2 Solns prob set 2

2 Solns prob set 2 - BILD 2, Multicellular Life ­...

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Unformatted text preview: BILD 2, Multicellular Life ­ ­Problem set #2 solutions Page 1 Problem set #1, Question 5b: An action potential is a relatively large and very transient change in the transmembrane potential of a cell. Although all cells have a resting membrane potential, only certain types of cells ­ ­mostly neurons and muscle fibers ­ ­have the voltage ­gated ion channels that produce action potentials. During an action potential, a depolarization of the plasma membrane causes voltage ­gated Na+ and voltage ­gated K+ channels to open, providing a pathway through which these two ions can the membrane. The driving force on Na+ pushes it into the cell, and the driving force on K+ pushes it out of the cell. Early in an action potential (i.e., during the rising phase), the inward movement of Na+ through voltage ­gated Na channels, which open faster than K channels, causes the cell membrane to become less inside ­negative and even inside ­positive. Later in an action potential (i.e., during the falling phase and after ­hyperpolarization), the outward movement of K+ through voltage ­gated K channels returns the membrane to inside ­ negative and eventually to its resting potential. Problem set 2 1. a. The term ectotherm refers to any animal whose primary source of heat is its environment, so the body temperature of ectotherms matches the environmental temperature. (All metabolic reactions produce some heat, but ectotherms can't/don't use that heat to regulate their body temperature.) The term endotherm refers to any animal that can use the heat produced by metabolic reactions to keep its body temperature warmer than that its surroundings. b. Some ectotherms ARE thermoregulators that can hold their body temperatures above the ambient air temperature. They accomplish this feat behaviorally by moving into or out of sunshine and physiologically by manipulating the color of their skin to make it more or less able to absorb heat energy. c. An endotherm can hold its body temperature warmer than its surroundings by increasing the rate of its metabolic reactions, thus increasing its internal production of heat. 2. In a counter ­current exchange system, two streams of fluid flow very close to one another, but in opposite directions (see Figure 40.12 in your textbook). For example, an artery and a vein may be located very close to one another and parallel to each other. If the amount of a variable (e.g., heat or some chemical species) is higher in one of the streams than in the other, that variable can move from the stream of fluid with the higher concentration to the stream of fluid with the lower concentration. For example, in a cool environment, arterial blood entering the leg of a metabolic homeotherm is likely to be warmer than venous blood that has traveled through the leg and has thus been exposed to the cool environment. If heat is transferred from the artery to the vein, the heat never enters the leg and can be returned to the major body mass, which is the BILD 2, Multicellular Life ­ ­Problem set #2 solutions Page 2 trunk of the body. Thus, this arrangment helps to conserve heat in the vital organs such as the heart, digestive tract, and respiratory system. 3. a. The basal metabolic rate is the amount of energy per unit of time that is required to keep an animal alive, but that does NOT include energy used for moving around, reproducing, or other extra activities. It includes the energy for processes such as membrane ion pumps, contraction of the heart muscle ­ ­basically energy for all of the processes that keep you alive while you sleep or are being a couch potato. b. Because activity level is explicitly omitted from the basal metabolic rate, knowing that an animal is very active doesn't tell you anything about its basal metabolic rate. c. Larger mammals have a higher basal metabolic rate: they have more cells and a bigger mass. However, if we remove the effect of body size from our consideration of this question (which we do by dividing the basal metabolic rate by the animal's body mass ­ ­ we call that normalizing for body size), we discover that the NORMALIZED basal metabolic rate is highest for the smallest mammals. That is, it takes a lot more energy to keep a gram of mouse alive than it does to keep a gram of elephant alive. 4. a. The equilibrium potential for an ion is calculated using the Nernst equation for that ion (see Figure 48.7). In this case, ENa = 62 log 140/10 = 62 x (1.146) = 71 mV. b. The Na current during the rising phase of an action potential could bring the membrane potential to +71 mV, because the peak of the action potential MAY reach ENa, but cannot become more inside positive than that value. 