Lab 2: Cell Membrane Structure and Function

Question 1

define: independent variable

Answer 1

• what is being manipulated

Question 2

define: dependent variable

Answer 2

• what is being measured (changes depending on independent variable)

Question 3

define: hypertonic

Answer 3

When the concentration of solute is greater outside than inside, water flows out= causes cell to shrivel and shrink in size

Question 4

define: hypotonic

Answer 4

When the concentration of solute is greater inside the cell than outside, water flows in= causes cell to swell and burst

Question 5

define: isotonic

Answer 5

when the concentration of both the solute and solvent is equal so theres no net movement

Question 6

We assigned 10 eggs to each treatment group. Why is it important to replicate an experiment?

Answer 6

It is important to replicate an experiment multiple times to determine if the data was by chance or if the data actually displays the normal case. We should draw conclusions rom our experiments and be confident in our data

Question 7

When chicken eggs are placed in a vinegar solution (20% acetic acid), the acid removes calcium from the shells, leaving behind only a thin, flexible membrane. How will the visible appearance of the following eggs be expected to change?
Egg A: 0% sucrose solution
Egg D: 30% sucrose solution
Egg H: 70% sucrose solution

Answer 7

• Egg A: The egg will look similar if not identical to a normal egg.
• Egg D: The egg will have lost its calcium shell at 30% sucrose. It may still appear opaque and milky in color.
• Egg H: The egg will have lost its calcium shell and now appear translucent, thus allowing you to see the yolk outline.

Question 8

Is the cell membrane permeable to water? Explain.

Answer 8

Yes. Water molecules can easily move across the membrane due to their small size.

Question 9

Is the cell membrane permeable to sucrose? Explain.

Answer 9

No. Sucrose molecules are too large to move into the membrane. However, the water in the sucrose solution can move across the membrane.

Question 10

Solution B is made up of a 10% sucrose solution. Is this solution hypertonic, hypotonic, or isotonic when compared to the inside of the egg? Clearly justify your answer by referring to the experimental results and comparing the total concentration of solutes inside and outside the egg.
Results: egg gains 1.30g in weight

Answer 10

• hypotonic
• The final weight of the egg has increased meaning there was initially a higher concentration of solutes inside the egg.

Question 11

Solution F is made up of a 50% sucrose solution. Is this solution hypertonic, hypotonic, or isotonic when compared to the inside of the egg? Clearly justify your answer by referring to the experimental results and comparing the total concentration of solutes inside and outside the egg.
Results: egg loses 1.20g in weight

Answer 11

• hypertonic

- The final weight of the egg has decreased meaning there was initially a lower concentration of solutes inside the egg.

Question 12

If you wanted to make a sucrose solution that is isotonic to the inside of the egg, what concentration of sucrose would you use? Justify your answer by referring to the experimental results. See google doc for the experimental results.

Answer 12

I would use a solution with a sucrose concentration between solution D & E (30%-40% sucrose) because these solutions gave data points that were closest to a net change of 0 in the weight of the egg. However it would be a solution slightly closer to solution D because it gave the data point closest to 0 change in weight. So, I would use a solution with a concentration of 31-33% sucrose.

Question 13

define: osmosis

Answer 13

Osmosis is the movement of water or other solvent through a plasma membrane from a region of low solute concentration to a region of high solute concentration, tending to equalise the concentrations of the solutes.

Question 14

define: lyse

Answer 14

explode from being over inflated with water

Question 15

Why does lysis happen and how is lysis time related to membrane permeability?

Answer 15

Water moves into the red blood cell. Water follows the place of higher solute concentration. The more permeable the RBC membrane, the faster the lysis time will be as water and solute rushes in

Question 16

Molecules that are highly soluble in oil have ____. Why?

Answer 16

low polarity
- Non polar molecules are not attracted to other molecules and thus they will not be attracted to the water based polar solutions surrounding the cell or the phosphate heads of the phospholipid bilayer membrane.

