English Botany 5-1 PDF

❑ Temperature plays a major role in the rate at which the respiratory reactions occur. For example, when air temperatures rise from 20°C (68°F) to 30°C (86°F), the respiration rates of plants double and sometimes even triple. ❑ Respiration is a catabolic process that takes place in the cytoplasm an...

❑ Temperature plays a major role in the rate at which the respiratory reactions occur. For example, when air temperatures rise from 20°C (68°F) to 30°C (86°F), the respiration rates of plants double and sometimes even triple. ❑ Respiration is a catabolic process that takes place in the cytoplasm and mitochondria of cells. The energy is released, with the aid of enzymes, from simple sugar and organic acid molecules. ❑ In aerobic respiration, stored energy release requires oxygen; CO2 and water are by-products of the process. ❑ Anaerobic respiration and fermentation do not require oxygen gas, and much less energy is released. The remaining energy is in the ethyl alcohol, lactic acid, or other such substances produced. Some released energy is stored in ATP molecules. Temperature, available water, and environmental oxygen affect respiration rates. ❑ Glycolysis, which occurs in the cytoplasm, requires no molecular oxygen; two phosphates are added to a 6-carbon sugar molecule, and the prepared molecule is split into two 3-carbon sugars (GA3P). Some hydrogen, energy, and water are removed from the GA3P, producing pyruvic acid. There is a net gain of two ATP molecules. Hydrogen ions and electrons released during glycolysis are picked up by NAD, which becomes NADH. ❑In aerobic respiration, which occurs in the mitochondria, pyruvic acid loses some CO2, is restructured, and becomes acetyl CoA. Energy, CO2, and hydrogen are removed from the acetyl CoA in the citric acid cycle, which involves enzyme-catalyzed reactions of a series of organic acids. Aerobic respiration Anaerobic respiration 2 2 ❑ NADH passes the hydrogen gained during glycolysis and the citric acid cycle along an electron transport system; small increments of energy are released and partially stored in ATP molecules, and the hydrogen is combined with oxygen gas, forming water in the final step of aerobic respiration (electron transport chain). Several of the electron carriers in the transport system are cytochromes. They are very specific and, as electrons flow along the system, they can transfer their electrons only to other specific acceptors. When the electrons reach the end of the system, they are picked up by oxygen and combine with hydrogen ions, forming water. ❑ Hydrogen removed from glucose during glycolysis is typically combined with an inorganic ion in anaerobic respiration. The hydrogen is combined with the pyruvic acid or one of its derivatives in fermentation. Both processes occur in the absence of oxygen gas, with only about 7% of the total energy in the glucose molecule being released, for a net gain of two ATP molecules. ❑ Two molecules of NADH and two ATP molecules are gained during glycolysis when two 3-carbon pyruvic acid molecules are produced from a single glucose molecule. Another molecule of NADH is produced when the pyruvic acid molecule is restructured and becomes acetyl CoA prior to entry into the citric acid cycle. ❑ In the citric acid cycle, acetyl CoA combines with 4-carbon oxaloacetic acid, producing first a 6-carbon compound, next a 5- carbon compound, and then several 4-carbon compounds. The last 4-carbon compound is oxaloacetic acid. Two CO2 molecules are also released during this process. ❑ Some hydrogen removed during the citric acid cycle is picked up by FAD and NAD; one molecule of ATP, three molecules of NADH, and one molecule of FADH2 are produced during one complete cycle. Energy associated with electrons and/or with hydrogen picked up by NAD and FAD is gradually released as the electrons are passed along the electron transport system; some of this energy is transferred to ATP molecules during oxidative phosphorylation. ❑ Energy used in ATP synthesis during oxidative phosphorylation is believed to be derived from a gradient of protons formed across the inner membrane of a mitochondrion, while electrons are moving in the electron transport system by chemiosmosis. ❑ Altogether, 38 ATP molecules are produced during the complete aerobic respiration of one glucose molecule; two are used to prime the process, so there is a net gain of 36 ATP molecules. ❑ In addition to photosynthesis and respiration, other metabolic pathways are required for growth, development, reproduction, and survival. Essential products of additional pathways include nucleotides, proteins, chlorophylls, and fatty acids. Secondary metabolites include alkaloids, phenolics, and terpenoids. ❑ Conversion of sugar produced by photosynthesis to fats, proteins, complex carbohydrates, and other substances is termed assimilation. Digestion