Biomolecules Lecture 12: Lipids (Part II)

Multiparagraph Summary

Lipids are a diverse group of biomolecules that play critical roles in energy storage, membrane structure, and signaling. Fatty acids are carboxylic acids with long hydrocarbon chains and can be classified as saturated, monounsaturated, or polyunsaturated. Saturated fatty acids have no double bonds, allowing tight packing and solid form at room temperature, with melting points increasing with chain length. Unsaturated fatty acids contain one (monounsaturated) or more (polyunsaturated) cis double bonds that introduce kinks, reducing packing efficiency and lowering melting points. Common examples include stearic acid (saturated), oleic acid (monounsaturated), and linoleic acid (polyunsaturated).

Triacylglycerols, composed of a glycerol backbone esterified with three fatty acids, are the main form of energy storage in adipocytes. Glycerophospholipids form the structural basis of biological membranes, featuring a glycerol backbone, two fatty acid chains, and a polar head group such as choline. Their amphipathic nature allows them to spontaneously form bilayers, critical for compartmentalization in cells.

Sphingolipids are another essential class of membrane lipids, particularly abundant in the central nervous system. Derived from the amino alcohol sphingosine, sphingolipids include ceramides, phosphosphingolipids (e.g., sphingomyelin), and glycosphingolipids (e.g., cerebrosides, globosides, and gangliosides). Glycosphingolipids are involved in cell recognition, immune function, and act as receptors for toxins. Defective degradation of gangliosides, as seen in Tay-Sachs disease, leads to fatal neurological deterioration.

Steroids are lipids with a four-ring structure and include cholesterol, steroid hormones, and vitamin D. Cholesterol maintains membrane fluidity and serves as a precursor for steroid hormones such as glucocorticoids, mineralocorticoids, and sex hormones. These hormones regulate metabolism, salt and water balance, reproductive functions, and gene expression via steroid-receptor complexes. Vitamin D, derived from cholesterol, is critical for calcium homeostasis, bone health, and immune modulation. Deficiency causes rickets, while excess can cause kidney stones.

Lipoproteins, composed of a hydrophobic lipid core and polar lipid-protein shell, transport triacylglycerols and cholesterol in the bloodstream. Low-density lipoproteins (LDL) carry cholesterol and are central to cardiovascular health; defects in LDL metabolism increase the risk of atherosclerosis. Plant sterols, structurally similar to cholesterol, reduce cholesterol absorption and are used therapeutically to manage serum cholesterol levels.

Bullet-Point Summary

Fatty Acids

• Carboxylic acids with long hydrocarbon chains.

• Saturated: no double bonds, solid at room temp, melting temp ↑ with chain length.

• Monounsaturated: 1 cis double bond, kinked chain, lower melting temp.

• Polyunsaturated: >1 cis double bonds, further reduced melting temp.

• Examples: Stearic acid (saturated), Oleic acid (monounsaturated), Linoleic acid (polyunsaturated).

Triacylglycerols

• Glycerol backbone + 3 fatty acids.

• Main energy storage in adipocytes.

Glycerophospholipids

• Glycerol backbone, 2 fatty acids, polar head group (e.g., choline).

• Form biological membranes.

Sphingolipids

• Derived from sphingosine.

• Types:

  • Phosphosphingolipids: sphingomyelin.

  • Glycosphingolipids: cerebrosides, globosides, gangliosides.

• Functions: membrane components, cell recognition, toxin receptors, immune interactions.

Gangliosides

• Contain sialic acid; naming: GM (mono), GD (di), GT (tri), GQ (quad).

• Impaired breakdown → Tay-Sachs disease (fatal neurological deterioration).

Steroids

• Four fused rings, may have methyl, alkyl, or hydroxyl groups.

• Cholesterol: membrane fluidity and precursor of steroid hormones.

• Steroid hormones: glucocorticoids, mineralocorticoids, and sex hormones.

• Vitamin D: cholesterol derivative, regulates calcium, bone health, immune function; deficiency → rickets.

