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Activity 7 The Water Cycle
Activity 7
The Water Cycle
GOALS
What Do You Think?
In this activity you will:
According to William Shakespeare, Caesar used part of his
last breath to utter the words “Et tu, Brute” (even you,
Brutus). His last breath would have been partly made up
of water.
• Measure the amount of
water transpired by a plant.
• Describe the processes that
take place in the water cycle.
• Provide examples of how
human activities can affect
the water cycle.
• Model the effects of acid rain
on an ecosystem.
• Is it possible that the molecules of water that Caesar
exhaled many centuries ago, are still a part of today’s
environment?
• Is it possible that these molecules could become a part
of you?
Write your answer to these questions in your Active Biology
log. Be prepared to discuss your ideas with your small group
and other members of your class.
For You To Do
Plants absorb water through their roots and return water to
the atmosphere through the process of respiration. In this
activity you will measure the amount of water transpired by a
plant over a period of time.
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A Vote for Ecology
1. You will first construct a very simple
“meter” to measure the loss of water.
Place the tip of a 0.1 mL pipette into
a piece of plastic tubing about 40 cm
long.
2. Submerge the tubing and the
attached pipette under water in the
sink or a tray. Fill both with water.
Make sure that all the air is drawn
out of the tube and pipette. Leave the
assembly under water while doing
the next step.
3. Choose a branch from a plant.
Suggested plants include Coleus and
Zebrina. Submerge the end of the
branch under water and make a
small, slanted cut. This step is very
important to ensure that no air
bubbles are introduced into the
xylem cells and the water will flow
easily. Do not get the leaves wet. If
you do, dry them gently with a paper
towel before you begin your
experiment.
4. While the branch and tubing are still
under water, push the freshly
trimmed end of the branch into the
open end of the plastic tube. The end
Be careful
when cutting
the plant.
Cut away
from yourself.
Report any
injuries.
of the branch should be about 1.5—
2.0 cm in the tube. There should be a
very tight fit between the stem of the
branch and the tube.
5. Bend the tubing into a U shape, as
shown in the diagram. Clamp the
tubing with the branch and the
pipette onto a ringstand.
6. Once the “meter” is set up, wait
about five minutes to make sure the
plant is transpiring. After this initial
waiting period, read the water level
at “0 minutes.” Then read the water
level in the pipette every three
minutes for a total of 30 minutes.
a) Record your observations in a
table.
7. At the end of your experiment, cut
the leaves off the plant stem. Find the
area of the leaf surfaces. You can do
this by arranging all the cut-off
leaves on a grid. Trace the edge of
the leaves on to the grid. Count all of
the grids that are completely within
the tracing and estimate the number
of grids that lie partially within the
tracing.
a) Record the area of the leaves in
square centimeters.
leaves
pipette
stem
b) Calculate the water loss per square
centimeter of leaf surface. Divide
the water loss at each reading by
the leaf surface area you
calculated.
c) Calculate and record the average
loss per square centimeter for the
class.
plastic
tubing
d) Graph the loss per square
centimeter over time.
Wash your hands after
the activity.
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Activity 7 The Water Cycle
e) Assuming that your plant stem
continued transpiration at the
same rate, estimate the total
volume of water that might be
transpired in 24 h.
8. Use the results of your experiment
to answer the following questions.
a) List some of the factors that
might affect the results of this
experiment. Explain how each
factor might affect your results.
b) How could you improve the design
of the experiment to reduce errors?
c) Consider what factors could affect
the rate of transpiration of a plant.
Develop a hypothesis and design
an investigation to test your
hypothesis.
The Water Cycle
Water is necessary to life in many ways. Land plants absorb water from
the soil and land animals drink water or obtain it from their food.
Water constantly bathes organisms that live in ponds, lakes, rivers, and
the oceans. Other organisms rely on water to carry nutrients to their
cells and organs.The cytoplasm in cells is mainly water.
Every day about 1200 km3 of water evaporates from the ocean, land, plants, and ice caps. An
equal amount of precipitation falls back on the Earth.
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Bio Words
water (hydrologic) cycle:
the cycle or network of
pathways taken by water
in all three of its forms
(solid, liquid, and vapor)
among the various places
where is it temporarily
stored on, below, and
above the Earth’s surface
evaporation: the process
of changing from a liquid
to a gas
condensation: the process
of changing from a gas to
a liquid
precipitation: water that
falls to the Earth’s surface
from the atmosphere as
liquid or solid material in
the form of rain, snow,
hail, or sleet
runoff: the part of the
precipitation appearing in
surface streams
groundwater: water
contained in pore spaces
in sediments and rocks
beneath the Earth’s
surface
infiltration: the
movement of water
through pores or small
openings into the soil and
porous rock
aquifer: any body of
sediment or rock that has
sufficient size and
sufficiently high porosity
and permeability to
provide an adequate
supply of water from
wells
long cycle
t
cycle
condensation
ens
ns
precipitation
THE WATER
CYCLE
cond
con
ondensation
atm
water vapor
life processes
precipitation
evaporation
evaporation
vap
va
runoff
infiltration
groundwater
The volume of water in the biosphere remains fairly constant through
time. In fact, the water that you used today has been around for
hundreds of millions of years. It has probably existed on the Earth’s
surface as a liquid, a solid, and a vapor. However, water is always
moving from place to place. It is forever changing from one state to
another.This complicated movement of the Earth’s water is called the
water cycle or hydrologic cycle. Some of the pathways of this
cycle are shown in the diagram above.
One of the largest reserves of water on Earth is found in the oceans.
