  We report that a cell-sized water droplet (CWD) with a diameter of several
tens of microns can serve as a micro-biochemical reactor. Since the droplet
inner surface is covered by a phospholipid layer, it provides an environment
similar to that in living cells. The CWD is manipulated with laser tweezers and
we monitor the time-development of biochemical reactions in a single CWD by
fusing two droplets which contain a substrate and an enzyme, respectively. As
actual examples of enzymatic reactions, we present results on the reaction of
calcein with esterase and on the expression reaction of green fluorescent
protein.