Question 17

It is important to note that polar and non polar molecules do not actually repel each other. Why would someone think this?

Answer 17

• The reason they separate is that polar molecules attract eachother. Because they attract eachother they exclude non polar molecules (ex: oil).

Question 18

define: partition coefficient. What does the coefficient mean?

Answer 18

Measure of solubility of a solute molecule in oil relative to its solubility in water. A value close to 1 means that the solute has a high solubility in oil and a low solubility in water. Meaning it does not have many strong polar groups

Question 19

Why do you think a solute’s solubility in oil affects its ability to cross the RBC membrane? Explain, with reference to polarity.

Answer 19

A solute that is very soluble in oil vs water can cross the RBC membrane more easily because solutes soluble in oil have low polarity. Low polarity means it is not very attracted to the polar solutions surrounding the cell or to the phosphate heads in the phospholipid bilayer membrane. Thus, a solute very soluble in oil can move in and out of solution and across the RBC membrane easily.

Question 20

How do very polar molecules and strongly-charged ions cross cell membranes?

Answer 20

Proteins that exist in the cell membrane can help polar molecules and strongly charged ions cross the cell membrane.
Channel proteins: act like pores allowing ions and polar molecules to exit and enter (req. no energy)
Proton pumps: use ATP to force ions through the cell membrane against their concentration gradient

Question 21

Why might temperature increase membrane permeability?

Answer 21

When temperature increases the cell membrane loses some of its rigidity because the phospholipids become less compact, increasing permeability (layer “melts”). Proteins denature due to the heat. transport proteins can no longer control what enters or leaves the cell.

Question 22

Do you understand the idea of experimental design given in lab prep exercise B?

Answer 22

• see lab sheet

Question 23

How do you calculate the total magnification of a microscope?

Answer 23

total magnification = eye piece magnification (10) x objective magnification (4, 10, or 40)

Question 24

Are red blood cells permeable to NaCl? Justify your answer using the results.
Results: lysis time was longer than 8min and the RBC appeared shrivelled under a microscope

Answer 24

No. Red blood cells are not permeable to NaCl or the cells would have lysed instead of shrivelled. NaCl is too big of a molecule to pass through the cell’s phospholipid bilayer.

Question 25

Why does a tube with .15M NaCl and a tube of .3M ethylene glycol have the same total solute concentration?

Answer 25

NaCl splits into 2 separate solute particles when placed in water (Na+ and Cl-).

Question 26

What is a negative control and why is it important?

Answer 26

A group in which no response is expected. Helps us get rid of complicating factors that may interfere with our results (confounding variables).

Question 27

Define: denature

Answer 27

What happens to a protein when it loses its structure.

Question 28

Do you understand why we chose the specific solution to test our hypothesis in part B? Do you understand why we chose the specific solution to use in our negative control group?

Answer 28

• see lab paper

Question 29

Explain why we cannot draw conclusions from treatment groups whose paired negative controls give a result

Answer 29

We cannot definitively say that the variable we are testing is the reason why a reaction was observed. We know that the negative control we used should not have elicited a response but the negative control did. There is an unknown variable that is causing a result (example: the protein denatures causing it to lyse).

Question 30

state all the steps of the scientific method

Answer 30

• ask a research question
• choose variables (ind + dep)
• compose hypothesis
• design experiments to test hypothesis
• gather results and objective observations
• discuss results and revise hypothesis based on your conclusions

Question 31

What does it mean when a solute reaches equilibrium across 2 sides?

Answer 31

there is an equal amount of solute on both sides and movement of solute still occurs across the sides BUT the net movement of molecules is zero

Question 32

state the differences and define: cell membrane channels, carriers and pumps

Answer 32

all are proteins but

• channels: no energy req; allow solutes to travel according to gradient
• carrier: gated by ATP; change shape to pick up solute and move to other side
• pump: requires ATP; changes shape to pick up solutes and move to other side