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Plant Processes
OVERVIEW
INSTRUCTOR:
UNIT: Performance of Technical Skills Related to Plant and Soil Science and Technology
LESSON: Plant Processes
IMS REFERENCE: IMS #8386
TOPIC NOTES
Plants are an important part of our lives. Just look around you. Most likely, somewhere within
eyesight you will see a plant.
Plants surround us everywhere. They grow
naturally in open fields and spaces. We
plant them in our yards and
neighborhoods. We bring them into our
homes and offices. We grow them for their
beauty. We eat them because they taste
good and provide us with nutrition.
As you look at a plant, you may notice that
it appears stationary and basically lifeless.
It moves only if touched or blown by the
wind. You cannot see the plant actually
growing. You may notice, however, that
over a few days or weeks the plant gets
taller or has more leaves.
Although a plant may appear rather
stationary and life- less, the plant is
actually full of life! Inside the plant, many
physiological* processes are constantly
occur- ring. Chemical reactions in the
leaves produce sugar molecules for the
cells throughout the plant. Water
molecules move throughout the plant,
carrying nutrients for the cells. Cell
particles convert the sugar molecules into
energy for cell division and expansion.
This topic describes several of the plant processes that give a plant life. These processes include
photosynthesis, respiration, transpiration, and translocation. Each process affects the production,
use, and storage of food reserves within the plant. Each is also necessary for the plant to grow,
survive, and become an important part of our own growth and survival.
PHOTOSYNTHESIS
Photosynthesis (photo – light; synthesis – putting together) is the single most important biological
process required for human existence. It provides food and oxygen for our survival.
Photosynthesis is the process by which green plants, algae, and certain classes of bacteria capture
solar energy and use it to make simple sugar molecules. Plants use these simple sugar molecules
as an energy source for growth and development.
* Underlined words are defined in the Glossary of Terms.
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Photosynthesis takes place within the chloroplasts of plant cells. An individual chloroplast
contains the stroma, thylakoids, and numerous chlorophyll pigments.
The stroma is a gel-like material within the chloroplast. Throughout the stroma are the thylakoids.
Thylakoids may be stacked in piles called grana, or in long singular forms called stroma
thylakoids. Stroma thylakoids connect the grana to each another. Thylakoids (from the Greek
word, thylako, “sac” or “pouch”) contain the chlorophyll pigments that capture light energy.
Additional pigments called carotenoids are also present. They assist the chlorophyll in harvesting
light.
Chlorophyll pigments absorb mostly blue and red wavelengths of light. They reflect green
wavelengths. This gives them their green color. Carotenoids absorb only blue and violet
wavelengths. They reflect red, yellow, orange, and green wavelengths of light.
All green plant tissue is capable of photosynthesis. However, most photosynthesis occurs in plant
leaves. Cells within plant leaves have the highest concentration of chloroplasts. Plant leaves also
contain stomates and mesophyll (specialized tissue), which allow gases and water molecules to
move freely in the spaces surrounding the cells.
Carbon dioxide gas from the air passes into the plant through the numerous stomates located on
the undersides of the leaves. Guard cells next to each stomate close the stomate in conditions of
low-light, low-moisture, and/or high carbon dioxide concentrations that cause the photosynthesis
process to slow down or stop.
Plant roots absorb water and minerals from the soil. Xylem tubes in the stems carry the water and
minerals to the leaves. In the presence of light energy, six carbon dioxide molecules combine
with six water molecules. This reaction produces one simple sugar molecule, along with six
molecules of water and six molecules of oxygen.
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The plant uses the water and oxygen molecules produced during photosynthesis for other
chemical process occurring at the same time as photosynthesis. Excess molecules pass through
the open stomates of the plant leaf and escape into the air. Water within the plant transports the
sugar molecules from the leaves to other parts of the plant. The sugar molecules then convert into
molecules of carbohydrates, fats, proteins, cellulose, and lignin that are important for plant
growth and development.
Light reactions and dark reactions
make up the photosynthetic process.
Light reactions occur within the
thylakoids. Dark reactions occur
within the stroma.
During the light reactions, the
chlorophyll within the thylakoids
change the light energy into chemical
energy. Light splits the water
molecules, which then results in a
release of energy. The oxygen
produced during photosynthesis
results from the split water molecules.
