This is regulated by the passage of ions between different cellular compartments through ion channels or biochemical signals initiated by metabotropic receptors. Excitability emerges as a biophysical consequence of charge separation across biological membranes. Interfaces are formed by biomembranes that bind regions with different ionic compositions. In all cells, excitability is underpinned by the thermodynamics of interfaces. Here, we invoke this extended definition of excitability to explore cellular phenomena that specifically drive whole-organism behaviour across prokaryotic and eukaryotic cells.ĭuring eukaryogenesis, cells evolved new mechanisms of excitability to sense and react rapidly to their environment. An excitable system often undergoes a characteristic excursion through state space, before returning to its original state after a refractory period has elapsed. These can also propagate spatially, with or without attenuation. More generally, however, an excitable dynamical system is capable of supporting not only all-or-nothing responses that are independent of the stimulus amplitude, but also other signal types. Traditionally, the term ‘excitability’ has been associated with neurons and their electrical properties, often manifesting as rapid changes in membrane potential and electrical spiking activity known as action potentials. An initially weak reaction can be amplified nonlinearly to trigger a strong or impulsive response. This article is part of the theme issue ‘Basal cognition: conceptual tools and the view from the single cell’.Ĭellular excitability is the capacity to generate dynamic responses to stimuli, often over millisecond timescales. This comprehensive new perspective on the evolution of excitability enriches our view of eukaryogenesis and emphasizes behaviour and sensing as major contributors to the success of eukaryotes. We conjecture that together with an increase in cell size, these new forms of excitability greatly amplified the degrees of freedom associated with cellular responses, allowing eukaryotes to vastly outperform prokaryotes in terms of both speed and accuracy. The innovations include a vastly expanded repertoire of ion channels, the emergence of cilia and pseudopodia, endomembranes as intracellular capacitors, a flexible plasma membrane and the relocation of chemiosmotic ATP synthesis to mitochondria, which liberated the plasma membrane for more complex electrical signalling involved in sensing and reacting. But what are the key events and innovations during eukaryogenesis that made all of this possible? Here we describe the ancestral repertoire of eukaryotic excitability and discuss five major cellular innovations that enabled its evolutionary origin. These advances enabled new and more complex forms of cellular behaviour in eukaryotes, including directional movement, active feeding, mating, and responses to predation. Eukaryotes, in particular, evolved radically new ways to sense and react to their environment. All living cells interact dynamically with a constantly changing world.
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