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11 Dec 2019

The main function of a cell membrane is to allow substances to enter and leave the cell. Which is a correct description of transport through a cell membrane?

a. Diffusion, or passive transport, occurs when molecules diffuse from a lower concentration to a higher concentration.

b. Facilitated transport occurs when proteins that extend through the cell membrane provide a channel through which certain substances can diffuse more rapidly than by passive diffusion.

c. The lipid bilayer opens up and allows the ions or molecules to enter the cell, then closes.

d. Active transport occurs when molecules diffuse from a higher concentration to a lower concentration.

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Collen VonLv2

13 Dec 2019

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Proteins and electrolytes can be transported into cells in two ways: either through diffusion through the plasma membrane, or facilitated diffusion through a channel. Some proteins or electrolytes can be transported through both mechanisms. We can make a simplified model of this by modeling two membranes in parallel with each other, and assuming 1D diffusion through both. Assume that protein A can be transported through the membrane in either method (i.e., direct diffusion or facilitated diffusion) (NOTE: this picture doesnât really describe the system well. Imagine the red circles can move through the membrane AND/OR through the channel). Outside of the cell, the concentration of protein A is 100 nM. Inside the cell, the concentration is 10 nM. Assume the thickness of the plasma membrane is 10 nm. The diffusion constant for direct diffusion of Protein A in the membrane is 1x10^-10 cm2 /s, and the diffusion constant for facilitated diffusion is 5x10^-10 cm2 /s. The partition coefficient Î¦ for Protein A in the plasma membrane is 0.01, and the partition coefficient Î¦ for Protein A in the channel is 0.05.

A. Assume that initially the channel is closed, so Protein A can only get into the cell via direct diffusion through the membrane. Calculate the steady-state flux of Protein A across the membrane. Assume 1-D diffusion, steady state diffusion, and no convection.

B. Assume that the channel is now open, and acting in parallel to direct diffusion. Assume 1-D diffusion through both the membrane and the channel, assume steady state diffusion, and assume no convection.

C. Determine a general expression for the effective diffusion coefficient through the system in Part B, and show how these two pathways behave like resistors in parallel.

Below, is a completed lab. After completing this lab, I was asked the following question:

What changes would you make, if any, if you did this lab again?

I'm having a hard time thinking of what I would do different. Please help!

Observations & Questions for Part 1

Record your observations and your time and temperature data in tables. Use one table for the pure water and one table for the salt solution.

Data Table 1: Pure Water and Salt Solution
Seconds	Distilled H20 Room temp	Distilled H20 Ice bath	Saltwater Room temp	Saltwater Ice bath
0	23	23	17	23
30	23	10	20	7
60	23	4	22	1
90	22.5	1	22	0
120	22.5	0	22	0
150		0	22	-1
180		-1	22	-1
210		-1	22	-2
240		-2		-2
270		-1		-2
300		-1		-2
330		-1		-2
360		-1
390		0
420		0
450		0
480		0
510		0
540
570
600
630
660

Make two graphs of your data. On one graph plot the data from the pure water. On the other graph plot the data from the salt solution. On both plot temperature on the y-axis and time on the x-axis.

A. Record the freezing point of the pure water and the freezing point of the salt solution.

Freezing point of salt Sol: -2 degree Celsius

Freezing point of pure H2O: 0 degree Celsius

B. How do these two freezing points compare?

The freezing point of salt water is lower than the freezing

point of fresh water. Salt in the water lowers the freezing point

of water.

C. What are some practical applications of freezing point depression, boiling point elevation, and vapor pressure lowering?

Some practical applications of freezing point depression

are antifreeze in a radiator and salt on the road used to melt

ice in the winter. Some practical applications of boiling point

elevation are a sealed container and possibly a pressure cooker.

Some practical applications of vapor pressure lowering are freeze

drying and steam engines.

Questions - Part 2

To what biological structure is the dialysis bag comparable? How is it similar? How is it different?

The plasma membrane is comparable to the dialysis bag.

The similarity between them is that they are both semipermeable.

The difference is the dialysis bag acts as a permeable membrane,

however, purely based on its pore size, the small molecules are

able to get through but the bigger molecules are unable to. The

plasma membrane is a more complex system that uses both active

and passive transport to allow the molecule to move through.

In biological systems if a cell is placed into a salt solution in which the salt concentration in the solution is lower than in the cell, the solution is said to be hypotonic. Water will move from the solution into the cell, causing lysis of the cell. In other words, the cell will expand to the point where it bursts. On the other hand, if a cell is placed into a salt solution in which the salt concentration in the solution is higher than in the cell, the solution is said to be hypertonic. In this case, water will move from the cell into the solution, causing cellular death through crenation or cellular shrinkage. In your experiment is the Karo

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