What is biophysics?

Science and Profession

A young boy skips a stone across a still pond, and a startled frog jumps into the water. Physics studies nature in the arc of the stone, the rippling of the water, the sound of the splash, and the surprising motions of the atoms and molecules within the stone and the water of the pond. Biology, on the other hand, studies the boy and the frog—their cells, nerves, muscles, and senses—which are very different from the dead mass of the stone and the still water of the pond. Despite their immense differences, the boy, the frog, the stone, and the pond have the same atoms and obey the same basic laws of nature. Biophysics enters, for example, when the boy hears the sound of the splash, one of many meeting grounds between biology and physics as they merge into one knowledge. Since biological systems are chemical and mathematics is the language of physics, biophysics has significant overlap with biochemistry and biomathematics.

Biology is the scientific investigation of the laws of life. In particular, biology studies both the structure and the function of cells and organisms such as viruses, bacteria, plants, and animals, including their communities. It studies the means by which life nourishes and maintains itself and by which it perpetuates itself by genetic transmission, reproduction, and evolution. Medical science applies this knowledge in the service of humankind. Physics is the scientific investigation of the laws of nature. Physics has two major divisions, experimental and theoretical physics. Instruments are the tools of the experimental physicist, and mathematics is the tool of the theoretical physicist. Distinct from biology, physics restricts its investigations to inanimate objects. At the human level, nature appears as matter and waves; physics studies the properties of both and their interactions. For the arena of biophysics, the most useful branches of physics are atomic, energetic, fluid, and electromagnetic physics.

Broadly taken, atomic physics studies atoms and their nuclei and molecules (which are isolated atom groups) and their formation into solids, liquids, and gases. Atoms consist of electrons circulating around a tiny nucleus of protons and neutrons. Electrons have negative charges and give the atoms their distinctive shapes and chemical, biological, and medical properties. Protons and neutrons are similar except that protons have a positive charge and neutrons have no charge. The protons hold the electrons within the atoms by electrical forces, while nuclear forces bind protons and neutrons within the nucleus. The exchange of electrons between and among atoms determines the chemical properties of materials, including biological and medical materials.

Most atoms are neutral, with an equal number of electrons and protons. Hydrogen, with one electron and one proton, is the smallest atom and a common biological constituent, along with carbon, nitrogen, and oxygen, which possess six, seven, and eight electrons, respectively. When an electron is missing from an otherwise neutral atom, the atom becomes a positive ion. Adding an electron converts a neutral atom to a negative ion. Ions are abundant in biological fluids. The hydrogen positive ion, along with calcium, potassium, and sodium positive ions and the chlorine negative ion, are important biological atomic ions. In most electronic devices, electrons produce electrical activity, but in living organisms and humans, ions govern this activity.

Since atomic nuclei do not play a significant role within living organisms, the smallest matter particles of biological importance are atoms. Groups of atoms make biological molecules, deoxyribonucleic acid (DNA), carbon dioxide, water, and bones, for example. Some atomic nuclei are unstable and release energetic particles and waves. These radioactive products are usually damaging to cells and their DNA, but under controlled conditions, they are useful, for example, as radioactive tracers.

Energetic physics studies the basic forces of nature. The electromagnetic force, together with gravity and the nuclear forces, is one of the known fundamental forces of nature. In biological materials, other forces, such as the osmotic force, are complex manifestations of the electrical force. These forces allow matter to interact—to hold, pull, and push other matter and to exchange energy and momentum. While the prime biological interaction among matter is electrical, the major interaction between matter and waves is electromagnetic. An example is the absorption of the electromagnetic wave—light—by the eye to form visual images.

Fluids are groups of atoms or molecules that move easily; included in this definition are both liquids and gases. Biological fluids are important for the transportation of materials across cell membranes, for blood circulation, and for respiration. Fluid physics investigates fluid motions under the influence of various forces. Confined fluids develop pressures as a result of the forces between the fluid particles. Under a pressure difference, fluids flow toward the lower pressure. Thus, blood flows because of the blood pressure generated by the heart and arteries.

Electromagnetic physics studies electrical, magnetic, and electromagnetic fields in detail. Charge motion occurs when an electrical voltage acts across conducting materials, whether in an electronic device or in a biological system. In the body, biochemical activity generates voltages across nerve cell membranes, allowing the nerves to serve as the body’s electronic network.

