Signalling Lipids
High-Yield Summary
- Terpenes are built from repeating 5-carbon isoprene (C₅H₈) units; classified by unit count (monoterpene=2/10C ... tetraterpene=8/40C, polyterpene=many). Precursors to steroids and vitamins A/D/E/K.
- Terpenoids = chemically modified terpenes (rearranged skeleton or added O-containing groups), still derived from the terpene base.
- Steroids share a rigid 4-ring core (3 cyclohexane + 1 cyclopentane); diversity comes from oxidation state/functional groups. Steroid hormones: high-affinity receptors, active at low concentration, alter gene expression via intracellular receptors.
- Cholesterol is a two-way membrane fluidity buffer — increases fluidity at low temp (wedges between tails), decreases fluidity at high temp (restrains tail movement) — and a precursor to steroid hormones, vitamin D, bile acids.
- Prostaglandins: local (autocrine/paracrine) signals derived from arachidonic acid, regulate cAMP; roles in smooth muscle contraction, temperature, sleep-wake, inflammation/fever/pain. NSAIDs inhibit cyclooxygenase (COX), blocking their synthesis.
- Fat-soluble vitamins: A (vision/growth/immunity), D (bone metabolism/immunity, made in skin via sunlight), E (antioxidant), K (blood clotting/bone proteins). Provitamin A = carotenoids, converted to active vitamin A.
Key Terms
- Isoprene
- 5-carbon (C₅H₈) building-block unit of all terpenes.
- Terpenoid
- Chemically modified terpene — rearranged carbon skeleton and/or added oxygen-containing groups.
- Steroid
- Lipid with a 3-cyclohexane + 1-cyclopentane fused ring core; hormone diversity comes from oxidation state/side groups.
- Prostaglandin
- Autocrine/paracrine signaling molecule derived from arachidonic acid; regulates cAMP.
- Cyclooxygenase (COX)
- Enzyme converting arachidonic acid into prostaglandins; the target NSAIDs inhibit.
- Provitamin A
- Carotenoids (from colorful vegetables) that the body converts into active vitamin A.
Terpene Classes
- Monoterpene
- 2 isoprene units, ~10 carbons.
- Sesquiterpene
- 3 isoprene units, ~15 carbons.
- Diterpene
- 4 isoprene units, ~20 carbons.
- Triterpene
- 6 isoprene units, ~30 carbons.
- Tetraterpene
- 8 isoprene units, ~40 carbons.
Fat-Soluble Vitamins A/D/E/K
| Vitamin | Main Function |
|---|---|
| A (retinal/retinol/retinoic acid) | Vision, cell growth, bone development, immune function |
| D (D2, D3) | Bone metabolism, immune health; skin-synthesized via sunlight |
| E (tocopherols) | Antioxidant — protects membranes from oxidative damage |
| K (phylloquinone, menaquinones) | Activates blood clotting factors and bone proteins |
Common MCAT Trap
- Cholesterol's fluidity effect is bidirectional and temperature-dependent — don't memorize it as "always increases fluidity." Low temp: increases fluidity (prevents tight packing). High temp: decreases fluidity (restrains movement).
- NSAIDs don't block arachidonic acid production — they block cyclooxygenase, the enzyme converting arachidonic acid into prostaglandins, one step downstream.
- Terpenes and terpenoids are precursors/intermediates in steroid hormone synthesis pathways, not steroids themselves — keep the two families conceptually separate even though they connect.
Quick Recall
What structural feature defines all terpenes?
What are the three defining functional properties of steroid hormones?
What enzyme do NSAIDs inhibit, and what does that block?
Besides membrane structure, what three molecules is cholesterol a precursor for?