
Carbon
dioxide
Water
vapor
Oxygen
Water
Food produced and sent to
other parts of plant
Food converted to energy to
be used by all parts of plant
Photosynthesis
Respiration
FIGURE 5.1 COMPARISON OF PHOTOSYNTHESIS AND RESPIRATION IN PLANTS.
Dry peas, beans, or other seeds
Gas sampling bottle, 250 ml
Glass plate
Glass terrarium or jar with lid
Logger Pro software
Microwave oven
PC or Macintosh computer
Peas, beans, or other seeds soaked for 24 hours
Petroleum jelly (Vaseline)
Photoflood lamp
Potato, apple, or other plant organ void of chlorophyll
Ring stand
Sodium pyrogallate (see Instructors’ Notes
for preparation)
Split rubber stopper
Universal lab interface
Various plants, such as Pelargonium (Geranium)
or Zebrina
Vernier CO2 gas sensor
Water-filled heat shield to control temperature generated from lamp
Procedure for Exercise A
The Vernier CO2 gas sensor monitors the concentration
levels of CO2 gas in air. In this exercise, you will use the
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gas sensor to monitor changes in the levels within a closed
chamber as your test plant either consumes CO2 via photosynthesis or produces CO2 via respiration. When the gas
sensor is connected to a computer, you can easily observe
the changes in levels over time and record your results.
Once you are familiar with the system, you will be able
to pose your own questions, formulate your own
hypotheses, design experiments, predict the outcomes,
collect data, and interpret the results.
1. Detecting net photosynthesis. Connect the CO2
gas sensor to your computer according to the manufacturer’s instructions. Place a large plant in a glass
terrarium and insert the gas sensor probe. Cover the
terrarium with a lid to prevent the air inside the
chamber from mixing with the air outside. Arrange
the lamp to shine light on the plant. Since
photoflood lamps often generate intense heat, you
may need to place a water-filled heat shield between
the lamp and the terrarium. Monitor the levels of
CO2 for 10–60 minutes. Note the change in the levels and the rate of change. What is happening? What
does this tell you about what is happening biologically? Record your results in worksheet 5-1 at the
end of this laboratory topic.
Try changing the light intensity by moving the
lamp further from the plant or inserting filters. What
happens to the rate of change? What happens if you
turn off the light entirely? Record your results in
worksheet 5-1.
2. Detecting respiration in plants. Cut a fresh
potato into 1 cm3 cubes and place approximately ten
of them in a 250-ml sampling bottle. Connect the
CO2 gas sensor with the split rubber stopper. Monitor the change in the levels of CO2. What do you
observe? Is the potato respiring? Is the potato alive?
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Now place dry peas, beans, or other seeds in the
sampling bottle. Monitor the CO2 levels. Compare
these results with peas, beans, or seeds soaked in
water overnight. Is there any difference? What is happening within the seed?
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Try other plants, or change the conditions, such
as temperature. You may try to start with a higher initial CO2 level by exhaling into the sample chamber.
Pose your own questions and design your own experiments. What affects photosynthesis? When is the
consumption of CO2 the greatest?
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Cut another fresh potato into 1 cm3 cubes. This
time, microwave the cubes just long enough to kill
any living tissue (approximate 30–40 seconds). Place
the cubes in the sampling bottle and monitor the
CO2 levels as before. Now what do you observe?
How do these results compare with those of the fresh
potato cubes?
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Try other tissues, such as cut pieces of an apple
or a carrot. What do you observe?
Record your results in worksheet 5-1.
3. Do plants need oxygen? We mentioned that plants
are aerobic, but is this really true? What would happen if plants were deprived of oxygen? Do seeds need
oxygen to germinate, grow, and survive?
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Bell jar
To
computer
Ring
stand
Cotton
Wire mesh
platform
CO2
sensor
Inverted
beaker
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TRANSPIRATION
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Remove the CO2 sensors, and place each setup
in a warm location in the lab. Cover the jars with
paper, or place them in the dark. Observe the seeds
after several days. What do you see? What is the effect
of O2 on germination? Speculate on the correlation
between germination and respiration. Why was it
important to keep the seeds in the dark?
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Soaked
seeds
Na pyrogallate
Culture
dish
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Glass
plate
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For this activity, we will use sodium pyrogallate to remove oxygen from our test chamber. The
setup is illustrated in figure 5.2. Make a platform out
of wire mesh, and place it in a 15-cm-diameter culture
dish. Cover the platform with moist cotton. Place seeds
(peas or beans) that have been soaked in water
overnight on the moist cotton. Add 200 ml of freshly
prepared sodium pyrogallate in the bottom of the culture dish below the platform. Suspend the CO2 sensor over the seeds with a ring stand. Cover the whole
setup with a bell jar sealed to a bottom glass plate with
petroleum jelly. The cord to the sensor must extend
under the lip of the bell jar. Monitor the CO2 levels
over time. As a control, establish a similar setup with
another set of soaked seeds on moist cotton. Instead of
sodium pyrogallate, fill the bottom of the culture dish
with water. Again monitor the CO2 levels.
