Answer:
When a muscle cell contracts, the myosin heads each produce a single power stroke.
Explanation:
In rest, attraction strengths between myosin and actin filaments are inhibited by the tropomyosin. When the muscle fiber membrane depolarizes, the action potential caused by this depolarization enters the t-tubules depolarizing the inner portion of the muscle fiber. This activates calcium channels in the T tubules membrane and releases calcium into the sarcolemma. At this point, <em>tropomyosin is obstructing binding sites for myosin on the thin filament</em>. When calcium binds to the troponin C, the troponin T alters the tropomyosin by moving it and then unblocks the binding sites. Myosin heads bind to the uncovered actin-binding sites forming cross-bridges, and while doing it ATP is transformed into ADP and inorganic phosphate which is liberated. Myofilaments slide impulsed by chemical energy collected in myosin heads, <u>producing a power stroke</u>. The power stroke initiates when the myosin cross-bridge binds to actin. As they slide, ADP molecules are released. A new ATP links to myosin heads and breaks the bindings to the actin filament. Then ATP splits into ADP and phosphate, and the energy produced is accumulated in the myosin heads, which starts a new binding cycle to actin. Z-bands are then pulled toward each other, thus shortening the sarcomere and the I-band, and producing muscle fiber contraction.
The signaling molecule for flowering might be released earlier than usual in a long-day plant exposed to flashes of red light during the night.
Higher plants are sessile organisms that sense and respond to environmental stimuli such as light and chemical cues by changing their morphology.
The signaling pathway uses a complex network of interactions to coordinate biochemical and physiological responses such as flowering, fruit ripening, germination, photosynthetic regulation, and shoot or root development.
These signals are first recognized by receptors and transmitted through complex networks to the cell nucleus.
The signal is transduced to the nucleus by one of several systems involving GTP-binding proteins (G proteins) that change activity upon GTP binding, a protein kinase cascade that sequentially phosphorylates and activates various proteins and Membrane ion channels that alter the ionic properties of cells.
This signal is manifested in the nucleus as a change in the activity of DNA-binding proteins, transcription factors that specifically interact with and regulate the regulatory regions of genes.
Thus, detection of environmental signals is transmitted through transduction pathways, and changes in transcription factor activity can coordinate expression changes in gene portfolios to guide new developmental programs.
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Cytokinesis. The final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions.
