Download 4. Transpiration Discussion/Lab 2

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
The celery to the left is undergoing transpiration. Evaluate how transpiration
relates to the flow of water through the celery’s xylem?
Analyze how 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.” Analyze how this property of water relates
to plant nutrient absorption.
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
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?”