Uptake is cheap, breach is not: an ATP budget for nanoplastic–membrane interaction

Mechanical action of plastic particles on the cell membrane is repeatedly proposed as a route to cellular damage and to depletion of ATP, but the energy budget of that route has never been evaluated. Here I assemble it from four channels and from measured membrane constants. The bending work of engulfing one spherical particle is 8πκ ≈ 503 kBT ≈ 26 ATP and is independent of particle radius. Converting measured tissue burdens (33.7–47 730 µg per gram) into particle numbers gives 6×102 to 7×106 particles per cell; engulfing all of them costs at most 1.8×108 ATP, which is 0.18 s of the cell’s ATP turnover. The membrane-repair channel is bounded from above and cannot reach 10−3 s of turnover at any realistic lesion count. By contrast, a single pore of 5 nm radius admits a diffusion-limited flux of 1.9 × 107 Ca2+ per second, which at the 1:1 stoichiometry of the plasma-membrane Ca2+-ATPase consumes 0.2–2 % of the cell’s entire ATP turnover from one hole, and over 30 s consumes 16 % of the cell’s standing ATP pool. The largest channel, however, does not lie on the demand side at all: the measured fall in mitochondrial ATP production in primary human monocytes exposed to polystyrene particles reaches 50–60 %, equivalent to 5–6×108 ATP per second of lost supply, or 26–31 times one open pore, and unlike a pore it is persistent. The budget implies that the mechanical cost of taking particles in is negligible across the entire measured exposure range, and that the energetic weight sits in the ionic consequence of membrane failure and in the loss of ATP-producing capacity. The decisive mechanical quantity is therefore the probability of failure, governed by the classical wrapping criterion Rmin = √ 2κ/(w − σ), in which the chemical state of the particle surface enters through a single quantity—the adhesion energy w. For membrane tensions of 0.01–0.05 mN m−1 and adhesion energies of the same order there exists a regime of frustrated wrapping that is the mechanical candidate for breach. The adhesion energy of weathered nanoplastic on a membrane has never been measured; together with number-based rather than mass-based dosimetry, these are the two quantities that would close the budget.