Biophysicist
Biophysics is a quantitative approach, but it does not replace descriptive biology, and the two inform each other.
Biophysicist (or biological physicist) is a scientist who uses the concepts, mathematics, and experimental methods of physics to study biological systems. The field of study is biophysics. Biophysicists work at scales from single molecules to organelles, cells, tissues, organisms, and populations. They study questions such as how macromolecules (DNA, RNA, and proteins) fold and interact, how molecular machines generate force and movement, how ion channels conduct current, and how networks of interacting molecules give rise to behavior at the level of the cell. Biophysicists are trained in physics, chemistry, biology, mathematics, or engineering, and they work in universities, research institutes, hospitals, and the pharmaceutical and biotechnology industries.
Conceptual Framework: How Do Physical Laws Govern Living Systems?
Biophysics is a quantitative approach, but it does not replace descriptive biology, and the two inform each other. Biophysicists treat a cell as a physical and chemical system that obeys the laws of thermodynamics, statistical mechanics, fluid dynamics, electromagnetism, and, where relevant, quantum mechanics. Living systems are open and operate far from thermodynamic equilibrium, continually consuming energy (for example from ATP hydrolysis or ion gradients), so many processes are described by non-equilibrium thermodynamics, and not by equilibrium thermodynamics alone. At the same time, equilibrium concepts remain useful for many molecular problems, such as binding and folding of proteins in solution.
Thermal Fluctuations and Low-Reynolds-Number Hydrodynamics
At the scale of a cell, physical effects are different from those in everyday life. Molecules are subject to constant thermal motion (Brownian motion), so processes are stochastic. At low Reynolds number, viscous forces dominate over inertia, and swimming microorganisms and molecular motors move in a regime in which momentum is irrelevant. Typical thermal energy at body temperature is kBT ≈ 4.3 × 10−21 J (about 4.3 pN·nm, or 2.6 kJ/mol at 310 K), which is the unit in which many molecular energies and forces are measured.
Thermodynamics of Biomolecules
A central concept is the free energy landscape of a molecule. At constant temperature and pressure, the direction of a process is set by the change in Gibbs free energy:
where ΔH is the change in enthalpy, T is the absolute temperature, and ΔS is the change in entropy. A process is spontaneous if ΔG < 0. For a reaction, the equilibrium constant is related to the standard free energy change by:
For example, a binding reaction with _ΔG_° = −40 kJ/mol at 310 K has K = exp(40,000 / (8.314 × 310)) ≈ exp(15.5) ≈ 5 × 106, which corresponds to a dissociation constant of about 0.2 μM (in 1 M standard state units). In protein folding, the native state is typically more stable than the unfolded state by only about 20 to 60 kJ/mol, which is a small difference that results from large opposing contributions. The unfolded chain has a large conformational entropy, and the native state is stabilized by the hydrophobic effect, hydrogen bonds, van der Waals contacts, and electrostatic interactions. The folding of a protein is described by a funnel-shaped energy landscape, in which the many unfolded conformations are guided toward a smaller number of low-energy states. This explains how folding can occur on a timescale of microseconds to seconds, although the number of possible conformations is astronomically large (Levinthal's paradox). The native state of many proteins is a minimum of the free energy, but not for all. Some proteins are intrinsically disordered, some fold by interaction with chaperones, and some are kinetically trapped.
Experimental Methods and Subfields: How Do Biophysicists Measure Biomolecular Dynamics?
The tools of biophysics are chosen to measure properties at the scale of the problem.
