Lecture 5: 19/09

Question 1

How do cells apply stress/strain to actin networks?

Answer 1

Myosin - an actin motor protein

Multiple myosins form a mini filament

Question 2

How do myosins move along the actin filaments?

Answer 2

Hydrolysis driven power stroke (cross bridge cycle)

Not processive at the single head level; the superstructure of myosin mini filaments allows it to energetically overcome thermodynamics and become processive

Question 3

Explain the cytoskeleton components?

Answer 3

Actin: the frame
Crosslinkers: solidify the frame
Actin + Myosin: contract the frame

Question 4

How does myosin change mechanics?

Answer 4

Generate tension between actin filaments

Question 5

What are the roles of myosin and crosslinkers in an actin network?

Answer 5

Crosslinked actin with active myosin significantly increases the storage modulus (i.e., stiffness) compared to entangled, myosin, and crosslinked states, respectively.

Myosin + crosslinker = stiffer network (strain stiffening)

Question 6

What is the function that represents G”/G’?

Answer 6

G”/G’ = tan(delta)

Question 7

Explain the relationships between G’, G’’, and frequency in viscoelastic materials.

Answer 7

Low frequency: G”(w) > G’(w), more fluid like
High frequency: G’(w) > G”(w), more solid like

There is a crossover as frequency increases from a fluid-like to a solid-like domain

Question 8

What determines the frequency crossover between solid and fluid like?

Answer 8

The crosslinking energy and timescale (duration).
Crosslinking determines storage or loss of strain energy.

Question 9

Describe the vector relationships between G’, G”, and G*?

Answer 9

G’ in the x-direction
G’’ in the y direction
G* is the vector addition (hypotenuse) of G’ and G”, and delta is the angle between G’ and G*

Question 10

What does the loss tangent describe?

Answer 10

Loss tangent = G”/G’ = loss/storage
Described the relative energy that is lost vs stored

LT > 1: principally viscous
LT < 1: principally solid

Question 11

How does the ratio to myosin/actin impact stiffness and stress (K’)?

Answer 11

Increasing the myosin concentration increases the tension.
Increasing the myosin concentration increases the stiffness.
Increasing the myosin concentration increases the solidity (elasticity) as there is less frequency dependence (more flat)

Question 12

Do myosin and the rheometer have the same effect?

Answer 12

Internal and external stress both stiffen with the same X (scaling exponent) in regards to the differential storage modulus K’.

Myosin is an internal source, rheometer would be external.

Question 13

What is K’?

Answer 13

Differential elastic modulus that measures stiffness as a function of stress

Question 14

Describe the scaling coefficient of frequency as it relates to K’?

Answer 14

K’(w) ~ w^x
x = 0 for solids
x = 1 for liquids

Question 15

Why can we just look at the relationship between frequency and storage modulus to determine stiffness and solidity?

Answer 15

Elasticity is not time dependent (so G’ can show us this information), but viscosity is

Question 16

What determines if the stress is stored or lost?

Answer 16

• Crosslinkers in the system (sliding vs tension)
• Can the filaments move on the timescale of the stress application

If the crosslinkers are bound for longer than the (timescale) force applied, the energy will be stored