Oscillating electric charges produce electromagnetic waves. Thus nuclei produce gamma rays and atoms generate x-rays and ultraviolet, visible, and infrared waves; electronic devices generate a variety of microwave and radio waves. Individual electromagnetic waves appear as packets, called photons, which carry both energy and momentum. Photon energy and momentum decrease dramatically in going from gamma rays to radio waves. Gamma rays and x-rays carry high energy and momentum and are very destructive if encountered by molecules within cells. Both of these radiations easily penetrate soft tissues, so that their damage may permeate an entire organism. (Low x-ray dosages, however, give relatively safe images of the body’s structure.) Even ultraviolet rays carry sufficient energy to damage biological organisms. Since ultraviolet rays are not very penetrating, they mainly damage skin cells.

Visible photons carry enough energy to be useful to life as it has evolved on the earth and not so much as to be damaging. Infrared photons produce heat. Individual microwave and radio photons have essentially no biological effects, but both can cause damage if the total energy that they carry creates excess heat, as with a defective microwave oven, or if the electric or magnetic fields in the waves produce undesirable biological effects. The level at which such effects occur has not been clearly established.

Senses are the means by which organisms know their surroundings. Light (an electromagnetic wave) and sound (a matter wave in air) are two physical stimuli. Vision and hearing are the biological responses to these stimuli. Biophysics of the senses studies vision, hearing, and other senses, including the orientational, chemical, somatic, and visceral senses.

Color vision is an extraordinary phenomenon. The human eye evolved under direct and reflected sunlight. Light absorbed differently by three color pigments in the retina signals to the brain the colors of an illuminated scene. Lack of one or two of the pigments produces different forms of color blindness. Normal humans can distinguish roughly twenty thousand different colors. The response of the eye peaks at yellow green, where sunlight has its maximum energy at the earth’s surface. Matching detector response to source output is characteristic of any efficient electronic detector. Indeed, evolution has made the eye so efficient that a dark-adapted eye can respond to perhaps only one visible photon. This superb detector is at the limit allowed by the laws of physics.

The ear is another exquisite sense organ fashioned by evolution. Hearing picks up sound waves. Speech is one prime source for sound, so human hearing matches the human vocal range. Although human hearing extends over a very wide range, from about twenty to twenty thousand cycles per second, it is so precise that the ear can tune in to single tones. A possible explanation for this paradoxical behavior is that evolution has shaped the ear as a mechanical traveling wave amplifier.

Perceived sensations move from a sense organ, such as the eye, by electrical signals conducted over the nervous system to the animal brain. The response to these signals triggers other nerve impulses to the muscles, which contract and move the animal, such as a frog jumping into a pond. This is the arena of electrical biophysics, which investigates the effects of electrical and magnetic fields in living organisms.

Nerve impulses are electrical signals within the nerve. A stimulus at the end of a nerve initiates chemical changes that produce the electrical motion of ions. Tunnel-like proteins on the membrane surface channel the ions across the membrane. The resulting local change in charge propagates along the length of the nerve; in this way, for example, sound in the ear sends signals to the brain. This electrical activity produces low-frequency electrical waves that can be detected in various parts of the body, such as by brain wave monitors and electrocardiograms (EKGs or ECGs).

Typical nerve voltages occur in pulses somewhat smaller than one hundredth of a volt, lasting several thousandths to hundredths of a second. These pulses involve the conduction of sodium and potassium positive ions across the membrane through the protein ion channels. The result is ionic communication in the nervous system. The human brain is part of that system and generates electrical waves with frequencies of about 0.5 to 50 cycles per second, with voltages of hundredths to tenths of a volt when picked up by external electrodes attached to the scalp.

Electrocardiograms pick up electrical activity in the heart. The beating heart displays time traces with narrow spikes of uniform height. These spikes represent electrical signals that trigger the heart muscle to contract at a continuous, seemingly rhythmic beat of about once a second. The electrical stimulus, however, is decidedly not rhythmic but is instead a staccato beating. A mathematical technique called Fourier analysis shows that such spikes have a wide range of rhythmic frequencies, from zero to about ten cycles per second.

Danger would ensue if the electrical activity of the heart became a pure rhythm. If this happens, the frequency range collapses to around six cycles per second, too fast for the heart to follow. Deadly ventricular fibrillations may follow. The heart beats in shallow, spasmodic pulses, and sudden cardiac arrest results. Here medical physics saves lives in the form of heart pacemakers and defibrillators. A pacemaker delivers mild electrical current to speed up a chronically slow heart rate, while a defibrillator provides a sharp electrical jolt to restore the normal heartbeat when fibrillation threatens cardiac arrest. Without treatment, patients identified as candidates for sudden cardiac arrest have only a 60 percent change of living a full year. The odds rise to 90 percent, however, for those who receive jolts from a defibrillator.

The latest generation of defibrillators is quite sophisticated. They can coax ventricular tachycardia back to a slower, normal beat by delivering mild electrical currents but also deliver ever-stronger stimulation and, if needed, a sharp jolt to prevent cardiac arrest. Some devices deliver a positive pulse immediately followed by a negative one. This requires less energy from the power source and produces less tissue damage in the patient.

