What is Biophysics?
Biophysics is that branch of knowledge that applies the
principles of physics and chemistry and the methods of
mathematical analysis and computer modeling to
understand how biological systems work.
Biophysics is a molecular science. It seeks to explain
biological function in terms of the molecular structures and
properties of specific molecules. The size of these
molecules varies dramatically, from small fatty acids and
sugars (~1 nm = 10-9 m), to macromolecules like proteins
(5-10 nm), starches (>1000 nm), and the enormously
elongated DNA molecules (over 10,000,000 nm = 1 cm
long but only 2 nm wide; imagine a piece of string 45 miles
long!).
These molecules, the sole building blocks of living
organisms, assemble into cells, tissues, and whole
organisms by forming complex individual structures with
dimensions of 10, 100, 1000, 10,000 nm and larger.
Proteins assemble into the casein micelles of milk, which
aggregate to form the curd of cheese; proteins and
ribonucleic acids assemble into ribosomes, the machinery
for building proteins; lipids and proteins assemble into cell
membranes, the external barriers and internal surfaces of
cells; proteins and DNA wind up into chromosomes, the
carriers of the genetic code; and so on.
Consequently, much effort in biophysics is directed to
determining the structure of specific biological molecules
and of the larger structures into which they assemble.
Some of this effort involves inventing new methods and in
building new instruments for viewing these structures.
Many of the exciting new developments in biological
microscopy, described here under Resources in
Biophysics, are part of this effort.
What Does Biophysics Study?
The biological questions with which biophysics is
concerned are as diverse as the organisms of biology:
• How do linear polymers of only 20 different amino
acids fold into proteins with precise threedimensional
structures and specific biological
functions?
• How does a single enormously long DNA molecule
untwist and exactly replicate itself during cell
division or direct the production of proteins?
• How are sound waves, or photons, or odors, or
flavors, or touches, detected by a sense organ and
converted into electrical impulses that provide the
brain with information about the external world?
• How does a muscle cell convert the chemical
energy of ATP hydrolysis into mechanical force and
movement?
• How does the cell membrane, a lipid barrier
impermeable to water-soluble molecules,
selectively transport such molecules through its
non-polar interior?
Biophysics seeks to answer these questions using an
eclectic approach. The specific molecules involved in a
biological process are identified using the techniques of
chemical and biochemical analysis. Their molecular
structures and interactions are determined using the
spectroscopic techniques of physics and chemistry. And
the relationship between biological function and molecular
structure is investigated using highly precise and
exquisitely sensitive physical instruments and techniques
that are able to monitor the properties or the movement of
specific groups of molecules, or in exciting new
developments, are able to view and manipulate single
molecules and to measure their behavior.
Biophysics explains biological functions in terms of
molecular mechanisms: precise physical descriptions of
how individual molecules work together like tiny machines
to produce specific biological functions. Some of these
biophysical mechanisms, many involving detailed
molecular models, are described in detail under Resources
in Biophysics.
Selected Topics in Biophysics:
Biophysics is a diverse and eclectic field, and consequently
difficult to categorize. For the purposes of this summary of
educational resources in biophysics, biophysics is divided
into three parts or topic areas: molecular structures,
biophysical techniques, and biological mechanisms. Each
topic area is defined here and an attempt is made to
indicate how these areas are interrelated within the field of
biophysics. An annotated list of specific resources,
available as text files or web sites, for each topic area is
then provided on subsequent pages.
Molecular Structures:
Biophysics explains the biological functions of cells,
tissues, and organisms in terms of the structure and
behavior of biological molecules. Genes, the basic
elements of biological information, reflect the molecular
structures of the enormously large, linear DNA
(deoxyribonucleic acid) molecules of which they are made.
The behavior of enzymes, hormones, and antibodies
reflects the molecular structures of proteins and the organic
chemistry of the functional groups of the amino acid side
chains.& The surface and barrier properties of biological
membranes reflect the ability of lipids to aggregate into
flexible two-dimensional bilayers with hydrophobic cores
and polar surfaces.
Information about the molecular structures and biophysical
properties of proteins, nucleic acids (DNA and RNA), lipids,
and carbohydrates is available on the Select Topics in
Biophysics page.
Biophysical Techniques:
The characterization of molecular structure, the
measurement of molecular properties, and the observation
of molecular behavior presents an enormous challenge for
biological scientists. A wide range of biophysical
techniques have been developed to study molecules in
crystals, in solution, in cells, and in organisms. These
biophysical techniques provide information about the
electronic structure, size, shape, dynamics, polarity, and
modes of interaction of biological molecules. Some of the
most exciting techniques provide images of cells,
subcellular structures, and even individual molecules. It is
now possible, for example, to directly observe the biological
behavior and physical properties of single protein or DNA
molecules within a living cell and determine how the
behavior of the single molecule influences the biological
function of the organism.
Information about the wide variety biophysical techniques
available to study the structures, properties, and functions
of molecules both in the test tube and in living biological
systems is available on our Select Topics in Biophysics
page.
Biophysical Mechanisms:
Much of the scientific success of biophysics depends upon
its ability to develop detailed physical mechanisms to
explain specific biological processes. The double helical
structure of DNA, for example, provides a framework for an
explanation of how genetic material is replicated and of
how genetic mutations arise: specific proteins mediate the
unwinding of the DNA duplex and the assembly of a new
strand based on complementary base pairing of the four
DNA bases, guanine with cytosine and adenine with
thymine; mismatch of one of these base pairs generates a
complementary strand with a single base substitution (a
mutation). The value of this, and a variety of other
biophysical mechanisms, is unlimited for human knowledge
in general and for biomedical research in particular.
Molecular descriptions of a variety biological functions
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