Biology 11 – Dalesandro Lab #4 – Osmosis October 17, 2013

Instructions:

-First, obtain your petrie dishes and potato slices from Mr. D.

-Dry your slices with the paper towel and then trace the outline of the slices on your paper.

-Place the slices into petrie dishes, one slice per dish.

-Cover one slice with distilled water, one slice with salt water, and one slice with sugar water. Ensure your slices are completely submerged!

-Wait 25 minutes. While waiting, measure your traced drawings and record their sizes.

-After 25 minutes, remove the slices from the petrie dishes and dry them with paper towel. Trace their outlines on a new sheet of paper.

-Measure the new outlines and compare to the original tracings.

-Dispose of the potato slices, empty your petrie dishes, and clean your station.

Lab write-up sections:

Purpose – To investigate osmosis and osmotic changes in potato slices.

Materials – Petri dishes, salt water, sugar water, distilled water, potato slices, paper towel.

Procedure – Y.O.W.

Results – Table showing size changes (if any); answers to the questions below.

Observations – Tracings of potato slices before and after the period of immersal.

Conclusion – Y.O.W.

Due date: One week from today (Oct. 24, 2013).

Questions: 1) Describe the size changes you observed in each of the the potato slices over 25 minutes (if any).

2) Explain how osmosis was responsible for the changes in size.

3) In what direction was the osmotic gradient in each submerged potato slice? Explain. Osmosis Experiment in a Potato

Materials - if done in class 8 pencils (use classroom pencils) 9 Potato Activity Sheets 24 potato slices (not furnished) in 1% salt solution 1 box of toothpicks 4 sheets of paper towel for blotting potato slices (not furnished - blot the slices of potato before giving them to the student groups) 8 250mL bottles of deionized water 8 250mL bottles of 8% NaCl solution 8 250mL bottles of 30% sucrose solution 1 500mL bottle of extra deionized water 1 500mL bottle of extra 8% NaCl solution 1 500mL bottle of extra 30% sucrose solution 8 #1 ziploc bags containing: 3 small petri dishes 3 lids for petri dishes (labeled #1- water; #2- salt; #3- sucrose) 1 sheet of paper towel (not furnished)

Preparation: For this activity, new potatoes work best (redskin). Each potato slice should be long as possible (at least 6cm), about 1.5cm wide, and depth should be as thin as possible. Store potato slices in the 1% salt solution provided, until ready to be used by students. The class is divided into 8 groups. Have each group get one pencil for the group. • Give each group 1 Potato Activity Sheet and a ziploc bag of supplies, and one 250mL bottle of each solution, and 3 toothpicks. • A GTF should get the 24 potato slices, remove them from the salt solution, blot them and prepare to distribute three slices to each group. • Another GTF should give instructions while a GTF and teacher monitor student groups to make sure instructions are followed accurately.

Have each group do the following: • Place the 3 petri dishes on the appropriate sections of the Potato Activity Sheet. • Place the three potato slices on the activity sheet (after it has been blotted to remove excess solution). • Trace the three slices on the appropriate sections of the Potato Activity Sheet. • As soon as a slice is traced, place it in the bottom of the petri dish that is next to its tracing. • Pour the water in petri dish #1 in the first section, so that the potato slice is submerged.. • Pour the salt solution in petri dish #2 in the middle section, so that potato slice is submerged.

EXPLAIN TO STUDENTS: This is an 8% salt solution. That means that 8 parts of the solution are salt and 92 parts are water. (Think of pennies in a dollar. 8% means 8 pennies out of 100 pennies.)

• Pour the sucrose solution in petri dish #3 in the bottom section, so that the potato slice is submerged.. EXPLAIN TO STUDENTS: This is a 30% sucrose (sugar) solution. That means that 30 parts of the solution are sucrose and 70 parts are water. (30% would equal 30 pennies out of 100)

• Gently place the lids over the petri dishes. • Carefully slide the paper and cups to one section of the work area. DO NOT leave the petri dishes close to the edge of the area where they might be knocked over or spilled. • Record the Start Time at the bottom of the page. • Leave the experiment undisturbed until instructed to return to check the results.

