How do you answer each of the questions for the pre-lab about transpiration? LAB

Question

How do you answer each of the questions for the pre-lab about transpiration?

LAB Introduction:

The amount of water needed daily by plants for the growth and

maintenance of tissues is small in comparison to the amount that is lost

through the process of transpiration (the evaporation of water from the

plant surface) and guttation (the loss of liquids from the ends of vascular

tissues at the margins of leaves). If this water is not replaced, the plant will

wilt and die.

The transport of water up from the roots in the xylem is governed by differences in water potential

(the potential energy of water molecules). These differences account for water movement from cell

to cell and over long distances in the plant. Gravity, pressure, and solute concentration all

contribute to water potential. The movement itself is facilitated by osmosis, root pressure, and

adhesion and cohesion of water molecules.

The overall process: Minerals actively transported into the root accumulate in the xylem,

increasing solute concentration and decreasing water potential. Water moves in by osmosis. As

water enters the xylem, it forces fluid up the xylem due to hydrostatic root pressure. But this

pressure can only move fluid a short distance. The most significant force moving the water and

dissolved minerals in the xylem is upward pull as a result of transpiration, which creates a negative

pressure. The pull on the water from transpiration is increased as a result of cohesion and adhesion

of water molecules.

The details: Transpiration begins with evaporation of water through the stomates (stomata), small

opening in the leaf surface which open into air spaces that surround the mesophyll cells of the leaf.

The moist air in these spaces has a higher water potential than the outside air, and water tends to

evaporate from the leaf surface (moving from an area of high water potential to an area of lower

water potential). The moisture in the air spaces is replaced by water from the adjacent mesophyll

cells, lowering their water potential (since the cytoplasm becomes more concentrated). Water will

then move into the mesophyll cells by osmosis from the surrounding cells with higher water

potentials, including xylem. As each water molecule moves into a mesophyll cell, it exerts a pull on

the column of water molecules existing in the xylem all the way from the leaves to the roots. This

transpiration pull is caused by (1) the cohesion of water molecules to one another due to hydrogen

bond formation, and (2) by adhesion of water molecules to the walls of the xylem cell which aids in

offsetting the downward pull of gravity.

The upward transpirational pull on the fluid in the xylem causes a tension (negative pressure) to

form in the xylem, pulling the walls of the xylem inward. The tension also contributes to the lowering

of water potential in the xylem. This decrease in water potential, transmitted all the way from the

roots, causes water to move inward from the soil, across the cortex of the root, and into the xylem.

Evaporation through the open stomates is a major route of water loss in plants. However, the

stomates must open to allow the entry of CO2 used in photosynthesis. Therefore, a balance must be

maintained between the gain of CO2 and the loss of water by regulating the opening and closing of

stomates and also affect the rate of transpiration. 2

PRE- Lab Questions:

1. What is your hypothesis?

2. What are the independent and dependent variables?

3. List the controls.

4. Why will you let the potometer equilibrate for 5 minutes before recording the time zero

reading?

5. Why will you have to find the surface area of the leaf? (Please don’t answer “so you know the amount per surface area”).

6. What will compiling data from other groups represent?

7. From where will the leaf lose its water and how do you know?

Explanation / Answer

1)Hypothesis in this experiment is plants will lose water through transpiration ,transpiration in plants is controlled by water potential.This transpiration will pull water from the potometer into the plant

2)Independependent Variable is the time.Read the potometer every 3 minutes for 30 minutes total

Dependent Variable is the amount of water loss measured in mL/min/m2.Read the water level and find the change

3) This would be the normal room condition plant any change may occur if change occurs in their environmental condition

4) To obtain accurate results it is important that the water in the xylem vessels is continuous with the water in the potometer ..This is why the shoot is cut underwater and placed into the bung of the potometer which is filled with water.The shoot should then be left for 5 minutes to allow it to equilibrate.

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