A series of key cellular chemical reactions starting and ending with oxaloacetic acid. Also called the citric acid or tricarboxylic acid cycle, it produces energy in the form of ADENOSINE TRIPHOSPHATE (ATP) and is the last stage in the biological oxidation of fats, proteins, and carbohydrates. Named after Sir Hans Krebs, a German biochemist working in England in 1900, who won the Nobel Prize for his discovery.
(citric acid cycle) a complex cycle of enzyme-catalysed reactions, occurring within the cells of all living animals, in which acetate, in the presence of oxygen, is broken down to produce energy in the form of *ATP (via the *electron transport chain) and carbon dioxide. The cycle is the final step in the oxidation of carbohydrates, fats, and proteins; some of the intermediary products of the cycle are used in the synthesis of amino acids. [Sir H. A. Krebs (1900–81), British biochemist]
the periodic sequence of events in sexually mature nonpregnant women by which an egg cell (ovum) is released from the ovary at four-weekly intervals until *menopause. The stages of the menstrual cycle are shown in the diagram. An ovum develops within a *Graafian follicle in the ovary. When mature, it bursts from the follicle and travels along the Fallopian tube to the uterus. A temporary endocrine gland – the corpus luteum – develops in the ruptured follicle and secretes the hormone *progesterone, which causes the lining of the uterus (endometrium) to become thicker and richly supplied with blood in preparation for pregnancy. If the ovum is not fertilized the cycle continues: the corpus luteum shrinks and the endometrium is shed at *menstruation. If fertilization does take place the fertilized ovum becomes attached to the endometrium and the corpus luteum continues to secrete progesterone, i.e. pregnancy begins.... menstrual cycle
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