| Biophysical Technique | Primary Physical Observable | Spatial / Force Resolution | Temporal Resolution | Representative Biological Application |
|---|---|---|---|---|
| Cryo-Electron Microscopy (Cryo-EM) | Electron scattering potential in vitreous ice | 1.2 – 3.5 Å | Static freeze-trapped states (ms plunge) | Atomic reconstruction of ribosomes and ion channels |
| NMR Spectroscopy | Nuclear spin relaxation and chemical shifts | 0.2 – 2.0 Å (atomic coordinates) | Picoseconds to seconds | Solution dynamics of 10–50 kDa globular proteins |
| Optical Tweezers | Momentum transfer from focused laser trap | 0.1 – 1.0 nm; 0.1 – 100 pN | ~10 μs – 1 ms | Single-molecule kinesin stepping and DNA unzipping |
| Single-Molecule FRET (smFRET) | Non-radiative dipole-dipole energy transfer | 2 – 10 nm (~0.2 nm precision) | ~1 ms (camera) to ns (photon counting) | Real-time conformational switching of ribozymes |
| Patch-Clamp Electrophysiology | Transmembrane ionic current across gigohm seal | Single ion channel (~1–100 pS) | ~10 – 100 μs | Gating kinetics of voltage-gated Na+ and K+ channels |
| All-Atom Molecular Dynamics (MD) | Classical Newtonian atomic trajectories | < 0.01 Å (numerical precision) | 1 – 4 fs integration step (up to ms total) | Ligand binding pathways and membrane permeation |
Structural Biophysics
The three-dimensional structures of biomolecules are determined at atomic or near-atomic resolution by several methods:
- X-ray crystallography obtains a diffraction pattern from a crystal of the molecule, and the pattern is converted to an electron density map. It is limited by the need for crystals, and the molecule is held in a crystal lattice.
- NMR spectroscopy determines structures of smaller proteins (traditionally up to about 30 to 50 kDa, with larger sizes with specialized labeling) in solution, and it reports on dynamics over a range of timescales.
- Cryo-electron microscopy (cryo-EM) images molecules that are frozen in a thin layer of vitreous ice and reconstructs the structure from many particle images. Since the "resolution revolution" of the 2010s, which came from direct electron detectors and improved software, it has given structures at resolutions better than 3 Å for many complexes, and it is used for large and flexible assemblies and for membrane proteins. Jacques Dubochet, Joachim Frank, and Richard Henderson received the 2017 Nobel Prize in Chemistry for its development.
- Small-angle X-ray scattering (SAXS) gives low-resolution information about the shape of molecules in solution, and cryo-electron tomography images structures inside cells.
These methods do not "overcome" the diffraction limit of light by a trick. They use radiation or electrons with wavelengths much shorter than visible light, and the resolution of each method is limited by other factors, such as the quality of the crystal, the signal-to-noise ratio, and radiation damage. Structures are the basis for the study of mechanism and for structure-based drug design. Machine learning methods for structure prediction, notably AlphaFold, now predict the structures of many proteins with high accuracy, and the experimental and the computational methods complement each other.
Single-Molecule Biophysics
Single-molecule methods observe individual molecules, which allows the measurement of distributions, rare states, and sequences of events that are hidden in the average of an ensemble.
- Optical tweezers use a tightly focused laser beam to trap a micrometer-sized bead that is attached to a molecule, and they apply and measure forces from about 0.1 to over 100 piconewtons with sub-nanometer resolution. They were used to observe the steps of kinesin along a microtubule (about 8 nm per step, with forces of the order of 5 to 7 pN), the stepping of myosin on actin, the unwinding of DNA by helicases, and the rotation of ATP synthase. Arthur Ashkin received a share of the 2018 Nobel Prize in Physics for optical tweezers.
- Magnetic tweezers apply forces and torques to molecules by magnetic fields, and they are widely used to study the supercoiling of DNA.
- Atomic force microscopy (AFM) scans a sharp tip over a surface to image the topography of molecules and cells, and it can also pull on molecules to measure the forces that unfold proteins (single-molecule force spectroscopy). High-speed AFM images molecules in action.
- Förster resonance energy transfer (FRET) at the single-molecule level (smFRET) measures the distance between two dyes in the range of about 2 to 10 nm, from the efficiency of energy transfer:
- Super-resolution fluorescence microscopy (such as STED, PALM, and STORM) overcomes the diffraction limit of light by using properties of fluorophores, and it was recognized with the 2014 Nobel Prize in Chemistry (Eric Betzig, Stefan Hell, and W. E. Moerner).
Membrane Biophysics and Electrophysiology
Membrane biophysics studies the physical properties of the lipid bilayer and the function of membrane proteins. Topics include the phase behavior of lipid mixtures, the formation of ordered domains (sometimes called "rafts", although their size and existence in living cells are debated), membrane elasticity and curvature, and the insertion and folding of membrane proteins. Membrane tension and bending energy are described by continuum models such as the Helfrich model.
