Lipids and Fats

Lipid:

Lipid molecules are all hydrophobic since they contain fewer polar O-H bonds and more nonpolar C-H bonds. Lipids are used for storing energy and building membranes.

Fats:

Fats are formed by excess carbohydrate and store energy in both animals and plants. The most common fat is called triacylglycerol or triglyceride, which combined with a glycerol molecule and three fatty acids.

*Glycerol – three carbon alcohol.

*Fatty acid – long hydrocarbon chain containing a single carboxyl group.

Fatty acid can be either saturated or unsaturated. Saturated fatty acids only contain single bonds and make these fatty acids fit more closely together. (Like lard and butter.) Unsaturated fatty acids contain double bonds between carbon atoms, and make the fats in a form of liquid. (Like olive oil and sunflower oil.)

When a glycerol molecule reacts with three fatty acids, three –H atoms from the hydroxyl groups combined with three –OH from the carboxyl groups, and form three water molecules. The bonds between a glycerol and fatty acids are called ester bonds.

Phospholipids:

Phospholipids are made of one glycerol molecule, two fatty acids and a polar phosphate group. Since the phospholipids have both polar and nonpolar parts, they can form a bilayer and act as the main components in cell membrane. The hydrophilic head (polar phosphate group) can mix with water whereas the hydrophobic tails (nonpolar fatty acids) only can mix with each other in the center of bilayer.

Sterols:

Sterols contain four fused hydrocarbon rings and few different functional groups. Cholesterol is an important steroid component of cell membranes. Cholesterol is often converted into vitamin D and bile salts.

Waxes:

Waxes are another lipid containing long-chain fatty acids attached to alcohols or carbon rings.

Waxes are usually used to form waterproof coatings

on plants or animals.

Biological Macromolecules

There are four major classes of macromolecules in living organisms: carbohydrates, lipids, proteins, and nucleic acids. Macromolecules are large molecules which composed of a large number of repeating subunits; you can call them polymers (except lipids).
Reactions of Polymers:
Dehydration reaction (condensation reaction, anabolic reaction):
monomer(-OH) + monomer(-H) + energy → polymer + H2O
Hydrolysis (catabolic reaction):
polymer +H2O → monomer(-OH) + monomer(-H) + energy
*Enzymes can act as catalysts in both reactions.


Carbohydrates:Carbohydrates are produced by plants through the process of photosynthesis, and act as the energy source, structural compounds and raw materials. Carbohydrates can be classified into three groups: monosaccharaides, disaccharides, and polysaccharides.

Monosaccharaide: monosaccharaides are simple sugars which contain a single chain of carbon (three to seven carbon atoms), multiple hydroxyl groups, and a carbonyl group(C=O). If the carbonyl group is attached to the end of the carbon chain, it must be an aldose; if it is located on the carbon chain, it is ketoses. Aldoses and ketoses are isomers if they contain same number of carbon atoms. Ex: glucose, galactose and fructose are isomers which have the same chemical formula C6H12O6.

Disaccharide: disaccharides are sugars contain two monosaccharaides which attached to one another by glycosidic linkages. There are three major disaccharides: maltose, sucrose, and lactose.
α-glucose + α-glucose → maltose (held by an α 1-4 glycosidic linkage)
Ex: Beer
α-glucose + α-fructose → sucrose (held by an α 1-2 glycosidic linkage)
Ex: Table sugar
α-glucose + α-galactose → lactose
Ex: Milk

Polysaccharide: polysaccharides are monosaccharide polymers and held by glycosidic linkages. They can either store energy or act as a structural compound.
  1)Energy storage: excess α-glucose molecules are linked by α 1-4 glycosidic linkage in the main chain and α 1-4 glycosidic linkage at the branched points. Starch and glycogen can both be digested into single glucose by enzymes.
Starch: (in plant, monomer: α glucose) starch is a mixture of two different polysaccharides- amylose (unbranched) and amylopectin (branched).
Glycogen: (in human and animals, monomer: α glucose) the structure of glycogen is similar to amylopectin (branched). Glycogen can be transferred to energy during physical exercise.


polysaccharides

 2)Structural compound: unlike starch and glycogen, cellulose and chitin cannot be digested by the enzymes in human body, thus they are used to build cell walls or hard exoskeleton.
Cellulose: (in plant, monomer: β glucose) the monomers in cellulose are held by β1-4 glycosidic linkage. Because cellulose is a straight chain polymer and neither coiled nor branched, also have the hydroxyl groups formed hydrogen bonds between parallel molecules, cellulose can produce micro fibrils to build cell walls.
Chitin: (in animals, monomer: N-acetylglucosamine) chitin can be found in the hard exoskeleton of insects or crustaceans, also in some medical applications.