Question

You are investigating the effects of ATP, Ca²⁺, and other ions on skeletal muscle contraction, and are conducting two experiments. First using glycerinated muscle, you can compare the ability of muscle to contract under 3 experimental conditions (1: ATP; 2: KCl+MgCl₂; and 3: ATP+KCl+MgCl₂). Second, using fresh muscle, you compare the ability of muscle to contract under another 3 experimental conditions (1: ATP; 2: Ca²⁺; and 3: ATP+Ca²⁺). For each experiment, what results would you expect regarding the amounts of muscle contraction that would occur under each of the 3 experimental conditions? What is the importance of ATP and Ca²⁺ in muscle contraction?

Answer 1

In Experiment 1

Consider solution are placed on slide which consist of thread of muscle fibre, so that we could easily observe the changes such as change in the length of the fibre or shortening of fibre or may be colour change.

• when KCl and MgCl2 is added alone, no changes were found, the combination of KCl and MgCl2 with ATP will increase the strength of muscle contraction, this happens due to myosin finds greater affinity towards these ions. ATP can cause muscle contraction by itself. Sacromeres in the muscle fibre shortened in the length and colour change may observe

One cycle of a single myosin ATPase head expends 1 molecule of ATP, but it takes thousands of these myosin heads, each producing about 5 power stroke per second (1 ATP/stroke), to produce a noticeable or useful contraction of the whole muscle.

In Experiment 2

• Ca2+ is not one of the requirements to cause contraction, but ATP is important for muscle contraction. Presence of Ca triggers contraction by reaction with regulatory proteins in which absence of ca prevent interaction of actin and myosin.

When a muscle fiber is relaxed, the concentration of calcium ions is less in the sarcoplasm. As a muscle action potential travels along the sarcolemma and into the transverse tubule system, the ions release channels open in the sarcoplasmic reticulum membrane. As a result calcium ions flood in the sarcoplasm around the thick and thin filaments which in turn binds troponin (muscle protein) that causes the change in shape.

The globular head of myosin acts as an ATPase enzyme that hydrolyzes ATP molecule. The energy derived from the hydrolysis of ATP is used by myosin to bind to the exposed active site on the actin filament to form a cross bridge.