AP Biology Notes Outline Enduring Understanding 2.C

Transcription

1 AP Biology Notes Outline Enduring Understanding 2.C Big Idea 2: Biological systems utilize free energy and molecular building blocks to grow, to reproduce and to maintain dynamic homeostasis. Enduring Understanding 2.C: Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis. Learning Objectives: Essential Knowledge 2.C.1: Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. (2.15) The student can justify a claim made about the effect(s) on a biological system at the molecular, physiological or organismal level when a given scenario in which one or more components within a negative regulatory system is altered. (2.16) The student is able to connect how organisms use negative feedback to maintain their internal environments. (2.17) The student is able to evaluate data that show the effect(s) of changes in concentrations of key molecules on negative feedback mechanisms. (2.18) The student can make predictions about how organisms use negative feedback mechanisms to maintain their internal environments. (2.19) The student is able to make predictions about how positive feedback mechanisms amplify activities and processes in organisms based on scientific theories and models. (2.20) The student is able to justify that positive feedback mechanisms amplify responses in organisms. Essential Knowledge 2.C.2: Organisms respond to changes in their external environments. (2.21) The student is able to justify the selection of the kind of data needed to answer scientific questions about the relevant mechanisms that organisms use to respond to changes in their external environments. Bozeman Instruction Videos: 2.C.1 - # 018: Positive and Negative Feedback Loops 2.C.2 - # 019: Response to External Environments Required Readings: Textbook Ch. 1 (pp. 11) Textbook Ch. 40 (pp. 868) Textbook Ch. 36 (pp. 777) Textbook Ch. 39 (pp. 832 & 834) Textbook Ch. 46 (pp & 1016) Textbook Ch. 45 (pp & pp. 988; 990) Textbook Ch. 39 (pp & pp ) Textbook Ch. 41 (pp. 1122) Textbook Ch. 40 (pp ) Animated Content Review Videos: Temperature Regulation in Animals: Plant Response to Drought: Lactation in Mammals: Onset of Labor in Childbirth: Maintenance of Blood Glucose Homeostasis: Diabetes Mellitus: Thyroid Function & Control of Metabolism & Development: Darwin s Experiments on Phototropisms in Plants: Went s Experiments on Phototropisms in Plants: Photoperiodism in Plants: Shivering and Sweating in Humans: Chemotaxis in Bacteria: Practicing Biology Homework Questions: Questions #26-29 L. Carnes

2 Essential Knowledge 2.C.1: Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. Organisms use negative feedback mechanisms to maintain their internal environments by returning the changing condition back to its target set point, while positive feedback mechanisms amplify responses. Examples of negative feedback responses include temperature regulation in animals and plant responses to drought; examples of positive feedback mechanisms are the onset of labor in childbirth and ripening of fruit. Alterations in feedback mechanisms can have deleterious effects, including diabetes and Graves disease in humans and the inability of plants to tolerate water stress during drought. Negative Feedback Mechanisms: Negative feedback mechanisms maintain dynamic homeostasis for a particular condition (variable) by regulating physiological processes, returning the changing condition back to its target set point. Feedback Inhibition mechanism of homeostasis whereby a change in a physiological variable (D) that is being monitored triggers a response that counteracts the initial fluctuation (A). Final product (D) inhibits first enzyme in sequence (A) and shuts reaction down. Temperature regulation in animals is an illustrative example of a negative feedback loop, whereby dynamic homeostasis for a particular condition (temperature) is regulated by physical processes (hypothalamic control). What is the condition being monitored? What is the set point? How is homeostasis maintained if set point is exceeded? How is homeostasis maintained if temperature drops below the set point? WHY is this an appropriate example of a NEGATIVE feedback loop?

