● `color{violet}("Amino acids")` are `color{violet}("organic compounds")` containing an `color{violet}("amino group")` and an acidic group as substituents on the same carbon i.e., the `color{brown}("α-carbon.")`

● Hence, they are called `color{brown}("α-amino acids.")`

● They are substituted `color{violet}("methanes.")`

● There are `color{brown}("four substituent groups")` occupying the `color{violet}("four valency positions.")`

● These are `color{violet}("hydrogen, carboxyl group, amino group")` and a variable group designated as `color{violet}("R group. ")`

● Based on the nature of `color{violet}("R group")` there are many `color{violet}("amino acids.")`

● However, those which occur in proteins are only of twenty one types.

● The `color{brown}("R group")` in these `color{violet}("proteinaceous amino acids")` could be a `color{violet}("hydrogen")` (the amino acid is called glycine), a `color{violet}("methyl group")` (alanine), `color{violet}("hydroxyl methyl")` (serine), etc.

● The chemical and physical properties of `color{violet}("amino acids")` are essentially of the `color{violet}("amino, carboxyl")` and the R functional groups.

● Based on number of `color{violet}("amino and carboxyl groups,")` there are `color{violet}("acidic")` (e.g., glutamic acid), `color{violet}("basic")` (lysine) and `color{violet}("neutral")` (valine) `color{violet}("amino acids.")`

● Similarly, there are `color{violet}("aromatic amino acids")` (tyrosine, phenylalanine, tryptophan).

● A particular property of `color{violet}("amino acids")` is the ionizable nature of `color{violet}(–NH_2)` and `color{violet}(–COOH)` groups.

● Hence in solutions of different pHs, the structure of `color{violet}("amino acids changes.")`

● B is called `color{brown}("zwitterionic form.")`

● `color{violet}("Lipids")` are generally `color{brown}("water insoluble.")`

● They could be `color{violet}("simple fatty acids.")`

● A `color{brown}("fatty acid")` has a `color{violet}("carboxyl group")` attached to an `color{violet}("R group. ")`

● The `color{violet}("R group")` could be a `color{violet}("methyl" (–CH_3),)` or `color{violet}(("ethyl" (–C_2H_5))` or higher number of `color{violet}(–CH_2 )`groups (1 carbon to 19 carbons).

● For example, `color{brown}("palmitic acid")` has 16 carbons including `color{violet}("carboxyl carbon. ")`

● `color{brown}("Arachidonic acid")` has 20 carbon atoms including the `color{violet}("carboxyl carbon.")`

● `color{violet}("Fatty acids")` could be saturated (without double bond) or unsaturated (with one or more `C=C` double
bonds).

● Another `color{violet}("simple lipid is glycerol")` which is `color{brown}("trihydroxy propane.")`

● Many `color{violet}("lipids")` have both `color{violet}("glycerol")` and `color{violet}("fatty acids")`.

● Here the `color{violet}("fatty acids")` are found esterified with `color{violet}("glycerol.")`

● They can be then `color{brown}("monoglycerides, diglycerides")` and `color{brown}("triglycerides. ")`

● These are also called `color{brown}("fats")` and `color{brown}("oils")` based on melting point.

● `color{violet}("Oils")` have lower melting point (e.g., gingely oil) and hence remain asoil in winters.

● Some `color{violet}("lipids have phosphorous")` and a `color{violet}("phosphorylated organic compound")` in them.

● These are `color{violet}("phospholipids.")`

● They are found in `color{violet}("cell membrane.")`

● `color{brown}("Lecithin")` is one example.

● Some tissues especially the `color{violet}("neural tissues")` have lipids with more complex structures.

● `color{violet}("Living organisms")` have a number of carbon compounds in which`color{violet}(" heterocyclic rings")` can be found.

● Some of these are nitrogen bases – `color{brown}("adenine, guanine, cytosine, uracil")`, and `color{brown}("thymine. ")`

● When found attached to a sugar, they are called `color{brown}("nucleosides.")`

● If a `color{violet}("phosphate group")` is also found esterified to the sugar they are called `color{brown}("nucleotides. ")`

● `color{violet}("Adenosine, guanosine, thymidine, uridine")` and `color{violet}("cytidine")` are `color{violet}("nucleosides.")`

● Adenylic acid, thymidylicacid, guanylic acid, uridylic acid and cytidylic acid are nucleotides.

● `color{violet}("Nucleic acids")` like `color{brown}("DNA ")`and `color{brown}("RNA")` consist of nucleotides only.

● `color{violet}("DNA and RNA")` function as genetic material.