Thesis (pdf) - Espci

1
General introduction
In recent years, research in biology has evolved towards a more quantitative
analysis. The use of physical approaches have improved the understanding
of many biological processes including membrane and cytoskeleton interactions,
cell division, mechanotransduction and cell movement. In vitro approaches
which try to identify the minimum amount of components needed
to reproduce particular phenomena in living systems, are also an important
tool for biological research.
The study of biology is so vast that spans several orders of magnitude in
both force and length. Typical length scales range from a few nanometers for
a molecular motor, to a few meters corresponding to the size of a mammalian.
Forces range from a few piconewtons 1 (stall force of a molecular motor), to
several newtons (earth attraction on a human being is of about ∼ 500 N).
We will focus on the cellular and sub-cellular scale. A cell has a typical
diameter of 10 µm, and contains many organelles and proteins, including
molecular motors, which are in the range of a few nanometers, capable of generating
forces of piconewtons, and they are the smallest entities that we will
consider. Our unit scale of energy is thus ∼ pN × nm, which is of the order
of the thermal energy kBT ∼ 4 pN nm (the energy released by hydrolization
of ATP is of about 8 kBT ). As a consequence we expect that thermal fluctuations
play an important role at these microscopic scales. Moreover, living
organisms continuously consume energy (ATP), which maintains them in an
out of equilibrium steady state. Thermal equilibrium state corresponding to
the death of the organism. Any physical approach at this scale must explicitly
take into account these fluctuations and can not be described in terms of ener-
1 1 pN = 10 −12 N