NAME: Michael Timson

DATE:

FORM: L6-4

LAB: #7

SUBJECT: Biology

TEACHER’S NAME: Miss Sarjeant

TITLE: WATER POTENTIAL

AIM: To determine the effect of sugar solution of varying concentrations of potato cylinders

INTRODUCTION:

Plant cells are often exposed to external solutions of concentrations that differ from

the concentration of their cytoplasm or vacuoles. This can have large effects on the cell

causing it to either gain or lose its water. This process is known as osmosis. Osmosis is

described as the spontaneous net movement of water molecules from a region of high

concentration to a region of low concentration through a partially permeable. In other words,

the water molecules move in the direction that tends to equalize the solute concentrations on

both sides. As biological membranes are semipermeable, osmosis plays a vital role in

biological systems. These membranes are impermeable to large and polar molecules, such

as ions, proteins, and polysaccharides, while being permeable to non-polar

and/or hydrophobic molecules like lipids as well as to small molecules like oxygen, carbon

dioxide, nitrogen, and nitric oxide. Water molecules travel through the plasma or cell

membrane by diffusing across the phospholipid bilayer via small transmembrane proteins

similar to those responsible for facilitated diffusion and ion channels known as aquaporin.

Osmosis provides the primary means by which water is transported into and out of cells.

Instead of concentration, the term water potential is often used as

concentration generally refers to solute. The symbol for water potential is Ψ (psi). Water

potential is the potential energy of water per unit volume relative to pure water in reference

conditions and it quantifies the tendency of water to move from one area to another. The

water potential of a solution also measures how freely the water molecules can move and

how much pressure is being applied to it. A solution that has a high water potential is a

solution that is said to have a water and is under pressure. A solution that contains a lot of

dissolve solutes and is highly concentrated is said to have a low water potential and thus is

not under pressure. Pure water at normal atmospheric pressure has a water potential of 0 and

as more solute is dissolved, the lower the water potential. Pressure potential is based on

mechanical pressure, and is an important component of the total water potential within

plant cells. Pressure potential increases as water enters a cell. As water passes through

the cell wall and cell membrane, it increases the total amount of water present inside the cell,

which exerts an outward pressure that is opposed by the structural rigidity of the cell wall. By

creating this pressure, the plant can maintain turgor, which allows the plant to keep its

rigidity. As water leaves the cell wall and cell membrane, it decreases the total amount of

water present inside the cell and no longer exerts an outward pressure that is opposed by the

structural rigidity of the cell wall. The cell therefore loses its turgor and becomes flaccid.

The amount by which the dissolved solute lowers water potential of a solution is

known as the solute potential and is represented by the symbol Ψ s. As it lowers water

potential, solute potential is always a negative value. The more solute there is the, the more

negative the solute potential. The amount by which pressure increases the water potential of a

solution is known as the pressure potential and is represented by Ψp. As it increases water

potential, pressure potential is always positive. The overall water potential of a solution can

calculated using the equation: Ψ = Ψp + Ψs

APPARATUS/MATERIALS

6 Petri dishes, 2 large potatoes, Cork bora, Stop watch, Sucrose, Distilled water, Paper

towel, graph paper, Scalpel

METHOD:

Six petri dishes were first labelled A-F. Volumes of sucrose were then added to each

boiling tube, with the exception of petri dish labelled A, and each was diluted with different

volumes of distilled water to obtain the sucrose concentration percentages 0.0, 0.2, 0.4, 0.6,

0.8 and 1.0 respectively. Using the cork bora, 30 potato cylinders were then extracted from

two large potatoes and each cylinder was then measured, using a graph paper, and cut to 5cm

in length. Five potato cylinders were then simultaneously placed into each petri dish for 30

minutes while immediately starting the stopwatch. Each petri dish was then covered using

another. After 45 minutes, the potato cylinders were simultaneously retrieved from the petri

dishes and re-measured. Their changes in length were recorded and a graph of percentage

change in length of the potato cylinders for each sucrose concentration was plotted.

RESULTS

AVERAGE PERCENTAGE CHANGE IN LENGTH OF POTATO STRIPS IMMERSED IN

DIFFERENT CONCENTRATIONS OF SUCROSE SOLUTIONS AFTER 30 MINUTES

Concentration Of Sucrose Soln. (Moles/

Dm-3)

Percentage Change In Length Of 5 Potato

Strips In Each Petri Dish (%)

Average %

Change In Length

1 2 3 4 5

0.0 2.3 2.7 2.5 2.4 2.6 2.5

0.2 0 0 -1.0 0 1 0.0

0.4 -2.5 -2.3 -2.6 -2.6 -2.5 -2.5

0.6 -4.0 -5.0 -7.0 -4.0 -5.0 -5.0

0.8 -7.5 -7.0 -8.5 -7.5 -7.0 -7.5

1.0 -10.5 -11.0 -9.0 -8.0 -11.5 -10.0

DISCUSSION:

From the results obtained, it was observed that the potato cylinders placed into

solution B, composed of 0.2 mol dm-3 sucrose displayed no changes in length. This

observation thus suggest that both environments, the water potential within the potato and the

solution, were equal. This was a result of the equal net movement of water molecules into and

out of the potato tissue/cells .This meant that the osmotic potential inside the cells comprising

the plant tissues of the potato cylinders was equivalent to that of the osmotic potential of the

external environment, solution B.

The potato cylinders in solution A, unlike solution B, underwent a positive change in

length of 2.5% as displayed on the graph. This means that the potato cylinder acquired an

increase in length a result of the net movement of water from solution A, being pure water,

having the higher water potential, into the potato tissues, having the lower potential. Thus the

solute potential of solution A was lower than that of the cells. The solute potential of the

potato tissues thus had greater negative value than that of the solution.

In solutions C-F, a negative percentage change in length of the potato was obtained

indicating that the potato strip decreased in length as a result of the loss of water from within

the potato tissue. As the sucrose concentration gradually increased as displayed on the graph,

the potato length gradually decrease giving the lowest percentage increase of -10% or 10% as

the highest decrease in length. Based upon these results, the potato tissues lost water as it

possessed the higher water potential than the various solutions. This was a result of the solute

concentration being greater in the external environment and hence the freedom of the water

molecules was reduced as they are attracted to the water molecules. This imbalance then

created a water potential gradient that caused water to move from within the cells inside of

the potato cylinders into the external environment via osmosis. As a result of this, the potato

cells would have become flaccid.

Based upon the straight line graph obtained, it is seen that as the number of moles of

sucrose gradually increased, the greater the decrease in length of the potato strip. The

gradient of this graph was calculated to be 12.5. This means that the potato strip had a change

in length of 12.5% per mole dm-3 of sucrose.

Within this experiment there were various sources of errors. Different potatoes were

used in acquiring the potato cylinders and thus not all of them had the same natural water

potential thus introducing an unwanted manipulated variable. Due to the external temperature

of the experiment being above the average room temperature, it may have decreased the rate

at which osmosis would have taken placed under the 45 minute thus causing the results to be

slightly inaccurate. Before conducting the experiment, all apparatus were washed thoroughly

to ensure that there are no residue of chemicals on the apparatus when conducting the

experiment that would affect the changes in length of the potato cylinders.

CONCLUSION: Within the limits of experimental error, it can be concluded that as the water

potential of the solution decreased, there was a decrease in length of the potato cylinders as a

result of osmosis.