3 Plant responses to water limitations are an illustrative example of negative feedback. For plants, balancing water uptake against water loss is important because severe water deficit can be deadly. Fortunately, plants have control systems that enable them to cope with extreme water deficits. Most of these control systems help the plant conserve water by reducing the rate of transpiration. Water deficit in the leaf causes guard cells to lose turgor, which assist in closing stomata. It also stimulates increased synthesis of ABA (abscisic acid) in the leaf, a hormone that acts on guard cell membranes. ABA is the primary internal signaling molecule that enables plants to withstand drought. When a plant begins to wilt, ABA is produced and accumulates in leaves and causes stomata to close rapidly, reducing transpiration and preventing further water loss. By affecting second messengers such as calcium, ABA causes potassium channels in the plasma membrane of guard cells to open, leading to massive loss of potassium ions from the cells. As potassium ions exit the cell, water follows via osmosis. The accompanying osmotic loss of water reduces guard cell turgor and leads to closing of the stomatal pores. What is the condition being monitored? What is the set point? How is homeostasis maintained if temperature drops below the set point? WHY is this an appropriate example of a NEGATIVE feedback loop? Positive Feedback Mechanisms: Positive feedback mechanisms amplify responses and processes in biological organisms. The variable initiating the response is moved farther away from the initial set-point. Amplification occurs when the stimulus is further activated which, in turn, initiates an additional response that produces system change. Illustrative examples include lactation in mammals, the onset of childbirth during labor and fruit ripening in plants. Positive Feedback a physiological control mechanism in which a change in some variable (Z) triggers mechanisms that amplify the change. In this example, the product of the reaction (Z) enhances the reaction of one of the enzymes (black arrow), increasing the rate of production of the product.

4 Lactation in mammals is an example of a positive feedback mechanism. Review the diagram to the right What variable initiates the response? How is the variable moved further away from the initial set point? When is the response amplified? Why is this an appropriate example of POSITIVE feedback? A complex interplay of local regulators and hormones induces and regulates labor. A series of strong, rhythmic uterine contractions bring about birth. Estradiol (produced in the ovaries) induces oxytocin receptors in the uterus. Oxytocin stimulates the uterus to contract and stimulates the placenta to make prostaglandins. Prostaglandins stimulate more contractions of the uterus (cause more prostaglandins to be produced by the placenta) and cause more oxytocin to be produced, which amplifies the contractions thereby forcing the fetus out of the uterus and through the vagina. What variable initiates the response? How is the variable moved further away from the initial set point? When is the response amplified? Why is this an appropriate example of POSITIVE feedback? In most plants, a burst of ethylene production in the fruit triggers the ripening process. The enzymatic breakdown of cell wall components softens the fruit, and the conversion of starches and acids to sugars makes the fruit sweet. The production of new scents and color helps advertise ripeness to animals which eat the fruit and disperse the seeds. A chain reaction occurs during fruit ripening: ethylene triggers ripening, and ripening triggers more ethylene production. The result is a huge burst in ethylene production (a gas) which spreads from fruit to fruit. Why is this an example of positive feedback? This is an amplified response the variable initiating the response is moved further away from the initial set point amplification occurs when the stimulus is further activated which, in turn, initiates additional response that produces a system change.

5 Alterations in Feedback Mechanisms: Alteration in the mechanisms of feedback often results in deleterious consequences. Illustrative examples include Diabetes mellitus in response to decreased insulin and Graves disease (hyperthyroidism). Two types of cells in the pancreas secrete insulin and glucagon, which are antagonistic hormones that help maintain glucose homeostasis: Insulin reduces blood glucose levels by promoting the cellular uptake of glucose and slowing glycogen breakdown in the liver and promoting fat storage. Glucagon increases blood glucose levels by stimulating the conversion of glycogen to glucose in the liver and by stimulating the breakdown of fat and protein into glucose. A disruption in glucose homeostasis can be quite serious, affecting the heart, blood vessels, eyes, and kidneys. Diabetes mellitus, perhaps the best-known endocrine disorder is caused by a deficiency of insulin or a decreased response to insulin in target tissues. It is marked by elevated blood glucose levels because cells are unable to take up enough glucose to meet metabolic needs. This high level of glucose in blood exceeds the capacity of the kidneys to reabsorb the nutrient and sugar is excreted in the urine. Type I diabetes mellitus (insulin-dependent diabetes) is an autoimmune disorder in which the immune system destroys the beta cells of the pancreas, thereby destroying a person s ability to produce insulin. Type II diabetes mellitus (non-insulin-dependent diabetes) is characterized either by a deficiency of insulin or, more commonly, by a failure of target cells to respond normally to insulin. Insulin is produced, but target cells often fail to take up glucose form the blood, and blood glucose levels remain elevated.