Carbon fixation occurs through a
series of reactions that make up the
dark reactions. The term “dark
reactions” is misleading in that
darkness is not a requirement. Most of the dark reactions occur during the day. During the dark
reactions in the stroma, light is not necessary for these re- actions to proceed.
In addition to producing sugar, the dark reactions also produce amino acids and lipids that are
essential to plant life. The dark reactions of photosynthesis provide the raw materials to produce
almost everything a plant needs!
RESPIRATION
Respiration is a complex cellular process that converts the simple sugar molecules produced
during photosynthesis into useful energy. Through a continuous series of many individual
reactions, one simple sugar molecule oxidized by six oxygen molecules converts into six
molecules of carbon dioxide and six molecules of water. This chemical conversion of the simple
sugar molecules into carbon dioxide and water also produces useful energy.
The chemical process of respiration is nearly the opposite of photosynthesis. During
photosynthesis, car- bon dioxide and water combine to form simple sugar molecules. The simple
sugar molecules (C6H12O6) produced during photosynthesis serve as stored energy and are
extremely important for the growth and development of plant cells.
The sugar molecules are of limited value until their structure is converted into useful energy. This
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Plant Processes
occurs during respiration in which the simple sugar molecules break down into molecules of
carbon dioxide and water and release the stored energy.
The following chart shows is a brief comparison of photosynthesis and respiration:
Photosynthesis
Respiration
Food:
Created
Used
Energy:
Stored
Released
Location:
Chloroplast
Mitochondrion
Oxygen:
Released
Used
Carbon dioxide:
Used
Released
Sunlight:
Required
Not Required
The
by photosynthesis
and respiration
show the
importance
of water,of water, oxygen, and
Theequations
equationsrepresented
represented
by photosynthesis
and respiration
show
the importance
oxygen,
carbon
for plantand
growth
and development.
A plant
about
25%
thesugar manufactured
carbon and
dioxide
fordioxide
plant growth
development.
A plant
usesuses
about
25%
ofofthe
sugar
manufactured
during
photosynthesis
for
energy.
The
remainder
of
the
manufactured
food
during photosynthesis for energy. The remainder of the manufactured food reserve is stored throughout
reserve
is stored throughout the plant.
the plant.
Respiration
release
(for(for
cellcell
use)use)
occur
continually.
Photosynthesis
ceases during
Respirationand
andenergy
energy
release
occur
continually.
Photosynthesis
ceasesthe
during the night, in
night, in the absence of light. Carbon dioxide and water molecules produced in the plant cells by
the absence of light. Carbon dioxide and water molecules produced in the plant cells by respiration are
respiration are important for the photosynthetic reactions during the daylight hours. Excess
important
for and
the water
photosynthetic
reactions
the daylight
Excess the
carbon dioxide and water
carbon
dioxide
molecules not
used in during
the chemical
reactionshours.
escape through
molecules
not
used
in
the
chemical
reactions
escape
through
the
stomates
and
into
the atmosphere.
stomates and into the atmosphere.
Respirationoccurs
occurs
mostly
within
mitochondrion
of a plant
cell. Mitochondria
Respiration
mostly
within
the the
mitochondrion
of a plant
cell. Mitochondria
(pl) are (pl)
oftenare often referred to
as
the
“powerhouses”
of
a
cell.
They
consist
largely
of
two
membranes:
an
outer
membrane
and a folded
referred to as the “powerhouses” of a cell. They consist largely of two membranes: an outer
inner membrane
called
the membrane
cristae. The
first
ofThe
respiration
occurs
just outside
of the mitochondria
membrane
and a folded
inner
called
thephase
cristae.
first phase
of respiration
occurs
just
outside
of the mitochondria
whileoccur
the second
and thirdareas
phases
occurthe
in different
areas within
while
the second
and third phases
in different
within
mitochondria.
the mitochondria.
The first phase of respiration is glycolysis. It occurs in the cytoplasm of a cell, near the mitochondria.
The
first phase
of respiration
glycolysis.down
It occurs
in the
cytoplasm
of asmall
cell, near
the to pass through the outer
Glycolysis
is the
process ofis breaking
sugars
into
molecules
enough
mitochondria.