The oceans contain about 97% of the Earth’s water. Other surface
water includes lakes, rivers, estuaries, marshes, and swamps. By
contrast, the atmosphere holds less than 0.001% of the Earth’s water.
This means that rapid recycling of water must take place between the
Earth’s surface and the atmosphere.
By absorbing heat energy from the Sun, some of the water on the
Earth’s surface changes to water vapor by evaporation. It rises
upward into the atmosphere until it reaches a point where the
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Activity 7 The Water Cycle
temperatures are low enough for the water vapor to condense to
form tiny droplets of liquid water.This process is called
condensation.These droplets of water are light.They collect around
dust particles forming clouds or fog.They remain suspended in the
atmosphere as clouds or fog and are supported by rising air currents
and winds.When conditions are right, the droplets come together to
form larger drops or sometimes ice crystals. Once the mass of the
droplet or ice crystal can no longer be supported by air currents,
precipitation occurs. Precipitation may take the form of rain, hail,
sleet, or snow.
Snow falling high in the
mountains or in the polar
regions of the Earth may stay
frozen there for years.
Gradually, as layers of snow
accumulate, the bottom layers
of snow turn to ice, forming
glaciers. Sometimes the snow
or ice at the surface of the
Earth can change directly back
into water vapor.This process
is called sublimation.
Other precipitation lands on
the surface of the Earth and
flows along the surface as
runoff.The ground runoff
gathers in streams, lakes, and
oceans, and the cycle then
repeats itself.
Approximately 1.7% of the water on Earth is stored
in the polar icecaps, glaciers, and permanent snow.
However, some of the
precipitation seeps into the Earth to form groundwater.This
process is called infiltration. Sometimes the rock under the surface
is very permeable.That is, water flows easily through it. In this case,
some of the groundwater may seep to the surface, forming individual
springs. Aquifers are large accumulations of underground water.They
can provide an excellent source of water from wells. Groundwater
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Bio Words
transpiration: the
emission of water vapor
from pores of plants as
part of their life processes
flow, although measurable, is much slower than the flow in streams
and rivers.That is because the passageways through the pore spaces in
the materials beneath the Earth’s surface are very small. Nonetheless,
regardless of its speed, groundwater eventually also returns to the
rivers, lakes, and oceans. And, the water cycle continues.
Plants and animals also play a very important role in the water cycle.
Water enters living organisms by osmosis. However, through cellular
respiration, water is released back into the atmosphere. As you saw in
this activity, plants, especially broadleaf trees and shrubs play a major
role in the water cycle through the process of transpiration.
Transpiration is the loss of water through the leaves of a plant.
The Human Impact on the Water Cycle
The Earth’s water supply remains constant, but humans can interfere
with the water cycle. As population increases, living standards rise,
and the industry and economy grow, humans place a greater demand
on the supply
of freshwater.
The amount of
freshwater
needed increases
dramatically,
yet the supply
of freshwater
remains the same.
As more water is
withdrawn from
rivers, lakes, and
aquifers, local
resources and
future water
supplies are
threatened.
A person can probably exist on about 4 L (four liters is about
one gallon) of water a day for drinking, cooking, and washing.
At present in the United States, people use almost 6000 L a day
for their needs and comforts.These include recreation, cooling,
food production, and industrial supply.
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A larger
population and
more industry
also mean that
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Activity 7 The Water Cycle
more wastewater is discharged. Domestic, agricultural, and industrial
wastes include the use of pesticides, herbicides, and fertilizers.They
can overload water supplies with hazardous chemicals and bacteria.
Poor irrigation practices raise soil salinity and evaporation rates.
Urbanization of forested areas results in increased drainage of an area
as road drains, sewer systems, and paved land replace natural drainage
patterns. All these factors put increased pressure on the water
equation.
Pollutants that are discharged into the air can also affect the water
cycle. Sulfur and nitrous oxides from the burning of fossil fuels,
combustion in automobiles, and processing of nitrogen fertilizers enter
the atmosphere.They combine with water droplets in the air to form
acids.They then return to the surface of the Earth through the water
cycle as acid precipitation.
Reflecting on the Activity and the Challenge
In this activity you observed one of the
processes that take place in the water
cycle. You learned that a great amount
of water is transpired by a living plant.
You also read about some of the other
processes that are involved in the water
cycle. The water cycle is very complex,
and at any stage humans can have a
significant impact. Perhaps the
environmental issue you have chosen
involves one part of the water cycle.
1. Name and describe at least four processes that take place in the water cycle.
2. What is the energy source that drives the water cycle?
3. How has the water cycle determined partly where people live in the United
States?
4. What would happen to the planet if the hydrologic cycle stopped functioning?
5. Describe three ways in which humans can have a negative effect on the water
cycle.
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Inquiring Further
Environmental models
Environmental models allow scientists
to study what could happen to the
plants and animals in an area if changes
occurred. Models help check predictions
without disrupting a large area.
Build an ecocolumn to research how
acid rain affects an ecosystem.
(You will be allowed to use household
vinegar as the acid.) An ecocolumn is
an ecological model that is especially
designed to cycle nutrients.
Record the procedure you will use.
Have your teacher approve your
procedure before you create your
model.
ECOCOLUMN
1.
Using scissors,
remove the top
and bottom
of a plastic bottle.
(see bottle in
the middle)
From a second
bottle, make
a cut just before
the point at
which the bottle
narrows.
(see bottle on
the left)
Slide part 1 into
part 2 as shown.
Then make a
seal with silicone
or tape. Next
stack the structure
on top of part 4.
(See assembly
in the middle.)
2.
2.
1.
3.
4.
4.
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Active Biology
A sample of a
more complex
ecocolumn.
You decide on the
design for your
own ecocolumn.