takes place within plant cells with the aid of enzymes. During digestion, large insoluble molecules are broken down by hydrolysis to smaller soluble forms that can be transported to other parts of the plant. ❑ Growth is defined as an “irreversible increase in volume due to the division and enlargement of cells.” As cells mature, they differentiate into forms adapted to specific functions. ❑ Development is a change in form as a result of growth and differentiation combined. Cells themselves assume different shapes and forms, which adapt them to the problem involved in their total volume increasing at a greater rate than their total surface area. Many growth phenomena are influenced by hormones, which are produced in minute amounts in one part of an organism and usually transported to another part, where they have specific effects on growth, flowering, and other plant activities. Vitamins are organic molecules that function in activating enzymes; they are sometimes difficult to distinguish from hormones. The major known types of plant hormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene. ❑ Darwin and his son noted in 1881 that coleoptiles bend toward a light source. Frits Went, in 1926, followed up on the Darwinian observations and demonstrated that something in coleoptile tips moved out into agar when decapitated tips were placed on it. He called the substance auxin and showed that it could cause coleoptiles to bend. Went’s experiment with oat coleoptiles. A. A germinated oat “seed” with an intact coleoptile. B. The tip of a coleoptile was cut off, placed on a small agar block, and left for an hour or two. C. When this agar block was placed squarely on a decapitated coleoptile, growth was vertical. D. When the agar block was placed off center so that only half of the decapitated coleoptile was in contact with it, the tip bent away from it. The leaf within the coleoptile was pulled up slightly to support the agar block. This experiment demonstrated that something affecting growth diffused from the coleoptile tip into the agar and from the agar to the part of the decapitated coleoptile touching the block. How a bioassay for auxin is made. A. Coleoptile tips are cut and placed on a measured portion of agar. B. for a set period of time. C. The coleoptile tips are then removed, and the agar is cut into blocks of specific size. An agar block is placed off center on a decapitated coleoptile held by a clamp; the leaf within the coleoptile is pulled up slightly to support the agar block. After another set period, the angle of curvature is measured. D. The angle of curvature is compared to that produced when a similar agar block containing a known amount of auxin is placed off center on another coleoptile for the same period of time.  Three groups of plant hormones that promote the growth of plants have been found; others are known only for their inhibitory effects. Auxins stimulate the enlargement of cells and are involved in many other growth phenomena. Synthetic auxinlike compounds (2,4,5-T & 2,4-D) have been used as weed killers and defoliants. IAA or auxin (indole acetic acid) stimulate the formation of roots on almost any plant organ. Monocot plants tends to be the least sensitive to auxins. ❑ Acetyl coenzyme A, which is vital to the process of respiration, functions as a precursor in the synthesis of GA. ABA is synthesized in plastids, apparently from carotenoid pigments. ❑ Gibberellins promote stem growth without corresponding root growth and flowering; cytokinins promote cell division and can be used to stimulate the growth of axillary buds. Abscisic acid causes buds to become dormant and apparently helps leaves respond to excessive loss of water. Ethylene gas hastens ripening of fruits and is used commercially to ripen green fruits. ❑ Senescence is the breakdown of cell parts that leads to the death of the cell. ❑ Plant movements, such as nutations (spiraling growth) and nodding (the bent hypocotyl – growing tip seedling through the soil), twining, contractile, and nastic movements (turgor changes or pressure in special cells - nastic include sleep and contact movements), are growth movements that result primarily from internal stimuli.  