Lipids (Fatty Acid and Glycerol)

Lipids are organic substances:
- Insoluble in water.
- Soluble in organic solvents.
- Related to fatty acids.
- Utilized by living cells.
Include fats, waxes, sterols, fat-soluble vitamins, mono-, di-, or triglycerides, phospholipids, etc.
Unlike carbohydrates, proteins, and nucleic acids, lipids are not polymeric molecules.
Play a great role in cellular structure.
Chief source of energy.

Lipid: organic compound such as fat or oil.
Lipids consist of repeating units called fatty acids.
Fatty acids: organic compounds with the general formula $CH3(CH2)nCOOH$ , where $n$ usually ranges from 2 to 28 and is always an even number.
Fatty acids are chains of carbon and hydrogen atoms with an acid group $(COOH)$ and a methyl group $(CH3)$ on the other end.

Fatty acids are carboxylic acids (or organic acid), usually with long aliphatic tails (long chains), either unsaturated or saturated.

Lack carbon-carbon double bonds.
Have higher melting points compared to unsaturated acids of the corresponding size due to their ability to pack their molecules together, leading to a straight rod-like shape.

Indicated when a fatty acid has more than one double bond.
Often, naturally occurring fatty acids possess an even number of carbon atoms and are unbranched.
Contain a cis-double bond(s) which create a structural kink that disables them to group their molecules in a straight rod-like shape.

Biologically, a lipid is a macrobiomolecule that is soluble in nonpolar solvents.
Nonpolar solvents: typically hydrocarbons used to dissolve other naturally occurring hydrocarbon lipid molecules that do not (or do not easily) dissolve in water.
Examples: fatty acids, waxes, sterols, fat-soluble vitamins (vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, and phospholipids.

Triglyceride: an ester derived from glycerol combined with three fatty acid molecules.
Shows the chemical reaction of how glycerol combines with three fatty acids to form a triglyceride (triester of glycerol).
A triglyceride molecule can be formed from any combination of fatty acids.

Illustrates the structure of a lipid bilayer with polar heads (hydrophilic) and nonpolar tails (hydrophobic).

Oily or greasy nonpolar molecules, stored in the adipose tissue of the body.
Heterogeneous group of compounds, mainly composed of hydrocarbon chains.
Energy-rich organic molecules, which provide energy for different life processes.
Class of compounds characterized by their solubility in nonpolar solvents and insolubility in water.
Significant in biological systems as they form a mechanical barrier dividing a cell from the external environment, known as the cell membrane.

Glycerophospholipids are the main structural component of biological plasma membrane as the cellular plasma membrane and the intracellular membranes of organelles, both in animal and plant cells.

Triglycerides, stored in adipose tissue, are a major form of energy storage in both animals and plants.
Major source of energy because carbohydrates are fully reduced structures.

Lipid signaling is a vital part of the cell signaling.
Lipid signaling may occur via activation of G-protein-coupled or nuclear receptors, and members of several different lipid categories have been identified as signaling molecules and cellular messengers.

Supplies energy.
Carries Vitamins A, D, E, and K through the body.
Promotes healthy skin.
Promotes normal cell growth.
Acts like a “cushion” and heat regulator to protect your heart, liver, and other vital organs.
Helps you feel full longer.
Adds flavor to food.

Two main classes:
- Nonsaponifiable lipids
- Saponifiable lipids
Saponification: process that involves the conversion of fat, oil, or lipid into soap and alcohol by the action of heat in the presence of aqueous alkali $(NaOH)$ .
During saponification, ester reacts with an inorganic base to produce alcohol and soap.
Generally, it occurs when triglycerides are reacted with potassium or sodium hydroxide to produce glycerol and fatty acid salt called ‘soap’.

Comprises one or more ester groups, enabling it to undergo hydrolysis in the presence of a base, acid, or enzyme.
Include waxes, triglycerides, sphingolipids, and phospholipids.

Nonpolar and polar lipids.
- Nonpolar lipids: namely triglycerides, are utilized as fuel and to store energy.
- Polar lipids: that could form a barrier with an external water environment, are utilized in membranes; comprise sphingolipids and glycerophospholipids.
Fatty acids are pivotal components of all these lipids.

Non-metallic.
Tetravalent.
Has 3 naturally occurring isotopes ( $12C$ and $13C$ – stable, $14C$ – radioactive).
Has several allotropes, best known are graphite, diamond, and amorphous carbon.
Has a high melting point and can easily combine with oxygen at elevated temperatures.
Acts as an excellent hardener for iron and yields various steel alloys.
The radioactive isotope of carbon is $C-14$ , which is used to date ancient objects of organic origin.

Important for all known living systems, and life could not exist without it.
Available in the form of hydrocarbons other than food and wood such as fossil fuel, methane gas, and crude oil.

Carbon dioxide $(CO2)$ : an essential element present in the air and in the water for sustaining life on earth.
Photosynthesis by green plants takes their energy from the sun in order to break down water into oxygen and hydrogen.
Living organisms that cannot photosynthesize are bound to rely on other living organisms in order to consume their minimum requirements of carbon dioxide molecules.
A balance of carbon and oxygen is necessary for the survival of almost all living organisms on this planet.