24 
Proteins, Workhorses Proteins are intensely 
of the Cell studied by biomedical 
researchers because 
these molecules are 
involved in nearly every 
biological function. 
Proteins include the 
enzymes that allow the 
chemical reactions 
necessary to life to take 
place efficiently. They 
also include many of the 
hormones that regulate 
growth and development. 
They are important 
components of the cell’s 
physical structure, 
making up half of the 
cell’s dry weight. They 
help transmit messages 
from nerve cells and 
work in muscle cells to 
convert chemical energy 
into mechanical energy, 
which permits movement. 
Proteins in the cell 
membrane control 
molecules entering and 
leaving the cell. 
Hemoglobin proteins in 
red blood cells transport 
oxygen through the 
bloodstream, and 
antibody proteins fight 
infection. 
Proteins are made of 
strings of amino acids 
ordered according to 
instructions contained in 
the DNA. A protein’s 
primary structure is this 
linear chain of amino 
acids; however, almost 
immediately after it is 
created the chain springs 
into helices, sheets, or 
other shapes that form 
the protein’s secondary 
structure. The shapes 
then fold and coil fur- 
ther into a complex, 
three-dimensional 
structure. This is the 
active form of the protein 
that can bind to, and 
interact with, other 
molecules. 
The many proteins that 
are enzymes act as 
catalysts, speeding up 
reactions without being 
permanently altered 
themselves. Without 
enzymes, many 
