Biology  5/e   Raven/Johnson
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Actin: All eukaryotic cells have a network of protein fibers that crisscrosses their cytoplasm. Actin filaments, microtubules, and intermediate filaments are the three types of protein fibers that comprise this cytoskeleton. Actin molecules spontaneously form their filament structures. These actin filaments play a major role in determining cell shape and also function in cell movement and the contraction of muscle cells.

ATP: ATP, adenosine triphosphate, is the universal energy currency of all living organisms and cells. Energy must be supplied if work is to be done, and whenever work is performed by a cell, the necessary energy is supplied by breaking one of the high-energy phosphate bonds of ATP. This releases energy (to do work) and leaves ADP (adenosine diphosphate) and P_i (inorganic phosphate). There is a continual cycling between ATP and ADP + P_i in living cells. Cellular respiration uses the energy released from oxidized foodstuffs to synthesize ATP from ADP + P_i. Without ATP, a cell could not work and therefore, the organism could not live; it could not carry out active transport or contract muscles, for example.

Bone: Bone is a type of specialized connective tissue, one of the four major types of vertebrate tissues. Bone is similar to another type of connective tissue, cartilage. In bone, the collagen fibers are coated with calcium phosphate salts, so that bone is strong without being brittle.

Striated muscle: Striated muscle is another name for skeletal and cardiac muscle. It is responsible for movement of the vertebrate skeleton and beating of the heart. The fibers in a striated muscle can contract simultaneously and exert considerable force. It gets its name from the pattern of alternating light and dark bands seen microscopically.

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