Glycolysis
is
the
process
of
breaking
down
sugars
into
molecules
small
enough
to be broken down
layer of the mitochondria. As sugar is commonly stored as starch, starch molecules
must
pass through the outer layer of the mitochondria. As sugar is commonly stored as starch, starch
themselves before glycolysis can proceed.
molecules must be broken down themselves before glycolysis can proceed.
Asugar
sugarmolecule
molecule
undergoes
chemical
reactions
during glycolysis.
to the formation
A
undergoes
manymany
chemical
reactions
during glycolysis.
In additionIn
to addition
the
of
small
molecules,
a
small
amount
of
energy
is
released
(about
2%
of
the
amount
available
within a
formation of small molecules, a small amount of energy is released (about 2% of the amount
sugar molecule).
available
within a sugar molecule).
Oncethe
thesmaller
smallermolecules
molecules
enter
mitochondria,
the second
and phases
third phases
of respiration begin. Most
Once
enter
thethe
mitochondria,
the second
and third
of respiration
begin.
of the
energy
in the
smallmolecules
sugar molecules
is released
during
these
phases.The
Thesecond phase of
of theMost
energy
stored
in stored
the small
sugar
is released
during
these
phases.
second
phaseisofthe
respiration
is the Krebs cycle.
respiration
Krebs cycle.
The
stage
of respiration
occurs
near the
outer
inside theinside
mitochondria.
TheKrebs
Krebscycle
cycle
stage
of respiration
occurs
near
themembrane,
outer membrane,
the mitochondria. Unlike
Unlike
glycolysis,
the
Krebs
cycle
does
not
produce
a
final
product.
Instead,
it
consists
ofaaseries
series of reactions that
glycolysis, the Krebs cycle does not produce a final product. Instead, it consists of
of reactions that produce a few high-energy ATP molecules and many molecules that carry highproduce a few high-energy ATP molecules and many molecules that carry high-energy electrons to the
energy electrons to the third phase of respiration.
third phase of respiration.
The Krebs cycle stage of respiration also produces compounds used for creating amino acids and
The Krebs cycle stage of respiration also produces compounds used for creating amino acids and other
essential compounds. It is within the Krebs cycle that the carbon atoms in the sugar molecule form carbon
dioxide. Carbon dioxide produced during respiration is a waste product, released by the cell.
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Plant Processes
other essential compounds. It is within the Krebs cycle that the carbon atoms in the sugar
molecule form carbon dioxide. Carbon dioxide produced during respiration is a waste product,
released by the cell.
The third phase of respiration is the electron transport system. It occurs near the cristae of the
mitochondria. In this phase, the high energy electrons produced in the Krebs cycle are passed
along a chain of electron transport carriers. The movement of electrons along the chain is
responsible for the formation of large amounts of ATP.
In addition to energy, the electron transport system also produces the water molecules. Oxygen is
essential for respiration. It is the final acceptor of electrons in the electron transport system.
Without oxygen, the respiration process cannot function properly.
TRANSPIRA TION
Water is important to a plant. It is a necessary component of photosynthesis and a product of
respiration. It also gives a plant rigidity by keeping the plant cells expanded and turgid.
Water functions as the medium for transporting the nutrients absorbed from the soil up into the
plant. It also serves as the medium for transporting manufactured food reserves from higher
concentrations in the leaves to lower concentrations throughout the plant.
Water functions to moderate the temperature surrounding the plant. This occurs as a result of
transpiration. Transpiration is the evaporation of the water vapor released into the atmosphere by
the leaves and other parts of the plant. Evaporation is actually a cooling process.
Although a plant cannot survive without water, only a
small portion of the water absorbed through a plant’s roots
actually becomes part of the plant. For example, of the
water supplied to the soil around a corn plant, only a
fraction of 1% of the water re- mains within the plant.
About 25% of the water evaporates directly from the soil.
Approximately 74% of the water passes through the plant
and evaporates into the atmosphere.
Without the process of transpiration, plant roots would not
be able to draw up large amounts of water. This is
important because water from the soil contains nutrients
that are essential for the plant’s survival. Water absorbed
by the roots moves upward through the plant within the
xylem tissue.
