AP Biology Notes Outline Enduring Understanding 2.C

L. Carnes

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.

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. 1015 & 1016) Textbook Ch. 45 (pp. 982-983 & pp. 988; 990) Textbook Ch. 39 (pp. 825-826 & pp. 839-840) Textbook Ch. 41 (pp. 1122) Textbook Ch. 40 (pp. 871-872)

Practicing Biology Homework Questions: Questions #28-32

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. 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).

http://bcs.whfreeman.com/thelifewire/content/chp41/41020.html Plant responses to water limitations are an illustrative example of negative feedback. ABA (abscisic acid) is the primary internal signaling molecule that enables plants to withstand drought. When a plant begins to wilt, ABA 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. The accompanying osmotic loss of water reduces guard cell turgor and leads to closing of the stomatal pores.

http://www.phschool.com/science/biology_place/labbench/lab9/guard.html

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 the onset of childbirth during labor and fruit ripening in plants. 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.

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.

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, 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.

http://bcs.whfreeman.com/thelifewire/content/chp50/5002002.html 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.

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 thyroid-stimulating 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.

http://www.biologyinmotion.com/thyroid/index.html

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.

http://bcs.whfreeman.com/thelifewire/content/chp38/3801s.swf (Darwin’s Experiments)

http://bcs.whfreeman.com/thelifewire/content/chp38/3802002.html (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.

Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year

http://bcs.whfreeman.com/thelifewire/content/chp39/3902002.html

Photoperiodism, such as flowering, 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.

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.

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.

Torpor in Animals: 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. Torpor is a physiological state in which activity is low and metabolism decreases; an adaptation that enables animals to save energy while avoiding difficult and dangerous conditions.

• Hibernation: a physiological state 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: a physiological state 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.