Lecture 8: 17/10 Flashcards
How can we measure single myosin forces and movement? How can we measure pN forces in general?
Atomic force microscopes, optical tweezers, magnetic tweezers
What are properties of magnetic tweezers?
Can readily rotate, exerts forces on all magnetic particles, very short distances
What are properties of AFM?
Precisely defined stress & strain, needs mechanical contact
What are properties of flow stretch?
Precisely defined stress & strain, needs flow exposures
What are the properties of optical tweezers?
Precisely defined stress & strain, no contact heating can be the issue
How is the ray optics regime defined?
Particle»_space; Wavelength of light
Refractive index of particle > surrounding fluid
The diffraction of light gives the particle a kick/transfer of momentum.
What happens if particle is not in center of beam intensity?
Asymmetry in intensity, creates asymmetry in force
What is Fata Morgana?
Optical illusion causing objects to appear to float when warmer air is on top of cooler air
Index of refraction
n = c/v
A measure of how much slower light locally is
What happens when light passes into a material of higher index of refraction?
The speed decreases, momentum is conserved
What is the equation for the energy of a photon?
E = (h * c) / lambda
What is the equation for momentum transfer between different refractive indices?
delta(p) = delta(n) * (h/lambda)
p2 - p1 = (n2 - n1) * (h/lambda)
Calculate the total momentum change per second of a 100mW 800 nm going from water (n=1.3) to glass (n=1.5)?
n1 = 1.3, n2 = 1.5, h = plank’s, lambda = 800 nm, P = 100mW
delta(p) of one photon = (n2 - n1) * (h/lambda)
E of one photon = hc / lambda
Photons / Time = Power / (E of one photon)
Total Momentum per time = (delta(p) of one photon) * (photons/time)
Explain what leads us to 3D single beam optical tweezers?
We must create not only a radial gradient, but an azimuthal (z) gradient.
When z gradient > scattering force -> the bead is trapped?
What is the formula for Stoke’s Law? How is it used for optical traps?
F = 6pirviscosityvelocity
It is used to determine at what velocity the particle pops out of the laser trap; This allows us to create a force vs velocity plot to calibrate the system.
When the flow causes the bead to pop out of the trap, you have exceeded the trap force
Continuously apply force until the bead pops out, plug this force in to calculate for bead velocity at this point.
What are the types of optical trap applications?
- Measuring strength or force exerted by motors
- Measuring unfolding or stretching forces
- Measuring relative forces
How does ATP concentration impact the single displacement vs time of particles (i.e., how does it influence myosin power stroke)? how does it impact the force?
more ATP = faster cycling
more ATP = greater power (force magnitude is same, applied over shorter duration)
more ATP does not influence step size
Duration increases as ATP concentration decreases
Duration is determined by the peak width
What is the average myosin step size obtained?
11nm
(smaller than 40nm, not influenced by ATP concentration)
How does an optical stretcher work?
Two divergent lasers shining in the middle, the particles get stretched by the lasers, which can be used to characterize how deformable different cells are (% percentage of cells vs. optical deformability)
How does an optical rotator work?
If you rotate the laser, you can rotate the sample/particle.
Makes use of non-uniform cell structures.
What is bio optical neuron guidance (BONG)?
Using a femtosecond laser to steer neuron extension and growth structure. Holding on to substructure of growth cone, uses laser to fake a chemoattraction. (?)
What are the similarities and differences between magnetic and optical traps?
Similarities:
- Both effective at applying known forces to small particles
Differences:
- Optical traps are more precise in virtually all aspects due to the laser component
- Multi-pole magnetic traps are powerful but complex
- Magnetic traps are useful for long slow measurements or where there is too much scattering/heating
What moduli do you expect for the cytoplasm vs cortex?
Cytoplasm - less stiff (Pa)
Cortex - more stiff (kPa)
What information does the trap & bead displacement vs time tell us?
The difference in amplitude between the optical trap position and trapped bead position tells us the resistance to displacement (higher amplitude means more resistance).
In phase: elastic solid
90 degrees out of phase: viscous fluid
