ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Forty-Three Review Answers 1. A tissue is a group of structurally and functionally similar cells (such as smooth muscle tissue). A collection of tissues is an organ (such as the stomach). These organs are further grouped into organ systems (such as the digestive system) to carry out the principal activities of the body. 2. Epithelial cells can be flat, cuboidal, columnar, or stretchy. They are held together tightly at the edges to form a protective sheet of tissue for the purposes of lining or covering body structures. Mucous membranes line internal cavities such as the digestive tract, are kept moist, and function in secretion, absorption, and protection. Cutaneous membranes are typically dry and cover the outside of the body, functioning primarily in support, but also in secretion and some absorption. 3. Endocrine glands are ductless glands that secrete their materials into the bloodstream (such as the adrenal gland). Exocrine glands secrete their materials out through ducts (such as a salivary gland). 4. A connective tissue is a supporting tissue composed of living cells embedded in a nonliving matrix. The three categories are defensive, structural, and sequestering. 5. A ligament is connective tissue consisting of tough layers of collagen laid down in parallel to form a strong connection between two bones. Cartilage and bone differ in that cartilage is avascular, and the collagen in bone is impregnated with calcium salts, making it harder and less resilient than cartilage. Blood fits the definition of connective tissue because it consists of living cells (blood cells) in a nonliving matrix (plasma). 6. Muscle is derived from mesoderm. Actin and myosin are abundant in muscle. The three categories of muscle cells are smooth, skeletal, and cardiac, the last two of which are striated. 7. Skeletal muscle cells develop as a consequence of several cells fusing end to end, hence the increased number of nuclei. Intercalated discs in cardiac muscle tissue represent areas of gap junctions between cells, which permits the myocardium to contract as a unit. 8. Neurons can be classified functionally as sensory, conveying sensory information to the brain, or motor, conveying a response message from the brain to an effector (muscle or gland). Support cells in the nervous system are collectively referred to as glia, although the "support cells" of the peripheral nervous system are considered to be the Schwann cells in their protective wrap around nerve axons. 9. Homeostasis is the maintenance of constant extracellular conditions within the body. A negative feedback loop prevents the departure of a controlled variable such as pH or temperature from its normal, or ideal, value. Changes in the controlled variable trigger responses that oppose the change and restore the variable to its ideal level. An example of negative feedback is the regulation of body temperature. Positive feedback refers to a condition in which a change in a controlled variable actually causes the body to drive the controlled variable even further away from the initial value. An example of positive feedback is the contractions of the uterus during childbirth.