Phospholipids + food tests + colorimeter NOT FINISHED Flashcards
draw the simplified structure of a phospholipid
https://studymind.co.uk/wp-content/uploads/2022/03/Screen-Shot-2022-03-29-at-1.19.17-AM.png
Describe the structure of phopholipids
A phospholipid has a glycerol molecule bonded to two fatty acid molecules
The glycerol molecule is also bonded to phosphate
Phosphate is negatively charged (this part of the phospholipid molecule is polar)
What can be said about the phosphate group in a phospholipid
Phosphate is negatively charged (this part of the phospholipid molecule is polar)
Because of this, the phosphate group is hydrophilic (it attracts water molecules)
draw and describe over simplified structure of phospholipid
https://www.ncbi.nlm.nih.gov/books/NBK539825/bin/phospholipids__wiki.jpg
polar hydrophilic head
tails represent two fatty acid molecules
The fatty acid molecules are non-polar and hydrophobic
describe the two regions within phospholipids
Phospholipids contain a both a hydrophobic and hydrophilic region
Phospholipids contain a both a hydrophobic and hydrophilic region
what does this result in
Phospholipids contain a both a hydrophobic and hydrophilic region
This means that phospholipids behave very differently in water compared to triglycerides
Describe how phospholipid molecules behave in water
In water, phospholipid molecules position themselves so that the hydrophilic head groups can interact with the water molecules (facing outwards towards the water molecules) but not with fat, while the hydrophobic tails cluster/orients itself, well away from water molecules
The structure formed is called a phospholipid bilayer
Describe the function of phospholipids
Due to the hydrophilic phosphate head and hydrophobic fatty acid tails on phospholipids a phospholipid bilayer structure is formed when in water
This property of phospholipids is extremely useful as it allows phospholipids to form membranes that we find both around cells and within cells
centre of bilayer is hydrophobic so water-soluble substances cant pass through it - the membrane acts as a barrier to those substances
In a food test, how do you prepare the food sample
1) Get a piece of food and break it up using a pestle and mortar (and a small amount of distilled water)
2) Transfer the ground up food to a beaker and add some distilled water.
3) Stir the mixture with a glass rod, so that the chemicals in the food dissolve in the water.
problem - mixture is going to be full of solid food particles - these could make the test results difficult to see
4) Filter the solution/mixture using a funnel lined with filter paper to remove suspended food particles.
we carry out our tests on the filtrate (food solution which passes through the filter)
how to test for starch
1) Prepare a food sample and transfer 3cm^3 of the sample to a test tube.
2) Then add 1cm3 of a solution containing iodine and potassium iodide to the sample and gently shake the test tube to mix the contents.
Observe the colour change
3) In the presence of starch, the iodine solution turns a blue-black colour …
in the absense of starch, the iodine solution remains orange
then the colour of the iodine solution will change from orangey-brown to blue-black
how to test for protein
1) Prepare a food sample and transfer 3cm^3 of the sample to a test tube.
2) Add 3cm3 of dilute soldium hydroxide solution and mix
3) Next add ten drops of dilute copper (II) sulfate solution and mix again
In the presence of protein, the solution will turn purple or lilac
If protein is absent, the solution will remain blue
extra:
sometimes the sodium hydroxide solution and the copper (II) sulfate solution are premixed
This is called biuret solution
this test actually detects peptide bonds
so you get a positive result with proteins as these contain peptide bonds
however a solution of amino acids would give a negative result -as it does not/this would not contain peptide bonds (only separate molecules)
how to test for lipids
1) Prepare a sample of food, but do not filter the solution. This is because lipid molecules can stick to filter paper.
In this case, leave the food mixture for a while for the lipid particles to settle
2) Transfer 3cm^3 of the sample to a test tube
3) Add 3cm^3 of ethanol and gently shake the test tube to mix the contents. Allow the mixture to settle.
4)Add 3cm^3 of water to another test tube.
5) Very gently, pour the test tube containing ethanol and the food sample into the test tube containing water
shake the solution gently to mix the contents
6) If lipids are present, then a white cloudy emulsion forms.
If the solution stays clear, then lipids are not present
SAFETY - Ethanol is highly flammable, so this experiment must not be carried out near any flames
safety precautions
safety goggles need to be worn
any spills must be cleaned up with plenty of water
two types of sugars
reducing sugars
non-reducing sugars
what is a reducing sugar
Reducing sugars can donate an electron to another molecule
examples of reducing sugars
All monosaccharides are reducing sugars
Some disaccharides are reducing sugars e.g. maltose and lactose
how to test for reducing sugars
1) Prepare the food sample and transfer 3cm^3 of the sample to a boiling tube.
