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Joined 2026-09-21 Message
Incoming links: Single-molecule experiment
Past exam of the biological physics course of the University of Oxford 2015 8 iii Solution 2026-10-25
Single-molecule experiments expose distributions and ordered events that an ensemble average can hide. Several distinct biological systems make this advantage concrete.
- In titin single-molecule force spectroscopy, individual sawtooth drops reveal sequential domain unfolding and contour release. An ensemble stretching curve can smooth away that sequence and obscure variation in unfolding thresholds. Force-dependent waiting-time distributions also distinguish stochastic transitions from a single deterministic breaking strength.
- In a patch clamp recording of one ion channel, discrete open-current levels and closed intervals reveal single-channel conductance, gating dwell times, and ion-channel open probability. An ensemble current alone cannot distinguish a few frequently open channels from many rarely open ones, or reveal a brief intermediate state with the same clarity.
- In Single-molecule observation of F1 rotation, a probe on the motor shaft reports angular progression and pauses associated with the catalytic cycle. Rotation demonstrates mechanical coupling directly; individual stepping and dwell-time statistics can connect chemical turnover to angular motion, whereas bulk ATP consumption alone cannot establish the motor's trajectory.
- In DNA unwinding measured by magnetic tweezers, individual extension changes resolve the waiting time and extent of open promoter complex formation by RNA polymerase. Separate binding, opening, and later movements can otherwise merge into one population-averaged response.
These methods also have limitations: the attached probe or applied load may perturb the molecule, inactive or heterogeneous molecules may be overrepresented, and finite temporal resolution can hide fast steps. Repeated independent trajectories and appropriate controls are needed to connect a single observed event to a reproducible mechanism. The gain is access to trajectories and heterogeneity, rather than an assumption that one molecule alone represents every molecule.
Single-molecule experiment 2026-10-14
A single-molecule experiment resolves a response attributable to an individual molecule, rather than only an average over many molecules. Optical detection need not resolve its constituent chemical atoms in an image: repeated photon absorption by the same molecule can produce a measurable response. One detected photon is not automatically one different molecule.
The measured channel matters. Direct photon absorption detection measures a change in transmitted light. Fluorescence detection collects photons emitted after excitation; a fluorescence excitation spectrum varies the illuminating frequency while observing that emission. Both can probe excitation, but their backgrounds and readouts differ. A labeled particle cartoon explains the interpretation; it supplies no experimental evidence by itself.
Such measurements expose differences that a population average can conceal. Suppose two groups of molecules switch between different internal quantum states. Their average response could vary smoothly even though each molecule changes in discrete steps. Resolving an individual response tests that interpretation directly, subject to the measurement's time resolution, sensitivity, and particle assignment. The following experiment makes that assignment through spectral selection and controlled perturbations.