Stomates on the leaf surfaces allow water vapor to escape
from the plant and into the atmosphere. Stomates are usually found on both sides of a leaf, with a
greater number on the bottom side. However, much variation exists among plants. Hot and dry
conditions may cause the stomates to close. This prevents the plant from losing excessive water.
As water within a plant evaporates through open stomates, it is replaced with more water. This
results in water being “pulled” through the plant. Water molecules adhere to each other and move
up the xylem in response to this pull. The cause of this transpirational pull is the water potential
gradient that exists from the soil through the plant and into the atmosphere.
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Water potential is a measure of the energy available in a solution to cause water molecules to
move. Water potential is recognized by the symbol Ψ and is usually expressed in terms of
pressure (such as in the unit of bars). Two main factors responsible for Ψ values are solute
concentration and pressure.
Solutes are particles such as sugars that are present in the water. As solute concentration
increases, Ψ values decrease. However, as pressure increases, so do Ψ values.
Water potential is greatest in the soil and lowest in the atmosphere, with intermediate values
along the plant pathway. Water moves in the direction of lowest water potential. Therefore, water
moves through the plant and out into the air. The drier the air, the lower its water potential.
Wilting occurs when more water is transpired than can be absorbed by the roots and moved
through the plant. That is why the occurrence of wilted plants is more common during the
summer than in the fall. Fortunately, a wilted plant usually returns to a healthy state with the
addition of water. A point does exist when the damage caused by a lack of water is too severe for
a plant to recover, regardless of the amount of water added later. This is called the permanent
wilting point. Wilting also occurs if the level of available moisture in the soil becomes less than
that required to keep up with the rate of plant transpiration.
Plants lose more water on hot, dry days than on cool, damp days. High temperature, low
humidity, and wind are environmental conditions that increase the water loss of plants.
In addition to climatic conditions, other factors influence transpiration rate and the amount of
water loss through evaporation. Plants with larger leaves have a greater surface area for
transpiration to occur. In addition, the larger leaves contain a greater number of stomates from
which water evaporates.
Leaf structure also affects transpiration rates. Succulent leaves have a thick cuticle, or waxy
covering, that prevents water loss. Pines and conifers have modified leaves (needles and scales,
respectively) that have a smaller surface area from which water loss can occur.
TRANSLOCATION
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Not all plant parts are capable of producing their own food supply. However, all plant cells
require a supply of energy to survive and function. Fruits, roots, and newly formed leaves are
examples of plant parts that either do not photosynthesize their own food or do not
photosynthesize enough food to meet their needs.
In order for these plant parts to receive sufficient food, sugars move from plant parts, such as the
leaves, where they are made. A region of a plant that produces and moves sugars is a “source”
site. The receiving region of the plant is the “sink” site. For example, a leaf is the source site from
which sugars may be moved to a developing fruit, which is the sink site. This process of moving
sugars originally produced by photosynthesis is called translocation.
Translocation occurs in vascular tissue called the phloem. Like the xylem, the phloem starts at the
roots and continues throughout the plant. However, unlike xylem tissue, phloem tissue remains
alive and flow occurs in many different directions. In woody plants, the phloem is located in the
inner layer of bark.
Special cells known as source cells located near chloroplasts transfer the sugars made in
photosynthesis to the phloem. No source cell is more than a few cells away from the phloem.
Cells involved in unloading sugars from the phloem are called transfer cells.
The specific pathways involved in translocation are very complex and not yet fully understood.
The most probable mechanism of phloem transport is the creation of a water potential gradient in
and across the phloem.
In translocation, when source cells actively load sugar into the phloem at the source, the water
potential decreases in the phloem due to the increased solute concentration. Water rushes in due
to the new lower water potential gradient, and a pressure develops. This pressure drives the flow
of water and sugar through the phloem to its destination — the “sink”— where the sugars are
then unloaded.
At the sink site, growing tissues use the sugar as
a source of energy. In addition, sink sites exist
for storage purposes. For storage, sugar is most
commonly changed into starch. Starch is stored
in the chloroplasts of leaves and in storage
organs.
Starch is broken down into sugars when energy
is needed. Many plants living in cold climates
translocate starch into their roots for winter
storage. During the spring as the plant begins to
grow again, the starch converts into sugar and moves upward throughout the plant.
