OSMOSIS

MEMBRANES ARE SELECTIVE PERMEABLE Many substances can diffuse (move) across biological membranes, but some are too big or too strongly charged to cross the lipid bilayer. If a substance can diffuse (move) across the membrane, the membrane is said to be permeable to it. If it cannot cross the membrane it is said to be impermeable. Because most biological membranes only let certain things in and out, it is said to be selective permeable.

OSMOSIS Water is very special, it can move across most membranes very easily, even when other things cannot. OSMOSIS is the DIFFUSION OF WATER through a selectively permeable membrane. Water moves from the area of high concentration to the area of low concentration until equilibrium is reached.

TYPES OF SOLUTIONS Isotonic Solutions = when the concentration of two solutions is the same. Hypotonic Solutions = when comparing two solutions, the solution with the lesser concentration of solutes. Hypertonic Solutions = when comparing two solutions, the solution with the greater concentration of solutes.

WITHIN RED BLOOD CELLS WHAT HAPPENS TO THE CELL IN A HYPOTONIC SOLUTION? WHAT HAPPENS TO THE CELL IN A HYPERTONIC SOLUTION?

GET THINKING… Because cells are filled with salts, sugars, proteins, and other molecules, it will almost always be hypotonic to fresh water. In which direction will water move? How can this affect living organisms? Fortunately, most cells do not come in contact with fresh water. Instead, cells are bathed in fluids such as blood, which are isotonic. Other cells that do come in contact with fresh water, such as plants and bacteria, are surrounded by a cell wall. The cell walls prevent the cell from getting too big.

WHAT HAPPENS TO MOLECULES THAT ARE TOO BIG BUT STILL NEEDED? A few molecules, such as glucose, can pass through the cell membrane much quicker than they should. How can a big molecule like glucose move about so quickly? Cell membranes have protein channels that make it easy for certain molecules to cross the membrane. The channels help the molecules to move across the membrane.

FACILITATED DIFFUSION Facilitated diffusion is the process that helps move specific molecules across the cell membrane fast, through diffusion (going from high concentration to low concentration) by using protein channels to facilitate/help the job without using energy.

GET THINKING AGAIN… Why would cells need to move certain molecules, such as glucose, quickly across the membrane? (Hint: think of the importance of glucose in living organisms).

DIFFUSION VS OSMOSIS

DEMONSTRATING OSMOSIS Dialysis tubing is used in medical procedures when patients need their blood filtered. It replaces the job that human kidneys usually do. Starch is a large carbohydrate that cannot move across a semi- permeable membrane because it is too large. Iodine is a small ion that dissolves in water easily.

MATERIALS 1 test tube Iodine solution and Starch solution 250 ml beaker 1 dialysis tubing Tweezers or forceps

PROCEDURES 1.Place 3 drops of iodine solution and 3 drops of starch solution into a test tube. Set it aside and do nothing further with this mixture. Use it as a comparison. 2.Pour a solution of iodine and water into your large beaker to the 200 ml mark. 3.Tie one end of your dialysis tubing, fill it with starch water, and then tie the other end. 4.Carefully rinse the dialysis tubing to remove any starch from the outside. 5.Carefully place the tied dialysis tubing into the beaker with the iodine solution and make observations on the data page. 6.Make a hypothesis of the direction of the movement of iodine or starch. Wait 20 minutes (set a timer to remind you when the time is up). Hypothesis 7.Use the forceps (tweezers) to remove the dialysis tubing and make observations on the data page for the dialysis tubing color and the iodine solution color. 8.When done, clean up your area and put the dialysis tubing in the trash can.

OSMOSIS