of their conjugated chain. An exception is lycopene, which has no ring structure and does not have vitamin A activity. Preformed vitamin A is found only in animal-derived food products, whereas dietary carotenoids are present primarily in oils, fruits, and vegetables.
The 11-cis-retinaldehyde (retinal) form of vitamin A is required by the eye for the transduction of light into neural signals necessary for vision (Saari, 1994). The retinoic acid form is required to main-
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4 Vitamin A ."
Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc . Washington, DC: The National Academies Press,
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As of 2013, the National Science Education Standards have been replaced by the Next Generation Science Standards (NGSS), available as a print book, free PDF download, and online with our OpenBook platform.
The NGSS offer a detailed description of the key scientific ideas and practices that all students should learn by the time they graduate from high school. The standards are based largely on the 2011 National Research Council report A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas.