Download Celery Model of Water Transport

Celery Model of Water Transport
Background Information
Just like other living things, plants are dependent upon water for survival. For
example, water is a necessary reactant for photosynthesis and it acts as a solvent
in which gases, minerals, and other solutes enter plant cells and move from cell to
cell and organ to organ. Plants obtain water from the soil through their roots.
Water is transported throughout the plant via the xylem, a type of vascular tissue,
from the roots through the stems to the leaves--where the water is utilized as a
reactant for photosynthesis. Plants only use about 1% of the water they obtain
from the soil for photosynthesis. The other 99% is evaporated from the leaf
surface into the atmosphere in a process known as transpiration.
Similar to humans sweating, plants use transpiration to cool down as well as
assist the flow of water and nutrients from the soil to their leaves. As the plant
loses water from its leaves via transpiration, it causes a pressure difference, which
effectively pumps more water from the soil through the plant.
Within the xylem, a process known as capillary action assists in moving water
from the roots through the stems to the leaves. Capillary action is defined:
Capillary action is the ability of a liquid to flow in narrow spaces (like the
xylem of plants) without the assistance of, and in opposition to, external forces
like gravity. It occurs because of intermolecular forces (hydrogen bonds)
between the water and the surrounding solid surface (xylem). If the diameter
of tube (xylem) is sufficiently small, then the combination of surface tension
(which is caused by cohesion within the water) and adhesive forces between
the water and its container (xylem) act to lift the liquid. In short, capillary
action is due to the pressure of cohesion and adhesion, which cause the liquid
to work against gravity.
Capillary action works because water molecules are attracted to each other
(cohesion) and to other materials (adhesion). These two processes work
together so that effectively the molecules of water “climb” up the stem of a plant.
Capillary action is driven by transpiration as it uses a pressure change, which
allows the water to overcome gravity and be drawn up through the xylem to
provide water and nutrients for the entire plant.
Materials
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Celery stalk with leaves
Food coloring
1000 mL beaker
Vegetable peeler
Cutting board
Scalpel
Microscope slide
Compound microscope
Methods
Day 1 (or prepared in advance by teacher)
1. Fill a1000 mL beaker with water and color it with your choice of food
coloring.
2. Obtain a celery stalk and carefully cut off the base of the stalk (the non-leafy
end) with a scalpel and place it in the beaker of colored water.
3. Leave the celery stalk in the water overnight.
Day 2
4. Carefully use the vegetable peeler to peel back the flesh on the rounded
side of the celery stalk.
5. Observe how far the colored water has traveled up the celery stalk.
6. Prepare a microscope slide with the longitudinal peeling and observe under
the compound light microscope.
7. Sketch the longitudinal section of your celery stalk as visible to the naked
eye and as viewed with the microscope. Label the vascular bundle,
including the xylem and phloem.
8. Carefully use the scalpel to cut a thin, sagittal section (cross-section) of
your celery stalk.
9. Prepare a microscope slide with the sagittal peeling and observe under the
compound light microscope.
10. Sketch the sagittal section of your celery stalk as visible to the naked eye
and as viewed with the microscope. Label the vascular bundle, including
the xylem and phloem.
Longitudinal Section Sketch
Sagittal Section Sketch
Naked Eye
Naked Eye
Compound Light Microscope
Compound Light Microscope
Making Connections with Everyday Objects
Observing Adhesion and Cohesion
1. Obtain a clean, dry penny.
2. Using the appropriate (labeled) water pipette, count how many drops of
water will fit on the penny before spilling over. Sketch and record below.
3. Clean and dry the penny.
4. Using the appropriate (labeled) rubbing alcohol pipette, count how many
drops of rubbing alcohol will fit on the penny before spilling over. Sketch
and record below.
Penny & Water
Penny & Rubbing Alcohol
# drops _____
# drops _____
5. Explain any differences between your observations of the penny with water
and the penny with rubbing alcohol.
6. Explain how adhesion and cohesion relate to this example.
Observing Capillary Action
1. Fill a petri dish with water.
2. Carefully place a straw or capillary tube in the petri dish.
3. Sketch what you observe in the box below and fill in the text box with the
appropriate terms.
