Osmosis

1. Osmosis The passive transport of water across a selectively permeable membrane is called osmosis. In order to fully understand this concept, you must know the difference between solute and solvent and the difference between “free” and “bound” water molecules.

2. water When salt is added to water, it readily dissolves forming a solution of saltwater. salt Any substance that dissolves in another is called a solute. Any substance that does the dissolving is the solvent. salt In this case, salt is the solute and water is the solvent.

3. Na Na Na Na Na Na Na Na Na Na Na Na Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Sodium Chlorine Table salt is known chemically as sodium chloride. Let’s look at what makes up a crystal of table salt. A crystal of salt is composed of many sodium and chlorineions. Ions are atoms that have a charge. The sodium ions have a positive(+) charge and the chlorine ions have a negative (–) charge.

4. salt crystal Let’s look at what happens to a crystal of salt as it dissolves in water. When a crystal of salt enters water, the (-) ions are attracted to the positive poles of water molecules. water molecules

5. salt crystal Let’s look at what happens to a crystal of salt as it dissolves in water. When a crystal of salt enters water, the (-) ions are attracted to the positive poles of water molecules. The (-) ion will then dissociateitself from the salt crystal and become surrounded by other water molecules.

6. salt crystal Conversely, the (+) ions are attracted to the negative poles of water molecules.

7. salt crystal Conversely, the (+) ions are attracted to the negative poles of water molecules. The (+) ion will also dissociate and become surrounded by water molecules.

8. Dissolved ions salt crystal Conversely, the (+) ions are attracted to the negative poles of water molecules. The (+) ion will also dissociate and become surrounded by water molecules. As long as there is sufficient water, this process will continue until all of the ions are dissolved.

9. Bound water molecules Ions Free water molecules When water molecules are clumped around ions, they are considered “bound.” When water molecules are not bound, they are called “free.”

10. Selectively permeable membrane Bound water molecules cannot pass through a selectively permeable membrane. Free water molecules however are permeable.

11. Selectively permeable membrane In osmosis, water flows from the side of the membrane that has the greatest number of free water molecules to the side with the fewest number. Net flow stops when the free water concentration in both areas is equal. Equalibrium

12. Selectively permeable membrane Which side of the membrane shown has the greatest number of free water molecules? Answer: The right side has 10 free water molecules. The left side only has 4 free water molecules.

13. Selectively permeable membrane In which direction will the free water molecules flow through the membrane? Answer: Flow will go from the right side (the most concentrated in free water) to the left side (the least concentrated).

14. Selectively permeable membrane When will net flow stop? Answer: When equilibrium is reached (7 free water molecules on each side).

15. Selectively permeable membrane Will free water molecules continue to flow back and forth across the membrane? Answer: Yes, but at equal rates.

16. Barriers Membrane Salt In this example of osmosis, an equal amount of water and salt are placed in a U-tube apparatus. In the middle of the U-tube are two barriers which prevent the two salt solutions from mixing. In addition, there is a membrane that only allows the flow of free water molecules. Water Salt Water

17. Membrane What do you hypothesize will happen to the fluid level on each side of the membrane when the barriers are removed? Answer The fluid levels on both sides stay the same. This is because both volumes of water have an equal concentration of free water molecules, so no net movement is observed. Salt Water

18. hypertonic hypotonic Membrane This time we will double the amount of salt on the right side of the U-tube. The diluted water on the left side is said to be hypotonic (have less dissolved solutes) compared to the concentrated water on the right. Dilute Saltwater Concentrated Saltwater The concentrated water on the right is said to be hypertonic (have more dissolved solutes) compared to the diluted water on the left.

