Organism level systems Study Guide

Plant hormones, also known as plant growth regulators, are chemical substances produced by plants that control various aspects of growth and development, including cell division, elongation, differentiation, and responses to environmental stimuli. Humans have learned to harness these powerful chemicals for a variety of agricultural and horticultural purposes. Auxins, gibberellins, and ethene are key plant hormones widely used. Auxins, primarily indoleacetic acid (IAA), are responsible for cell elongation, apical dominance, and root development. They are used in rooting powders to promote root formation in cuttings, as selective weed killers by causing excessive growth in broad-leaved plants, and to prevent fruit drop by delaying the formation of the abscission layer. Gibberellins play a crucial role in seed germination, stem elongation, and fruit development. They are employed to speed up malting in brewing, promote seed germination, and increase the size of fruits, particularly grapes, through stem elongation. Ethene (ethylene) is a gaseous hormone that promotes fruit ripening, senescence (ageing), and abscission (shedding of leaves or fruit). It is used commercially to ripen fruits like bananas and tomatoes after they have been transported unripe, allowing for better storage and distribution. Understanding how these hormones work and their specific applications allows for greater control over plant morphology, yield, and timing of production, enhancing efficiency and profitability in plant cultivation. The careful application of these hormones requires knowledge of their concentration-dependent effects, as too much or too little can have adverse outcomes.

The kidneys play a crucial role in maintaining the body's water balance, a process known as osmoregulation. They do this by controlling how much water is reabsorbed back into the bloodstream and how much is excreted in the urine. When a person is well-hydrated, the kidneys filter the blood and produce a large amount of dilute urine, meaning less water is reabsorbed. Conversely, if a person is dehydrated, the kidneys work to conserve water. They increase the reabsorption of water back into the blood, resulting in a small amount of concentrated urine. This regulation is vital for maintaining the correct concentration of salts and other substances in the blood, which is essential for normal cell function. The process involves filtration of blood in the glomeruli to form a 'filtrate', followed by selective reabsorption in the kidney tubules. Water reabsorption is primarily controlled by a hormone called Antidiuretic Hormone (ADH), which influences the permeability of the kidney tubules to water. If ADH levels are high, more water is reabsorbed; if ADH levels are low, less water is reabsorbed.