these concepts [ 29 – 32 ]. The basis of this discussion is the observation that a living
cell consists of about 70 % water. The rest of the cell is composed mostly of
macromolecules that are composed of thousands of atoms. First, Sect. 1.6.1
describes the basic macromolecules, which are the chemical building blocks of the
various components of a cell. Next Sect.1.6.2identifies the various components
that make up a cell and describes their different functions depending on where they
are found in a body. Then Sect.1.6.3describes the major categories and functions
of biological tissues, which are ensembles of similar cells.
1.6.1 Macromolecules.
Macromoleculesare large molecules that are made up of selections from a set of smaller
molecular chains such as monomers, glycerin, and fatty acids. The four basic groups of
macromolecules found in living organisms are proteins, carbohydrates, nucleic acids,
and lipids. Proteins, carbohydrates, and nucleic acids are known aspolymersbecause
they are made up of many monomers. Monomers include amino acids for forming
proteins, nucleotides that form nucleic acids, and simple sugars such as glucose and
fructose that are the building blocks for carbohydrates. Lipids are not polymers.
After water, proteins make up the second largest component in human cells,
tissues, and muscles.Proteinsconsist of one or more long chains of interconnected
monomers called amino acids.Amino acidsare small organic molecules composed
of amine (–NH 2 ) and carboxylic acid (–COOH)functional groups(groups of atoms
or bonds that are responsible for the characteristic chemical reactions of a molecule)
plusmolecular side chains(called R groups) that are specific to each amino acid.
The basic elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen.
Amino acids are connected by covalent bonds (referred to aspeptide bonds) to form
chains of molecules calledpolypeptide chains. A protein consists of one or more of
these polypeptide chains that are twisted, wound, and folded upon themselves to
form a macromolecule. Although about 500 amino acids are known, there are only
20 naturally occurring amino acids.
Proteins are an essential substance of living organisms and participate in almost
every process within cells. Common types of proteins include the following:
- Enzymesfor catalyzing chemical reactions that are vital to metabolism
- Structural proteins(such as keratin found in hair andfingernails and collagen
found in skin) that provide support for maintaining cell shape. Note thatcol-
lagenis the most abundant protein in the human body and is found in the bones,
muscles, skin and tendons. - Antibodiesthat bind to foreign particles or cells and assist in their elimination
from the body - Transport proteinsthat carry compounds or chemicals from one location to
another throughout living systems. An example of a transport protein ishe-
moglobinthat carries oxygen through the blood.
1.6 Structures of Biological Cells and Tissues 15