A vesicle containing synaptic proteins must travel from a neuron's cell body to an axon terminal one meter away. Which motor protein and cytoskeletal track would accomplish this, and why can't diffusion do the job?
ADynein walking along actin filaments toward the minus end; diffusion is excluded from axons
BKinesin walking along microtubules toward the plus end; diffusion over meter-scale distances would take years
CMyosin II walking along microtubules; myosin generates the most force per ATP
DKinesin walking along actin filaments; actin extends the full length of the axon
Kinesin is the anterograde motor that walks toward the plus end of microtubules — the cell periphery, including the axon tip. Microtubules, not actin, form the long-distance highway in axons, with their plus ends oriented toward the terminal. Diffusion at cellular temperatures can cover a few micrometers per second, but over one meter it would take years; directed motor transport covers it in days. Dynein moves in the opposite direction (retrograde, back to the cell body), and myosin walks along actin for local transport.
Question 2 Multiple Choice
During the myosin power stroke, at which step does ATP hydrolysis actually provide energy for movement?
AATP binds to myosin, causing it to release from actin — ATP binding provides the detachment energy
BATP hydrolysis (ADP + Pi release) occurs while myosin is detached and re-cocks the head into the high-energy conformation; the power stroke fires when Pi is released after rebinding actin
CATP hydrolysis occurs as myosin pivots during the power stroke itself
DEnergy comes from the electrostatic attraction between myosin and actin, not from ATP
This is the key misconception addressed by the topic. ATP hydrolysis does not directly power the pivot — instead, it re-cocks the myosin head into a high-energy 'cocked' conformation while myosin is detached from actin. The power stroke (the pivot that moves actin) is driven by the release of inorganic phosphate (Pi) after the head rebinds actin, releasing the stored conformational energy. ATP binding (before hydrolysis) actually causes myosin to release from actin — it provides detachment, not movement.
Question 3 True / False
Without motor proteins, large cells like neurons could not distribute organelles and vesicles effectively because diffusion alone is too slow over distances greater than a few micrometers.
TTrue
FFalse
Answer: True
Diffusion is efficient over short distances (micrometers) but scales poorly — diffusion time scales as distance squared, so a 1,000-fold increase in distance means a 1,000,000-fold increase in time. A vesicle diffusing 1 meter down an axon would take years. Motor proteins on cytoskeletal tracks provide directed, active transport that covers the same distance in days. This is why large, polarized cells like neurons are absolutely dependent on motor proteins for their function.
Question 4 True / False
Kinesin moves toward the minus end of microtubules, delivering cargo from the cell body toward the cell periphery.
TTrue
FFalse
Answer: False
Kinesin moves toward the plus end of microtubules, which points toward the cell periphery. In axons, plus ends face the terminal, so kinesin performs anterograde transport (cell body → axon tip). Dynein moves toward the minus end (retrograde transport, axon tip → cell body). This directionality is determined by the motor's mechanochemical structure, not by cargo type.
Question 5 Short Answer
Why is processivity — the ability to take many steps without detaching from the filament — especially important for kinesin, and how does kinesin's two-headed structure enable it?
Think about your answer, then reveal below.
Model answer: Kinesin must transport cargo over long distances (up to a meter in axons) without dropping it. If kinesin detached after every few steps, cargo would be lost and delivery would fail. Kinesin's two heads alternate: while one head is bound to the microtubule, the other swings forward to the next binding site before the trailing head releases — a hand-over-hand mechanism that keeps at least one head attached at nearly all times. This coordination produces high processivity (hundreds of steps per run). Myosin II, by contrast, operates in large ensembles during muscle contraction where individual non-processivity is acceptable because many motors are always engaged.
Processivity is a functional requirement shaped by the task. Long-distance cargo transport demands processive motors; ensemble force generation (muscle contraction) can use non-processive ones. The two-headed coordination of kinesin is the structural solution to the processivity requirement, directly linking molecular structure to cellular function.