Lipid Structure and Function

Lipids

1. Substance in origin that are insoluble in water

2. Not polymeric

3. Can be divided in three roles:

   1. Energy storage (Adipocytes in breast tissue)

   2. Structure components (Lipid bilayer)

   3. Cell signalling (Cortisol)

Fatty Acids

Structural Features of Lipids:

1. Long chains of carboxylic acids (4-36 carbons long)

   1. More complex animals range around C16 - C18

2. Can be either saturated or unsaturated

Saturated

Structural Features of Fatty Acids:

1. Don’t have any double bond present

2. The more carbons, the higher melting point.

3. Unbranched

4. Fully saturated with H atoms

5. Has a higher flexibility and melting point (carbon - carbon bonds can be twisted and rotated)

Unsaturated fatty acids

Structural Features of Fatty Acids:

1. Contain at least 1 double bond in hydrocarbon chain

   1. The more double bonds, the lower the melting point

   2. If the length of the carbon chain increases, the melting point also increases given that the double bonds is held constant

2. Can either be cis or trans (mostly are cis)

3. Has a lower melting point

4. Majority of biological fatty acids

Essential Fatty Acids

Linoleic acid (w-6) and a-Linoleic acid (w-3) are termed as ______ because the human body is incapable of synthesizing them and must be obtained through diet.

a-Linoleic acid (w-3)

The precursor of Docosahexaenoic acid (DHA), an important component for the human brain.

Storage Lipids

Also called triglycerides or fats, this is the most abundant class of lipids, as they serve as energy stores in animals, namely in adipocytes; it consists of a Glycerol esterified to three fatty acid residues

Hydrogenation

Reactions of Triacylglycerols

Reduces double bonds to produce solid, saturated fatty acids. Usually done in large scales using hydrogen gas over a catalyst (Pt or Pd)

Saponification

Reactions of Triacylglycerols

1. Process of making soap from fats or oils, one of the oldest chemical reactions known to humans. 

2. Fats or oils are reacted with a base to form a surfactant (soap) in a hydrolysis reaction.

Waxes

Reactions of Triacylglycerols

1. Esters of fatty acids and alcohols

2. The fatty acids in waxes are usually saturated

3. Highly insoluble in water

Glycerophospholipids

Structural Lipids

1. Also called phosphoglycerides

2. Major lipid component of animal biological membranes

3. Amphipilic molecules (have both polar and non-polar molecules)

4. Have glycerol as their backbone molecule

5. Most have phosphatidic acid as parent compound

Sphingolipids

Structural Lipids

1. Usually found in biological membranes

2. In contrast to glycerophospholipids, sphingolipids have sphingosine as their backbone molecule

3. Sphingosine is synthesized by the body by reacting palmitoyl-CoA with an amino acid.

Ceramides

Derivatives of Sphingosine

Makes up approximately 30-40% of the lipids found in skin cells

Choline Sphingomyelin

Derivatives of Sphingosine

Usually found in the cell membranes of tissues in the nervous system

Glycosphingolipids

Structural Lipids

1. Found in outer plasma membrane

2. Consists of a ceramide (sphingolipid) connected to one or more fatty acid residue

3. Plays important roles in cellular adhesion, cellular recognition and signalling

Glycolipid

Structural Lipids

1. Also called Sulfolipids

2. Major lipid component in plant cells

3. Makes up approx 70-80% of plant lipids in chloroplasts

Eicosanoids

Signalling Lipids

Also called Paracrine hormones (hormones that act only on nearby cells or organs)  due to being derived from Arachidonic (a 20-carbon polyunsaturated fatty acid)

Prostaglandins

Eicosanoids

1. Have a 5-carbon ring

2. Found in almost all tissues in the human body

Thromboxanes

Eicosanoids

1. Have a 6-carbon ring

2. Produced by platelets

Leukotrienes

Eicosanoids

1. Produced by immunocompetent cells such as leukocytes, mast cells, basophils, eosinophils, & neutrophils

2. Plays a vital role in the contraction and relaxation of the smooth muscles of the Lungs

Steroids

Signalling Lipids

1. Large, terpene-based (unsaturated hydrocarbons formed by linking two or more isoprene units) class of lipids

2. Consists of four rings fused together

3. Most common steroid and precursor of all animal steroids is cholesterol

4. As with most other lipids, cholesterol is amphipathic, with a polar head group (-OH) and a non-polar body (steroid nucleus)

Cholesterol

Steroids

1. Synthesize by all animal cells by Acetyl-CoA

2. Regulates fluidity of the membrane in animal cell plasma membrane

3. Vital part of lipoproteins (carriers of cholesterol) and is the precursor of all sex and steroid hormones

4. Also functions as a structural lipid in the epidermis of the skin

5. Excess cholesterol can have adverse side effects

Lipoprotein

Cholesterol

Transport cholesterol, triacylglycerols, and other hydrophobic biomolecules

Gangliosides

Glycosphingolipids

1. Ceramides with oligosaccharides attached

2. Used in cellular signaling and recognition

Cerebroside

Glycosphingolipids

1. Ceremides with a monosaccharide attached

2. Found in membrane of nerve cells