Macropinosome formation occurs as a localized sequence of biochemical activities 
and associated morphological changes, which may be considered a form of signal 
transduction leading to the construction of an organelle. Macropinocytosis may 
also convey information about the availability of extracellular nutrients to 
intracellular regulators of metabolism. Consistent with this idea, activation of 
the metabolic regulator mechanistic target of rapamycin complex-1 (mTORC1) in 
response to acute stimulation by growth factors and extracellular amino acids 
requires internalization of amino acids by macropinocytosis. This suggests that 
macropinocytosis is necessary for mTORC1-dependent growth of metazoan cells, 
both as a route for delivery of amino acids to sensors associated with lysosomes 
and as a platform for growth factor-dependent signalling to mTORC1 via 
phosphatidylinositol 3-kinase (PI3K) and the Akt pathway. Because the 
biochemical signals required for the construction of macropinosomes are also 
required for cell growth, and inhibition of macropinocytosis inhibits growth 
factor signalling to mTORC1, we propose that signalling by growth factor 
receptors is organized into stochastic, structure-dependent cascades of chemical 
reactions that both build a macropinosome and stimulate mTORC1. More generally, 
as discrete units of signal transduction, macropinosomes may be subject to 
feedback regulation by metabolism and cell dimensions. This article is part of 
the Theo Murphy meeting issue 'Macropinocytosis'.