This process occurs in maple trees. During the winter, the maple roots act as a sink. In the spring
they become a “source” of sugar, which moves upward throughout the tree. The “sap”
translocated upwards from the roots is often harvested and processed into maple syrup.
What directs the process of translocation? One factor that determines a region of the plant as a
sink is the rate at which sugars are removed from the phloem. When a sink area has received as
much or more sugar than contained within the phloem sap, translocation into that area will cease.
Another source of control is believed to be plant hormones involved in growth regulation. The
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hormones may not only promote the formation of new growth regions (sink sites), but may also
be released from the sink as a signal to increase the flow of translocation into that area.
STORAGE OF FOOD IN PLANTS
As discussed previously, the simple sugars manufactured during photosynthesis function as stored
energy needed for building new cells and tissues. This stored energy exists in the form of
carbohydrates, fats, and proteins. Food manufactured through photosynthesis, but not used for
tissue development or energy, is stored within the plant. Plants store food in their seeds, roots,
stems, and other parts.
Carbohydrates are composed of carbon, hydrogen, and oxygen. They consist of starches, sugars,
and fructosans (a chain of fructose sugar molecules in many grasses). Carbohydrates comprise
almost all the total dry weight of a plant. Starch, usually the most abundant food stored in plants,
is found in great quantities in grain and tuber crops.
Fats are also composed of carbon, hydrogen, and oxygen. However, these elements exist in
different proportions as compared to carbohydrates. Fats and oils are stored in many types of
plant seeds. Peanuts and sunflowers are two examples of crops with high contents of fats and oils
stored in their seeds.
Proteins are used in plant cell production. Carbon, hydrogen, and oxygen comprise 75% to 85%
of the protein molecule. Proteins also contain nitrogen, sulfur, and may contain phosphorus. The
seeds of many crops contain high levels of protein.
Plants are similar to animals in that digestion and assimilation occur during food manufacturing.
During digestion, starches, fats, and proteins change into soluble forms for plant use. During
assimilation, carbohydrates, fats, proteins, and simpler nitrogenous foods convert into living
material (protoplasm).
SUMMARY
Although a plant may often appear rather stationary, many physiological processes are constantly
occur- ring within the plant. These physiological processes produce food molecules for the cells
and convert them into energy for cell division and expansion.
Photosynthesis is the process by which plants capture solar energy and use it to make simple
sugar molecules. Respiration is the complex cellular process that converts the simple sugar
molecules produced during photosynthesis into useful energy.
Transpiration is the evaporation of the water vapor released into the atmosphere by the leaves and
other parts of the plant. It is important for moving water throughout a plant. Water is the medium
that functions to move or translocate food reserves and nutrients throughout the plant.
The simple sugars manufactured during photosynthesis function as stored energy needed for
building new cells and tissues. This stored energy exists in the form of carbohydrates, fats, and
proteins. Food manufactured, but not used for tissue development or energy, is stored within the
plant. Plants store food in their seeds, roots, stems, and other specialized parts.
ACKNOWLEDGEMENTS
Kelly Coleman, Graduate Student, Department of Agricultural Education, Texas A&M
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University, researched and compiled the information for this topic.
Keith Zamzow, Curriculum Specialist, Instructional Materials Service, Texas A&M University,
edited and reviewed this topic.
Vickie Marriott, Office Software Associate, Instructional Materials Service, Texas A&M
University, edited and prepared the layout and design for this topic.
Christine Stetter, Artist, Instructional Materials Service, Texas A&M University, prepared the
illustrations for this topic.
REFERENCES
Kramer, Paul J. and John S. Boyer. Water Relations of Plants and Soils. New York, NY:
Academic Press.
Miller, Kenneth R. and Joseph Levine. Biology. Englewood Cliffs, NJ: Prentice Hall.
Salisbury, Frank B. and Cleon W. Ross. Plant Physiology. Belmont, CA: Wadsworth Publishing
Co.
GLOSSARY OF TERMS
Amino acid - The structural unit of a protein.
ATP (adenosine triphosphate) - An energy-storing molecule that contains three phosphate groups.
Energy is released when a phosphate group is removed.
Carotenoid - Yellow to red pigments found widely in plants and animals.
Cytoplasm - The area between the nucleus and cell membrane of a cell.
