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WATER BALANCE IN PLANTS AND HUMANS Ch 44 and 39 I. Purpose of maintaining water balance To keep an organism in osmotic balance o Land animals and plants are hypotonic to their environment and would tend to lose water o They have mechanisms to prevent excess water loss II. Water balance in Mammals Involves the kidney A. Structure of the mammalian kidney Composed of millions of nephrons Each nephron filters blood and reabsorbs water and ions Each nephron consists of a long tube The long tube and the millions of nephrons increases surface area for water reabsorption Each nephron is surrounded by capillaries to increase surface area for water reabsorption B. parts of a nephron Glomerulus: a mass of capillaries surrounded by Bowmans capsule that filters fluid from blood. Fluid contains urea, water, ions, glucose, amino acids Loop of Henle: reabsorbs molecules back into blood o Water: by osmosis o ions, glucose and amino acids: by active transport Collecting tubule: collects waste to be transported to ureter and bladder C. How kidneys filter Renal artery brings blood to kidney Blood travels to the Glomerulus where it is filtered, removing ions, urea, glucose and amino acids Water follows because blood has become hypotonic In the loop of Henle, ions like Na+ are reabsorbed back into the blood by active transport Water follows because filtrate has become hypotonic In the collecting tubule, the filtrate leaves the kidney through the ureter to the bladder http://www.sumanasinc.com/webcontent/ animations/content/kidney.html http://www.mhhe.com/cgibin/netquiz_get.pl?qfooter=/usr/web/hom e/mhhe/biosci/genbio/animation_quizzes/a nimate_64fq.htm&afooter=/usr/web/hom e/mhhe/biosci/genbio/animation_quizzes/a nimate_64fa.htm&test=/usr/web/home/m hhe/biosci/genbio/animation_quizzes/ani mate_64q.txt&answers=/usr/web/home/m hhe/biosci/genbio/animation_quizzes/ani mate_64a.txt D. Endocrine Control of Water balance 1. Dehydration: high blood osmolality Means not enuf water in blood Osmoreceptors in hypothalamus pick this up and: Produce Sensation of Thirst act on pituitary to secrete ADH We drink acts on kidney Tubules to Reabsorb more Back into Blood Blood osmolality lowers and we get negative feedback to hypothalamus URINE IS MORE CONCENTRATED! 2. Excess water from overhydration: low blood osmolality Osmoreceptors in hypothalamus detect this and cause pituitary to secrete less ADH which means less water reabsorption into blood URINE IS DILUTE! E. Other mechanisms used by land Animals Exoskeleton in insects Dead skin in mammals Scales Animals that live in deserts often are nocturnal III. Water Balance in land plants Plants must balance the rate of photosynthesis with the rate of transpiration Both processes require open stomata Problem: if water is limited, excessive transpiration will cause water column to break and plant will die Problem: if stomata close to prevent excessive transpiration, photosynthesis will stop and plant will die Mechanisms to prevent excess transpiration in plants that live in dry areas (see notes ch 36): Thicker waxy cuticle on plants control of stomata opening in plants sunken stomata stomata located on lower surface of leaves