18.1: Introduction

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The cell as it appears in a microscope was long thought to be a bag of liquid surrounded by a membrane. The electron microscope revealed a cytoskeleton composed of thin and thick rods, tubes and filaments. Other intracellular structures and organelles are enmeshed in these microfilaments, intermediate filaments and microtubules. We will compare the molecular compositions of these structures and their subunit proteins. In aggregate, they account for organelle location in cells, the shapes of cells, and cell motility. Cell motility includes the movement of cells and organisms, as well as the internal movements of organelles (e.g., vesicles) and other structures in the cell. Of course, these movements are not random…, and they require chemical energy! A long and well-studied system of cell motility is the interaction of actin and myosin during skeletal muscle contraction. We will first consider a paradox suggesting that ATP was required for contraction BUT ALSO for relaxation of muscle fibers. Then we look at experiments that resolve the paradox. Animals control skeletal muscle contraction, but some muscles contract rhythmically or with little or no control on the part of the animal - think cardiac muscles of the heart, or smooth muscles like those in digestive and circulatory systems. We will also look at the role of calcium ions and regulatory proteins in controlling the response of skeletal muscles our commands, and finally, at the elasticity of skeletal muscles.

Learning Objectives

When you have mastered the information in this chapter, you should be able to:

1. Compare and contrast roles of cytoskeletal structures in different kinds of cell motility.

2. Distinguish the roles of microfilaments, microtubules and intermediate filaments in the maintenance and alteration of cell shape and structure.

3. Suggest how ciliary and spindle fiber microtubules can maintain their length.

4. Explain how spindle fiber microtubules can change their length.

5. Propose an experiment to show which part of a motor protein has ATPase activity.

6. Define the actin-myosin contraction paradox.

7. Outline the steps of the contraction cycle involving myosin and actin.

8. Compare and contrast mucle and flagellar structure and function.

9. Explain why smooth muscles do not show striations in the light microscope.

10. Outline the structure of a skeletal muscle, from a whole muscle down to a sarcomere.

11. Propose alternate hypothesis to explain hereditary muscle weakness involving specific proteins/genes, and suggest how you might test one of them.