Join TSR now and get all your revision questions answeredSign up now

AS Biology Help

    • Thread Starter
    Offline

    3
    ReputationRep:
    I am stuck with these two questions below. I was wondering if anyone know some answers and please explain. I will be grateful

    1) Explain, in terms of their solubility in different solvents and knowledge of the emulsion test, why i would not see a false positive due to ; monosaccharides and disaccharides, starch, nucleic acid and Protein.

    2) The emulsion test is qualitative. How could it be made the basis for a semi-quantitative test (giving an indication of whether lipids are high, medium or low concentration) or even a fully quantitative test for lipids.

    I know that monosaccharides dissolve in everything so they won't form a precipitate with ethanol, but none of the others
    Offline

    3
    ReputationRep:
    1. Monosaccharides and disaccharides are not soluble in alcohol and nucleic acids and proteins are not soluble full stop, so they wouldnt dissolve in the alcohol and create a positive white result.
    2. Place the samples in a colorimeter and see how much light is absorbed/transmitted.
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by typicalvirgo)
    1. Monosaccharides and disaccharides are not soluble in alcohol and nucleic acids and proteins are not soluble full stop, so they wouldnt dissolve in the alcohol and create a positive white result.
    2. Place the samples in a colorimeter and see how much light is absorbed/transmitted.
    Thank you so much!!
    Offline

    3
    ReputationRep:
    A small degree of modification to Virgo's reply; An emulsion is a translucent appearance of a mixture of lipid (fat [or oil when liquid]) and water; the oil splits into tiny droplets that mix with the water to produce a cloudy mixture.

    Alcohol does not really come into the equation (unless we are referring to a binge on a Saturday night lol!). Monosaccharides e.g. glucose, fructose and disaccharides e.g. lactose, maltose, are all soluble in water, so they will not produce an emulsion i.e. will not give a false positive result.

    Proteins and nucleic acids are not soluble in water as they are v large molecules, they will not emulsify (so no false positive), rather they will form a precipitate of undissolved powder-like material that will settle at the bottom of the container e.g. a test tube.

    You cannot really say not soluble "full stop" as solubility depends on the solvent, the prevailing conditions such as temperature, pressure and pH (the latter determines whether a solute is charged or neutral).

    Emulsification in the human body refers to formation of a suspension of lipid in the water-based (aqueous) contents of the duodenum (first part of the small intestine - shaped like an upside-down "U") which is aided by bile secreted into the duodenum from the liver via the bile duct; this emulsification greatly increases the surface area of lipid exposed to the enzyme pancreatic lipase from the pancreas to break down (digest) the lipid into glycerol and FFA (free fatty acids) hich can then be absorbed into the blood.

    Hope this addition is useful!
 
 
 
Poll
Which party will you be voting for in the General Election 2017?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Quick reply
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.