Question

muscle contraction interactive and video
in the sliding filament model of muscle contraction, what does calcium bind to and what is its function?
it binds to troponin, causing it to move tropomyosin so that myosin heads can attach to actin.
it binds to myosin, causing it to move tropomyosin so that troponin can bind to actin.
it binds to actin, causing the muscle to relax.
it binds to myosin, causing the muscle to contract.
none of these.

Explanation:

In the sliding filament model of muscle contraction, calcium ions ($\text{Ca}^{2+}$) play a crucial role. They bind to troponin, a protein complex associated with the thin filaments (actin). When $\text{Ca}^{2+}$ binds to troponin, it causes a conformational change in troponin. This change then moves tropomyosin (another protein that covers the myosin - binding sites on actin in the resting state) out of the way. As a result, the myosin heads can attach to the actin filaments, which is a key step in muscle contraction.

Answer:

It binds to troponin, causing it to move tropomyosin so that myosin heads can attach to actin.