Electrophysiology measures the electrical properties of cells and channels. In the patch clamp technique, developed by Erwin Neher and Bert Sakmann (Nobel Prize in Physiology or Medicine, 1991), a glass pipette is sealed against a patch of membrane, and the current through a single channel can be measured. Single-channel currents are of the order of picoamperes. For example, a channel with a conductance of 20 pS at a driving force of 100 mV passes 2 pA, which corresponds to about 12 million ions per second (2 × 10−12 C/s divided by 1.6 × 10−19 C per elementary charge). The resting potential of a cell and the equilibrium potential of an ion are described by the Nernst equation:
which at 37 °C for a monovalent cation is about 61.5 mV × log10([ion]out / [ion]in). The generation and propagation of action potentials in neurons was described by Alan Hodgkin and Andrew Huxley in a mathematical model (the Hodgkin-Huxley model, 1952), in which the membrane is an electrical circuit with a capacitance and voltage-dependent conductances for sodium and potassium:
where m, h, and n are gating variables. They received the Nobel Prize in Physiology or Medicine in 1963. The model was developed from voltage-clamp measurements of the squid giant axon, and it was proposed before the channels were known as molecules.
Molecular Motors and Mechanics
Molecular motors convert chemical energy into mechanical work. Kinesin and dynein move along microtubules, myosin moves along actin filaments, and the bacterial flagellum motor, ATP synthase, and the DNA and RNA polymerases are rotary or linear machines. The work done by a motor in one step is the product of the force and the distance, and it is compared with the energy from ATP hydrolysis, which is about 50 to 60 kJ/mol under cellular conditions (about 80 to 100 pN·nm per molecule). For kinesin, a force of 6 pN over 8 nm is about 48 pN·nm, which corresponds to an efficiency of about 50 percent, although the value depends on the load and the conditions. Models of motors, such as Brownian ratchets, describe how directed motion arises from thermal fluctuations that are rectified by energy input.
The mechanics of polymers, such as DNA and the cytoskeleton, is described with the worm-like chain model, in which the stiffness is characterized by the persistence length, which is about 50 nm for double-stranded DNA and about 17 μm for an actin filament. Cell mechanics, motility, and the forces of cells on their surroundings are studied in mechanobiology.
Computational and Theoretical Biophysics: How Do Simulations Reconstruct Atomic Trajectories?
Many events in biomolecules occur on timescales and length scales that are difficult to observe directly, and computation fills part of the gap.
All-Atom Molecular Dynamics and Quantum Mechanics/Molecular Mechanics
In molecular dynamics (MD), the positions of all atoms in a system are propagated by numerical integration of Newton's equations of motion:
where U is a potential energy function, the force field, which is parameterized from quantum chemical calculations and experimental data. The time step is about 1 to 4 fs, which is limited by the fastest motions (bond vibrations), so a simulation of a microsecond requires about 109 steps. Simulations on supercomputers and on special-purpose hardware (such as Anton) reach microseconds to milliseconds for systems of 105 to 106 atoms, and in some cases longer. Results are used to study the folding of small proteins, the binding of drugs, the opening of ion channels, and the interactions of proteins with membranes. Enhanced sampling methods (such as replica exchange, umbrella sampling, and metadynamics) improve the exploration of the energy landscape. Because classical force fields are approximate, simulations are validated against experiments. Where chemical bonds break or form, or electrons are important, quantum mechanics/molecular mechanics (QM/MM) methods treat the active site quantum mechanically, and the rest of the system classically.
Coarse-Grained Models and Statistical Polymer Physics
Coarse-grained models group several atoms into one particle, which reduces the number of degrees of freedom and allows longer times and larger systems, such as viral capsids, membranes, and chromatin, with the cost of lower detail. The statements that MD gives the "exact" trajectory are not correct, since it is limited by the force field, the sampling, and the time scale.
Statistical mechanics and polymer physics are also used in theoretical work, for example in models of the folding funnel, of the physics of chromatin and the nucleus, of liquid-liquid phase separation and biomolecular condensates, and of the physical limits to sensing and signaling, such as the limit on the precision with which a cell can measure the concentration of a ligand, which is set by the random arrival of molecules at receptors (the Berg-Purcell limit).
Systems and Network Biophysics: How Do Molecular Interactions Generate Cellular Behavior?
Systems biophysics studies how interactions among many molecules produce behavior at the scale of the cell.