Diagnostic and Treatment Techniques

Perhaps the most obvious example of the influence of physics upon biology and medicine is in instruments. Physicists and engineers continually fashion new instruments based on novel developments in physics, and many of these find important applications in biology and medicine. This area of biophysics changes continually. As the complexity of instrumentation is reduced to the routine, the necessity for the involvement of physicists disappears. Biology and medicine take over the new tool.

The optical microscope is an example of a valuable instrument taken into biology and medicine. Edmund Beecher Wilson (1856–1939) used a microscope to draw the first primitive pictures of the cell in 1922. Only six cell constituents were clearly shown. Today, advanced instruments such as the electron microscope have provided a more detailed picture of the cell, with its dozens of specialized structures. Microscopes have long been a staple of medicine and are now supplemented by fiberoptic technology. Thin fibers guide light inside the patient’s body and allow a physician to heal lesions and diseased sections with lasers.

X-ray analysis is another valuable tool of the biophysicist. X-rays have wavelengths that match the distances between atoms in molecules. Thus, molecules produce distinctive x-ray patterns. Computational analysis allows a scientist to determine molecular structure from these patterns. In the early 1950s, scientists including Rosalind Franklin, James D. Watson, and Francis Crick used x-ray analysis to determine the double helix structure of DNA, allowing the genetic code to be cracked.

Sophisticated computer analysis of a patient’s three-dimensional x-ray patterns provides startling and accurate imaging of the body’s interior without intrusion. Magnetic resonance imaging (MRI) provides complementary three-dimensional internal images, using microwave resonance in a high magnetic field produced by superconducting magnets. MRI produces images by picking up radio signals from the hydrogen atoms that permeate all body tissues. The spinning hydrogen nuclei are aligned by the strong magnetic field. A pulse of radio waves disorients the nuclei, which emit a distinctive radio signal as they reorient to the magnetic field. In comparison, ultrasonography is a surprisingly simple tool, creating images with high-frequency sound.

The use of lasers in medicine has become widespread. A medical laser is created by choosing a suitable laser wavelength to offer desirable penetration and absorption within the human tissue involved in a given procedure, along with effective beam delivery and tissue removal systems. A partial list of medical laser applications includes retinal attachment, corneal alterations to adjust vision, dental drilling, the removal of surface lesions and stains on the skin, pulsed lithotripsy to break kidney stones into fragments, laser angioplasty to repair and unclog blood vessels, gynecological surgery, and bloodless incision for all types of procedures. Of importance to the physician are ease of operation of the laser equipment, reliability, reasonable cost, and, above all, effectiveness.

Body tissues are mainly composed of water. The water molecule absorbs strongly in the infrared spectrum and is transparent to the visible spectrum. Hemoglobin (in the blood) and melanin (in the skin) play important roles in laser-tissue interactions. Both absorb strongly, but differently, in the visible and near infrared ranges. The bulk of medical procedures that use lasers rely on rapid, selective heating of the target body tissue. For example, bloodless laser surgery requires rapid heating and vaporization of body tissue in the cut. The pulse duration is adjusted so that a thin layer of nearby tissue is heated to cause coagulation and stop bleeding.

An exciting, but experimental, cancer treatment that involves lasers is photodynamic therapy. Safe, light-sensitive dyes, such as the porphyrins, are injected into animals that have tumors. The dyes are absorbed by the tumors, and the tumor is exposed to intense laser light. The dyes alter to a toxic form, and tumor destruction is produced.

Lasers also make possible many research activities. One extraordinary application is the use of the laser’s intense beam to act as optical tweezers. The beam can capture one living cell for study and can move organelles within the cell.

A final application is within molecular biophysics, which deals with the molecular constituents of living cells. Here biophysics applies quantum physics to determine the physical structure and biological behavior of the molecules that make up the human body.

Individually, all the molecules studied are inanimate and dead. With the aid of modern computers, physics accurately describes the behavior of the smaller cell constituents. In principle, quantum theory appears to be capable of describing the most complex molecules, including DNA, the basis of the genetic code. This code instructs the assembly of amino acids and proteins, and therefore the structure of an organism. In this task, quantum theory appears to be limited only by computational complexity.

When these dead molecules assemble as a cell, life begins. Viruses are such an assembly, but they inhabit the border region between large, inanimate molecules and the smallest living cells. The Escherichia coli (E. coli) virus is about a hundred atoms across and contains approximately one hundred thousand atoms. With the virus, dead physics meets live biology and medicine has its most elementary protagonist. Here all the sciences are challenged with the unanswered question of how life arises and survives.

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