18 WJEC Biology AS Level - Marianne Izen - 2nd Edition Flashcards
Biosensor
A device that combines a blomolecule, such as an enzyme, with a transducer, to produce an electrical signal which measures the concentration of a chemical.
Polymer
A large molecule comprising repeated units, monomers, bonded together.
Monomer
Single repeating unit of a polymer.
What is important to say when describing the Benedict’s test?
When describing the Benedict’s test, do not forget to say:
a. The sample is mixed with Benedict’s reagent and strongly heated.
b. The colour change is from blue to green/orange/red.
Describe Benedict’s test
Equal volumes of Benedict’s reagent and the solution being tested are heated to at least 70°C.
If a reducing sugar, such as glucose, is present, the solution will change colour from blue through green, yellow and orange until finally a brick-red precipitate forms.
This test does not tell you the actual concentration of reducing sugar, so it is described as a qualitative test.
How can we detect sucrose using Benedict’s test?
Some disaccharides, such as sucrose, are non-reducing sugars and give a negative result with Benedict’s test, i.e. the solution remains blue.
Sucrose can only be detected if it is first broken down to its constituent monosaccharides, for example by heating with hydrochloric acid.
Benedict’s reagent needs alkaline conditions to work, so alkali is added.
Benedict’s reagent is then added and heated as before.
If the solution now turns red then a non-reducing sugar was initially present.
What is another way of detecting sucrose apart from using the Benedict’s test?
Another way of detecting sucrose would be to use sucrase, an enzyme that hydrolyses sucrose into glucose and fructose.
The Benedict’s test will then give a positive result.
However, enzymes are specific.
Sucrase will only hydrolyse sucrose, so other non- reducing sugars still give a negative result.
How do we do a quantitative measurement of glucose and what is its importance?
Giving an actual value to the concentration of sugar present is much more useful.
This is described as a quantitative measurement, and using a biosensor, an accurate measurement may be obtained.
This is important in monitoring medical conditions such as diabetes, where an accurate measurement of the concentration of blood glucose is required.
What are polysaccharides?
Polysaccharides are large, complex polymers.
They are formed from very large numbers of monosaccharide units, which are their monomers, linked by glycosidic bonds.
Describe why glucose is stored as starch and glycogen?
Glucose is the main source of energy in cells and it must be stored in an appropriate form.
It is soluble in water and so it would increase the concentration of the cell contents, and consequently draw water in by osmosis.
This problem is avoided by converting the glucose into starch in plant cells or glycogen in animal cells.
Starch and glycogen are storage products.
They are polysaccharides.
They are more suitable than glucose for storage because:
- They are insoluble so they have no osmotic effect.
- They cannot diffuse out of the cell.
- They are compact molecules and can be stored in a small space.
- They carry a lot of energy in their C-H and C-C bonds.
Describe the main features of starch
Starch is the main store of glucose for plants.
Starch grains are found in high concentrations in seeds and storage organs such as potato tubers.
Starch is made of a-glucose molecules bonded together in two different ways, forming the two polymers, amylose and amylopectin.
Explain why is the Benedict’s test called a semi-quantitative test.
If two solutions with different concentrations have the same treatment in the Benedict’s test, the more concentrated solution will have a greater colour change.
The test does not actually measure the concentration, but indicates which solution is more concentrated.
It can, therefore be described as a semi- quantitative test.
Define the three terms:
- qualitative
- semi-quantitative
- quantitative
Qualitative - tells you if a molecule is present.
Semi-quantitative - tells you the relative concentrations of solutions, but no actual values.
Quantitative - gives a numerical value for the concentration.
What will happen if a cell absorbs too much water?
If a cell absorbs too much water, its solutes would not be at the appropriate concentration for the cell’s reactions.
If too much water were absorbed into an animal cell, it would burst.
