Inside every cell, logistics is in full swing: tiny protein motors, like couriers, haul cargo along a network of microtubules. One of the main workhorses is kinesin, which steps using the energy of ATP (the cell's main energy currency). For a long time, researchers tried to evaluate it like a car: how much fuel it burns, how fast it gets there. But different researchers used different yardsticks, and the big picture didn't come together.
The authors of the new study decided to set things straight. They built a unified model of the "kinesin-cargo" system and derived formulas for four types of efficiency — from simple efficiency to tricky information-thermodynamic measures. The model accounted for the motor's discrete steps and its coupling to the cargo. The comparison yielded a surprise: by all these measures, kinesin operates at low efficiency, sometimes just a few percent.
Such low efficiency is not a bug but, perhaps, a feature. If the motor isn't chasing economy, then evolution has aimed it at other priorities: reliability, delivery speed, or the ability not to damage fragile cargo. The work urges scientists to stop measuring everything with one yardstick and to look for other, more suitable metrics — for example, how quickly and accurately the motor finds its target. After all, a courier who saves fuel but is late is useless.
Kinesin can take up to a hundred steps per second without leaving the microtubule, like a person running without lifting their feet off the ground.
In the story "A Journey into the Cell," sci-fi author Greg Egan describes nanobots working on principles of molecular motors — almost like kinesin.