Hypotheses - Bigger molecules will diffuse more slowly across a membrane until it becomes too big to cross the membrane at all while a molecule's shape may be important in facilitated diffusion. In addition, higher concentration gradient will increase rate of diffusion including osmosis. To determine concentration of overall solute in potato cells, wait until the potato cylinders reach equilibrium and measure the change in weight. Since the solute concentration of outside solution is known, one can determine the concentration of solute in potato cells by the change in weight after it reaches equilibrium
Hazards - NaOH may cause eye or skin burns upon contact. Harmful if swallowed or inhaled. Use gloves and goggles as precaution.
Procedures
I. Influence of molecule size on diffusion across a semi-permeable membrane
Procesure 1: Observe effect of molecule size on diffusion across a semi-permeable membrane
Use cellulose membrane (dialysis tubing) to simulate a cell membrane. Cellulose membrane has tiny pores in it. Use Iodine, and phenolphthaleine as indicators.
Iodine - starch indicator. Changes from light brown to dark blue for positive
Phenolpthalein - pH indicator. turn red in basic solutions
1. Obtain four pieces of string and two pieces of water soaked dialysis tubing about 15 cm long
2. Seal one end of each bag by folding over about 1cm of the end and tying it tightly with a piece of string. The ends of the tube must be sealed tightly to prevent leaks
3. Roll the untied end of each tube between your thumb and finger to open it and form a bag.
4. Fill one tube with 10ml of water and add 8 drops of phenolpthalein. Seal the open end of the bag by folding the end and tying it securely.
5. Fill the other bag with 10ml of starch solution. Seal the open end of the bag by folding the end and tying it securely.
6. Carefully rinse the outside of each bag in tap water.
7. Fill a beaker with 200ml of tap water and add 15 drops of 1 M NaOH. Submerge the dialysis bag containing phenolphthalein in the beaker
8. Fill a beaker with 200 ml of tap water and add 60 drops of iodine. Submerge the dialysis bag with starch in the beaker.
9. Observe color changes in the two bags contents and the surrounding solutions
10. After 30 minutes, record in table 1 the color inside and outside the bags. Answe questions in your notebook.
II. Effect of Steepness of Concentration Graident on the rate of Osmosis (Diffusion of Water).
Bag A - hypotonic
Bag B - isotonic
Bag C and D - hypertonic
Procedure 2: Oberve Osmosis in Different Conditions and observe the effect of steepness in concentration graident on the rate of osmosis.
1. Obtain 8 pieces of string and 4 pieces of pre-soaked dialysis tubing (15cm long). Seal one end of tube by tying it tightly like before.
2. Open the other end of the tube by rolling it between your thumb and finger.
3. Bag A - 10 ml 1% Sucrose. Outside 50% sucrose
4. Bag B - 10 ml 1% sucrose, Bag C - 10 ml 25% sucrose, Bag D - 10 ml 50% sucrose outside 1% sucrose
5. Try to carefully remove any excess air for each bag. Tie folded ends securely, rinse bags with tap water, then check for leaks. Quickly blot excess water from outside bags.
6. Place dry paper towel on scale and tare. Weigh each bag to nearest 0.1g. Record these initial weights in table 2
7. Place bag B,C, and D in large beaker filled with 1% sucrose. Record time
8. Place bag A in empty beaker and will with just enough 50% sucrose to cover the bag. Record time.
9. Remove bags from beakers at 15 minute intervals for one hour. Always blot dry, and weigh them to the nearest 0.1g. Avoid leaks and quickly return bags to their respective containers.
10. For each 15-min interval, record the total weight of each bag in table 2. Then calculate and record the change in weight
III. Determine the Concentration of Water in Living Plant Cells.
Procedure 3: Determine the Concentration of Solute in living plant cells
1. Locate 5 beakers in the back of the room with 5 concentrations of salt solution
2. The initial weights of potato cylinders that you see in solution were recorded. Copy in table 3
3. Carefully remove a potato cylinder from its solution. Quickly shake dray and place on scale to measure weight. Be sure to first place a dry paper towel and tare, before adding the potato cylinder. Record the result and quickly replace the cylinder back to its respective beaker. Note the time you recorded the final weights.
4. repeat for all potato cylinders.
5. Calculate the change in weight and change in time (total time) for each cylinder and record in Table 3. Calculate rate of osmosis (change weight/change time) for each cylinder and record.