6 In vertebrate animals, thyroid hormone, secreted by the thyroid gland, regulates both homeostasis and development. In humans and other mammals, thyroid hormone regulates bioenergetics; helps maintain normal blood pressure, heart rate, and muscle tone; and regulates digestive and reproductive functions. The hypothalamus secretes TSH-releasing hormone (TRH), which stimulates the anterior pituitary to secrete thyroidstimulating hormone (TSH). TSH then stimulates the thyroid gland to synthesize and release the thyroid hormones T3 and T4. These hormones exert negative feedback on the hypothalamus and anterior pituitary by inhibiting the release of TRH and TSH. Too much or too little thyroid hormone in the blood can result in serious metabolic disorders. Excessive secretion of thyroid hormone can lead to high body temperature, profuse sweating, weight loss, irritability, and high blood pressure. Grave s disease is an autoimmune disorder in which the immune system produces antibodies that bind to the receptor for TSH and activate sustained thyroid hormone production. Protruding eyes, caused by fluid accumulation behind the eyes, are a typical symptom. Essential Knowledge 2.C.2: Organisms respond to changes in their external environments. Organisms respond to changes in their internal and external environments through behavioral and physiological mechanisms, such as phototropism and photoperiodism in plants, hibernation and migration in animals, taxis and kinesis in animals/bacteria, and shivering and sweating in humans. Phototropism & Photoperiodism in Plants: Response to information and communication of information are vital to natural selection in plants. In phototropism, changes in the light source lead to differential growth, resulting in maximum exposure of leaves to light for photosynthesis. In photoperiodism, changes in the length of night regulate flowering and preparation for winter. In plants, physiological events involve interactions between environmental stimuli and internal molecular signals. These responses are often caused by hormones. Plant hormones help coordinate growth, development, and responses to stimuli. Hormones are chemical signals that coordinate the different parts of an organism. They are produced by one part of the body and transported to another. Any growth response that results in curvatures of whole plant organs toward or away from a stimulus is called a tropism. Phototropism is the growth of a shoot in a certain direction in response to light. Positive Phototropism: growth toward light. Negative Phototropism: growth away from light.

7 (Darwin s Experiments) (Went s Experiments) The term auxin is used for any chemical substance that promotes cell elongation in different target tissues in plants. Auxin is responsible for phototropisms due to unequal distribution of the chemical along the stem of the plant. Plant stems bend toward light as a result of increased cell elongation on the side of the stem away from the light source. Auxins stimulate elongation of cells within young developing shoots: 1) Auxins produced in apical meristems (tip of plant) activate proton pumps in plasma membrane. 2) Increased movement of H + ions lowers ph and the cell wall and causes it to become more acidic. 3) This weakens the cell wall, allowing turgor pressure to expand the cell wall, resulting in elongation.

8 Photoperiods, the relative lengths of night and day, are the environmental stimuli plants use most often to detect the time of year. Photoperiodism, such as flowering in plants, is a physiological response to a change in length of night and day. Long day plants flower only if a period of continuous darkness is shorter than a critical period (summer plants). Short day plants require a period of continuous darkness longer than a critical period in order to flower (early spring and fall plants). Short-day plants are actually long-night plants, meaning what the plant measures is the length of the night. Long-day plants are actually short-night plants. Many flowering plants (angiosperms) use a photoreceptor protein, such as phytochrome or cryptochrome, to sense seasonal changes in night length, or photoperiod, which they take as signals to flower. Long-day plants flower when the night length falls below their critical photoperiod. These plants typically flower in the northern hemisphere during late spring or early summer as days are getting longer. In the northern hemisphere, the longest day of the year (summer solstice) is on or about 21 June. After that date, days grow shorter (i.e. nights grow longer) until 21 December (the winter solstice). This situation is reversed in the southern hemisphere (i.e., longest day is 21 December and shortest day is 21 June). Short-day plants flower when the night lengths exceed their critical photoperiod. They cannot flower under short nights or if a pulse of artificial light is shone on the plant for several minutes during the night; they require a continuous period of darkness before floral development can begin. Natural nighttime light, such as moonlight or lightning, is not of sufficient brightness or duration to interrupt flowering. Shivering and sweating in humans is a physiological response to changes in temperature controlled by the hypothalamus and negative feedback loops.

