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Observing Osmosis in Different Molarity Solutions

Evy Geitzenauer

Partners: Reed Lewis, Conor O’Rourke, Joe Kelly, Megan Kelly, Morgan Zangs

Lahr

AP Biology Period 2

31 October 2018

Observing Osmosis in Different Molarity Solutions

Purpose

The purpose of this lab was to determine the molarities and observe the rate of osmosis occurring within different colored solutions using potato cylinders. Osmosis is the diffusion of water through a selectively permeable membrane. The water moves from a region of higher water potential to a region of lower water potential (Foglia). In distilled water, the concentration is highest.  When solutes are dissolved in water, the water concentration decreases. In this lab, the concentration of sucrose was trying to be determined in each of the unknown colored substances.

Theory

Cell transport is the movement of materials across cell membranes, either inside or out of the cell (Lahr). Cellular transport is important to cells because that is how they receive nutrients as well as dispose waste products. Without it, cells would not function. Through cell transport, they can also communicate with other cells using cell signaling. There are two types of cell transport, active and passive. When there is no energy required for the movement of materials, that is called passive transport. The most common types of passive transport are osmosis, diffusion and facilitated diffusion. Active transport, on the other hand, requires energy and moves molecules against the gradient, from an area of low concentration to an area of high concentration (Lahr). In this lab, osmosis occurred, therefore passive transport was being studied.

As the concentration of sucrose increases, the percent mass change will also increase in the potato. In a hypertonic solution the concentration is on the outside of the cell, in this case, the potato. This higher concentration forces water to rush from the inside of the cell to the outside. Sucrose is a hypertonic solution, meaning there is a larger amount of particles dissolved within the solution. Opposing hypertonic, is hypotonic. Hypotonic solutions have a higher concentration on the inside of cell, making the solution rush into the cell, therefore making it swell. If a cell is placed in a hypotonic solution, the potato mass increases. In a isotonic solution, the concentration is equally divided between the inner and outer parts of the cell. Therefore, cells can move in and out of the cell freely. Potato/dialysis tubes are a good model of osmosis because they have a semipermeable membrane and also contain a lot of water, which helps create a good visual model.

Materials

In this lab, potatoes were used to model the cells. 6 beakers were equally filled with different unknown concentrations of sucrose. A cork borer was then used to carve out 6 equal sized potato cylinders. The potato cylinders were then weighed to determine the initial mass. Once the mass was calculated, the potato cylinders were placed in the 6 different solutions. After 24 hours, the potatoes were taken out and weighed. Once they were weighed, the initial mass and the mass after 24 hours was calculated and compared using percent mass change.

Data

Table 1: Mass of Potatoes before and after osmosis with calculated mass change

POTATO #

COLOR

MASS

BEFORE (g)

MASS AFTER 24 HRS (g)

PERCENT MASS CHANGE (%)

1

Purple

0.78

0.58

-25.64%

2

Blue

0.79

0.67

-15.18%

3

Green

0.82

0.66

-19.51%

4

Yellow

0.78

0.9

15.38%

5

Orange

0.78

0.6

-23.07%

6

Red

0.74

0.99

33.78%

Table 2: Exact/Correct Molarities of Solution

MOLARITY (M)

COLOR

1M

Blue

.8M

Orange

.6M

Purple

.4M

Green

.2M

Yellow

0M

Red

Graph 1: Molarity and Percent Mass Change of Solutions

Observations

It was observed throughout the lab, that after 24 hours, some potato cylinders were not fully submerged in the sucrose solutions. It was also observed that the potato soaked in the purple solution was a darker potato with a brown color, while the other potatoes were a lighter cream color.

Calculations

% mass change= ((final mass- initial mass)) / initial mass) x 100

-25.64% = ((.58-.78) / .78) x 100

-15.18% = ((.67-.79)) / .79) x 100

-19.51% = ((.66- .82)) / .82) x 100

15.38% = ((.99-.78)) / .78) x 100

-23.07% = ((.6-.78)) / .78) x 100

33.78% = ((.99-.74)) / .74) x 100

Discussion

The purpose of this lab was to calculate the molarities of the sucrose in the 6 unknown solutions. The percent change in mass of the potato cylinders was calculated after the cylinders sat for 24 hours. Based off of the data that was collected, the highest molarity was the orange solution, which was found to be at 1.0M. The yellow solution was observed to be 0.2M. The blue solution was 0.4M. The green solution was 0.6M. The orange solution was 0.8. Lastly, the purple was 1.0M.

The mass of the potato cylinders varied depending on the concentration of the sucrose. When the potato was in a concentration of 0.6M or higher, the potato was in a hypertonic solution, making the potato lose water. When the potato was in a solution below 0.2M, water rushed in. Therefore, the potato gained mass and was said to be in a hypotonic solution. The data collected in this lab was not fully accurate compared to the exact/correct molarities shown in Table 2. Based off of the data collected, when the molarity was highest, the percent mass change decreased. This shows that when a cell is placed into a higher concentration/molarity, the mass does not vary as much. Throughout the majority of this lab, osmosis was shown through the potato cylinders and solutions.

During this lab, a few limitations occured that could have had effects on the data collected. After the 24 hours, it was observed that some of the potatoes were not floating because the length was too long. This could have had effect on the osmosis process. Another variable was the potatoes initial mass. Not all the potatoes had the same initial mass, therefore the calculations of mass differed. Another limitation that could have affected our data was the temperature. If the temperature varied throughout the 24 hours, the osmosis could have been interrupted. A way to improve this lab could be to observe a larger sample size, rather than just one potato cylinder. With a larger sample size, the data could be more accurate. Another way to change this lab to continue research would be to try using a different “cell”, other than the potato, to see if the results or outcome differed.

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