Osmosis Question

[grayfalcon89]

Hey guys. I'm re-learning about osmosis in the school and I received this paper to answer. You can say I'm asking for homework answer but really, I can still get full credit by saying something "scientific" (the teacher doesn't check this ardently). But I really would like to know what's the right answer so I'm posting here.

 

1. When plant cells lose sufficient water, the internal pressure of the cell membrane pressing against the cell wall is (reduced, increased) and they (lose, gain) turgor pressure.

 

My answer: reduced and lose.

My explanation (this is unnecessary but here's why I think is): I know that in plant cells, cell walls restrict expansion by creating more turgor pressures. So, if the water decreases (which will be "shrinking"), the plant cells will lose turgor pressure and the internal pressure will decrease as a result.

 

2. The cells observed in tap water probably were at osmotic equilibrium; however, they were not immersed in an isotonic solution. Reconcile these two statements.

 

My answer: Both osmotic equilibrium and isotonic solution are similar in a way that there both exist equal amount of solute concentrations in both sides of the cell. But in an isotonic solution, the amount of solute concentrations is always same. In osmotic equilibrium, there is constant movements of molecules moving back and forth to have same amount of concentrations on both sides but there is no "net" change in concentration. Because of cell walls, the plant cells in tap water can do going back and forth movements without expanding, and it is doing this because the tap water is hypotonic (not isotonic).

 

3. When salt molecules are dissolved by water, sodium and chloride ions are formed. They then become surrounded by water molecules. How does this relate to the fact that neither ion can pass through the cell membrane?

 

My answer: This relates to the fact that the cell membrane is hydrophilic at the head and hydrophobic at the tail. When sodium and chloride ions dissolve in water, they become surrounded by water, and thus, become part of water molecule. So, they'll go through the head of cell membrane which likes the head but won't be able to pass through because the tails do not like water molecules.

 

I hope these are right. I had to use internet to find answers (I have really bad biology book from school; there isn't much detail!!). If I'm wrong, please tell me. I'll appreciate greatly. If I'm correct, still tell me so I know I'm doing it right.

[ecoli]

2. The cells observed in tap water probably were at osmotic equilibrium; however, they were not immersed in an isotonic solution. Reconcile these two statements.

 

My answer: Both osmotic equilibrium and isotonic solution are similar in a way that there both exist equal amount of solute concentrations in both sides of the cell. But in an isotonic solution, the amount of solute concentrations is always same. In osmotic equilibrium, there is constant movements of molecules moving back and forth to have same amount of concentrations on both sides but there is no "net" change in concentration. Because of cell walls, the plant cells in tap water can do going back and forth movements without expanding, and it is doing this because the tap water is hypotonic (not isotonic).

I was taught that even isotonic solutions in a cell, there is dynamic equillibrium between the cell and it's surroundings, with a net movement of zero. However, I can't think of a better answer to this question, perhaps because it's 3 in the morning, but it should be 4 due to DST.

 

My answer: This relates to the fact that the cell membrane is hydrophilic at the head and hydrophobic at the tail. When sodium and chloride ions dissolve in water, they become surrounded by water, and thus, become part of water molecule. So, they'll go through the head of cell membrane which likes the head but won't be able to pass through because the tails do not like water molecules.

 

No, the ions do not become part of the water molecules, they are simply surronded by the water molecules in an organized way. http://www.chemistry.ohio-state.edu/~grandinetti/teaching/Chem121/lectures/Solution%20Chemistry/SolutionChemistry.html

 

I believe the reason they do not pass through still has to do with the size of the ions, which are rather large.

[grayfalcon89]

What? I don't understand what you mean for #2.

 

As for #3, that was my initial size (it would be too fat to go through). I'm not sure which one's really true though. It sounds (from the site) that they are independent and just are floating around the water. But I learned from the school that they form ionic bond so it's more than just "floating around."

 

Now I'm honestly confused.. Is it size or hydrophilic/hydrophobic?

 

By the way, since you didn't comment, I take #1 is right..?