Lipids and Plant Pigments: Structure and Function
Two very different biomolecule families, sphingolipids and carotenoids, are both defined by structural features that show up constantly in exam questions: a small, fixed set of stereocentres in the sphingosine backbone, and a long conjugated polyene chain that decides both a carotenoid color and its chemistry.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Counting stereocentres in a sphingolipid backbone
Sphingomyelin, like other sphingolipids, is built on a sphingosine backbone: a long-chain amino alcohol bearing a defined stereocentre at C₂ (the carbon carrying the amide-linked fatty-acid nitrogen) and a second defined stereocentre at C₃ (the carbon carrying a free hydroxyl group). The remaining backbone carbon that carries the phosphocholine head group, C₁, is a simple -CH₂-O⁻ linkage: it carries two hydrogens and is not a stereocentre. The long hydrocarbon tail from the fatty acid and from the sphingosine chain itself is typically unbranched and carries no additional stereocentres. Counting carefully residue by residue like this, rather than trying to count every carbon bearing an oxygen substituent, gives exactly 2 chiral carbons for sphingomyelin — the C₂ (amide-bearing) and C₃ (hydroxyl-bearing) backbone carbons, and no others.
β-carotene: a long conjugated polyene decides both its color and its chemistry
β-carotene is a long chain of alternating single and double bonds (a polyene) capped at both ends by a methylated cyclohexene ring. That extended conjugation is what makes it coloured at all: the conjugated system absorbs light in the BLUE-GREEN region of the visible spectrum (its many closely-spaced π-to-π* energy levels put the absorption there), and the light that is NOT absorbed — predominantly red, orange and yellow wavelengths — is transmitted or reflected, which is why carrots and similar vegetables appear orange, not that β-carotene absorbs red/yellow light itself. The same electron-rich polyene system also makes β-carotene an excellent quencher of singlet oxygen (an unusually reactive, excited electronic state of O₂): the polyene reacts with singlet oxygen to form a carotenoid epoxide, safely dissipating the excess energy rather than letting singlet oxygen go on to damage other cellular molecules — this quenching behaviour makes β-carotene an ANTIOXIDANT, not an oxidising agent, despite reacting with an oxygen species. Finally, the long, almost entirely hydrocarbon polyene chain (with no polar functional groups) makes β-carotene strongly lipophilic (fat-soluble), which is exactly why it partitions into the oil phase, not the water phase, when carrots are cooked in an oil-and-water mixture such as a curry.
OlympIQ exam lens
- ✓Sphingomyelin has exactly 2 chiral carbons: C₂ (amide nitrogen) and C₃ (free OH) of the sphingosine backbone; the phosphocholine-bearing C₁ (a -CH₂-O⁻ carbon) is not a stereocentre
- ✓β-carotene ABSORBS blue-green light and therefore appears orange/red/yellow (the colours it does NOT absorb), not the other way round
- ✓β-carotene reacts with singlet oxygen to form a carotenoid epoxide — this quenching action makes it an antioxidant, not an oxidising agent
- ✓β-carotene is strongly lipophilic (an almost purely hydrocarbon polyene) and partitions into the oil phase when cooked in oil and water together
Do not describe β-carotene as absorbing the same colours it visually displays. A pigment appears the colour of the light it does NOT absorb — β-carotene absorbs blue-green light and transmits/reflects red-orange-yellow, which is why it looks orange, the reverse of saying it absorbs red/yellow light.
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