Download Sample Free Response Biochem Answers

Sample Free-Response Questions: Biochemistry Experimental Design Test
Grid in question
1. A sample of human DNA contains 31% thymine. What percentage of the DNA is cytosine? (2)
If the sample contains 31% T, then it also contains 31% A. A+T = 62%. That means G+C = 38%,
and C = ½ of that: 19%.
2. In the 1950's, Frederick Sanger discovered a method for sequencing the amino acids in a
polypeptide. He compared the overlapping regions to deduce the polypeptide's primary
structure. Given the overlapping sequences below, what is the primary structure of this 10
amino acid segment of the enzyme lysozyme? (2)
Arg-cys-glu
lys-val-phe-gly
cys-glu-leu-ala-ala
Lys-val-phe
phe-gly-arg-cys
Line up pieces:
Arg-cys-glu
cys-glu-leu-ala-ala
phe-gly-arg-cys
Lys-val-phe
lys-val-phe-gly
lys – val – phe – gly – arg – cys – glu – leu – ala - ala
3. Describe and discuss four aspects of protein structure. (8)
Primary structure – sequence of amino acids, as determined by transcription of DNA and
translation of RNA at ribosomes
Secondary Structure – the hydrogen bonding between the peptide bond carbonyl and the
hydrogen of the amine nitrogen between amino acids in different parts of the polypeptide chain.
Common structures include alpha helices and beta pleated sheets.
Tertiary Structures – non-polar, polar and ionic interactions between the R groups of different
amino acids in the polypeptide chain. Covalent cross linkages between cysteines (disulfide
bridges) are also part of tertiary structure.
Quaternary structure – the association of different subunits of a protein, such as the four distinct
polypeptides found in hemoglobin
4. Water has a number of significant properties that are essential for life. Describe and discuss
three of them. (6)
Water is highly polar, and dissolves polar covalent molecules and many ionic compounds easily.
Water has a high specific heat, which resists fast changes in temperature, which is important for
maintaining homeostasis
Water forms hydrogen bonds with electronegative elements such as N and O; responsible for the
stability of DNA double helix
Water has a higher density than ice, which is very unusual. This property allows ice to float, and
insulates the water underneath, preventing the oceans from freezing
The attractive forces between water molecules result in high levels of adhesion to charged
surfaces, which is essential for transpiration in plant (capillary action).
5. The Nobel prize for medicine was won in 1947 by Gerty Cori and her husband Carl. They isolated
two enzymes that convert glucose phosphate into glycogen. Glycogen is a polysaccharide,
composed of glucose molecules bonded together in two ways, called 1”4 and 1”6 bonds.
a) Explain why the two different enzymes are needed for the synthesis of glycogen from
glucose phosphate (2)
There are two separate linkages in glycogen – the 14 linkage and the 16 linkages.
Specificity in protein design requires two separate proteins – the active site is specifically
tailored for only one dehydration synthesis.
b) The formation of side-branches increases the rate at which glucose phosphate molecules
can be linked on a growing glycogen molecule. Explain the reason for this (2)
The side branches provide more substrate for the 1 4 enzyme to grow the glycogen
molecule.
c) Curve A was obtained using heat-treated enzymes. Explain the shape of curve A. (2)
High temperature can denature enzymes, eliminating their catalytic function due to
permanent conformational changes in the secondary, tertiary and quaternary structure.
6. Lipase is a digestive enzyme that accelerates the breakdown of triglycerides in the small
intestine. In the laboratory the rate of activity of lipase can be detected by a decline in pH.
a) Design an experiment to measure the activity of lipase over a range of temperature values.
(2)
b) Indicate which variable is dependent and which one is independent. (2)
c) Explain what causes the pH to decline as a result of lipase activity. (2)
a) The activity of lipase is measured at five temperature: 10, 20, 30, 40 , and 50°C. A control
could be added for the mean body temperature of the organism that was being studied if it
maintained a constant body temperature. At least 3 trials should be used.
b) The dependent variable is the enzyme activity; the independent variable is the temperature.
c) The pH declines because the hydrolysis of triglycerides results in the formation of fatty acids.
7. The protein content of harvested wheat grain depends on the water content in the soil at the
time of sowing. In an experiment carried out in semi-arid soil in Queensland, Australia, over
several years, researchers measured the protein obtained from wheat sown in different
conditions
a) Outline the relationship between water content at planting time and protein content. (1)
Soil water content is negatively correlated with protein content.
b) Suggest why this relationship exists (2)
Plants grown in waterlogged soils may not have sufficient oxygen to maximize protein synthesis.