Unit 4 - Honors Osmosis Homework Solutions

Please check your answers with the selected HW solutions from last week found below - you should be prepared to discuss any discrepancies in class!

In addition, remember that our exam on Thursday will cover both cell transport and cell structures.

From Nov. 30th - Introduction to Osmosis

The following answers are to the questions about the U-tube on the back of the page.

1. The concentration of salt is higher on the left side than the right. Since salt can pass through the membrane, it will diffuse from high salt concentration on the left to low salt concentration on the right.

2. At equilibrium, the concentration of salt will be the same on both sides of the membrane.

3. Starch is a large molecule. Thus, it may be too large to pass through the membrane through simple diffusion.

4. Since the concentration of starch is higher on the left side, and water can pass through the membrane through osmosis, it will move from the right side to the left side of the U-tube.

5. The movement of water from the right to left side will cause the water on the left to rise, and the right to fall.

Dec. 1st - Effects of Osmosis on Cells
MC questions on the bottom of the page:
1. A
2. B
3. D - The cell membrane is not rigid! It consists of a bilayer of loosely bonded phospholipids.
4. D
5. A
6. B

Dec. 2 - Facilitated Diffusion and Transport Proteins.
Questions along the way:
1. Facilitated diffusion does not require energy because it consists of molecules moving from where they are in high concentration to low concentration. Since diffusion does not require energy, the proteins simply provide a channel through which diffusion can occur.
2. Channel proteins have a passage through them that allows molecules to pass through. Carrier proteins bond to specific molecules that fit their receptor site. They then pull these molecules through the membrane or open and close a gate to let them through.
3. Both types involve movement of molecules from where they are in high concentration to low. Simple diffusion occurs when the molecules can pass through the membrane without being blocked by their size or polarity. Facilitated diffusion uses transport proteins to move molecules through the membrane that are otherwise too big or insoluble to pass through.

Homework Answers:
1. Since diffusion is the process by which molecules move from high concentration to low concentration, it will continue to occur until everywhere is at the same concentration. This is the definition of equilibrium.

2. Two solutions are isotonic if they have the same solute concentration.

3. There are two ways that molecules can pass through a membrane using facilitated diffusion. A channel protein opens a passageway for molecules to diffuse through from high to low concentration. One type of carrier protein bonds to specific molecules and pulls them through the membrane. Another acts like a gate, opening first to one side of the membrane, and then to the other. Even though the proteins are needed for molecules to pass through the membrane, no energy is needed since the molecules still move from high to low concentration.

4.
a)Since the water concentration is higher on the side with lower solute concentration (the left), it will move to the side with lower water concentration, which is on the right. In summary, the water will move from left to right.
b) The concentration of starch will be the same on both sides of the membrane.
c) Since the bag in the 6% solution swells up, the unknown concentration inside must be greater than 6%. This indicates that the 6% starch solution is hypotonic to the solution inside the bag.

Since the bag in the 10% solution shrivels up, water is moving from inside the bag to the solution outside. This indicates that the 10% solution is hypertonic to the solution inside the bag.

This means the unknown concentration must be above 6% and below 10%

Dec. 3rd - Osmosis Problems
1. 10%
2. The solution is hypertonic. The mass of the potato goes down, which means water left the potato to go into the solution. This means the solution has higher concentration of salt compared to the potato.
3. Because water has moved from the potato to the solution surrounding it.
(The arrow should point OUT of the potato.)

4. The concentration is highest in the solution.
5. The water concentration is highest in the potato.
6. Water will flow from the potato into the solution since the solution has lower water concentration.

7.
160 mL starch/3000 mL water = 5.3% concentration of starch in solution.

8.
a) hypotonic
b) hypertonic
c) VERY close to isotonic (though still a bit hypertonic)

9. 13% starch concentration
10.
a) hypertonic
b) isotonic
c) hypotonic

11. Diffusion is the movement of particles from areas of high concentration to low concentration. Osmosis is the diffusion of water molecules across a membrane.