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I understand that glucose is soluble and hence it is not stored by cells but my teacher said that I also have to say that 'it would have an osmotic effect on the cell.' I don't understand what that means.

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The issue here can be explained by colligative properties:

In short, colligative properties are properties that depend upon the concentration of the molecules, but not upon the type of those molecules. And osmotic pressure is a colligative property.

That being said, have in mind that 10 molecules of a huge polysaccharide in a given volume or 10 molecules of a small monosaccharide in the same given volume will have the same osmotic pressure.

So, let's imagine that the cell needs a given amount of glucose. Let's say 1000 molecules, just for an oversimplified example. The osmotic pressure can be calculated by (assuming Van't Hoff factor is 1):

π = MRT

Where M is the molar concentration, R is the ideal gas constant and T is the absolute temperature.

Thus, the osmotic pressure created by those 1000 molecules can be calculated taking into account their molar concentration.

However, If we simply polymerize those 1000 glucose molecules into a huge polysaccharide (be it starch, glycogen or cellulose, it doesn't matter)...

1000 molecules of glucose → 1 molecule of starch

... the molar concentration will be 1000 times smaller, and the osmotic pressure will drop 1000 times.

As the actual numbers are way higher than this simplified example, we can clearly understand that cells avoid storing glucose to prevent a deadly osmotic pressure, in which the cell bursts due to the amount of water that moves in (specially animal cells, which have no walls that could prevent the membrane from rupturing).

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    $\begingroup$ Could you elaborate the 'deadly osmotic pressure (specially animal cells, which have no walls)'? $\endgroup$ – Tyto alba May 15 '17 at 9:28
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    $\begingroup$ @Tytoalba en.wikipedia.org/wiki/Cytolysis $\endgroup$ – user24284 May 15 '17 at 9:49
  • $\begingroup$ I wonder what torture what look like where every cell of the victim is ruptured at once? $\endgroup$ – Chloe May 15 '17 at 21:16
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    $\begingroup$ @Chloe a bad SF movie special effect where a deathray causes its target to melt into a puddle of goo? $\endgroup$ – Dan Neely May 16 '17 at 17:13
  • $\begingroup$ Sorry but I am a GCSE Biology student but is the osmotic effect the same as osmotic pressure and when you say the osmotic pressure do you mean pressure by glucose/starch molecules outwards from the cell? $\endgroup$ – Rishi Malhotra May 16 '17 at 19:20
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You most have misunderstood something... Cells do store glucose, however they do so by combining glucose molecules into longer storage molecules such as starch or glycogen. https://en.m.wikipedia.org/wiki/Glycogen

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    $\begingroup$ ...and they do this precisely to avoid osmotic effects. $\endgroup$ – Alan Boyd May 15 '17 at 8:49
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    $\begingroup$ @Jeppe and you seem to have missed the point of the question... $\endgroup$ – Mindwin May 15 '17 at 13:19
  • $\begingroup$ Then the question was poorly defined... as it did not specify storage method. $\endgroup$ – Jeppe Nielsen May 15 '17 at 14:13
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They do. Some cells can store glucose.

In human beings, liver cells and muscular cells do store glucose in the shape of glycogen which is a polymer of glucose (= a chain of glucose).

Muscular cells can use it during their contraction. The liver use it to put it in the blood to keep the blood sugar steady.

However it is not the preferred form of energy storage : the energy is stored in triglycerides (=fats).

A theory that explains this is that :

Glycogen is hydrophilic : it is very hydrated so it takes a lot of room Triglycerides are lipophilic : it takes less room for equivalent energy Therefore, glycogen storage are limited. In muscular cells, the glycogen storage is used after about 40 minutes (unless you are trained for endurance) and in the liver, the glycogen lasts about 24h.

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    $\begingroup$ Storing glycogen isn't storing glucose: it's storing something that can be made into glucose. $\endgroup$ – David Richerby May 15 '17 at 19:36
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You maybe asked the question Why cells do not store glucose? Answer: Cells actually store glucose in the form of glycogen. Glucose is soluble or dissolved and it has a higher osmotic effect so cells can swell up which means it can become larger due to storing glucose directly. i hope it is super clear to you and you might find helpful

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