Two components of the cytoskeleton—microtubules and microfilaments—generate cell movement (see Figure 4.21). Both of these structures consist of long protein molecules that can change their length or shape.
Practitioners of the martial arts can achieve amazingly coordinated high jumps, with the aim of focusing the force of the jump. For a human, such a movement requires years of training and practice.
Microtubules are components of the cytoskeleton
Microtubules are important intracellular effectors for changing cell shape, moving organelles, and enabling cells to respond to their environment. Microtubules gener-
ate forces by polymerizing and depolymerizing the protein tubulin. The spindle that moves chromosomes to the mitotic poles at anaphase is made up of microtubules (see Figure 9.8). Another example of microtubule involvement in cell movement is the growth of the axons of neurons in the developing nervous system. Neurons find and make their appropriate connections by sending out long extensions that search for the correct target cells. If polymerization of tubu-lin is chemically inhibited, these neurons do not extend.
Microtubules also generate the small-scale movements of cilia and flagella (see Figures 4.23 and 4.24). We have seen a number of the functions of these structures in the previous chapters of this book. Whereas many protists and small invertebrates use cilia for locomotion, larger multicellular animals typically use ciliated cells to move liquids and particles over cell surfaces. Many mollusks, for example, use cilia to circulate a current of water across their gas exchange and feeding surfaces. In humans, the cilia continuously sweep a layer of mucus from deep down in the lungs, up through the windpipe, and into the throat. The mucus carries particles of dirt and dead cells. We can then either swallow or spit out the mucus, and with it, the trapped detritus. Ciliated cells lining the female reproductive tract create currents that sweep eggs from the ovaries into the oviducts and all the way down to the uterus. Flagellated cells maintain a flow of water through the bodies of sponges, bringing in food and oxygen and removing carbon dioxide and wastes. Flagella power the movement of the sperm of most species.
Microfilaments change cell shape and cause cell movements
Microfilaments are proteins that change conformation as a means of generating forces. The dominant microfilament in animal cells is the protein actin. Bundles of cross-linked actin strands form important structural components of cells. The microvilli that increase the absorptive surface area of the cells lining the gut are stiffened by actin microfilaments (see Figure 4.22), as are the stereocilia of the sensory hair cells in the mammalian ear. Actin microfilaments can change the shape of a cell by polymerizing and depolymerizing. Microfilaments reach their highest level of organization in muscle cells, which generate large-scale movements.
Together with the protein myosin, actin microfilaments generate the contractile forces responsible for many aspects of cell movement and changes in cell shape. The contractile ring that divides an animal cell undergoing mitosis into two daughter cells is composed of actin microfilaments in association with myosin. The mechanisms that many cells employ to engulf materials (endocytosis; see Figure 5.15) also rely on nets of actin and myosin beneath the plasma membrane.
Certain cells in multicellular animals travel within the body by amoeboid motion, which is generated by the activity of actin microfilaments and myosin. During development, many cells migrate by amoeboid motion. Throughout an animal's life, phagocytic cells circulate in the blood, squeeze through the walls of the blood vessels, and wander through the tissues by amoeboid motion. The mechanisms of amoeboid motion have been studied extensively in the protist for which this type of movement was named—the amoeba (see Figure 28.4).
Amoeboid motion is accomplished by the cell extending a lobe-shaped projection called a pseudopod and then seemingly squeezing itself into that pseudopod. The cytoplasm in the core of the cell is relatively liquid and is called plasmasol, but just beneath the plasma membrane the cytoplasm is much thicker, and is called plasmagel. To form a pseudopod, the thick plasmagel in one area of the cell thins, allowing a bulge to form. Just under the cell surface, in the plasmagel, is a network of actin microfilaments that interacts with myosin to squeeze plasmasol into the bulge. As the microfilament network continues to contract, cytoplasm streams in the direction of the pseudopod. When the cytoplasm at the leading edge of the pseudopod converts to plasmagel, the pseudopod stops forming. Thus the basis for amoeboid motion is the ability of the cytoplasm to cycle through sol and gel states and the ability of the microfilament network under the plasma membrane to contract and cause the cytoplasmic streaming that pushes out a pseudopod.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.