Deterministic and Stochastic Network Architectures
Researchers build deterministic models (systems of ordinary differential equations, based on mass-action kinetics) and stochastic models (such as the Gillespie algorithm for chemical master equations) of gene regulatory networks, signaling pathways, and metabolism. Stochastic models are needed because many molecules are present in few copies per cell, so that noise in gene expression is large and affects cell fate. Examples are:
- Gene circuits and switches. The bistable genetic toggle switch and the repressilator oscillator in bacteria were constructed in the 2000s as test cases of the theory, and they helped to start synthetic biology.
- Oscillators. The circadian clock is a gene regulatory circuit with delayed negative feedback that oscillates with a period of about 24 hours, and the cell cycle is controlled by biochemical oscillations that involve the cyclin-dependent kinases.
- Chemotaxis. Bacterial chemotaxis shows perfect adaptation through integral feedback, which was analyzed with control theory.
- Quorum sensing and pattern formation. Reaction-diffusion systems, such as those proposed by Alan Turing in 1952, are used to model patterns in development.
The approach links biophysics to systems biology, and the two fields overlap, so that the boundary is not sharp.
Specialized Subdisciplines: How Does Biophysics Extend to Bioenergetics, Neuroscience, and Medicine?
Bioenergetics, Sensory Physics, and Clinical Biophysics
- Bioenergetics and photobiology. The physics of photosynthesis, including energy transfer in light-harvesting complexes and electron transfer, and of the respiratory chain and ATP synthesis (chemiosmosis).
- Neurophysics and sensory biophysics. The physics of vision, hearing, and olfaction, including the sensitivity of the eye to single photons and the mechanics of hair cells.
- Biological imaging and medical physics. Methods such as magnetic resonance imaging, ultrasound, and optical coherence tomography were developed with physical principles and are used in medicine.
- Quantum biology. Quantum effects such as coherence in photosynthetic energy transfer, tunneling in enzyme catalysis and electron transfer, and the proposed radical-pair mechanism in magnetoreception are studied. For many of these, the role of quantum effects in the function of the system in the cell is not settled.
- Biophysics of disease. The mechanisms of protein misfolding and aggregation in diseases such as Alzheimer's disease, and the mechanics of cancer cells.
Education, History, and the Profession: How Did Biophysics Evolve as a Scientific Discipline?
Historical Milestones and Academic Training
The field has roots in the nineteenth century, with the work of Hermann von Helmholtz on nerve conduction and vision and of Luigi Galvani on electricity in tissues. In the twentieth century, the discovery of the structure of DNA by Rosalind Franklin, Maurice Wilkins, James Watson, and Francis Crick with X-ray diffraction, the structures of hemoglobin and myoglobin by Max Perutz and John Kendrew, Erwin Schrödinger's book What Is Life? (1944), which influenced many physicists, and the work of Max Delbrück on phages brought physicists into biology. The Biophysical Society was founded in 1957. Many university programs in biophysics, in departments of physics, chemistry, biochemistry, or bioengineering, offer degrees at the undergraduate and graduate level. Training emphasizes mathematics, physical chemistry, and the experimental methods above. Biophysicists are employed in academic research, in the development of drugs and instruments, and in the analysis of data.
Frequently Asked Scientific Questions
How does a biophysicist differ from a biochemist or molecular biologist?
While biochemists focus primarily on chemical reactions, metabolic pathways, and covalent modifications, and molecular biologists investigate genetic information flow (DNA replication, transcription, and translation), a biophysicist quantifies the underlying physical forces, energetics, and mechanical dynamics of these processes. Biophysicists measure piconewton forces, femtosecond-to-millisecond conformational kinetics, and free-energy landscapes using physical instrumentation and statistical mechanics.
Why is thermal energy (kBT) the fundamental energy scale for biophysicists?
At physiological temperature (310 K), the thermal energy unit kBT equals approximately 4.3 × 10−21 J (4.3 pN·nm or 2.6 kJ/mol). Because non-covalent interactions such as hydrogen bonds (4–20 kBT) and ATP hydrolysis (~20–25 kBT) occur on this exact scale, biomolecular machines operate amidst intense stochastic Brownian collisions rather than deterministic macroscopic trajectories.
How do optical tweezers measure the force of a single motor protein?
Optical tweezers focus a near-infrared laser beam through a high-numerical-aperture objective to create a three-dimensional radiation-pressure trap capable of holding a micrometer-sized dielectric bead attached to a single motor protein like kinesin. As the motor steps along a microtubule, it pulls the bead from the trap center; a quadrant photodiode measures nanometer displacements, converting deflection into piconewton force via Hooke's law (F = −_κx_).