<p>The question: "A cell is immersed in a solution of sucrose and glucose whose individual concentrations are less than concentrations of the solutes in cytoplasm, but whose combined concentration is greater than concentration of solutes in cytoplasm. Does water enter the cell or leave the cell?"</p>
<p>The answer: leave the cell</p>
<p>Why is this the answer? Is osmosis irrespective of what kinds of solutes you have, and is only concerned with how many total you have? Is that rule about a solute diffusing down its own concentration gradient when mixed with more solutes apply only to diffusion?</p>
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Is osmosis irrespective of what kinds of solutes you have, and is only concerned with how many total you have?
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This is correct. I'm not sure if you're familiar with the concept of osmolarity, but osmotic pressure is dependent solely on the number of solutes and not the kind of solutes.</p>
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Is that rule about a solute diffusing down its own concentration gradient when mixed with more solutes apply only to diffusion?
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Yup. That is correct; at the very least when talking about cells.
You have to remember that large molecules like glucose and sucrose can only cross the cell membrane with relative difficulty compared to water. Thus, it's easier for the cell to try and create an isotonic environment by removing water via osmosis than bringing in large solutes.</p>
<p>The question is confusing and the answer is technically incorrect.</p>
<p>First, the correct answer to the question as written is "yes." Water is entering and leaving the cell. However, the NET flow of water is either:</p>
<ol>
<li><p>Into the cell if you assume the first condition described is correct (each solute has a greater concentration inside the cell than outside the cell)</p></li>
<li><p>Out of the cell if you assume the second condition is correct (the combined concentration outside the cell is greater than the combined concentration inside the cell)</p></li>
</ol>
<p>I don't see how both conditions can be met at the same time.</p>
<p>GoldShadow is quite correct about particles. Osmotic pressure is calculated by the number of particles dissolved in water, irrespective of what they are.</p>
<p>Technically, water also dissolved down its own concentration gradient, but we never talk about the "concentration" of water. The higher your solute concentration, the "lower" the "solvent concentration" when water is the solvent. Most students find it easier to think of osmosis as going "against" the gradient. Even though that's incorrect, it will produce the correct answer to a better worded version of the question given.</p>
<p>
[quote]
"A cell is immersed in a solution of sucrose and glucose whose individual concentrations are less than concentrations of the solutes in cytoplasm, but whose combined concentration is greater than concentration of solutes in cytoplasm. Does water enter the cell or leave the cell?"
[/quote]
</p>
<p>It's definitely a little vague/ambiguous, but I think what it's saying is that the concentration of glucose alone, or sucrose alone, is not equal to the total concentration of solutes in the cell. For example...
Concentration of glucose outside: 0.1M
Concentration of sucrose outside: 0.1M
Concentration of all solutes inside the cell: 0.15M</p>
<p>In such a case, neither the glucose nor the sucrose (individually) is more concentrated than the total solute inside the cell... but combined (ie 0.2M) they exceed the concentration of solute in the cell.</p>