Which of the following statements regarding contraction in skeletal and smooth muscle is TRUE? A. In skeletal muscle, calcium in
itiates contraction by binding to troponin, while in smooth muscle calcium initiates contraction by binding directly to myosin. B. In skeletal muscle, calcium initiates contraction by binding to troponin, while in smooth muscle calcium initiates contraction by binding to calmodulin. C. In skeletal muscle, calcium initiates contraction by binding to myosin light-chain kinase, while in smooth muscle calcium initiates contraction by binding directly to tropomyosin. D. All of the above are TRUE.
B. In skeletal muscle, calcium initiates contraction by binding to troponin, while in smooth muscle calcium initiates contraction by binding to calmodulin.
Explanation:
The Calcium ions (Ca2 +) have a key role in skeletal muscle contraction. Moreover, the Troponins are regulatory proteins that also play roles in skeletal contraction. In this muscle, troponin is attached to tropomyosin, when Ca2 + binds to troponin it induces a conformational change, thereby exposing this protein to its binding to myosin filaments on the actin filaments, which initiates muscle contraction.
On the other hand, calmodulin (calcium-modulated protein) is a protein which mediates the contractile state of the smooth muscle by binding Ca2+ ions. The function of this protein is to stimulate the development of force (contraction) in response to Ca2+ ions by activating the phosphorylation of myosin.
PHANERITIC TEXTURE - Igneous rocks with large, visible crystals because the rock formed slowly in an underground magma chamber. PORPHYRITIC TEXTURE - an igneous rock in which PHENOCRYSTS (large crystals) are surrounded by a fine groundmass (very small crystals).
A small portion of energy is passed from one trophic level to the next successive level. The rest is lost to the environment. The progressive loss of energy at each trophic level puts a natural limit on the total dry mass of living biomass that can exist at each successive trophic level. In a stable ecosystem, the biomass in any trophic level should be more than the successive level in order for it to support the successive level.