Pre lab assignment 1 osmosis and tonicity practice problems

OSMOSIS AND THE SCIENTIFIC METHOD

OBJECTIVES
Upon the completion of this exercise, the student should be able to:

1. formulate a scientific hypothesis.

2. design and understand a basic experiment.

3. distinguish between dependent and independent variables.

4. measure the mass of an object.

5. calculate volumes needed to dilute a solution and make the dilutions.

6. express data in tabular form, graph data, and calculate the slope of a line to determine rate.

7. determine the tonicity in living cells (isotonic point).

8. explain the importance of tonicity in living cells and organisms.

A REVIEW OF THE SCIENTIFIC METHOD
Scientists study natural phenomena by following the scientific method. For example, after observing a fruit fly, a biologist might use the steps in the scientific method as follows1:

Observation: Observe something in the natural world

The life cycle of a fruit fly is 30 days at 29 °C.

Question: Ask a question about how it works

How do changes in temperature affect the life cycle of a fruit fly?

Hypothesis: A possible explanation for the observation(s); Testable and falsifiable

A decrease in temperature to 18 °C will increase the time it takes the fruit fly to complete its life cycle.

Prediction : If my hypothesis is correct then this will happen. ("if.....then” statement)

If I decrease the temperature of a fruit fly's environment to 18 °C then the time it takes the fly to complete its life cycle will increase.

Experiment: Design a controlled way to test the hypothesis

Place 100 flies at 18 °C for one generation, and place 100 flies at 29 °C for one generation. Compare how long it takes each group to complete their life cycle.

Analysis of Results: Support or reject the hypothesis based on the results of the experiment.

Fruit flies placed at 18 °C have a longer life cycle than those placed at 29 °C.

Conclusions: What you conclude from your results

Decreasing the temperature of a fruit fly's environment to 18 °C will increase the time it takes the fruit fly to complete its life cycle.

1Harris, Dr. Katherine, Hartnell College Biology Tutorials: Scientific Method Tutorial. http://www.hartnell.edu/tutorials/biology/scimethod.html

Developing a hypothesis:
1. A hypothesis must be specific.
2. A hypothesis is a statement and is not written in the form of a question.

3. A good hypothesis should clearly state how the dependent variable is expected to be affected by the independent variable. There should only be one independent variable, and what it is should be stated in the hypothesis.

4. The hypothesis must be testable.

a. You must be able to perform an experiment to test your hypothesis.

b. You must be able to make observations or measurements to determine how the dependent variable changes as a result of the independent variable.

5. The hypothesis must be falsifiable. In other words, it must be possible for the hypothesis to be incorrect.

6. A hypothesis is written in language that is clear and simple. It should be obvious to the reader exactly what you are testing.

Designing the Experiment
A well-designed experiment should include the following:

Independent Variable – The factor that is changed or manipulated by the experimenter to see what effect occurs. Temperature is the independent variable in the example above.

Dependent Variable – The factor that is measured/assessed during the experiment, because it is expected to change due to changes in the independent variable. Time it takes the fruit fly to complete its life cycle is the dependent variable in the example above.

Experimental Group(s) – The group(s) where the independent variable is being tested. Fruit flies placed at 18 °C are the experimental group in the example above.

Control Group(s) – The group(s) that the experimental group(s) is compared to. Fruit flies placed at 29 °C are the control group in the example above.

In some cases, positive and/or negative controls are used to show that an experiment is working correctly.

Positive control – a sample, etc. that is treated in the same manner as the experimental group and is designed/expected) to give a positive result.

Negative control – a sample, etc. that is treated in the same manner as the experimental group and is designed/expected to give a negative result

To interpret the results, each experimental group can be compared to the positive and negative controls.
DIFFUSION, TONICITY, AND OSMOSIS

Cells must move materials through membranes and throughout the cytoplasm in order to maintain homeostasis. The movement is regulated because cellular membranes, including the plasma membrane and organelle membranes, are selectively permeable. Membranes are phospholipid bilayers containing embedded proteins; the hydrophobic characteristics of the phospholipid fatty acids limit the movement of water, polar solutes, and charged solutes across the membrane.

All matter above absolute zero (0 Kelvin; -213°C) has kinetic energy, which is displayed as random molecular motion. Although individual molecules move about at random, a net directional movement may occur in response to inequalities in concentration, pressure, or temperature. Molecules will move from a region of higher concentration, higher pressure, or higher temperature to regions of lower concentration, lower pressure, or lower temperature until an equal distribution of molecules is achieved. This passive movement is called diffusion; it does not require energy input. The difference between the higher concentration, pressure, or temperature and the lower concentration, pressure, or temperature is referred to as a gradient.

