26 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”function, as a protein that loses its native conformation loses its biolog-
ical activity (see Fig. 3.6). There are over 10,000 kinds of proteins in a
mammalian cell, and about half of these are enzymes. Enzymes are the
workhorse molecules. They perform a function called catalysis, which
speeds up a chemical reaction without itself being altered. All mole-
cules change from one form to another according to thermodynamics,
i.e., from one with a higher energy level to one with lower. Enzymes
accelerate this reaction by decreasing the energy barrier needed for the
change. In the living system chemical reactions take place as quickly as
milliseconds. Without enzymes these changes may take months or years,
too slow to cope with the immediate needs of life. Hence, almost all
biochemical reactions are aided by enzymes. These include the intake
of nutrients, digestion of food, harnessing of energy, DNA replication,
transcription, translation, and even the synthesis of enzymes themselves.
The storage of energy in the form of carbohydrate and fat, and the syn-
thesis of components of the physical structures of a cell are all the work
of enzymes.
Some proteins are important for signal transduction, i.e., for trans-
mitting a message from one part of the cell to another, such as from the
cell surface to the nucleus in a stepwise manner.^6 Other proteins are
regulators of genes. One type, called a transcription factor, activates a
gene and leads to the synthesis of a protein; the opposite type, called a
repressor, blocks the activation. Some proteins serve as cytoskeletons,
providing rigidity and motility to the cells. Others are constituents of the
physical framework of a cell, usually in combination with carbohydrates
and lipids. The different types of proteins are shown in Table 3.4.
In addition to the nucleus, there are other visible structures inside
a cell, collectively designated as the organelles. Mitochondria are the
power plants where carbohydrates are oxidized (called “respiration”)
with O 2 to make ATP (adenosine triphosphate), the energy currency of
a cell. The harnessing of energy from carbohydrates goes through four
stages. The first stage is the glycolytic pathway in the cytoplasm in which
a glucose molecule is turned into pyruvate. The second is the tricar-