Although all lipids are hydrophobic, some parts of some lipids known as phospholipids are hydrophilic, which proves very useful for cells in the body. Phospholipids have only two fatty acids chains linked to a glycerol molecule instead of three, which leaves one hydroxyl group free to form a covalent bond with a phosphate group. As the oxygen atoms in the phosphate group are more electronegative, they form a partial negative charge, and are in turn hydrophilic. Phospholipid bilayers (two layers in opposite direction) are what make up cell membranes, the phosphate group part of the molecule being in contact with water on each side of the cell, meaning that water cannot get in or out of the cell due to the hydrophobic fatty acid tails. Phospholipids create a spherical structure when surrounded by water so that the hydrophobic tails are at the centre. This means that proteins such as some hormones may be transported to the cell through water and be kept from that water through the layer of fatty acid tails surrounding it, with the hydrophilic phosphate groups surrounding the fatty acids.
Chromatography
Chromatography is a technique used to distinguish chemicals when they are in a mixture. Different solutes, from a dye to an amino acid, have different solubilities in different solutes, meaning that some solutes will dissolve more in a solvent than other solutes will. The process works by placing a sample of the solute on a piece of chromatography paper and allowing the solvent to soak up the paper. The result will be that the further up the paper a solute moves, the more soluble it is in the solvent, whereas the less soluble a solute is in that particular solvent, the lower up the paper it will move. This procedure can be used to distinguish between substances and recognise their properties with regard to certain solvents in order to make discoveries about their function or similar.
A particular solute will always move the same relative amount of distance to the total distance the solvent has moved up the paper in that particular solvent. For a different solvent the relative distance will be different, as solutes have different solubilities in different solvents. The calculation used to obtain this relative distance is: distance moved by the solute/distance moved by the solvent (solvent front). This gives the Rf (Relative Fraction or Relative Front).
Aims
- To understand the biochemical tests for lipids.
- To understand the use of chromatography.
Method
Refer to method sheet, any changes have been corrected on the sheet in pencil.
Results
Emulsion Test
- Olive oil will not dissolve in water.
- Propan-1-ol will dissolve in water, crating s slimy or stringy effect under observation.
- Olive oil dissolves in propan-1-ol.
- When solution of olive oil and propan-1-ol is added to the solution of propan-1-ol and water, an emulsion is created (solid suspended in water) and a cloudy white colour is formed; the solute has dissolved.
Grease Spot Test
The paper turned translucent when olive oil was placed upon it.
Chromatography
N.B. The colours in the Rf section correspond to the colours observed on the chromatography paper.
Discussion
Emulsion Test
An emulsifying agent is a substance that is soluble in both oil and water, allowing the two to mix. Therefore if a lipid, such as olive oil which is made of unsaturated fats, is dissolved in an emulsifying agent, such as propan-1-ol, the solution will be able to dissolve in water. This is exactly what the results reflect, and so it was the alcohol that allowed the oil to dissolve in the water. The prpan-1-ol is able to do this of its chemical structure. It is an alcohol with a hydrocarbon skeleton and a hydroxyl group ‘head’, which is hydrophilic. When the propan-1-ol is added to the olive oil, the olive oil’s triglyceride structure is broken down, which in turn means that although the fatty acid tails are still hydrophobic, they are balanced out by the hydrophilic nature of the hydroxyl groups with constitute the head of the propan-1-ol, meaning that the solution will dissolve in water, which the results confirm.
Grease Spot Test
As the paper turned translucent when olive oil was placed upon it, this indicates that the olive oil contains lipids.
Chromatography
The results of the experiment do seem to be accurate, as the colours found in each of the samples are generally true at face value. The colours found in the Green sample, for example, were found to be yellow and blue, there being more blue in the mixture than yellow. This composition and rough ratio of less yellow to more blue are true to the colours that come together to form the colour green. The colours found to make the Yellow sample also seem accurate, as they are very similar. However, the ratio of more orange to yellow dye in the mixture is surprising, especially as yellow is a primary colour and it would be expected to simply have one result of yellow, as the Red sample does of red. The X sample was found to contain more blue than red, and the Y sample was found to contain a mixture of more blue than yellow. This is where chromatography can be very useful, as it shows which solutes are in a mixture, which there are more of in a mixture, and which solutes dissolved more in that mixture. Using the Relative Front calculation was also very useful, as it meant that no matter how long the experiment lasted for, the relative solubility and amounts of solute would be the same for each sample. This can be very useful when identifying mixtures in biological compounds, as a particular molecule will have a particular Rf in a particular solvent. For example, when a protein is being looked at, and chromatography is used to separate out the amino acids in order to identify them, certain amino acids will always have the same Rf in a particular solvent, and therefore they can be easily identified.
Conclusion
Emulsion Test
From the results of this test, it may be concluded that a lipid, such as olive oil, is able to dissolve in water when an alcohol, such as propan-1-ol, is added to it. This is important when identifying lipids in a substance.
Grease Spot Test
From this test, it can be concluded that a substance is a lipid or contains a lipid if it is able to turn paper translucent.
Chromatography
From this test, it can be concluded that the solutes which have dissolved more in the dye or sample move furthest up the paper, and that those solutes which have dissolved less in the sample will move less up the paper, separating out the mixture into its components and amounts of components.
References
