Biological Science (7th Edition)
7th Edition
ISBN: 9780134678320
Author: Scott Freeman, Kim Quillin, Lizabeth Allison, Michael Black, Greg Podgorski, Emily Taylor, Jeff Carmichael
Publisher: PEARSON
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Chapter 45, Problem 5TYU
Summary Introduction
To review:
The significance of the data on sarcomere structure in inspiring the sliding-filament model and the reason why the observation that muscle cells contain many mitochondria and endoplasmic reticulum is justified when the molecular mechanism of muscular contraction is understood.
Introduction:
The sliding-filament model of muscle contraction was proposed by Huxley and Hanson in 1954. The muscle fiber (cell) is made up of smaller units known as myofibrils. These are striated due to the presence of dark and light bands. A repeating unit between two Z lines is known as a sarcomere. The sarcomere shortens and lengthens as the muscles contract and relax, respectively.
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The sliding-filament model of skeletal-muscle contraction assumes a sliding or slipping of interdigitating filaments of actin and myosin. Electron micrographs show that during contraction the actin and my-osin filaments remain of constant length while the distance between Z lines shortens. Explain how this happens in terms of the molecular structure of the muscle filaments. What is the role of regulatory pro-teins (troponin and tropomyosin) in contraction?
As mentioned in class, one additional major use of ATP in skeletal muscle (besides
powering the myosin heads) is the recycling of calcium ions back into the sarcoplasmic
reticulum after depolarization. The resting concentration of Ca++ in the muscle cell
cytoplasm is about 50-100 nM, and the spike concentration after depolarization is about
10-20 μΜ.
a) Consider a single sarcomere. What is the number of free calcium ions within the
sarcomere at rest? What is the number of free calcium ions after depolarization?
b) The major ion pump responsible for calcium ion recycling is SERCA (sarco/endoplasmic
reticulum calcium ATPase). SERCA uses one molecule of ATP to pump two calcium ions,
and the resting level can be restored in about 10-20 ms. How many molecules of ATP are
used in a single sarcomere for pumping calcium in a single "twitch"?
c) Assume that a single "twitch" is sufficient to drive one sarcomere from its fully extended
length (about 2.5 µm) to its fully contracted length (about 1…
Describe the movement of myosin along an actin filament. Outline the primary molecular steps and explain how this process is used by cells. How many actin monomers within an actin filament would a myosin molecule need to ratchet in order to contract a cell by approximately 1 um? How fast could a non-muscle cell make this contraction compared to a muscle cell if the relative rates of myosin walks on F-actin are 4.5 um/sec and 0.04 um/sec for myosin II and I respectively?
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- Skeletal muscle cells undergo contractions based on a molecular mechanism involving: 1)the sliding of actin and myosin filaments past each other without any change in filament length 2) the shortening of actin filaments following direct interaction with myosin 3) dynamic instability of microtubules 4)nonearrow_forwardA typical relaxed sarcomere is about 2.3 µm in length and contracts to about 2 µm in length. Within the sarcomere, the thin filaments are about 1 um long and the thick filaments are about 1.5 um long. (a) Describe the overlap of thick and thin filaments in the relaxed and contracted sarcomere. (b) An individual "step" by a myosin head in one cycle pulls the thin filament about 15 nm. How many steps must each actin fiber make in one contraction?arrow_forwardThe ability of myosin to walk along an actin filament may be observed with the aid of an appropriately equipped microscope. Describe how such assays are typically performed. Why is ATP required in these assays? How can such assays be used to determine the direction of myosin movement or the force produced by myosin?arrow_forward
- Skeletal muscle fibres are very metabolically active and are adapted to create a lot of ATP from aerobic cellular respiration. Additionally, cells require other things to stay alive. Explain how the following three organ systems contribute to allowing a muscle cell to carry out its roles: a) Respiratory system b) Cardiovascular system c) Gastrointestinal system asap please.arrow_forwardCertain multi-headed myosins bind cooperatively to actin filaments. The binding interaction is mainly electrostatic in nature, so the presence of additional salt (ions) in solution can interfere with binding; ions will tend to associate with charged residues on the two proteins, blocking electrostatic attractions that would otherwise take place. Briefly describe the expected shape of the binding curve for one of these myosins, and what will happen to the shape when the salt concentration increases.arrow_forwardWhile the sarcoplasmic reticulum of muscle was first described by nineteenth-century microscopists, its true significance was not ap-preciated until its intricate structure was revealed much later by the electron microscope. What could you tell a nineteenth-century microscopist to enlighten him or her about the structure of the sarcoplasmic reticulum and its role in the coupling of excitation and contraction?arrow_forward
- How many actin monomers within an actin filament would a myosin molecule need to ratchet in order to contract a cell by approximately 1 μm?arrow_forwardA typical relaxed sarcomere is about 2.3 μm in length and contracts to about2 μm in length. Within the sarcomere, the thin filaments are about 1 μmlong and the thick filaments are about 1.5 μm long.(a) Describe the overlap of thick and thin filaments in the relaxed and contracted sarcomere.(b) An individual “step” by a myosin head in one cycle pulls the thin filamentabout 15 nm. How many steps must each actin fiber make in one contraction?arrow_forwardsince sarcomeres within skeletal muscles are rigidly aligned with each other what do you think excessive stretch or compression (remember the basic structure of the sarcomere with overlapping thin and thick filaments and the length-tension relationship) will do to the force generation of a muscle contraction?arrow_forward
- You know from experience that skeletal muscle tires quite quickly, especially if there is not enough oxygen. Interestingly, shellfish such as clams can maintain a month-long contraction in the muscle that keeps the shell closed. No oxygen gets in this situation. These muscles have a different version of myosin called paramyosin. Knowing what you know about the cellular mechanism of muscle contraction, propose a hypothesis to explain how paramyosin might work.arrow_forwardHow does the structure of the microfilaments allow for linear contraction of a whole muscle cells?arrow_forwardWith regard to muscle contraction, which of the following is an INCORRECT statement with regard to the interactions of filaments that occur in the sarcomere? A. When muscles are relaxed tropomyosin blocks binding sites on actin subunits, which keeps cross-bridges from forming. B. The myosin heads conduct a power stroke motion to slide when bound to actin, to move the "thin" filaments towards the center of the sarcomere. C. During contraction, actin subunits are removed from the ends of the "thin" filaments to shorten actin polymers, thus reducing the length of the sarcomere. D. "Thick" filaments are anchored at the M-line, while "thin" filaments are anchored at the Z-line. E. Numerous myosin heads engage with the actin filaments simultaneously, such that there is no back-slipping during the contraction process.arrow_forward
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