Nonmembrane Bound Organelles 231 Ribosomes

As mentioned above, ribosomes are organelles used to help form proteins. Instead of being composed of, or contained in, a membrane, ribosomes are proteins combined with RNA molecules. The ribosome is made of two subunit pieces which come together to facilitate formation of a chain of amino acids into a peptide and then often into a larger protein. Ribosomes have a large subunit and a small subunit. A ribosome that is attached to the endoplasmic reticulum (ER) is called bound ribosome, while those ribosomes scattered in the cytoplasm or joined together in small clusters in the cytoplasm are called free ribosomes. Messenger RNAs from the cell's nucleus travel into the cytoplasm and encounter ribosomes. The ribosomes then bring together amino acids to form an amino acid chain, or peptide, according to the code of the mRNAs (see Sec. 7.5). If the peptide is formed on free ribosomes it is usually used for intracellular processes. Peptides formed by ribosomes bound to endoplasmic reticulum are often for use outside of the cell or in the cell membrane. The peptides, once formed in the RER, may be transferred to the Golgi apparatus for packaging into a vesicle for extracellular transport.

2.3.2 Cytoskeleton

As the name implies, most cells have a matrix of tubules and filaments, which can maintain or change the cells' shape as well as move substances around within the cytoplasm. There are three main types: microfilaments, intermediate filaments, and microtubules (Fig. 2.7).

Microfilament as the name implies, are small linear fibers which form much of the framework of a cell and are used to maintain structure. They are sometimes called actin filaments since they are composed

Figure 2.7 Cytoskeleton. The three types of cytoskeletal filaments are microfilaments with a diameter of around 7 nm, microtubules which may exceed 25 nm, and intermediate filaments with an intermediary size. The filaments have in common that they are all composed of repeating protein subunits. The cytoskeleton provides for the form of the cell, for transport within the cell, and, in some cells, for the motility of the cell. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

Figure 2.7 Cytoskeleton. The three types of cytoskeletal filaments are microfilaments with a diameter of around 7 nm, microtubules which may exceed 25 nm, and intermediate filaments with an intermediary size. The filaments have in common that they are all composed of repeating protein subunits. The cytoskeleton provides for the form of the cell, for transport within the cell, and, in some cells, for the motility of the cell. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

APPLICATION BOX 2.2 The Muscle Machine

Hugh E. Huxley elegantly described in 1969 what has been come to be known as the Sliding Filament Theory of muscle contraction. The theory describes the movement of a series of proteins in conjunction with calcium and energy from ATP.

The sliding between the linearly arranged proteins actin and myosin creates a subcellular machine. This machine changes the shape of the muscle cell, and when combined with other muscle cells, the shape of the whole muscle. Changing the shape of a muscle ultimately creates, in conjunction with bones, movement of a limb. It is fascinating to understand how the actions of the molecular muscle machine composed of actin, myosin, and other proteins translate into movement of an organism. This information of cell and molecular biology is useful in robotics and bionics, the combination of biology and technology, and in biomechanics in general. Muscle, generally working with a third-class lever within the skeletal system, is roughly 25% efficient when performing work. By comparison, an internal combustion engine operates at roughly 20% efficiency.

of the protein actin. Cell extensions filled with a network of cross-linked actin filaments are called microvilli (Fig. 2.8). They serve to increase surface area for absorption and secretion across the cell membrane.

A special type of microvilli are stereocilia. They are specialized cell protrusions with a rigid internal framework composed of actin on the apical membrane of hair cells. These hair cells are part of the inner ear (Fig. 2.9) and are responsive to sounds transmitted to the ear through the air. The hair cells are arranged on a platform called the basilar membrane. The membrane vibrates in different regions of the inner ear depending on the frequency of the sound wave. When a segment of the membrane vibrates, the hair cells resting on that membrane also vibrate. The stereocilia (hairs)

Microvilli

Figure 2.8 Microvilli. These are protrusions on a cell's apical surface that serve to increase surface area for absorption or secretion. For instance, intestinal lining cells have rich microvilli layers for the vast amounts of absorption occurring in the small intestine. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

Microvilli

Figure 2.8 Microvilli. These are protrusions on a cell's apical surface that serve to increase surface area for absorption or secretion. For instance, intestinal lining cells have rich microvilli layers for the vast amounts of absorption occurring in the small intestine. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

Figure 2.9 Stereocilia on hair cells within the spiral organ of the inner ear. Sound waves travel through the air to the chambers of the ear, move fluid of the ear, and cause a wave motion of the basilar membrane. The movement of the basilar membrane causes movement of the hair cells relative to their rigid stereocilia. The bending of the stereocilia causes activation or inhibition of signals in the cochlear nerve leading to the brain. This neuronal activity is detected as sound. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

Figure 2.9 Stereocilia on hair cells within the spiral organ of the inner ear. Sound waves travel through the air to the chambers of the ear, move fluid of the ear, and cause a wave motion of the basilar membrane. The movement of the basilar membrane causes movement of the hair cells relative to their rigid stereocilia. The bending of the stereocilia causes activation or inhibition of signals in the cochlear nerve leading to the brain. This neuronal activity is detected as sound. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

of these now vibrating hair cells are anchored into yet another membrane called the tectorial membrane. The other end of the stereocilia is tightly anchored to the inner surface of the hair cell membrane. Tension is applied to the stereocilia as the rest of the cell moves with the basilar membrane. When this movement occurs relative to the stereocilia, a signal is sent to the central nervous system, which then is interpreted as a specific sound.

