BIOL 1406

PreLab 5.2

How do substances move across a selectively permeable membrane?

A selectively permeable membrane is a membrane that allows some substances to pass through easily, while other substances pass through very slowly or not at all.  All cell membranes, including the plasma membrane, are selectively permeable.  In the exercise below, you will learn about 2 types of diffusion across selectively permeable membranes:

Use the interactive exercise below to learn more about simple diffusion and facilitated diffusion.

 

Your Turn
Assume there is a higher concentration of each of the following substances in the extracellular fluid surrounding a cell than in the cell’s cytoplasm. Predict whether the substance would be more likely to enter the cell by simple diffusion or by facilitated diffusion, and explain why.
Is O2 more likely to enter the cell by simple diffusion or facilitated diffusion:  

Explain why: 		Hint Check your answer.
Is glucose more likely to enter the cell by simple diffusion or facilitated diffusion:  

Explain why: 		Hint Check your answer.
Is Na+ more likely to enter the cell by simple diffusion or facilitated diffusion:  

Explain why: 		Hint Check your answer.
Is insulin (a protein) more likely to enter the cell by simple diffusion or facilitated diffusion:  

Explain why: 		Hint Check your answer.
Is estrogen (a lipid) more likely to enter the cell by simple diffusion or facilitated diffusion:  

Explain why: 		Hint Check your answer.
Are hydrogen ions more likely to enter the cell by simple diffusion or facilitated diffusion:  

Explain why: 		Hint Check your answer.



Dialysis tubing is composed of a selectively permeable membrane. However, its selectivity is much more limited than the selectivity of cell membranes. Microscopic holes, or pores, in the dialysis tubing allow substances to be separated only on the basis of size. Unlike with cell membranes, whether the substance is nonpolar, polar, or charged has little effect on its ability to cross the tubing. Molecules and ions that are smaller than the pores can move through the tubing, while those that are larger than the pores cannot. Dialysis tubing can be ordered from scientific supply companies in a variety of pore sizes. Dialysis is routinely used in biochemistry and molecular biology laboratories to separate and purify substances on the basis of size.

 

Your Turn
Of the substances listed in the exercise above (O2, glucose, Na+, insulin, estrogen, and hydrogen ions) which would not pass easily through the plasma membrane by simple diffusion, but probably would pass through the pores of dialysis tubing? (Hint: There are 2 correct answers.)
List the substances: Hint Check your answer.
Explain why: Hint Check your answer.


Diffusion of materials through cell membranes always involves net movement of a substance down its own concentration gradient (from higher concentration to lower concentration.) However, in some cases substances must be transported across cell membranes against their concentration gradient (from lower concentration to higher concentration.) This requires cellular energy and, therefore, a form of active transport. For example, the transfer of a phosphate group from an ATP molecule to a membrane protein may change the shape of the protein, causing it to pump a substance across the membrane against its concentration gradient. Whenever a substance is being moved against its concentration gradient, a form of active transport is required.
 

 

Your Turn
Simple diffusion, facilitated diffusion, and active transport allow the cell to be very selective about what is allowed to cross its membranes. Examine the figure below, and answer the questions.
Will glucose enter the cell by simple diffusion, facilitated diffusion, or active transport?

Explain why: 		Hint Check your answer.
Will K+ enter the cell by simple diffusion, facilitated diffusion, or active transport?

Explain why: 		Hint Check your answer.


 


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