Lipoproteins

• Transport triacylglycerols and cholesterol.

• LDL: major cholesterol carrier; defects increase atherosclerosis risk.

• Plant sterols reduce cholesterol absorption.

40 MCQs

1. Which fatty acid has no double bonds?
   A. Oleic acid
   B. Linoleic acid
   C. Stearic acid
   D. Arachidonic acid

2. Saturated fatty acids are typically:
   A. Liquid at room temperature
   B. Solid at room temperature
   C. Kinked in structure
   D. Polyunsaturated

3. Cis double bonds in fatty acids:
   A. Increase melting point
   B. Cause a kink in the chain
   C. Pack tightly together
   D. Make the fatty acid saturated

4. Polyunsaturated fatty acids:
   A. Have one double bond
   B. Cannot pack tightly together
   C. Are solid at room temp
   D. Are always trans

5. Triacylglycerols:
   A. Form cell membranes
   B. Store energy in adipocytes
   C. Contain a phosphate group
   D. Are amphipathic

6. Glycerophospholipids have:
   A. One fatty acid chain
   B. Two fatty acid chains + polar head
   C. Three fatty acid chains
   D. Only sphingosine backbone

7. Sphingomyelin is a type of:
   A. Triacylglycerol
   B. Phosphosphingolipid
   C. Steroid
   D. Glycosphingolipid

8. Cerebrosides are:
   A. Phospholipids
   B. Simple glycosphingolipids
   C. Cholesterol derivatives
   D. Steroid hormones

9. Gangliosides:
   A. Contain phosphate groups
   B. Contain sialic acid residues
   C. Are triacylglycerols
   D. Are saturated fatty acids

10. Tay-Sachs disease is caused by:
    A. Excess cholesterol
    B. Faulty N-acetylhexoseaminidase
    C. Defective LDL receptor
    D. Vitamin D deficiency

11. Glucocorticoids:
    A. Regulate blood pressure
    B. Inhibit allergic and inflammatory responses
    C. Regulate salt excretion
    D. Act as lipid transporters

12. Mineralocorticoids primarily:
    A. Regulate metabolism
    B. Regulate salt and water balance
    C. Control sex hormone levels
    D. Form lipid bilayers

13. Progesterone:
    A. Is a precursor for steroid hormones
    B. Regulates immune function
    C. Is a triacylglycerol
    D. Forms cell membranes

14. Androgens are:
    A. Female sex hormones
    B. Male sex hormones
    C. Steroid transporters
    D. Glycosphingolipids

15. Estrogens:
    A. Male sex hormones
    B. Female sex hormones
    C. Triacylglycerols
    D. Plant sterols

16. Vitamin D synthesis involves:
    A. Enzymatic hydroxylation only in kidneys
    B. UV light-induced B ring disruption
    C. Direct ingestion from cholesterol only
    D. Phosphate attachment

17. Active Vitamin D function:
    A. Reduces cholesterol absorption
    B. Increases intestinal Ca2+ absorption
    C. Inhibits steroid hormone action
    D. Forms membrane phospholipids

18. Deficiency of Vitamin D causes:
    A. Tay-Sachs disease
    B. Rickets
    C. Atherosclerosis
    D. Cushing’s syndrome

19. Lipoproteins are composed of:
    A. Only proteins
    B. Only hydrophobic lipids
    C. Hydrophobic core + polar lipid-protein shell
    D. Steroid hormones

20. LDL carries:
    A. Triacylglycerols only
    B. Cholesterol only
    C. Both cholesterol and triacylglycerols
    D. Sphingolipids

21. Plant sterols:
    A. Increase cholesterol absorption
    B. Block cholesterol absorption
    C. Are identical to cholesterol
    D. Are toxic to humans

22. Saturated fatty acids have:
    A. One cis double bond
    B. No double bonds
    C. Multiple double bonds
    D. Trans double bonds

23. Monounsaturated fatty acids:
    A. Solid at room temp
    B. Contain one cis double bond
    C. Contain multiple double bonds
    D. Are fully saturated