Electron - Negatively charged subatomic particle.
Evaporation - The process in which liquid water is converted into water vapor.
Hormone - Naturally produced compound that is made in one region of an organism and affects
another part of the same organism.
Lipid - Waxy or oily substance that is a storage form of chemical energy.
Mechanism - A process or technique for achieving a result.
Molecule - The smallest part of a substance that can exist separately and still retain its chemical
proper- ties and characteristic composition.
Physiological - Refers to the functions or processes that occur within the cells, tissues, and organs
of plants.
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Starch - A kind of carbohydrate manufactured by plants and stored in the seeds, roots, and fruit as
a reserve energy supply.
Turgidity - Being swollen and filled with fluid.
SELECTED STUDENT ACTIVITIES
FILL-IN-THE-BLANK: Complete the following statements.
1.
The evaporation of water from plants is called _______________________.
2.
Sugars are produced during the process of __________________________.
3.
The cause of the transpirational pull is the ____________________ gradient that
exists from the soil through the plant and into the atmosphere.
4.
A plant will ___________ when it has transpired more water than it can absorb
through its roots.
5.
Cells involved in unloading sugars from the phloem are called ___________ cells.
6.
The process of moving sugars from one site to another is called _____________.
7.
The result of ___________________ is the exact opposite of photosynthesis.
8.
Two main factors responsible for water potential are __________ concentration
and pressure.
9.
____________ pigments absorb mostly blue and red wavelengths of light.
10. Most of the process of respiration occurs within the ________ of the plant cell.
TRUE/FALSE: Circle the "True" if the statement is true or "False" if it is false.
11. Glycolysis involves the splitting of water molecules.
a. True
b. False
12. Plants usually do not recover once they have passed the permanent wilting point.
a. True
b. False
13. Carbon dioxide molecules enter a leaf through the stomates.
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a. True
b. False
14. Glycolysis occurs in the cristae of the mitochondria.
a. True
b. False
15. A large amount of energy is released during the electron transport system.
a. True
b. False
16. Water potential is greatest in the soil and lowest in the atmosphere.
a. True
b. False
17. Translocation is the process of capturing sunlight and manufacturing food
reserves.
a. True
b. False
18. Photosynthesis takes place within the chloroplasts of plant cells.
a. True
b. False
19. Water and nutrients absorbed from the soil move upward into the plant through
the phloem.
a. True
b. False
20. Dark reactions occur in a plant during the night when light is not available for
photosynthesis.
a. True
b. false
SHORT ANSWER/LISTING: Answer the following questions or statements.
21. Differentiate between a "source" site and a "sink" site within a plant.
22. List two reasons why transpiration is important to a plant.
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23. List examples of plant organs where food reserves are stored.
MULTIPLE CHOICE: Place the letter of the correct answer in the space provided at the
left of each number.
24. The light reactions of photosynthesis occur in the:
A. anthers
C.
B. Cladophylls
D.
stroma
thylakoids
25. On a hot sunny day, which of the following leaf types will most likely transpire
the most water?
26. Carbon fixation occurs during:
A. Photosynthesis
B. Respiration
C.
D.
Translocation
Transpiration
27. Which of the following organelles are commonly referred to as the "powerhouses"
of a cell?
A. Chloroplasts
C. Mitochondria
B. Mesophylls
D. Vessel Elements
ADVANCED ACTIVITIES
1.
Gently cover both sides of a leaf with plastic wrap. Place the entire plant in a
bright area. What happens to the leaf after a few days? What was the function of
the plastic wrap?
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2.
Fill two clear containers half full with water dyed with blue food coloring. Cover
each container with foil. Punch a hole in the center of the foil on each container.
Make each hole large enough for a celery stem to fit through. Insert a celery stem
in each container. Place one container in a normal lighted area. Place the other
container under very intense light conditions. Observe the process of water
movement within the celery stems.
3.
Attach a strip of aluminum foil to the upper surface of a green geranium or coleus
leaf with some paper clips. Place the plant in a sunny location (bright, indirect
light) for two to three days. Remove the partly covered leaf from the plant and
remove the foil. Soak the leaf in alcohol for a few hours. Place a few drops of
iodine on the leaf. Record your observations and explain your findings.
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