Q: What is the smallest contractile portion of a muscle? Read this paragraph and choose the correct option from the provided answers: 10.3 Muscle Fiber Contraction and Relaxation A sarcomere is the smallest contractile portion of a muscle. Myofibrils are composed of thick and thin filaments. Thick filaments are composed of the protein myosin; thin filaments are composed of the protein actin. Troponin and tropomyosin are regulatory proteins. Muscle contraction is described by the sliding filament model of contraction. ACh is the neurotransmitter that binds at the neuromuscular junction (NMJ) to trigger depolarization, and an action potential travels along the sarcolemma to trigger calcium release from SR. The actin sites are exposed after Ca++ enters the sarcoplasm from its SR storage to activate the troponin-tropomyosin complex so that the tropomyosin shifts away from the sites. The cross-bridging of myposin heads docking into actin-binding sites is followed by the "power stroke"the sliding of the thin filaments by thick filaments. The power strokes are powered by ATP. Ultimately, the sarcomeres, myofibrils, and muscle fibers shorten to produce movement. Choices: - filaments - cell - capillary - sarcomere A:
sarcomere
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Q: In how many basic was can substances cross the plasma membrane? Read this paragraph and choose the correct option from the provided answers: If a cell were a house, the plasma membrane would be walls with windows and doors. Moving things in and out of the cell is an important role of the plasma membrane. It controls everything that enters and leaves the cell. There are two basic ways that substances can cross the plasma membrane: passive transport and active transport. Choices: - one - three - four - two A:
two
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Q: What can be thought of as the most biologically productive regions on earth? Read this paragraph and choose the correct option from the provided answers: An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the ocean. Estuaries can be thought of as the most biologically productive regions on Earth, with very high biodiversity. Estuaries are zones where land and sea come together, and where fresh and salt water meet. Choices: - swamps - forests - coasts - estuaries A:
estuaries
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Q: What can prevent the passage of molecules and ions between cells, such as in the blood-brain barrier? Read this paragraph and choose the correct option from the provided answers: In addition to holding cells together, tight junctions play a role in the transport of materials. Tight junctions prevent the passage of molecules and ions through the space between cells. So these molecules and ions must actually enter cells (either by diffusion or active transport) in order to proceed through a tissue. This allows tight junctions to indirectly play a role over what substances are allowed into a specific cell. Tight junctions play this role in maintaining the blood-brain barrier. Choices: - fatty junctions - solid junctions - special junctions - tight junctions A:
tight junctions
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