Lipid Notes: Fatty Acids, Triglycerides, Phospholipids, Waxes, Steroids, and Cholesterol

Fatty Acids and Triglycerides

  • Fatty acids are a key component of many lipids. They consist of a carboxyl group joined to a hydrocarbon (alkyl) chain of variable length.

  • Structure details:

    • End group: carboxyl group –COOH

    • Rest of molecule: hydrocarbon tail (R) of variable length

    • Some fatty acids have carbon–carbon double bonds (C=C) in the tail; they are unsaturated. If there are no C=C bonds, the fatty acid is saturated.

  • Label the components of a fatty acid:

    • Carboxyl group at one end

    • Hydrocarbon tail (alkyl chain)

    • Potential C=C bonds in the tail for unsaturated fatty acids

  • Common example shown: Olive Oil (rich in unsaturated fatty acids, e.g., oleic acid)

  • Saturated vs. unsaturated:

    • Saturated: no C=C double bonds; straight chains; generally solid at room temperature

    • Unsaturated: one or more C=C double bonds; bends in the chain; generally liquid at room temperature

  • Fatty acids are key components of many lipids, notably triglycerides

  • Formation of triglycerides:

    • Triglycerides are formed when three fatty acids esterify to a glycerol backbone

    • Ester bond formation is a condensation reaction (esterification)

  • Triglyceride structure (general): glycerol backbone + three fatty acid tails

  • Triglycerides: functions and properties

    • Long-term energy store in organisms

    • Stored in the body as adipose tissue; provides insulation in cold habitats

    • Long hydrocarbon tails store a lot of energy; energy released when broken down

    • Hydrophobic and insoluble in water; do not affect a cell’s osmotic potential

  • Lipids overview note:

    • Lipids are insoluble in water

    • Found in fats, oils, waxes, hormones and other important molecules

  • Triglycerides form an emulsion in water (i.e., do not dissolve; form droplets)

  • Milk contains triglycerides (as part of milk fat)

Phospholipids

  • Function: Phospholipids make up the cell/plasma membrane of cells.

  • Structure: glycerol backbone with two fatty acid tails and a phosphate-containing head group (often with an additional small molecule attached, e.g., choline).

  • Labeling and meaning (phospholipid):

    • Glycerol backbone

    • Two fatty acid tails (hydrophobic)

    • Phosphate group (hydrophilic, head)

    • Head group (e.g., –CH2–CH2–N+(CH3)3 in phosphatidylcholine) depending on the molecule

  • Phospholipid bilayer (doodle and label concept):

    • A bilayer forms with hydrophobic tails facing inward and hydrophilic heads facing outward toward water

    • This arrangement creates a semi-permeable membrane

  • Membrane properties:

    • Hydrophobic tails are non-polar; most water-soluble molecules cannot pass through easily

    • The membrane regulates entry and exit of substances to/from the cell

  • Key terms:

    • HYDROPHOBIC: tail regions

    • HYDROPHILIC: head regions

    • LDL cholesterol: often referred to as "bad" cholesterol; can contribute to plaque formation when elevated

    • HDL cholesterol: often referred to as "good" cholesterol; helps transport cholesterol away from arteries

  • Phospholipids in the membrane: form the basic structure of the cell/plasma membrane

Waxes

  • Waxes are long-chain hydrocarbons.

  • Properties: very water-repellent and solid at room temperature.

  • Roles:

    • Plants: waxes in leaves/stems to prevent water loss

    • Some animals: waxes to clean and protect their bodies/hair

    • Bees: use wax to build honeycombs

Steroids and Cholesterol

  • Steroids: have a basic structure of four fused rings of carbon atoms; hydrophobic and insoluble in water

  • Roles:

    • Basis of many steroids, including testosterone and estrogen

    • Used to make other molecules, including some neurotransmitters (as part of broader biosynthesis pathways)

  • Cholesterol: a type of lipid steroid; a key component of cell membranes

  • Health note: too much LDL cholesterol can lead to fatty deposits in arteries, increasing the risk of heart disease and stroke

  • To reduce cholesterol: diets should be low in saturated fats and trans fats

  • LDL vs HDL overview:

    • LDL cholesterol: delivers cholesterol to tissues; associated with higher risk when elevated

    • HDL cholesterol: helps remove cholesterol from tissues to the liver; considered protective

Connections and Practical Implications

  • Lipids provide energy storage, insulation, and membrane structure; different lipid types have specialized roles (triglycerides for energy, phospholipids for membranes, steroids for signaling).

  • Dietary choices influence lipid profiles (saturated and trans fats increase LDL; some unsaturated fats can increase HDL or lower LDL).

  • Cholesterol is essential for membranes and steroid synthesis, but balance is important for health.

  • Real-world relevance: nutrition guidelines, cardiovascular health, and the role of lipids in cellular biology and hormone regulation.

Quick References (LaTeX)

  • Triglyceride formation (esterification):
    extGlycerol+3extR−COOH<br>ightarrowextTriglyceride+3extH2extOext{Glycerol} + 3 ext{R-COOH} <br>ightarrow ext{Triglyceride} + 3 ext{H}_2 ext{O}

  • General fatty acid structure:

    • Saturated: CH3-(CH2)_n-COOH

    • Unsaturated: CH3-(CH2)m-CH=CH-(CH2)_n-COOH

  • Phospholipid bilayer concept (described above)

  • Common lipid-associated terms:

    • LDL: low-density lipoprotein (bad cholesterol)

    • HDL: high-density lipoprotein (good cholesterol)