term lipid comprises a diverse range of molecules
and to some extent is a catch all for relatively water-insoluble or nonpolar compounds of biological origin,
including waxes, fatty acids
(including essential fatty acids),
fatty-acid derived phospholipids, sphingolipids, glycolipids and terpenoids,
such as retinoids and steroids. Some lipids are
linear aliphatic molecules, while others have ring structures. Some are aromatic, while others are
not. Some are flexible; while others are rigid.Most lipids have some polar character in addition to being largely
nonpolar. Generally, the bulk of their structure is nonpolar or hydrophobic ("water-fearing"),
meaning that it does not interact well with polar solvents like water. Another
part of their structure is polar or hydrophilic
("water-loving") and will tend to associate with polar solvents like
water. This makes them amphiphilic molecules
(having both hydrophobic and hydrophilic portions). In the case of cholesterol, the polar group is a mere -OH (hydroxyl or alcohol).Lipids in food include the
oils of such grains as soybean, from animal fats, and are parts of many foods such as milk, cheese, and meat. They also act as vitamin carriers as well.
Proteins compose over 50% of the dry weight
of an average living cell and are very complex macromolecules. They also play a
fundamental role in the structure and function of cells. Comprised mainly of
carbon, hydrogen, oxygen, and some sulfur,
they also may contain iron, copper, phosphorus, or zinc. Proteins
are large organic compounds made of amino acids arranged in a linear chain and
joined together by peptide bonds between the carboxyl and amino
groups of adjacent amino acid residues.
Most proteins fold
into unique 3-dimensional structures. Mainly four different structure of
protein they are
- Primary structure: the amino acid
- Secondary structure: regularly repeating local structures
stabilized by hydrogen bonds. The most
common examples are the alpha helix
and beta sheet. Because secondary structures
are local, many regions of different secondary structure can be present in the
same protein molecule.
- Tertiary structure: the overall shape of a single protein
molecule, the spatial relationship of the secondary structures to one another.
Tertiary structure is generally stabilized by nonlocal interactions, most
commonly the formation of a hydrophobic core,
but also through salt bridges,
hydrogen bonds, and disulfide bonds.
- Quaternary structure: the shape or structure that results from the interaction of more than
one protein molecule, usually called protein subunits in this context,
which function as part of the larger assembly or protein complex.