Biological molecules flashcards

Biological molecules underpins the whole of A-Level Biology. Students must understand how the four major groups of biological molecules are built, structured and function. Carbohydrates range from simple monosaccharides to complex polysaccharides such as starch, glycogen and cellulose. Lipids include triglycerides and phospholipids, which are central to membrane structure. Proteins are polymers of amino acids held together by peptide bonds, and their function depends on precise three-dimensional shape. Nucleic acids, DNA and RNA, store and transfer genetic information. Students should understand condensation and hydrolysis reactions, the role of water as a biological solvent, and how inorganic ions contribute to biological processes. Practical skills including Benedict's test, biuret test and iodine tests are also required.

1. Card 1

   What type of reaction joins two monosaccharides together?

   A condensation reaction, which releases a molecule of water and forms a glycosidic bond between the two sugar units.

2. Card 2

   How does the structure of cellulose differ from starch?

   Cellulose has beta-1,4-glycosidic bonds causing straight chains that form hydrogen bonds between fibres, whereas starch uses alpha-1,4 and 1,6 bonds forming coiled, branched chains.

3. Card 3

   What are the four levels of protein structure?

   Primary (amino acid sequence), secondary (alpha-helix or beta-pleated sheet), tertiary (overall 3D fold) and quaternary (multiple polypeptide chains together).

4. Card 4

   What bond forms between two amino acids during protein synthesis?

   A peptide bond forms between the amine group of one amino acid and the carboxyl group of another, releasing water in a condensation reaction.

5. Card 5

   What is the difference between a triglyceride and a phospholipid?

   A phospholipid has one fatty acid replaced by a phosphate group, making it amphipathic, whereas a triglyceride has three fatty acid chains esterified to glycerol.

6. Card 6

   Why is water described as a dipolar molecule?

   Oxygen attracts electrons more strongly than hydrogen, creating a partial negative charge on oxygen and partial positive charges on hydrogen, making water a dipolar molecule capable of hydrogen bonding.

7. Card 7

   What is the role of hydrogen bonds in maintaining protein structure?

   Hydrogen bonds form between amino acids in the polypeptide chain, stabilising secondary structures such as the alpha-helix and beta-pleated sheet.

8. Card 8

   What is the monomer of DNA and what are its three components?

   The monomer is a nucleotide, consisting of a deoxyribose sugar, a phosphate group and one of four nitrogenous bases (adenine, thymine, guanine or cytosine).

9. Card 9

   How does the structure of glycogen differ from starch?

   Glycogen is more highly branched than starch due to more frequent alpha-1,6-glycosidic bonds, allowing faster release of glucose, suiting its role in animal energy storage.

10. Card 10

    What is a reducing sugar and which reagent is used to test for it?

    A reducing sugar can donate electrons to reduce copper(II) ions. Benedict's reagent is used; a positive result produces a brick-red precipitate on heating.

11. Card 11

    What bonds are broken when a protein is denatured?

    Denaturation disrupts hydrogen bonds, ionic bonds and disulfide bridges that maintain tertiary structure, causing the protein to lose its shape and biological activity.

12. Card 12

    What is the significance of the base-pairing rule in DNA?

    Complementary base pairing (A with T, G with C) via hydrogen bonds ensures accurate DNA replication and faithful transfer of genetic information.

• Students often confuse condensation and hydrolysis reactions, incorrectly stating that hydrolysis releases water rather than using water to break bonds.
• A common error is assuming all polysaccharides are used for energy storage, overlooking that cellulose serves a structural role and cannot be digested by most animals.
• Students frequently state that denaturation destroys the primary structure of a protein, when in fact only the secondary, tertiary and quaternary structures are disrupted, leaving the amino acid sequence intact.

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