Energetics and forces in living cells
Proteins can act as exquisite nanomachines to produce or sense the motion associated with cell division, intercellular trafficking, muscle contraction, and countless other activities.
Physics Today 68, 2, 27 (2015); https://doi.org/10.1063/PT.3.2686
Alex Dunn is an assistant professor of chemical engineering and Andrew Price is a doctoral student in biophysics, both at Stanford University in Stanford, California.
Life is intrinsically mechanical. Animals run, fly, and swim. Plants move daily to track the Sun. Even microscopic organisms, first observed more than three centuries ago when Antoni van Leeuwenhoek trained his microscope on pond water, swarm and tumble. A look at our own cells reveals that subcellular components are in constant motion, which allows living cells to grow, divide, change shape, and move.
In addition to producing motion, our bodies must also sense it. Living cells respond to a wide variety of mechanical stimuli, including stretch, fluid flow, osmotic potential, and the stiffness of their surroundings. Our senses of hearing and touch require nerve cells to detect minuscule mechanical forces. And our ability to regulate blood pressure across meters of height depends on mechanosensitive arteries and arterioles distributed throughout the body.
More subtly, living tissues are remarkably sensitive to the mechanical cues provided by their surroundings. Stem cells grown on soft surfaces are primed to differentiate and form correspondingly soft tissues such as fat or nervous tissue, whereas cells grown on harder surfaces differentiate to form bone cells.1 On longer length scales, the growth and development of our organs require precise changes in shape, with tightly controlled tissue-level mechanical stresses and strains. The mechanosensory response is also apparent in everyday life: Consistent exercise, for instance, leads to increases in bone and muscle mass, and slacking off reverses the gains.
To make complex morphogenetic decisions, our cells must constantly communicate with each other. Much of the intercellular communication is through chemical signals, but growing evidence suggests that physical mechanisms provide significant control as well.2 The image above, a still from a video of the development of a fruit-fly embryo, exemplifies the complex orchestration among hundreds of cells. Individual cells are squeezed, pushed, and pulled across significant distances to form different parts of the developing body.
Although motion is a pervasive aspect of life, until recently biologists had little understanding of how living things produce, detect, and respond to mechanical cues at the cellular level. Only in the past decade have researchers learned key aspects of how living cells pull that off and how those different functions are integrated among groups of cells within tissues. Although the molecular details of how a cell works are complex, some relatively simple physical models provide powerful hints.
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