Results
I. Influence of molecule SIZE on diffusion across a semi-permeable membrane.
Answers & Comments
Explanation:
Hypotheses - Bigger molecules will diffuse more slowly across a membrane until it becomes too big to cross the membrane at all while a molecule's shape may be important in facilitated diffusion. In addition, higher concentration gradient will increase rate of diffusion including osmosis. To determine concentration of overall solute in potato cells, wait until the potato cylinders reach equilibrium and measure the change in weight. Since the solute concentration of outside solution is known, one can determine the concentration of solute in potato cells by the change in weight after it reaches equilibrium
Hazards - NaOH may cause eye or skin burns upon contact. Harmful if swallowed or inhaled. Use gloves and goggles as precaution.
Procedures
I. Influence of molecule size on diffusion across a semi-permeable membrane
Procesure 1: Observe effect of molecule size on diffusion across a semi-permeable membrane
Use cellulose membrane (dialysis tubing) to simulate a cell membrane. Cellulose membrane has tiny pores in it. Use Iodine, and phenolphthaleine as indicators.
Iodine - starch indicator. Changes from light brown to dark blue for positive
Phenolpthalein - pH indicator. turn red in basic solutions
1. Obtain four pieces of string and two pieces of water soaked dialysis tubing about 15 cm long
2. Seal one end of each bag by folding over about 1cm of the end and tying it tightly with a piece of string. The ends of the tube must be sealed tightly to prevent leaks
3. Roll the untied end of each tube between your thumb and finger to open it and form a bag.
4. Fill one tube with 10ml of water and add 8 drops of phenolpthalein. Seal the open end of the bag by folding the end and tying it securely.
5. Fill the other bag with 10ml of starch solution. Seal the open end of the bag by folding the end and tying it securely.
6. Carefully rinse the outside of each bag in tap water.
7. Fill a beaker with 200ml of tap water and add 15 drops of 1 M NaOH. Submerge the dialysis bag containing phenolphthalein in the beaker
8. Fill a beaker with 200 ml of tap water and add 60 drops of iodine. Submerge the dialysis bag with starch in the beaker.
9. Observe color changes in the two bags contents and the surrounding solutions
10. After 30 minutes, record in table 1 the color inside and outside the bags. Answe questions in your notebook.
II. Effect of Steepness of Concentration Graident on the rate of Osmosis (Diffusion of Water).
Bag A - hypotonic
Bag B - isotonic
Bag C and D - hypertonic
Procedure 2: Oberve Osmosis in Different Conditions and observe the effect of steepness in concentration graident on the rate of osmosis.
1. Obtain 8 pieces of string and 4 pieces of pre-soaked dialysis tubing (15cm long). Seal one end of tube by tying it tightly like before.
2. Open the other end of the tube by rolling it between your thumb and finger.
3. Bag A - 10 ml 1% Sucrose. Outside 50% sucrose
4. Bag B - 10 ml 1% sucrose, Bag C - 10 ml 25% sucrose, Bag D - 10 ml 50% sucrose outside 1% sucrose
5. Try to carefully remove any excess air for each bag. Tie folded ends securely, rinse bags with tap water, then check for leaks. Quickly blot excess water from outside bags.
6. Place dry paper towel on scale and tare. Weigh each bag to nearest 0.1g. Record these initial weights in table 2
7. Place bag B,C, and D in large beaker filled with 1% sucrose. Record time
8. Place bag A in empty beaker and will with just enough 50% sucrose to cover the bag. Record time.
9. Remove bags from beakers at 15 minute intervals for one hour. Always blot dry, and weigh them to the nearest 0.1g. Avoid leaks and quickly return bags to their respective containers.
10. For each 15-min interval, record the total weight of each bag in table 2. Then calculate and record the change in weight
III. Determine the Concentration of Water in Living Plant Cells.
Procedure 3: Determine the Concentration of Solute in living plant cells
1. Locate 5 beakers in the back of the room with 5 concentrations of salt solution
2. The initial weights of potato cylinders that you see in solution were recorded. Copy in table 3
3. Carefully remove a potato cylinder from its solution. Quickly shake dray and place on scale to measure weight. Be sure to first place a dry paper towel and tare, before adding the potato cylinder. Record the result and quickly replace the cylinder back to its respective beaker. Note the time you recorded the final weights.
4. repeat for all potato cylinders.
5. Calculate the change in weight and change in time (total time) for each cylinder and record in Table 3. Calculate rate of osmosis (change weight/change time) for each cylinder and record.
Results
I. Influence of molecule SIZE on diffusion across a semi-permeable membrane.