Movement of water into a model cell
In Exercise 1, as water moves by osmosis into a dialysis bag (our “model cell”), the pressure in the bag increases. You will quantify this pressure change by measuring the height of the water in a glass tube inserted into the bag (See Figure 1). A taller column contains more water, so it weighs more and exerts more downward pressure. 1. Work with one or two partners. Write your partners’ names here:
Partner’s name Discussion leader’s name
2. After looking at the osmometer set up in the Central Study Area, obtain the following materials: 20 cm of dialysis tubing 400 ml beaker scissors 10% sucrose solution string (with dye added) glass tube marking pencil ruler supporting clamps 4. Cut a piece of dialysis tubing 20 cm long. Soak the tubing under running water until soft. Tie a tight knot in one end of the dialysis tubing. Open the other end by rubbing it between your fingers. 5. While your partner holds the bag, carefully pour in 8-10 cm worth of 10% sucrose solution containing dye. (The dye is used as a tracer to let you see if any sucrose solution is leaking out of the bag.) Insert the glass tube into the dialysis bag. Tie the upper end of the bag around the glass tube by wetting a piece of string and wrapping it a few times around the tubing before knotting it. Try to minimize the air pocket left inside the bag. 6. Make sure that: (a) the filled portion of the bag is less than 10 cm long (b) the submerged end of glass tube is about 5 cm from the bottom of the bag (c) the air space inside the bag is as small as possible, (d) that top end of the tube is at least 15-18 cm above the fluid level in the bag, (e) there are no leaks. (There should be no green dye outside the tube.) 7. Hold the bag and glass near the upper knot and carefully rinse off the dialysis bag with water. Avoid squeezing the bag. If properly tied, no dye should leak out. 8. Fill the beaker with about 360 ml of room temperature water from the carboy and lower the dialysis bag into the beaker (See Figure 1). Support the upper end of the glass tube with clamps in such a way that (a) the water level in the beaker is about l.5 cm below the upper end of the fluid inside the dialysis bag, and (b) the bag is not touching the bottom of the beaker.