19. hypertonic hypotonic Membrane Now hypothesize what will happen to the water levels on each side of the membrane when the barriers are removed. Hint: Adding salt results in an increase of bound water molecules and a decrease in free water molecules. Dilute Saltwater Concentrated Saltwater Free Bound

20. Membrane Answer: The left side has a lower concentration of salt but a higher concentration of free water molecules. Thus the water will flow through the membrane from left to right. Dilute Saltwater Concentrated Saltwater

21. Membrane Let’s run the same experiment again but this time place a pressure gage on the right side. The pressure gage will give an indication of the amount of pressure the water on the left side exerts. We’ll now remove the barriers. Dilute Saltwater Concentrated Saltwater Now watch the red gage needle…

22. Membrane The red needle indicated an increase in water pressure. Notice the water level did not change. This is because the gage prevents the water from moving. Dilute Saltwater Concentrated Saltwater

23. Membrane The blinking red arrow does not represent water flow, but the direction of water pressure instead. Pressure as a result of osmosis is called osmotic pressure. Dilute Saltwater Concentrated Saltwater

24. Now let’s see what effect osmosis has on living cells. In this beaker of pond water is a microscopic one-celled organism called a paramecium. Paramecia are so small that you need a microscope to see them. Pond water

25. Notice how the two contractile vacuoles inside the paramecium expand and contract. Make a hypothesis as to why they are doing this. Pond water Answer: Water constantly enters the paramecium by osmosis. The contractile vacuoles expel the water from the cell.

26. What will happen to the paramecium if the contractile vacuoles fail to function? Answer: The paramecium will probably expand to the point that it lysis (breaks open). Pond water

27. Let’s add some salt to the pond water. salt The salty pond water is now hypertonic compared to the inside of the paramecium. Will water tend to flow out of the paramecium or into it? Salty Pond water Pond water Answer: Water will tend to flow out of the paramecium. H2O H2O

28. How will the added salt affect the rate at which the paramecium contracts its vacuoles? Answer: To conserve water, the paramecium will slow the rate at which its vacuoles contract. Salty Pond water Before salt was added After salt was added Compare the rates

29. Assume we now place the paramecium in distilled water. Distilled water is pure water. It does not have any impurities or salts dissolved in it. Distilled water has nothing but free water molecules. Salty Pond water Distilled water

30. Now with the paramecium in distilled water, hypothesize how the rate of vacuole contractions will change. Answer: Too much water is entering into the paramecium. The vacuoles must contract faster to expel the water. Distilled water In pond water In distilled water Compare the rates

31. Osmoregulation is the ability of an organism, like this paramecium, to regulate its internal environment via osmosis.

32. Cheek cells are placed in three different solutions below. Based on what happens to the cells, determine if the solution is hypotonic, hypertonic or isotonic to the cell’s internal environment. Refer to the definitions below. H2O H2O H2O H2O Isotonic: Having a solute concentration equal to that of another solution. Hypertonic: Having a higher concentration of solute than another solution. Hypotonic: Having a lower concentration of solute than another solution.

33. Isontonic Hypertonic Hypotonic Answer: Unlike paramecia, cheek cells lack contractile vacuoles to regulate osmosis. This makes cheek cells more susceptible to death by shrivelingin hypertonic solutions and swelling in hypotonic solutions.

34. Sea water is hypertonic to fresh water. Ocean fish like tuna cannot survive in fresh water. Sea water Fresh water Fresh water is hypotonic to sea water. Fresh water fish like brown trout cannot survive in sea water.

35. The cell below represents a typical plant cell with the following components: Nucleus Cell wall Chloroplast Central vacuole (acqueous) Mitochondria Golgi body Cytosol H2O H2O and out of the cell by via osmosis. Water moves into the cell…

36. Let’s place plant cells in isotonic, hypertonic and hypotonic solutions. Hypotonic Isotonic Hypertonic H2O H2O H2O H2O In an isotonic solution, water moves in and out at equal rates. In an hypertonic solution, water moves out of the cell. In an hypotonic solution, water moves into the cell.

37. In a hypertonic solution, plant cells are plasmolyzed, which is a condition where the plasma membrane pullsaway from the cell wall. In a hypotonic solution, plant cells become turgid, or swollen. The osmotic pressure that causes this is called turgor. Hypotonic Isotonic Hypertonic Plasmolyzed cell Turgid cell

38. C A B Match each plant’s state below to the plant cell’s condition in each solution. Answer: Hypotonic Isotonic Hypertonic Plasmolyzed cell Turgid cell

39. Hypotonic Isotonic Hypertonic Plasmolyzed cell Turgid cell C A B

40. Hypotonic Isotonic Hypertonic Plasmolyzed cell Turgid cell C A B Hypertonic solutions make plants very flaccid. Hypotonic solutions help keep plants firm and upright. Isotonic solutions make plants slightly wilted (flaccid).