The water molecules are attracted to the
straw by __________.
When one water molecule moves closer
to the straw molecules, the other water
molecules are pulled along because of
___________.
The celery to the left is undergoing transpiration. How does transpiration
relate to the flow of water through the celery’s xylem?
How do the properties of water assist in its flow through the xylem?
Phloem Transport
Sugars, which are formed by the plant during photosynthesis, are an essential
component of plant nutrition. Like water, sugar (usually in the form of sucrose,
though glucose is the original photosynthetic product) is carried throughout the
parts of the plant by the vascular system. Phloem, the vascular tissue responsible
for transporting organic nutrients around the plant body, carries dissolved sugars
from the leaves (their site of production) or storage sites to other parts of the
plant that require nutrients. Within the phloem, sugars travel from areas of high
osmotic concentration and high water pressure, called sources, to regions of low
osmotic concentration and low water pressure, called sinks. (Osmotic
concentration refers the concentration of solutes, or sugars in this case; where
the concentration of solutes is highest, so is the osmotic concentration).
The nutrient-rich regions that supply sugars for the rest of the plant are called the
sources. Sources include the leaves, where sugar is generated through
photosynthesis. When they are high in supplies, the nutrient storage areas, such
as the roots and stems, can also function as sources. In the sources, sugar is
moved into the phloem by active transport, in which the movement of substances
across cell membranes requires energy expenditure on the part of the cell.
Sinks
Sinks are areas in need of nutrients, such as growing tissues. When they are low
in supply, storage areas such as the roots and stems cane function as sinks. The
contents of the phloem tubes flow from the sources to these sinks, where the
sugar molecules are taken out of the phloem by active transport.
Pressure Flow
The mechanism by which sugars are transported through the phloem, from
sources to sinks, is called pressure flow. At the sources (usually the leaves), sugar
molecules are moved into the sieve elements (phloem cells) through active
transport. Water follows the sugar molecules into the sieve elements through
osmosis (since water passively diffuses into regions of higher solute
concentration). This water creates turgor pressure in the sieve elements, which
forces the sugars and fluids down the phloem tubes toward the sinks. At the
sinks, the sugars are actively removed from the phloem and water follows
osmotically, so that conditions of high water potential and low turgor pressure
are created, driving the pressure flow process.
Water is known as a “universal solvent.” Describe why this property is important to
plants.
Active and Passive Transport
The article on the previous page refers to active and passive transport (osmosis)-processes that move materials into and out of cells. Highlight examples of active
transport in pink. Highlight examples of passive transport (osmosis) in green.
Active and passive transport are described below:
Active transport:
A kind of transport wherein ions or molecules move against a concentration
gradient, which means movement in the direction opposite that of diffusion
(or osmosis), hence the movement from an area of lower concentration to
an area of higher concentration. Thus, this process will require expenditure
of ATP energy, and the assistance of a type of protein called a carrier
protein.
Passive transport (Osmosis)
Net movement of water molecules through a semipermeable membrane from an
area of higher water potential to an area of lower water potential; the tendency of
water to flow from a hypotonic solution (low concentration of dissolved substances)
to hypertonic solution (higher concentration of dissolved substances) across a
semipermeable membrane
CEJ Extension Activity
Design an experiment to determine if adding a solute to the water (such as salt or
sucrose) affects the rate water is transported by osmosis through the xylem of the
celery (rate of transpiration)?
Procedure
1. Work in groups of four. Your teacher will tell you whether you will be
working with salt or sucrose. Two group members should start filling 6
cups with 100 mL of each of the following 6 concentrations of your
assigned solute: 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5M. Label each cup with its
concentration.
2. The other two group members should obtain several stalks of celery.
3. Find the mass of each celery stalk.
4. Add a celery stalk to each cup with varied concentrations of solute.
5. Soak the celery in the solutions overnight.
6. Harvest the celery. Lightly roll each in a paper towel to dry the surface, and
then find and record their final mass.
5. Compute the percent change in mass by the formula:
% Change = (Final Mass– Initial Mass)/Initial Mass x 100
6. Complete a CEJ template to discuss the question, “Does adding a solute affect
the rate water is transported through the xylem?”