Physical Biology of the Cell

Extracellular Movement of Signaling Molecules

“…Hormones such as insulin are directly secreted into the bloodstream where they can reach almost all parts of the body leading to a rapid and even distribution. Typical measured flow velocities of blood in capillaries are on the order of 500 μm/s (Phillips et al 2009), similar to the velocities in the plant phloem discussed above (although blood flow in larger vessels such as the aorta can be up to two orders of magnitude higher (Bahlmann et al 2001)). At this velocity, insulin secreted from the pancreas can reach a muscle that is several centimeters away within seconds to minutes.…”

“…Let us consider a typical protein with a diameter of 5 nm (corresponding to a molecular weight of 40-50 kDa). The Stokes-Einstein equation describes the diffusivity D of a molecule with a radius R moving through a fluid with a viscosity η at a temperature T as D = k B T/6πηR (k B is the Boltzmann constant) (Berg 1993; Phillips et al 2009). A protein with a diameter of 5 nm moving through water at room temperature would therefore be expected to have a diffusion coefficient of approximately 100 μm 2 /s.…”

“…The time scale of diffusion increases with the square of the distance. More precisely, the average time t it takes molecules with a diffusion coefficient D to diffuse a distance L in one dimension is approximately t = L 2 /2D (Berg 1993; Phillips et al 2009). Stable proteins with a diffusion coefficient of 100 μm 2 /s could therefore easily traverse a one-dimensional field of 1 mm within approximately 2 hours.…”

“…The time it takes molecules to travel a distance L by directional transport is linear and not quadratic as in the case of diffusion (Phillips et al 2009). For example, flow velocities of 300-500 μm/s in human blood vessels (Phillips et al 2009) and in the vasculature of olive trees (Lopez-Bernal et al 2010) have been measured. A molecule moving by fluid flow in the blood system would be able to travel about 1 mm within 3 seconds, 1 cm within 30 seconds, and 1 meter within 40 minutes.…”

Flux Balance Analysis of Ammonia Assimilation Network in E. coli Predicts Preferred Regulation Point

“…After NH 3 ex diffuses into the cytoplasm, internal NH 3 (NH 3 in) is protonated into NH 4 + in, which serves as the substrate of GDH and GS [27], [28]. The permeation of NH 3 can be described bywhere denotes the ammonia assimilation flux, is the permeability coefficient [20], [22], is the surface area of E. coli cells [29], and is the cellular volume (personal communication with Dr. Yuan ).…”