Biophysics is a discipline at the interface of physics and biology where physical concepts and tools for observing and modelling physics are applied to biological phenomena.
Several fields of biology in its broadest sense have benefited from the advances made by biophysics. Ecology, species evolution, development, medicine, cell biology or molecular biology are some examples of the application of biophysical understanding.
An approach inherited from physics is used to:
- produce internal body images: MRI, radiography, treatment, detection of cancerous tumours: radiotherapy, positron emission tomography;
- highlight the structure of constituent elements of living organisms: DNA or proteins;
- to measure and manipulate more and more precisely the constituent elements of life. For example, it is possible to use optical clamps to move organelles or to unroll the double helix of DNA by measuring the applied force.
Modern biophysics can be divided into a few categories: medical biophysics1 (imaging, radiation, detection, optics), molecular biophysics (protein structure, protein-protein interactions, 3D structure of DNA), cellular biophysics2 (cell and component mechanics, modeling of genetic signaling networks), tissue biophysics3 (organ growth processes, biomechanics, collective migration phenomena) and environmental and population biophysics (biosphere environment components, evolution theory).
Physiologists, who were the first biophysicists, later demonstrated that the laws of physics are necessary and sufficient to explain living things. In the mid-19th century, a multidisciplinary school was set up in Berlin, based on figures such as Johannes Müller and Hermann von Helmholtz, and explored in particular the role of electrical currents in nervous processes, or physiological optics. At the beginning of the 20th century, Darcy Thompson published his magnum opus, Form and Growth, in which he showed how complex processes of embryonic form development can be explained by simple physical and mathematical principles, inspired for example by foam physics. During the 20th century, the theory of vitalism fell into disuse, and biophysics had as its general goal the characterization of living organisms by means of physical and chemical techniques. After the Second World War, several researchers, particularly at the University of Cambridge, revolutionized biophysics, using X-ray crystallography, for example, to discover the structure of DNA (James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin, the first three winning the Nobel Prize in Physiology or Medicine in 1962 for this discovery), electrophysiology to discover the propagation of the potential for action in nerves (Alan Lloyd Hodgkin and Andrew Huxley, winning the Nobel Prize in Physiology or Medicine in 1963), or the role of chemical processes in pattern formation in the embryo5 (Alan Turing in 1952).
Biophysics aims to explain biological phenomena by the same laws that apply to the rest of the world. In this respect, it is the direct heir to the physiology of the early 20th century. As with many other complex systems (plasmas, superconductors, etc.), biophysicists seek to develop theories adapted to the phenomena typical of the living world. In many cases, such theories highlight certain common points between observations that are very different in principle, and open up new perspectives. Living organisms happen to be part of the most complex and diverse physical systems available for our observation. However, there is a remarkable unity at the cellular level, already demonstrated by the first observations of cells under the microscope (Schleiden 1838, Schwann 1840, Virchow 1855). One of the main examples of universality in the physical and mathematical description of biological processes is the reaction-diffusion theory developed by Turing in 1952 to explain the ex nihilo formation of patterns such as stripes or peas in the fur of animals during their development. This theory, which is still the subject of intense research in developmental biology, is also applicable to describe chemical, ecological or geological processes.
The progressive discovery of the unity of the physical processes involved in all living cells has been an important driver for the development of biophysics. Physicists seek to explain most of the observations by proposing synthetic theories. The most important successes are obtained when several observations in different contexts, in different organisms, are linked to the same physical explanation.
Observation techniques developed mainly through advances in physics:
- nuclear magnetic resonance (NMR), which solves the three-dimensional structure of small molecules;
- magnetic resonance imaging (MRI);
- X-ray diffraction used in crystallography, which allows the structure of molecules of any size to be resolved, provided they form regular crystals;
- electronic paramagnetic resonance (EPR);
- surface plasmon resonance (SPR);
- mass spectrometry, which makes it possible to identify proteins;
- electrophysiology, which measures the electrical activity of cells, potentially of a single cell at a time using the Patch-clamp technique;
- biophotonics and fluorescence microscopy;
- microcalorimetry, which measures heat changes during a reaction, such as the binding of water molecules to a protein;
- microtensometry, which measures the interaction forces within a lipid bilayer;
- polymerase chain reaction (PCR), which has many applications in the field of DNA manipulation.
All this requires the manipulation and purification of these molecules using high-pressure liquid chromatography (HPLC), electrophoresis, crystallogenesis, flow cytometry, genetic engineering and techniques to obtain sufficient quantities of identical molecules, such as polymerase chain reaction.
The devices are not yet able to "see" a molecule but by "illuminating" a large number of identical molecules with controlled radiation, from X-rays to radio waves (NMR, EPR), it is possible to deduce their common structure by analyzing the re-emitted radiation.
The use of a fundamental theoretical model based on quantum physics, and therefore the use of computer tools, is essential and often linked to the Internet.
Re-emitted radiation is also used to locate these molecules in space; this is what is used in imaging. This often involves coupling the molecule of interest to a biophotonic fluorophore.
There are countless examples of the use of these techniques in medicine. For example, the decoded genome, AIDS and TAT protein, use of EPR. One discipline uses these different tools and techniques to apply them to medicine: structural genomics.