What do you observe? What is the correlation
between CO2 production and the availability of O2?
From what you observe, do you think the seeds need
O2 for respiration?
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Record your results in worksheet 5-1.
4. How does CO2 enter leaves? Where does CO2
enter a leaf? Does it enter through the epidermal cells,
or does it enter through the stomata?
a. Remove a healthy leaf from a Coleus plant. Make
an epidermal peel of the lower surface of the leaf
by barely scoring the surface with a razor blade
and peeling the outer layer of cells off. Mount
with a drop of water on a microscope slide and
examine under the microscope. Look for stomata, openings in the surface surrounded by a pair
of cells called guard cells (see fig. 3.2). Count
the number of stomata per cm2. Make another
epidermal peel of the upper surface of a leaf.
Count the number of stomata per cm2 for this
surface and compare. Which surface has more
stomata? (Usually in Coleus one surface has nearly
all of the stomata. Is this the case for your plant?
Which surface, upper or lower, has more?)
b. An alternative way to view the stomata on the
surface of a leaf is to make an impression of
the surface with cellulose acetate. Paint the surface of the leaf with cellulose acetate, and let dry.
Peel off the impression and observe with the
microscope. If you have difficulty removing the
impression, try wilting the leaf in a warm (not
hot) oven first and then remove the peel.
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Petroleum
jelly
FIGURE 5.2 APPARATUS TO DETERMINE RELATIONSHIP OF OXYGEN TO RESPIRATION IN GERMINATING SEEDS.
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c. Set up four Coleus plants for this experiment.
For the first plant, cover the upper surfaces of
all the leaves with petroleum jelly. Place the plant
in the test chamber, insert the CO2 sensor,
and monitor the levels of CO2 for approximately
10–120 minutes. For the second plant, cover
the lower surfaces of all the leaves and monitor
the CO2 in the test chamber. For the third plant,
cover both the upper and lower surfaces
and monitor the CO2. Finally, for the fourth
plant, leave all the leaves uncovered and monitor
the CO2. This is your control. Place all
four plants under high light intensity to
stimulate photosynthesis.
What do you observe? What is the relationship between the relative rates of CO2 uptake and
the number of stomata on the open surface?
Record your observations in worksheet 5-1.
From your experiment, do you conclude that
most of the CO2 enters the leaf through the epidermis or the stomata?
(Chlorophytum)—one plant grown for 4 days in the
dark and one grown in a greenhouse or under
grow lights
Procedure for Exercise B
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1. You will test for starch in the leaves of four plants.
Two of the plants have variegated leaves, and the
other two have solid green leaves. One of the variegated plants and one of the solid green plants were
grown for at least 4 days in the dark. The other two
plants (one of each type) were grown in the light,
either in the greenhouse or under grow lights.
2. Take a leaf from each plant. Mark the leaves from
plants grown in the light with a notch so you can
identify them later. Place each leaf in boiling water to
remove any anthocyanins (purple or red pigments).
Then place each leaf in hot ethanol to remove the
chlorophyll. This should be done in a well-ventilated
area. Finally, soak each leaf in iodine solution to
detect starch.
3. Draw an outline of each leaf on worksheet 5-2 at the
end of this laboratory topic. Indicate where starch
is detected. What do you conclude is the effect of
light on the production of starch? What is the relationship between the pattern of variegation and the
detection of starch? Why do you suppose this relationship exists? If a leaf has little to no starch, how
does the plant stay alive?
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EXERCISE B: Saving for
Another Day—Storing Starch
Some of the first carbohydrates produced via photosynthesis are simple sugars such as glucose. Glucose is highly
soluble in water, so many plants convert it to starch, the
insoluble storage polysaccharide. Laboratory Topic 13
explores starch in a variety of food crops. In this
activity, we will examine where starch is stored in a leaf
after photosynthesis.
Materials Needed for Exercise B
Beaker of boiling water
Beaker of hot ethanol in a double boiler
(Caution: ethanol ignites easily.)
Beaker of Lugol’s iodine solution
Plants with solid green leaves, such as wandering jew
(Tradescantia), geranium (Pelargonium), or ivy (Hedera)—one plant grown for 4 days in the dark and one
grown in a greenhouse or under grow lights
Plants with variegated leaves, such as Coleus, variegated
geranium (Pelargonium), or spider plant
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EXERCISE C: Transpiration
Another important process in plants is the movement of
water from the roots to the leaves. Transpiration of water
from the surface of the leaves is an important part of this
movement. Do plants transpire water at the same rates
under the same conditions? What conditions increase the
transpiration of water? When should you be more concerned about watering your houseplants or garden plants?
When should you water more, or less?
In this exercise, we will detect water vapor as it evaporates from leaves. Then we will compare plants under
different conditions to see what variables affect the transpiration of water. Finally, we will look at the relative
number of stomata in a given area of leaf and relate this
factor to transpiration rates.