Have students leave the potatoes undisturbed for about 20-30 minutes (no less than 15 minutes) before checking results. The longer the potatoes soak, the better (although no noticeable length changes occur after 30 minutes.

CHECKING POTATO RESULTS

Have each group do the following: • Remove the first potato slice from the water using a toothpick and very gently blot it on a paper towel. • Place the potato slice on the original drawing and compare. Note changes in the length of the potato. • Remove the slice from the salt solution using a toothpick, blot gently, and compare to original tracing. • Remove the slice from the sugar solution using a toothpick, blot gently, and compare to original tracing.

Students should observe the following: 1) The potato slice in the water is larger, indicating that more water molecules went into the potato than came out, because there was a higher concentration of water outside of the potato.

2) The potato slice in the 8% salt solution is smaller, indicating that more water molecules came out of the potato than went in, because there was a higher concentration of water inside the potato.

3) The potato slice from the 30% sugar(sucrose) solution is also smaller indicating that more water molecules came out of the potato than went in, because there was a higher concentration of water inside the potato.

EXPLANATION: Diffusion is the movement of molecules across a membrane from an area of higher concentration to an area of lower concentration. Osmosis refers specifically to the diffusion of water. In all three cases, water is moving across the membrane to establish equilibrium. All of these can be explained by the diffusion or osmosis of water molecules to equalize the concentration of the solutions. The water could go in or out of the potato to equalize the concentration of sugar or salt in the solution.

The potato slices that have absorbed water (the potato slice in water) seems stiff because it has absorbed water molecules. The potato slices that have lost water to their surroundings (the potato slices in salt & sucrose solution) seem soft because they have lost water molecules. Name: ______Block: ______Date:______Osmosis in a Potato

Purpose: To examine the effects of osmosis in potatoes.

Hypothesis: Discuss what will happen to the potato slices in each solution: water, 8% salt and 30% sugar. ______

Materials:  potato slices  deionized water  30% sucrose solution  paper towel (blotting)  8% NaCl solution  small petri dishes  lids for petri dishes (labeled #1- water; #2- salt; #3- sucrose)

Procedure: 1. Trace 3 potato slices on “Osmosis Experiment in a Potato- Results Sheet” and measure the size. 2. Place a petri dish on the sheet beside each potato, matching the numbers. 3. Put potato slice #1 into petri dish #1. Put slice #2 into petri dish #2. Put slice #3 into petri dish #3. 4. Pour water into petri dish #1, so that potato slice is submerged. Pour the 8% salt solution into petri dish #2, so that potato slice is submerged. Pour the 30% sugar (sucrose) solution into petri dish #3, so that potato slice is submerged. 5. Place lids on top of containers. 6. Allow containers to sit undisturbed for at least 20 - 25 minutes. 7. Remove potato slice #1 from petri dish #1, blot dry, and compare the length to the original tracing by placing the slice on top of your original drawing. Note any changes in length, as well as whether the potato seems hard or soft. (Do the same for potato slices #2 and #3).

Observations and Table of Results: Potato Slice Original Length Original Width Resulting Length Resulting Width (cm) (cm) (cm) (cm) #1 (deionised water) #2 (salt solution) #3 (sucrose solution)

Discuss the results (both size and texture). 1. The potato submerged in deionised water ______2. The potato submerged in the salt solution ______3. The potato submerged in the sucrose solution ______

Discussion Questions: Use the term “osmosis” to explain the changes seen in each potato. Hint: Think about water movement. Potato #1 – ______Potato #2 – ______Potato #3 – ______

Conclusion: Discuss the results of the experiment, including the accuracy of your hypotheses. ______Osmosis in a Potato Results Sheet

PETRI DISH # 1 POTATO TRACING #1 WATER

How has the potato changed its size? ______How does it feel different? ______

POTATO TRACING # 2 PETRI DISH # 2 8% SALT SOLUTION

How has the potato changed its size? ______How does it feel different? ______

POTATO TRACING #3 PETRI DISH # 3 ADD 30% SUCROSE 30% SUCROSE SOLUTION

How has the potato changed its size? ______How does it feel different? ______