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A&P

LAB: Membranę Transport

Objectiyes:

■    Explain the role of the celi membranę in passive transport.

■    Understand and explain the terms diffusion, osmosis, selectively permeable membranę, and equilibrium.

•    Predict and observe the effects mass and concentration on ratę of osmosis and diffusion.

•    Observe and explain the problems cells face if they become too large.

BąękŁround.

The absorption of nutrients, excretion of cellular wastes, and the exchange of respiratory gasses are life processes which depend upon the efficient transport of substances into, out of, and throughout living cells. The process of diffusion can be easily visualizcd by adding Lugol's iodine solution to a glass of water. Initially, the iodine remains in a smali area in the water, dying it a yellowish. Over time, the molecules of iodine collide with each other, and with molecules of water, and the iodine eventually disperses throughout the entire glass of water, resulting in a golden brown color in the water. Much like the iodine diffuses through the glass of water, many important substances move into and out of cells by diffusion. Diffusion is the movement of a substance through a concentration gradient from high to Iow concentration. It is an exa nipie of passive transport because it requires no energy on the part of the celi. For this reason, diffusion is one of the most common and efficient means by which substances are transported between cells and their environment.

The celi membranę is the selectively permeable barrier whose total surface area is important in regulating the substances that diffuse into or out of the celi. In order to represent the diffusion of materials into the celi, cubes of potato can be used. Iodine reacts with the starch in the potato to tum a blue-black color. This provides a elear visual as to how far the iodine diffuses into the potato in a given period of time. The distance can be measured and used to calculate ratę of diffusion (Ratę = distance/time).

Partides such as iodine are not the only things that diffuse to maintain a State of equilibrium. Water will also diffuse through osmosis to maintain equilibrium. Since not all particles are able to cross the semi-permeable membranę, it is important to maintain a relatively equal concentration of water in the intracellular and extracellular environments. If there is too much water inside the celi compared to the surrounding environment, water will leave the celi and the celi will shrivel, substantially disrupting cellular function. If there is too much water outside the celi, water will enter the celi and it may burst.

Męthpds:

PART I - Diffusion & Celi Size

1.    Obtain one beaker and one large and one smali potato cube.

2.    Place the potato cubes into the beaker and fili it with just enough water to cover the large cube.

3.    Place enough iodine into the beaker to tum it dark yellow or brownish.

4.    Put the beaker aside for 45 minutes to an hour and complete Part II.

PART II - Osmosis

1.    Obtain 4 beakers and label them 0%, 1%, 3% & 5% NaCl

2.    Prepare lOOmL of each of the Solutions for the appropriate beaker.

3.    Obtain 4 potato cyclinders. Rinse them quickly in tap water and dry them with a paper towel.

4.    Weigh each one and create a data table to record the initial weights of each cylinder as well as three morę weights after 15,30 and 45 minutes.

5.    Place each cylinder in one of the beakers of solution. (NOTĘ: Make surę to keep track of which cylinder goes in which beaker.)

6.    After 15 minutes, remove the cylinders from the beakers. Dry them off quickly, weigh each one and put it back into the cylinder. (AGAIN be surę to keep track of which cylinder came from which beaker and put it back into the correct beaker.

7.    Repeat step 6 after 30 minutes and 45 minutes.

8.    Since each potato cylinder had a different starting mass, create a second data table to record the % weight change after 15, 30 and 45 minutes.

% weight change = (weight at time V'- initial weight/initial weight) x 100