The leaflets of the leaf toward the bottom of this picture have folded upward in response to being bump. The other leaves Typical twining of a tendril produced by a of this sensitive plant have remained fully manroot plant. Note that the direction of expanded. coiling reverses near the midpoint. Effect of auxin applied to the base of a Gardenia cutting 4 weeks after application. Left. A cutting treated with auxin. Right. An untreated cutting. Apical dominance is the suppression of the growth of the lateral buds (also called axillary buds), each of which can form a branch with its own terminal bud. Apical dominance is believed to be brought about by an auxinlike inhibitor in a terminal bud. It is strong in trees with conical shapes and little branching toward the top. Apical dominance can be offset with an application of cytokinins to axillary buds. A Jeffrey pine tree. The trunk of this tree, which would normally be single, is forked because of the earlier removal of the terminal bud, allowing a lateral bud to grow. Effect of gibberellins on flowering. Cabbage plants on the right were grown in a warm greenhouse but did not grow tall and flower until treated with gibberellins. The short cabbage plants on the left ere not treated with gibberellins. Effect of ethylene on holly twigs. Two similar twigs were placed under glass jars for a week; at the same time, a ripe apple was placed under the jar on the right. Ethylene produced by the apple caused abscission of the holly leaves. Water conservation movement in a grass leaf when insufficient water to maintain normal turgor is available. A. The leaf when adequate water is available. B. The leaf after it has rolled up. C. Enlargement of a cross section of a rolled leaf showing the location of the large, thin-walled bulliform cells, which partially collapse under dry conditions and thus bring about the rolling of the leaf blade. ❑ Tropisms are permanent, directed growth movements that result from external stimuli, such as light (phototropism), gravity (gravitropism), contact (thigmotropism), and chemicals such as salt (chemotropism or halotropism). ❑ Turgor movements result from changes in internal water pressures; they may be very rapid or take up to 45 minutes to become visible. Turgor movements include contact movements and sleep movements in which leaves or flowers fold daily in what is known as a circadian rhythm. ❑ Garner and Allard’s were responsible for coining the term photoperiodism. ❑ Photoperiodism is a response of plants to the duration of day or night. Short-day plants will not flower unless the day length is shorter than a critical day length, and long-day plants will not flower unless the day length is longer. Intermediate-day plants have two critical photoperiods; the flowering of day-neutral plants is independent of day length. A bush monkey flower. The white, two-lobed structure in the center is the stigma. A. The stigma as it appears before pollination. B. The stigma 2 seconds after being touched by a pollinator; the lobes have rapidly folded together. A prayer plant (Maranta). A. The plant at noon. B. The same plant at 10 P.M., after “sleep” movements of its leaves have occurred. Photoperiodism. The poinsettia with red bracts received less than 8.5 hours of light per day. The green poinsettias received more than 10 hours of light per day and did not flower. It was believed that stem tips bent toward light because auxin was destroyed or inactivated on the exposed side, leaving more growth-promoting hormone on the side away from the light, causing the cells there to elongate more and produce a bend. A cyclamen plant that received light from one direction for several weeks. Note how all visible plant parts are oriented on the side that received light. This is an example of positive phototropism. An experiment illustrating the effect of subjecting one leaf of a short-day plant (cocklebur) to short days while the rest of the plant is exposed to long days. A. The short-day plant exposed to long days. No flowers were produced. B. The same plant exposed to the same long days while one leaf was covered with black paper 16 hours a day for a few weeks (mimicking short days). The plant flowered, presumably because some substance that initiates flowering was produced in the shaded leaf and then diffused or was carried to the stem tip where flower buds are produced. ❑ Phytochromes are light-sensitive pigments that occur in all higher plants and play a role in many different plant responses (flowering). Phytochromes occur in two forms, each of which can be converted to the other by the absorption of light. Day light generally results in more Pr being converted to Pfr (the active form) than vice versa. Pr absorbs red light and Pfr absorbs far-red light. Pfr will convert back to Pr over a period of several hours in the dark; Pr is stable indefinitely in the dark. ❑ Growth responses to the stimulus of gravity are called gravitropisms. The primary roots of plants tend to be positively gravitropic, while shoots forming the main axis of plants are typically negatively gravitropic. ❑ Dormancy is a period of growth inactivity in seeds, buds, bulbs, and other plant organs even when appropriate environmental conditions are met. Quiescence is a state in which a seed is unable to germinate unless appropriate environmental conditions exist. This Coleus plant was placed on its side the day before the photograph was taken. The stems bent upward within 24 hours of the pot being tipped over. This is an example of negative gravitropism.