The Mistake Bank
Class 12 - Chapter 14: Biomolecules
Life is complex, but the chemistry of life has rules. Don't break them.
Is Sucrose Reducing?
CarbohydratesScenario: Does Sucrose (Table Sugar) reduce Fehling's or Tollen's reagent?
Student thinks: "It's a sugar. It's a disaccharide like Maltose."
Answer: "Yes, it is a reducing sugar."
(The most common sugar trap!)
Check the Linkage!
In Sucrose, the glycosidic bond is formed between $C_1$ of Glucose and $C_2$ of Fructose.
Both reducing groups (Aldehyde and Ketone) are tied up in the bond.
Answer: Non-Reducing Sugar.
Denaturation of Proteins
ProteinsScenario: When you boil an egg (denaturation), what happens to the protein structure?
Student thinks: "The protein is destroyed completely."
Answer: "Primary, Secondary, and Tertiary structures are all destroyed."
Primary Structure Survives!
Denaturation breaks H-bonds and S-S bonds, destroying the specific shape (Secondary and Tertiary).
The sequence of amino acids (Primary Structure) remains intact.
Glucose & 2,4-DNP Test
Chemical TestsScenario: Glucose contains an aldehyde group ($-CHO$). Does it give the 2,4-DNP test?
Student thinks: "Aldehydes give 2,4-DNP test. Glucose is an aldehyde."
Answer: "Yes."
Cyclic Structure Dominates!
In solution, Glucose exists mostly in the cyclic hemiacetal form (Pyranose).
The free $-CHO$ group concentration is too low to react with weak nucleophiles like 2,4-DNP or Schiff's reagent.
Answer: No, Glucose does not give the 2,4-DNP test.
Amino Acids Migration
ZwitterionScenario: In an electric field at pH = Isoelectric Point (pI), where does the amino acid move?
Student guesses Cathode or Anode.
It doesn't move!
At the Isoelectric Point, the amino acid exists as a neutral Zwitterion (net charge = 0).
$$ H_3N^+ - CH(R) - COO^- $$
It does not migrate towards either electrode.
Base Pairing in DNA/RNA
Nucleic AcidsScenario: Which base is present in DNA but not in RNA?
Student often confuses Uracil and Thymine.
Answer: "Uracil is in DNA." (Reverse!)
Remember: "Apple in the Tree, Car in the Garage"
DNA: Adenine (A) pairs with Thymine (T).
RNA: Adenine (A) pairs with Uracil (U).
Answer: Thymine is unique to DNA.
Fructose Functional Group
CarbohydratesScenario: Is Fructose an Aldose or Ketose?
Student assumes since it reduces Tollen's reagent, it must contain an Aldehyde group.
Answer: "Aldose."
It's a Ketose!
Fructose is a Ketohexose ($C_2$ has a ketone group).
It reduces Tollen's reagent only because the base causes it to isomerize into Glucose/Mannose (Enediol rearrangement).
Confess Your Sins!
"Biochemistry is where Biology meets Chemistry. Did you get lost in the intersection?"
Did one of these catch you? Or do you have a different horror story from your last exam?
Scroll down to the comments section below and tell us:
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