Intermediate filaments (Fig. 2.7), intuitively, are in between the size of microfilaments and microtubules. These fibers give further shape to the cell. They are stable and durable and hence very prominent in cells that withstand mechanical stress. There are five different types of intermediate filaments. One of them, called keratin, gives strength to the cells of the skin (Freeman, 2005).

Microtubules (Fig. 2.7), are composed of repeating protein subunits called tubulin. Microtubules can grow by adding subunits at either end or shorten by the removal of subunits. In this way, a substance that is attached to the microtubule can be moved through the cytoplasm. Additionally, motor protein such as kinesin can transport vesicles, granules, and organelles such as mitochondria along a microtubule by forming and breaking bonds with the energy supplied by ATP. The energized kinesin molecule then "walks" along the length of the microtubule (Freeman, 2005; Viel and Lue, 2006).

(b) Flagellum

Figure 2.10 (a) Cilia and (b) flagella. These are common cell surface modifications composed of microtubules. Cilia often move extracellular material along the surface of cells, such as particles on top of airway cells. The sperm's flagellum (tail) is used to help the sperm head drive through the surface of an oocyte to accomplish fertilization. SEM, scanning electron micrographs. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

(b) Flagellum

Figure 2.10 (a) Cilia and (b) flagella. These are common cell surface modifications composed of microtubules. Cilia often move extracellular material along the surface of cells, such as particles on top of airway cells. The sperm's flagellum (tail) is used to help the sperm head drive through the surface of an oocyte to accomplish fertilization. SEM, scanning electron micrographs. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

Two frequent cellular appendages associated with microtubules are cilia and flagella. Cilia (Fig. 2.10a) are fine cellular foldings of the cell membrane typically associated with the apical (top) surface of a cell responsible for absorption and motility. Microtubules create movement of the cilia moving extracellular material along the surface of the cells. An example of this is the airways leading to and from the lungs. Mucus accumulates particulate matter inhaled from the air, and the cilia move the contaminated mucus toward the throat for swallowing and disposal by the digestive system. The cilia in the apical surface membrane appear as "fuzz" on a cell at the light microscopic level giving rise to the identification of "hair" cells in certain cases, not to be mixed up with the hairs (stereocilia) of the inner ear hair cells described above.

Microtubules are also the functional component of flagella (Fig. 2.106). Flagella are long cell processes or cell surface modifications. Flagella are longer than cilia and are used for motility by some cells. The most recognizable flagellated cell is the sperm. The sperm flagellum is used to help motivate the cell through the female reproductive tract. Inside the flagellum is an arrangement of microtubules in a figure-of-8 which represents the fused pairs of microtubule doublets. Projecting from these figure-8 microtubules are protein called dynein. The dynein arms make and break bonds with the adjacent microtubules in a spiraling

Microtubule triplet

Microtubule triplet

Longitudinal section of centriole

Figure 2.11 Centrosome. The centrosome includes the two perpendicularly oriented centrioles and the space immediately around them. Centrosomes and centrioles serve as microtubule-organizing centers in nondividing cells and are involved in facilitating mitosis in dividing cells. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

Longitudinal section of centriole

Figure 2.11 Centrosome. The centrosome includes the two perpendicularly oriented centrioles and the space immediately around them. Centrosomes and centrioles serve as microtubule-organizing centers in nondividing cells and are involved in facilitating mitosis in dividing cells. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

fashion, which produces a swishing of the sperm tail. There are nine figure-8 microtubule structures surrounding a central pair of microtubules in the typical eukaryotic flagella in a 9 + 2 arrangement.

Centrosomes (Fig. 2.11) are microtubule structures involved in movement of chromosomes (DNA) during cell division. Centrioles are paired structures in the centrosome. Each centriole is comprised of nine triplet microtubules. The centrioles are organizing centers for microtubule formation since microtubules originate there and are grown and shortened from there as needed in the cell. When cells divide, genetic material is packaged together to form chromosomes. These chromosomes are then divided between daughter cells when cen-trioles attached to the chromosomes pull the duplicated genetic material apart through microtubule shortening (see Sec. 8.2).

Essentials of Human Physiology

Essentials of Human Physiology

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.

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