Lipids and Lipoproteins Quiz

LIPIDS

• Lipids are a heterogeneous group of organic compounds that are relatively insoluble in water and other polar solvents, but soluble in nonpolar solvents (e.g., ether, chloroform, benzene, acetone).
• Lipids are esters of fatty acids with alcohols or substances capable of forming such esters and unlike proteins, nucleic acids, and polysaccharides, lipids are not polymers but rather small molecules.
• Lipids are widely distributed in nature, both in plants and animals.
• In the human body, lipids are primarily found in plasma, adipose tissue, and biological membranes.

Intended Learning Outcomes

• What are lipids?
• Biological importance of lipids
• Classification of lipids:
  • Simple lipids:
    • Fats and oils
    • Waxes
  • Compound lipids:
    • Phospholipids
    • Glycolipids
    • Lipoproteins
  • Derived lipids
    • Steroids
    • Terpenes
    • Carotenoids
    • Fatty acids
      • Saturated
      • Monounsaturated
      • Polyunsaturated

What are Lipids?

• The word "lipid" is derived from the Greek word "lipos," which means fat.
• Lipids are esters of fatty acids with alcohol or substances capable of forming such esters.

Biological Importance of Lipids

• Energy reservoir: Lipids are the chief energy storage form. Dietary lipids provide about 25% of daily caloric needs.
• Thermal insulator: Lipids in subcutaneous tissue protect against external temperature.
• Structural components of biological membranes and nerves. (glycolipids, phospholipids, sphingomyelins, cholesterol and lipoproteins) and help maintain membrane fluidity.
• Cell recognition and tissue immunity
• Aids in fat digestion and absorption (bile acids).
• Acts as regulators and modulators in cellular metabolism (steroid hormones and prostaglandins).
• Provides fat-soluble vitamins (A, D, E, and K) and essential fatty acids.
• Components of the electron transport chain: a complex cellular machinery for energy production.
• Fixation and protection of internal organs.

Classification of Lipids

• Simple lipids: fats, oils, and waxes.
• Compound lipids: phospholipids, glycolipids, and lipoproteins
• Derived lipids: Steroids, terpenes, carotenoids, fatty acids (saturated, monounsaturated, polyunsaturated)

Fatty Acids (FAs)

• Aliphatic monocarboxylic organic acids (R-COOH).
• Obtained from the hydrolysis of natural fats and oils.
• Amphipathic molecules with a hydrophilic COOH head and a hydrophobic hydrocarbon chain.
• General formula CH3(CH2)n-2COOH.
• Most FAs have an even number of carbon atoms (4-36).
• Commonly occurring FAs in animals have even numbers of C atoms (12-24).
• Classified into: short, medium, and long chain FAs based on the number of carbon atoms.
• Classified as saturated or unsaturated based on the presence of double bonds in the hydrocarbon chain.

Fatty Acids: Naming and Numbering

• Systematic names are derived from the parent hydrocarbon.
• Numbering of carbon atoms is from the carboxyl (COOH) end.
• The methyl carbon at the other end is omega (ω) carbon.

Saturated Fatty Acids (SFAs)

• SFAs have straight, single-bonded hydrocarbon chains (no double bonds).
• Common SFAs in animal cells include palmitic acid (16:0) and stearic acid (18:0).
• Have higher melting points.

Unsaturated Fatty Acids (UFAs)

• UFAs have one or more double bonds in their hydrocarbon chain.
• Most naturally occurring UFAs have cis-configuration, creating a bend in the chain.
• UFAs have lower melting points than SFAs.
• Important cis-UFAs include oleic acid (18:1) and linoleic acid (18:2).
• Trans fats are produced by partial hydrogenation of UFAs and are associated with health concerns.

A. Simple Lipids (Neutral Fats, Triglycerides (TGs))

• Esters of glycerol with three fatty acids.
• Classification: simple TGs (all three fatty acids are the same) vs mixed TGs.
• Natural fats are mixtures of mixed TGs.
• Oils are liquid at room temperature due to their high unsaturated fatty acid content, while fats are solid at room temperature due to their high saturated fatty acid content.
• Oil hydrogenation (hardening) converts some unsaturated fats to saturated fats, increasing their melting point.

A. Simple Lipids (Rancidity of Neutral Fats)

• Rancidity is the development of undesirable odor and taste (bad flavor) in fats and oils due to decomposition caused by factors such as high temperature, oxygen, moisture, humidity, light, and metal exposure.
• Hydrolytic rancidity: hydrolysis of triglycerides by lipase enzymes, releasing volatile short-chain fatty acids
• Oxidative rancidity: oxidation of unsaturated fatty acids by oxygen, yielding peroxides, aldehydes, and ketones (bad odors and tastes).
• Addition of antioxidants (e.g., vitamin E, phenols, and quinones) help decrease rancidity.

A. Simple Lipids (Partial Hydrogenation of Neutral Fats)

• Partial hydrogenation converts some cis-unsaturated fatty acids to trans-unsaturated fatty acids.
• This increases the melting points and makes the fats more solid.
• Trans fats, produced through partial hydrogenation, are associated with potential health risks.

B. Compound/Conjugated Lipids (Phospholipids, Glycolipids, Lipoproteins)

• Esters of fatty acids with alcohols containing additional groups (e.g., sulfur, phosphorus, amino group, carbohydrate).
• Phospholipids are esters of fatty acids with glycerol and a phosphorus-containing compound. Essential for biological membranes.
• Glycolipids are lipids with carbohydrate groups. Essential structural components of cellular membranes, particularly in plant cells.
• Lipoproteins are complexes of lipids and protein. Essential structural components in transport of lipids in blood

1. Phospholipids

• Phospholipids are esters of glycerol with a phosphorus-containing compound. Crucial for all biological membranes.
• The polar heads of phospholipids are attracted to water, while the nonpolar tails face each other. This causes them to form bilayers in water.
• Different types of phospholipids exist, each with a unique polar head group (e.g., lecithins, cephalins, inositides, cardiolipins).

1. Glycerophospholipids (Glycerophosphatides)

• Glycerophospholipids contain glycerol as the alcohol.
• Two fatty acids are attached to glycerol in ester linkages at positions 1 and 2.
• A phosphate group is attached to glycerol at position 3 in a phosphodiester linkage.
• A polar head group (e.g., choline, serine, ethanolamine, inositol) is attached to the phosphate.
• Key examples include Lecithins, Cephalins, Inositides, and Cardiolipins.

II. Sphingophospholipids

• Sphingophospholipids contain sphingosine as the backbone.
• A fatty acid chains attached to the sphingosine via an amide linkage.
• A phosphate group and a polar head groups (such as choline) is attached at position 3.
• Sphingomyelins are the most abundant sphingophospholipids.

2. Glycolipids

• Classification according to the alcohol backbone: Glyceroglycolipids and sphingoglycolipids.
• Glyceroglycolipids often lack phosphates, and in some plants, these make up 70-80% of the total membrane lipids in chloroplasts.
• Sphingoglycolipids (e.g., cerebrosides, globosides, gangliosides) contain a ceramide backbone and carbohydrate groups.
• Important in cell recognition, immunity, nervous system function, and blood group antigens.

3. Lipoproteins

• Lipoproteins are complexes of lipids and proteins that transport hydrophobic lipids in the blood.
• They are crucial for transporting lipids between different tissues.
• Various classes of lipoproteins exist, including chylomicrons, VLDL, IDL, LDL, and HDL, with variations in size, density, lipid, and protein content.