- Define and explain Biological Molecules in your own words
- Use key terms such as monomer accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
Beneath every heartbeat and every thought lies chemistry: molecules built, broken and recycled with exquisite precision. In this chapter you will classify the molecules of life, build proteins level by level, explain how enzymes achieve their astonishing specificity, and follow DNA as it copies itself before every cell division.
This lesson focuses on Biological Molecules: classify carbohydrates, lipids, proteins and nucleic acids, and test foods for them.
Classify carbohydrates, lipids, proteins and nucleic acids, and test foods for them.
Key ideas
The molecules of life
Carbohydrates (sugars and starch) provide energy and structure; lipids store energy and form membranes; proteins do the work of the cell as enzymes, carriers and structural fibres; nucleic acids (DNA and RNA) store and transmit genetic information. Each can be identified by a food test: Benedict's reagent turns orange-red with reducing sugars, iodine turns blue-black with starch, Biuret reagent turns purple with protein, and the emulsion test gives a milky layer with lipids.
Protein structure
Proteins are built from amino acids joined by peptide bonds. Primary structure is the amino acid sequence; secondary structure is the folding into alpha-helices and beta-pleated sheets held by hydrogen bonds; tertiary structure is the overall 3D folding; quaternary structure is the assembly of multiple polypeptides, as in haemoglobin with its four subunits. A protein's function depends entirely on its shape.
Key term — monomer: A small molecule that joins with others to form a polymer; for example, amino acids are the monomers of proteins.
Name the four classes of biological molecule and give one function of each.
Carbohydrates – energy source (e.g. glucose) or structure (cellulose); lipids – energy storage and membranes; proteins – enzymes, transport and structure; nucleic acids – storing and transmitting genetic information.
Answer: Carbohydrates – energy source (e.g. glucose) or structure (cellulose); lipids – energy storage and membranes; proteins – enzymes, transport and structure; nucleic acids – storing and transmitting genetic information.
- Claiming both DNA strands are brand new after replication Replication is semi-conservative: each daughter molecule keeps one original strand and gains one new strand.
- Saying high temperature kills enzymes Enzymes are not alive, so they cannot be killed. High temperatures denature them by disrupting the bonds that hold their 3D shape.
Practice
The solution turns purple (from blue), which indicates the presence of protein.
Primary structure is the sequence of amino acids in the polypeptide chain. Tertiary structure is the overall three-dimensional folding of that chain, held by hydrogen, ionic and disulfide bonds.
The double helix unwinds and the strands separate; each strand acts as a template with free nucleotides pairing by complementary base pairing (A–T, C–G); DNA polymerase joins the nucleotides to form two molecules, each with one old and one new strand.
Hydrogen and ionic bonds in the enzyme are disrupted, changing the shape of the active site so the substrate no longer fits precisely; the enzyme is denatured and the rate falls.
Quick check
Which of these best defines "monomer"?
A reaction rate is 6 units at 25 °C. Using the simplified Q10 = 2 model, what is the rate at 35 °C (before denaturation)?
- Biological Molecules: classify carbohydrates, lipids, proteins and nucleic acids, and test foods for them.
- The molecules of life: Carbohydrates (sugars and starch) provide energy and structure; lipids store energy and form membranes; proteins do the work of the cell as enzymes, carriers and structural fibres; nucleic acids (DNA and RNA) store and transmit genetic information.
- peptide bond: The covalent bond formed between two amino acids in a condensation reaction, linking them into a polypeptide chain.
- Watch out for: claiming both DNA strands are brand new after replication