5. a. The movement of Na+ ions into a neuron during an action potential (i.e., the Na current) makes the plasma membrane less inside ­negative (i.e., more inside ­positive), even overshooting 0 mV and becoming as much inside positive as the value of ENa. b. Ca2+ does not contribute to the propagation of an action potential. c. Diffusion does not contribute to an action potential. During an action potential, ions move because they are pushed by an electrochemical gradient. d. Membrane proteins play a major role in the propagation of an action potential. When voltage ­gated ion channels open, they provide a pathway through which Na+ and K+ ions can cross the plasma membrane of an axon. e. Primarily during the falling phase of an action potential, K+ ions move out of the axons, pushed by their driving force. Removing + charges from the inside of the cell, returns Vm to inside ­negative; during the after ­hyperpolarization, it becomes even more inside ­ negative than Vrest. BILD 2, Multicellular Life ­ ­Problem set #2 solutions Page 3 6. a. A Na+ current across the plasma membrane of the presynaptic axon terminal depolarizes the membrane, which opens voltage ­gated Ca channels. At some synapses, ion channels that permit Na+ to cross the postsynaptic membrane are opened when neurotransmitter binds to postsynaptic receptors, and the Na+ depolarizes the postsynaptic cell. b. A rise in intracellular free Ca2+ starts the process of exocytosis . During exocytosis the membrane of a docked vesicle fuses with the plasma membrane of the axon terminal, releasing the neurotransmitter within the synapse into the saline solution that fills the synaptic cleft. c. Neurotransmitter diffuses within the synaptic cleft, away from the site of the fused vesicle. Some neurotransmitter molecules may diffuse all the way across the synaptic cleft and bind to postsynaptic receptors, causing changes in the postsynaptic cell. d. In chemical synaptic transmission, the most important membrane proteins are probably the postsynaptic receptors. If these receptors don't bind neurotransmitter or don't function normally, then synaptic transmission will not take place. e. Synaptic vesicles are organelles unique to presynaptic axon terminals at chemical synapses. They contain neurotransmitter molecules and release these molecules into the synaptic cleft through the process of exocytosis. 7. a. The presynaptic cell is the cell that conducts an action potential to a chemical synapse. Only neurons can be presynaptic cells. b. A postsynaptic cell is the cell on the other side of the synaptic cleft. It receives the signal that crosses the synaptic cleft in the form of neurotransmitter molecules. Neurons, muscle fibers, and gland cells are all postsynaptic cells in different locations. c. A postsynaptic potential is a change in the membrane potential of the postsynaptic cell. It is caused by ions that move through ion channels in the membrane. At many synapses, the ion channels are contained in the receptor complex, so they are called ligand ­gated ion channels. d. An excitatory postsynaptic potential increases the probability of an action potential in the postsynaptic cell. e. An inhibitory postsynaptic potential decreases the probability of an action potential in the postsynaptic cell. 8. The sensory ending of the stretch receptor depolarizes in response to stretch of the quadriceps muscle. (More about sensory receptor potentials soon.) Depolarization of the axonal membrane elicits an action potential that travels along the axon (and along both branches of the axon within the CNS) to the terminals. At the terminals, the depolarization of the membrane opens voltage ­gated ion channels in the plasma membrane of the axon terminal, allowing Ca2+ to enter the terminal, and the increase in [Ca2+]cytoplasm, triggers exocytosis of neurotransmitter. BILD 2, Multicellular Life ­ ­Problem set #2 solutions Page 4 At the synapse onto a spinal motor neuron that sends its axon to the quadriceps muscle, the neurotransmitter increases the probability of action potentials, and action potentials travel along the axon to the axon terminal of the motor neuron, where the same kind of synaptic events occur as described in the previous paragraph. Spinal motor neurons release acetylcholine. When a postsynaptic receptor at this kind of synapse binds ACh, an ion channel opens that allows both Na+ and K+ to cross the membrane, but the ratio of the two ions makes this synapse excitatory. When action potentials are produced in the muscle fibers within the quadriceps muscle, the muscle contracts. The synapse made by the sensory neuron onto the interneuron in the diagram is also excitatory, and the probability of an action potential in the interneuron is increased. In the interneuron, action potentials cause the same set of events in the presynaptic axon terminal, but the neurotransmitter that is released into the synapse with the motor neuron that controls muscle fibers in the hamstring muscle (the antagonist to the quadriceps muscle) in inhibitory. That is, the probability of action potentials in the hamstring motor neurons is decreased, so the hamstring muscle relaxes.. ...
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This note was uploaded on 02/18/2012 for the course BILD 2 taught by Professor Schroeder during the Spring '08 term at UCSD.

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