IMAT 2019 Q32 [Permeability]

Beetroot is a root vegetable with cells that contain a red pigment. Normally the pigment cannot pass out of the cells because it cannot diffuse through their cell surface membranes. An investigation was carried out into the effect of various chemicals on the permeability of the cell surface membranes of beetroot cells.

1cm3 samples were cut from the beetroot and washed in running water for 20 minutes to remove any pigment that was released from the damaged cells.

Five experiments were carried out. In each experiment, one cube of beetroot was placed in liquid P and one cube placed in liquid Q, and the results were observed.

Which row shows the correct results and conclusions?

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proteins can be denatured by change in pH, so in HCl the membrane transport proteins no longer work and the red pigment leaks out. Phospholipids have a polar component so they can dissolve in a polar solvent like ethanol.


So phospholipids have a hydrophilic polar head and a hydrophobic non polar tail. Both water and ethanol are polar. Yet phospholipids dissolve in ethanol and not water, right? if so why? Say if we replace ethanol with water in row 3, would it still be correct?

Hi, I think it is because ethanol is a small, polar molecule that can penetrate the plasma membrane and make it more soft and malleable. This can lead to the breakdown of the membrane structure.

but an ethanol molecule is about 3 times as large as a water molecule

Yeah not smaller than water molecules but it can also increase membrane permeability. That means the lipid molecules become more disordered and the membrane becomes less stable. Not only this, it can also affect the function of membrane proteins by altering their conformation or disrupting their interactions with the lipid bilayer.
Seems like it can disrupt the lipid bilayer in several ways.

well yes lipids do dissolve in alcohols. but my question was, why don’t they do so in water as well? because it seemed to me that water has all the same characteristics that make alcohols dissolve lipids, yet, it doesn’t. for example water is polar, small, forms hydrogen bonds etc…

Well sorry it is beyond my knowledge but if you want to refer to the contents of ChatGPT here it goes…:joy: I hope this answers your question.

Water and alcohols have some similar properties, such as being polar and capable of forming hydrogen bonds. However, the key difference between water and alcohols when it comes to dissolving lipids (fats and oils) lies in their overall molecular structures and the nature of their interactions with lipids.
Here’s why water doesn’t dissolve lipids as effectively as alcohols:

  1. Molecular Structure: Water molecules have a small size and are highly polar due to their bent shape and the electronegativity of oxygen. Alcohols, on the other hand, also have polar characteristics but are typically larger molecules with a hydrophobic hydrocarbon tail. This hydrophobic tail makes alcohols more similar in structure to lipids than water is.
  2. Hydrophobic Effect: Lipids, including fats and oils, are hydrophobic molecules, meaning they have a strong aversion to water. Water molecules form a highly organized and hydrogen-bonded network around hydrophobic molecules, creating a cage-like structure. This results in a decrease in the entropy (disorder) of the water molecules surrounding the lipid, which is energetically unfavorable. In contrast, alcohols can partially shield the hydrophobic part of lipids due to their hydrophobic tails, which disrupts the water structure less than the complete immersion of a lipid in water.
  3. Hydrogen Bonding: While both water and alcohols can form hydrogen bonds, water’s ability to form multiple strong hydrogen bonds between its molecules leads to a highly cohesive and organized structure. This organization makes it difficult for water molecules to surround and dissolve lipids effectively, as they would rather interact with each other through hydrogen bonding.
  4. Polarity Difference: Alcohols often have a lower polarity than water, especially in the case of long-chain alcohols. This reduced polarity can make alcohols better suited for dissolving certain nonpolar or hydrophobic substances like lipids.

In summary, water’s strong hydrogen bonding and highly organized structure around hydrophobic molecules make it less effective at dissolving lipids compared to alcohols. Alcohols, while polar, have hydrophobic tails that can partially shield lipids from the highly organized water structure, allowing for better solubility of lipids in alcohols.

:raised_hands: ChatGPT :raised_hands:

seriously though, thanks!

Yes the :goat:. No problem!