Biology 1

Pre-Module Homework Assignment A

 

Background Information and Practice Problems

Solutions

You are made of approximately 68% water[footnoteRef:1]. Molecules like salt, amino acids, carbohydrates, enzymes, or any other polar or charged substances, are dissolved in water and make a solution. Water serves as the main ingredient in many solutions, especially biological solutions. A solution is a combination of a solvent, the main medium, and substances dissolved in the solvent, which are called solutes. In our bodies, the water inside and around our cells is the solvent in which various solutes are dissolved. Note even the name of intracellular water, cytosol, translates to “cell solvent”. [1: Mitchell, H.H. et al. 1945. J. Biol. Chem. 158:625-637.]

 

1. Which of the below best describes the relationship between solvents, solutes and solutions?

a) Solvent = Solute + Solution

b) Solute = Solvent + Solution

c) Solution = Solute + Solvent

2. The box illustrates a solution and both the solvent and solute are depicted by shapes. Which shape is the solute? Which shape is the solvent? Explain.

 

 

 

 

 

Solutions can be described based on how much of the solution is solute and how much of the solution is solvent. For example, in humans the plasma of blood is a solution composed of about 92% water as the solvent and 8% solutes like gases and nutrients (note that the percentages must add up to 100%).

 

3. Follow the steps below and use the diagram above to describe the % solute (circles) and % solvent (diamonds) in the solution.

a) Count up the circles: ____________

b) Count up the diamonds: ____________

c) Add up the circles and diamonds: __________

d) % solute = ____________

e) % solvent = = ____________

 

Solutes vs. non-solutes

Solutes, like sugars and amino acids, interact with water molecules by forming hydrogen bonds together. Formation of hydrogen bonds between water and solutes is what occurs when a substance dissolves into water. Molecules that do not form hydrogen bonds do not dissolve, such as lipids and other non-polar chemicals.

 

Semipermeable membranes

Cells are surrounded by a non-polar phospholipid bilayer. This lipid bilayer prevents most substances from entering or leaving the cell. Water is one of the few substances that can freely cross the membrane. Most polar solutes, like salt, sugar and amino acids, cannot freely cross the membrane and require help from a protein embedded in the membrane. In the diagram below, the dashed line represents the semipermeable membrane of a cell that is separating the cytosol from the extracellular environment. 

4: In the picture to the right:

a) Label the inside of the cell.

b) Label the outside of the cell.

c) Label the plasma membrane.

 

Osmosis

Water moving by diffusion across a membrane is called osmosis. Remember, diffusion is the movement of molecules from a high concentration to a lower concentration, in order to reach equilibrium. In the image below, the diamonds represent water and the circles represent solutes. Water will move by osmosis from the side with a higher concentration of water to a side with a lower concentration of water. In other words, water will move by osmosis from the side with a lower concentration of solute to the side with a higher concentration of solute. Water is attracted to solutes.

5. Follow the steps below and use the diagram to describe the % solute (circles) and % solvent (diamonds) in the extracellular solution. 

a) Count up the circles: ____________

b) Count up the diamonds: ____________

c) Add up the circles and diamonds: ____________

d) % solute = ____________

e) % solvent = ____________

6. Follow the steps below and use the diagram from question 5 to describe the % solute (circles) and % solvent (diamonds) in the intracellular solution.

a) Count up the circles: ___________

b) Count up the diamonds: ____________

c) Add up the circles and diamonds: ___________

d) % solute = _______

e) % solvent = ____________

 

7. From the calculations above, is there a higher concentration of water inside the cell or outside of the cell?

 

8. Predict: Will water flow into the cell or out of it? Explain.

 

 

 

9. Use the following diagram to answer the questions below. Start by filling in the blanks for the missing % concentrations for beakers B and C.

 

  

 

 

 

 

Beaker A Beaker B Beaker C

0 % Sugar ____% Sugar 40% Sugar

100 % Water 90% Water ____% Water

a) What is the solute concentration of:

· Beaker A?

· Beaker B?

b) What is the solvent concentration of Beaker C?

When we think about solutions in human biology, we mostly think about the intracellular cytosol compared to extracellular solutions like plasma and interstitial fluid. (Remember, we are about 68% water.) To describe the relative amounts of solutes on either side of a membrane, we use the terms hypertonic, hypotonic and isotonic. If the solutions have equal concentrations of solute, we say that they are isotonic. If the concentration of solute is not equal, hypotonic describes the side with a lower concentration of solute and hypertonic describes the solution with a higher concentration of solute. Use the questions below to practice using these terms.

10. The diagram to the right depicts a cell submerged in a solution. The cell membrane is permeable to water but not to sugar. Start by completing the % concentrations of the intracellular and extracellular fluids.

30% Sugar

___% Water

 

60% Sugar

___% Water

 

 

a) What is the cytosol in this example? (circle the

correct term below)

hypotonic hypertonic isotonic

b) How do you know?

 

c) Because the solutions are not isotonic, osmosis is going to occur. In what direction is water going to move?

 

d) Predict: What is going to happen to the size(mass) of the cell?

 

 

e) Predict: What is going to happen to the extracellular solute concentration?

 

 

 

Visualizing changes in solute concentrations: Graphing the data

Variables that change continuously can be visually represented in line graphs. An electrocardiogram of blood pressure as the heart beats, or blood insulin levels during a glucose tolerance test, are two common examples. Solute concentrations across membranes also change continuously over time due to osmosis and other factors, so line graphs are useful ways to visualize the changes.

Remember that line graphs need to have some basic components to them:

· Data. Usually data are collected at discrete points, and the individual points are plotted and then connected by a line.

· Legend. If more than one line is represented on a graph, a legend is needed to define the different lines.

· X-Axis. Horizontal (flat) axis representing the independent variable, the factor being tested according to the hypothesis.

· Y-Axis. Vertical (up and down) axis representing the dependent variable, which is measured, as is expected to change due to the independent variable.

 

One classic lab performed in general biology classes is to take regular chicken eggs and dissolve their shells by placing the eggs in an acidic solution (usually vinegar; you could do this at home!) overnight. The remaining soft membrane and white and yolk inside the egg make for a nice model of a cell that’s visible to the naked eye. This cell model can be used to observe osmosis and the effects of different solute concentrations on osmosis firsthand. Usually 3 eggs are used, and 1 each placed in hypotonic, isotonic, and hypertonic solutions.

Below are data collected from this experiment[footnoteRef:2]. Use the data and the graph to answer the following questions. [2: http://www.khaydock.com/articles/SSR%20June%202014%20027-036%20Haydock.pdf]

 

 

% Mass change Time (min) 0 M 0.8 M 2 M 0.00 0.00 0.00 0 15.00 0.20 0.01 -0.65 30.00 0.45 0.02 -1.4 45 0.75 -0.01 -1.95 60 1.1 0.01 -2.55

 

16. What solution molarity was the egg placed in represented by the yellow, starred points? the blue square points? the green circular points?

 

 

 

17. How long did it take the egg placed in the 0 M solution to increase its mass by about 1% due to osmosis?

 

18. How long did it take the egg placed in the 2 M solution to decrease its mass by about 1% due to osmosis?

 

 

 

 

 

19. Which line represents the hypotonic, isotonic, and hypertonic environments? Explain how this is evident from the graph.

 

 

 

 

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