2) Add 3cm^3 of Benedict’s reagent/solution to the sample.
Benedicts solution contains the copper ion Cu2+ - which makes the solution blue
3) Prepare a water bath so that it’s set to 75°C
4) Place the test tube in the water bath using a test tube holder and leave it in there for 5 minutes. Make sure the tube is pointing away from you.
If the solution remains blue then there is no reducing sugar present
However if a reducing sugar is present, this adds an electron to the copper 2+ ion
This now forms the copper 1+ ion and this forms a red precipitate
If there is only a very small amount of reducing sugar, then only a very small amount of red precipitate forms. and this causes the benedict’s solution to appear green (a lot of blue + little red)
if more reducing sugar is present, then the colour turns yellow
a higher level of reducing sugar produces an orange colour
IMPORTANT: The Benedict’s test only gives us a very approximate idea of the amount of reducing sugar
this is because the Benedict’s test only shows a narrow range of colour changes (and all humans perceive colour slightly differently)
Scientists say (therefore) that the Benedict’s test is semi-quantitative
_______
if a lot of reducing sugar is present, then we see a brick-red colour
5) If the food sample contains a reducing sugar, the solution in the test tube will change from blue to green, yellow or brick-red - it depends on how much sugar is in the food. (green - small amount of sugar is present, yellow - more sugar is present, brick red - a lot of sugar is present)
examples of non-reducing sugars
Some disaccharides are non-reducing sugars e.g. sucrose
how to test for non-reducing sugars
Important - We cannot test for non-reducing sugars directly
Instead, we need to break the glycosidic bond releasing the monosaccharides
Since all monosaccharides are reducing sugars, we can now test for them using Benedict’s solution
Situation - Solution - want to see if it contains a non-reducing sugar such as sucrose
- Check to see if the solution also contains any reducing sugar
If it does, we will need to take that into account later
Take a small amount of our unknown solution and carry out the Benedict’s test.
Note down any colour change which takes place
Next take a new boiling tube and add 3cm3 of our unknown solution
Add 3cm3 of dilute HCl
Gently boil the solution in the water bath for 5 minutes
If a non-reducing sugar is present, then the acid hydrolyses the glycosidic bonds releasing the monosaccharides
Next add 3cm2 of a dilute alkali e.g. NaOH solution
Then use pH paper to check that our solution is alkaline.
This is because the Benedict’s test cannot work under acidic conditions
Finally, add 3cm2 of Benedicts solution and then heat in boiling water for 5 mins
Note down any colour change
Working out what is in the solution
Unknown solution
First benedict’s test = blue -
Second benedicts test = orange
explain the results
First benedict’s test is blue - negative - colour remains blue
therefore no reducing sugar is present in the solution
Second Benedict’s test = orange (produces an orange colour)
Tells us that non-reducing sugar is present in solution
Working out what is in the solution
Unknown solution
First benedict’s test = green-
Second benedicts test = red
explain the results
Benedicts test first - green
This tells us that a very small amount of reducing sugar is present
Second benedict’s test - produces a red colour
This tells us that a non-reducing sugar is also present (quite a bit)
Working out what is in the solution
Unknown solution
First benedict’s test = red -
Second benedicts test = red
explain the results/suggest the problem
What does this tell us
The first Benedict’s test produced a red colour
Tells us that the solution contains a large amount of reducing sugar
In this case, we cannot test for a non-reducing sugar
That is because even if non-reducing sugar was present, we would not be able to see a colour change beyond red (no noticeable colour change)
This shows us that we can only test for a non-reducing sugar if there is either no reducing sugar present or only a very small amount
describe how to use a colorimeter to determine the concentration of glucose in a solution
READ
how to use the benedicts test to determine the concentration of a reducing sugar more accurately
if we left the test tube for several hours, the red precipitate forms on the bottom
this would allow us to see the blue Benedicts solution above it
This benedict’s solution will be less blue than it was before the test
This is because some of the Cu2+ ions have reacted and are no longer in solution
If we did the test with a greater concentration of glucose, then Benedicts solution would appear even less blue at the end as even fewer cu 2+ ions would remain in solution
With benedict’s test - the greater the concentration of glucose, the less blue the Benedict;s solution will be at the end
These changes in the blueness of the solution may be too subtle to detect by eye
So we quantify the blueness of the solution by using a machine called a colorimeter
describe how to use a colorimeter to determine the concentration of glucose in a solution
Before using a colorimeter, first filter off the red precipitate, leaving just the Blue Benedicts solution
how a colorimeter works