The cellular environment is aqueous, meaning that the solvent in which the solutes, such as salts and organic molecules, are dissolved is water. Water moves through membranes by a special form of diffusion called osmosis (osmosis is the movement of solvent, but in a biological system the solvent is water). Water is able to pass slowly through the membrane or more rapidly through specialized protein channels called aquaporins. Most other substances, such as ions, move through protein channels, while larger molecules, including carbohydrates, move via transport proteins or bulk transport.

Like solutes, water moves down its concentration gradient. Water moves from areas of high free water concentration (low number of solute particles) to areas of low free water concentration (high number of solute particles). Again, solutes decrease the concentration of free water, since water molecules surround the solute molecules.

Tonicity is a measure of the ability of a solution to cause water to move. It is influenced only by solutes that cannot cross the membrane. The terms hypertonic, hypotonic, and isotonic are used to describe solutions separated by selectively permeable membranes and refer to the relative concentration of the solute (not water). A solution with greater solute concentration as compared to the solution on the other side of the membrane is called a hypertonic solution; therefore, water will move into the hypertonic solution through the membrane by osmosis. A hypotonic solution has a lower solute concentration than the solution on the other side of the membrane; water will move down its concentration gradient into the more concentrated solution. Isotonic solutions have equal solute concentration on either side of the membrane. Water still moves back and forth across the membrane, but there is no overall or net change in the concentration on either side.

In non-walled cells, such as animal cells, the movement of water into and out of a cell is affected by the relative solute concentration on either side of the plasma membrane. As water moves out of the cell, the cell shrinks; if water moves into the cell, it swells and may eventually burst. In walled cells, including fungal and plant cells, osmosis is affected not only by the solute concentration, but also by the resistance to water movement in the cell by the cell wall. This resistance is called turgor pressure. The presence of a cell wall prevents the cells from bursting as water enters; however, pressure builds up inside the cell and affects the rate of osmosis. Turgor pressure is responsible for the support in many plants and for the ability for plant roots to draw water from the soil. Walled cells become turgid in hypotonic environments. In hypertonic environments, walled cells lose water, which causes the plasma membrane to separate from the cell wall, causing plasmolysis. In effect, plasmolysis is shrinkage of the cytoplasm, but not the cell itself.

Please watch this video on diffusion and osmosis and how potatoes can be used to determine the isotonic point of a solution.

https://edpuzzle.com/media/5ed0f7994f0c493f982e1dc1

Pre-lab Assignment: Introduction to Osmosis and Tonicity Practice Problems
In this pre-lab assignment, each dialysis bag represents a cell. Like the plasma membrane, the dialysis tubing is selectively permeable. The solutions are aqueous solutions, which means a solute (such as NaCl or sucrose) is in the solvent water. Water can move into or out of the dialysis bag or “model cell”, but the solute cannot. In this model system, the environment is the solution in the beaker surrounding the dialysis bag.

Student's Name Instructor

PRE-LAB ASSIGNMENT #1: OSMOSIS AND TONICITY PRACTICE PROBLEMS
To be completed before beginning week one of the Osmosis and the Scientific Method Lab.

Part I: Dialysis bags (“cells”) are placed in three different beakers. The cells, as well as the beakers, contain different aqueous solutions of NaCl. For each of the beakers, answer the questions below.

Beaker # 1

Cell # 1

Beaker # 2

Cell # 2

Beaker # 3

Cell # 3

Cell # 1 contains 15% NaCl Beaker contains 35% NaCl

Cell # 1 contains 30% NaCl Beaker contains 30% NaCl

Cell # 1 contains 50% NaCl Beaker contains 25% NaCl

Beaker #1:
a) The solution inside the cell contains 15% NaCl and % water.

b) The beaker contains 35% NaCl and % water.

c) The solution inside cell #1 is tonic to the solution outside cell #1.

d) The solution outside cell #1 is tonic to the solution inside cell #1.

e) In which direction will water move? (circle one below)

Into the cell Out of the cell No net movement of water

f) Draw an arrow or arrows on Beaker #1 above to show the movement of water.

g) Explain how you determined the movement of water.

h) Over time, what will happen to cell #1? (circle one below)

Shrinks Swells Remains the same size

Beaker #2:
a) The solution inside the cell contains 30% NaCl and % water.

b) The beaker contains 30% NaCl and % water.

c) The solution inside cell #2 is tonic to the solution outside cell #2.

d) The solution outside cell #2 is tonic to the solution inside cell #2.

e) In which direction will water move? (circle one below)

Into the cell Out of the cell No net movement of water

f) Draw an arrow or arrows on Beaker #2 above to show the movement of water.

g) Explain how you determined the movement of water.

h) Over time, what will happen to cell #2? (circle one below)

Shrinks Swells Remains the same size

Beaker #3:
a) The solution inside the cell contains 50% NaCl and % water.

b) The beaker contains 25% NaCl and % water.