9 Taxis & Kinesis in Animals: Environmental cues not only trigger some simple behaviors in animals, but also provide stimuli that animals use to change or orient both simple and complex movements in a particular direction. Kinesis: a change in activity or turning rate in response to a stimulus. Taxis: an oriented movement toward (positive) or away from (negative) some stimulus. A kinesis is a simple change in activity or turning rate in response to a stimulus. For example, sow bugs become more active in dry areas and less active in humid areas. Though sow bug behavior varies with humidity, sow bugs do not move toward or away from specific moisture levels. A taxis is an oriented movement toward or away from some stimulus. Trout and many other river fishes automatically swim or orient themselves in an upstream direction (toward the current). This keeps the fish from being swept away and keeps it facing the direction from which food will come. Chemotaxis in Bacteria: A dramatic example of rapid adaptation is the process of "chemotaxis", i.e. the ability to move toward nutrients (positive chemotaxis) and away from noxious compounds (negative chemotaxis). This is a valuable trait in bacteria, since it engages them to swim to sources of nutrients and away from toxic chemicals. Chemotactic responses are very rapid, they do not require active gene expression. Migration in Animals: Migration is a regular, long-distance change in location observed in a wide variety of birds, fishes, and other animals, generally in response to changing seasons. Many migrating animals tract their position relative to the sun, and adjust to changes in the time of day using their circadian clock, an internal mechanism that maintains a 24-hour activity rhythm or cycle. Other animals sense their position relative to the Earth s magnetic field, and can navigate without solar or celestial cues.

10 Nocturnal & Diurnal Activity in Animals: Nocturnal activity is an animal behavior characterized by activity during the night and sleeping during the day. Diurnal animals, such as squirrels and songbirds, are active during the daytime. Many times, these cycles are of adaptive value to the organism. Resource competition - Being active at night is a form of niche differentiation, where a species' niche is partitioned not by the amount of resources but by the amount of time (i.e. temporal division of the ecological niche). Hawks and owls can hunt the same field or meadow for the same rodents without conflict because hawks are diurnal and owls are nocturnal. This means they are not in-competition for each other's prey. Predation - Nocturnality is a form of crypsis, an adaptation to avoid or enhance predation. One of the reasons that lions prefer to hunt at night is that many of their prey species (zebra, atelope, impala, etc.) have poor night vision. Many species of small rodents are active at night because most of the dozen or so birds of prey that hunt them are diurnal. Water conservation - Another reason for nocturnality is avoiding the heat of the day. This is especially true in arid biomes like deserts, where nocturnal behavior prevents creatures from losing precious water during the hot, dry daytime. This is an adaptation that enhances osmoregulation. Animal Dormancy: Timing and coordination of behavior are regulated by various mechanisms and are important in natural selection. Behaviors in animals are triggered by environmental cues and are vital to reproduction and survival. Despite their many adaptations for homeostasis, animals may encounter conditions that severely challenge their abilities to balance their heat, energy, and materials budgets. For example, at certain seasons or times of day, temperatures may be extremely too high or low, or food may be unavailable. Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be closely associated with environmental conditions. Torpor: a DAILY FLUCTUATION in physiological state in which activity is low and metabolism decreases; an adaptation that enables animals to save energy while avoiding difficult and dangerous conditions (these organisms are easily awakened)! Hibernation: (winter dormancy) - a long-term physiological torpor in which metabolism decreases, the heart and respiratory system slow down, and body temperature is maintained at a lower level than normal during winter months. Estivation: (summer dormancy) - a long-term physiological torpor in which metabolism decreases, the heart and respiratory system slow down, and body temperature is maintained at a lower level than normal during summer months. DAILY TORPOR ANIMALS: bears, hummingbirds (daily), certain small mammals HIBERNATING ANIMALS: mice, ground squirrels, skunks, prairie dogs, bats ESTIVATING ANIMALS: mollusks, arthropods and insects, some reptiles and amphibians Effects of ambient temperature on metabolic rate, respiratory quotient, and torpor in an arctic hibernator What do the data conclude about the effect of ambient temperature on metabolic rate, respiratory quotient, and torpor in the arctic ground squirrel?