24. Glycerophospholipids form:
    A. Hormones
    B. Membrane bilayers
    C. Lipoproteins
    D. Vitamin D

25. Sphingolipids are abundant in:
    A. Liver cells
    B. CNS cells
    C. Adipocytes only
    D. Muscle fibers

26. Ceramides are:
    A. Triacylglycerols
    B. N-acyl derivatives of sphingosine
    C. Cholesterol precursors
    D. Steroid hormones

27. Phosphosphingolipid example:
    A. Ganglioside
    B. Cerebroside
    C. Sphingomyelin
    D. LDL

28. Globosides:
    A. Contain acidic oligosaccharides
    B. Contain neutral oligosaccharides
    C. Are triacylglycerols
    D. Contain phosphate

29. Gangliosides function as:
    A. Hormones
    B. Membrane receptors
    C. Energy storage
    D. Structural proteins

30. Cholesterol is:
    A. A triglyceride
    B. A steroid
    C. A glycosphingolipid
    D. A fatty acid

31. Steroid hormones are:
    A. Water-soluble
    B. Water-insoluble
    C. Triglycerides
    D. Phospholipids

32. Steroid hormones regulate gene expression via:
    A. Direct membrane insertion
    B. Binding to transcription factors
    C. Acting as energy molecules
    D. Forming sphingolipids

33. Glucocorticoid example:
    A. Aldosterone
    B. Cortisol
    C. Progesterone
    D. Estradiol

34. Mineralocorticoid example:
    A. Cortisol
    B. Aldosterone
    C. Progesterone
    D. Testosterone

35. Cholesterol derivatives include:
    A. Triacylglycerols
    B. Steroid hormones, vitamin D
    C. Sphingomyelins
    D. Cerebrosides

36. LDL receptor dysfunction increases:
    A. Vitamin D absorption
    B. Serum cholesterol
    C. Membrane fluidity
    D. Glucose uptake

37. GM series gangliosides contain:
    A. One sialic acid
    B. Two sialic acids
    C. Three sialic acids
    D. Four sialic acids

38. GD series gangliosides contain:
    A. One sialic acid
    B. Two sialic acids
    C. Three sialic acids
    D. Four sialic acids

39. GT series gangliosides contain:
    A. One sialic acid
    B. Two sialic acids
    C. Three sialic acids
    D. Four sialic acids

40. GQ series gangliosides contain:
    A. One sialic acid
    B. Two sialic acids
    C. Three sialic acids
    D. Four sialic acids

MCQ Answer Key

1. C – Stearic acid

2. B – Solid at room temperature

3. B – Cause a kink in the chain

4. B – Cannot pack tightly together

5. B – Store energy in adipocytes

6. B – Two fatty acid chains + polar head

7. B – Phosphosphingolipid

8. B – Simple glycosphingolipids

9. B – Contain sialic acid residues

10. B – Faulty N-acetylhexoseaminidase

11. B – Inhibit allergic and inflammatory responses

12. B – Regulate salt and water balance

13. A – Is a precursor for steroid hormones

14. B – Male sex hormones

15. B – Female sex hormones

16. B – UV light-induced B ring disruption

17. B – Increases intestinal Ca2+ absorption

18. B – Rickets

19. C – Hydrophobic core + polar lipid-protein shell

20. C – Both cholesterol and triacylglycerols

21. B – Block cholesterol absorption

22. B – No double bonds

23. B – Contain one cis double bond

24. B – Membrane bilayers

25. B – CNS cells

26. B – N-acyl derivatives of sphingosine

27. C – Sphingomyelin

28. B – Contain neutral oligosaccharides

29. B – Membrane receptors

30. B – A steroid

31. B – Water-insoluble

32. B – Binding to transcription factors

33. B – Cortisol

34. B – Aldosterone

35. B – Steroid hormones, vitamin D

36. B – Serum cholesterol

37. A – One sialic acid

38. B – Two sialic acids

39. C – Three sialic acids

40. D – Four sialic acids