Activity 1: The Effect of Cold on Characteristics Important to Fishes
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BackgroundDiffusion is the random molecular motion of molecules or atoms due to their kinetic energy. Generally things tend to diffuse from an area of higher concentration, pressure, or temperature towards an area of lower concentration, pressure, and temperature. In living organisms, concentration is generally the most important factor affecting the direction of diffusion, but temperature is most important in determining the rate of diffusion.
What is diffusion?
Why does decrease temperature decrease diffusion?
Temperature is a measure of how much molecular motion there is. Since diffusion is molecular motion, if there is more molecular motion, there is also more diffusion.
How does life depend on diffusion?
All chemical reactions that happen in cells depend on the diffusion of the molecules and enzymes involved in the reaction so that they collide with each other in just the right way for the reaction to occur. Everything your cells do depends on chemical reactions: synthesizing important molecules like ATP and protein, genetic control of what happens in a cell, converting molecules from one form to another such as the conversion of sugar to glycogen or fat, movement within a cell, muscle motion, and anything else involving energy, all depend on chemical reactions. Diffusion is also how small molecules like water and oxygen get into and out of cells through the membrane. When a larger molecule needs to be carried through a membrane it must first diffuse to the membrane and bind to a specific carrier molecule which will carry it across the membrane. The absorption of nutrients from the digestive system depends on diffusion. Waste molecules also diffuse to get out of cells. So if temperature slows down the rate of diffusion, the rate at which cells can get the required nutrients slows down, the rate at which cells can get rid of their toxic wastes slows down, and the rate at which their chemical reactions can occur slows down.
Why do polar fish need to conserve energy? How?
Evolution almost always favors genes that lead to the conservation of energy and matter since this cuts down on the amount of food an organism needs to survive and increases the amount of energy that can be devoted to passing its genes on to the next generation. Polar fishes conserve energy by generally being sedentary fishes who sit and wait in one area until food arrives. Most of them are benthic fishes that require no energy to sit in one place. The pelagic species, those that live elsewhere in the water column are neutrally buoyant so they don't need to suspend any energy to stay afloat at the level in which they live. Energy conservation also occurs at the molecular level in the structure of the enzymes which are designed to work at lower temperatures where there is less energy available for them to catalyze reactions. The fish that lack red blood cells and those that have fewer that the temperate fish also conserve energy because it takes less energy to pump the blood and because the hemoglobin and red blood cells do not have to be made. Since Notothenioid fish do not have glomeruli in their kidneys, the antifreeze glycoprotein are never expelled into the wastes from the blood so no energy needs to be used to reabsorb these glycoproteins. Your students might be able to come up with more examples as they go through this suite of activities.
The relationship between enzymes, chemical reactions, and temperature
Every reaction is regulated by an enzyme which is a specialized protein that acts as a catalyst. They do this by lowering the amount of energy (activation energy) that is needed for a reaction to occur. This is thrifty because it decreases the amount of energy that the organism needs and it protects the organism against the excessive heat that would have to be used if enzymes were not present. Each enzyme has a specific shape that is perfectly designed to bind to the reactants (substrates) in the reaction that it regulates. Each enzyme can be used over and over since it is not changed by the reaction it regulates. Most enzymes work best at the temperature typical for the organism in which it occurs. For example most human enzymes work best at about 37 C (body temperature). At higher temperatures there is more molecular motion so the enzyme can collide with the substrate molecules more often and cause more reactions to happen. This causes the rate of chemical reaction to increase.
At lower temperatures there is less molecular motion so there are less enzyme-substrate collisions so the chemical reaction rate decreases. Because of this, any cell activity will also decrease at colder temperatures. If temperatures are too high, the excessive molecular motion can break the strong disulfide bond or the weak bonds (H and ionic) between R-groups changing the shape of the enzyme so it no longer works (denaturing it). This would explain why the highest temperature in the enzyme activity may have had no reactions occurring.
Since an enzyme is a protein it is made out of amino acids. There are 20 kinds of amino acids each of which has different chemical properties due to its variable R-group. The interaction of these variable R-groups causes the strand of amino acids to fold into a globular shape. Because each protein has a different sequence of amino acids, these R-groups will interact in different ways causing the protein to have a different shape from all other proteins. Some of these R-groups are polar or ionic so they are attracted to water or other R-groups similar to them. Some are hydrophobic; they are repelled by water and tend to form the middle of the globular protein. Some form very strong disulfide binds with other amino acids in other parts of the protein giving them a sharp bend. These R-groups may also interact with the substrate molecules at a part of the enzyme called the "active site", making the substrates more reactive. This chemical and structural distinctiveness is what makes an enzyme specialized to catalyze one specific kind of reaction. POSSIBLY PUT A DIAGRAM HERE SHOWING IT.
How does polar fish enzyme structure enable them to work at low temperature.
This field is still being researched but ######### 1993? study of the enzyme lactose dehydrogenase? in an Antarctic fish (Trematomus bernacchii?) suggests that this parcticular enzyme in Antarctic fish has a slightly different sequence of amino acids which makes the enzyme more flexible and? exposes its active site so it is more accessible to the substrate. Both of these chemical adaptations would lower the amount of activation energy needed and would enable the reaction to occur at very low temperatures like -2 C. However this same flexibility would make the enzyme unable to function at high temperatures because the greater molecular motion it would cause would change the shape of the enzyme (denaturing it) so it wouldn't be able to work. This would explain why Antarctic fishes die when their temperature is raised above 6 C. Extension question: How can sunfish or goldfish in temperate region ponds where the temperature may fluctuate from 1-35 C survive this temperature range?
Why don't polar penguins and seals have to worry about this?
Since penguins and seals are homeotherms (warm-blooded organisms) and have wonderful layers of blubber and fur/feathers to act as insulation, they can maintain a very warm and constant temperature enabling their cells to keep a high metabolic rate at just the right temperature for their enzymes to work.
Cell membrane structure
According to the Fluid Mosaic Model, the cell membrane is mostly composed of a bilayer of phospholipids. Scattered through this bilayer is a mosaic of proteins with a variety of functions. Some membrane proteins are enzymes. Others carry important molecules in or out of a cell. Others serve as receptors to receive instructions(hormones, neurotransmitters, etc) from other parts of the body. These need to be able to change shape in order to do their job. DIAGRAM MIGHT HELP HERE.
How does nerve activity depend on diffusion, enzyme activity, and lipid malleability? See Atwood journal Dec 5
1. In order for most membrane proteins to work they need to be able to change shape. In order for this to happen, they and the lipid membrane around them, must be flexible. If a lipid has lost its fluidity or malleability, it will be harder for the proteins around them to change shape so it will be harder for them to do their job. The receptor proteins in the membrane of nerve cells need to change shape when a neurotransmitter binds to it. The carrier proteins involved in transporting ions in and out of nerves during and after a nerve impulse also need to be able to change shape. DIAGRAM MIGHT HELP HERE.
2. See the above description relating temperature and enzyme activity
3 See the above description relating diffusion to temperature
Why is dissolved oxygen important?
In aquatic ecosystems the source of oxygen is oxygen dissolved in the water. Organisms need oxygen so they can do aerobic respiration, a chemical process that occurs in cell mitochondria enabling them to make ATP. ATP is the "energy currency" of a cell. Cells use ATP to power almost everything they do.
How does temperature affect dissolved oxygen?
The lower the temperature the more dissolved oxygen. Insert a chart or graph here.
Some fish in Antarctica don't have red blood cells. Relate to dissolved oxygen. Lead students (through questions) about how this ties in with lower number of red blood cells and the resulting affect on blood viscosity and why that is important in the cold.
As blood gets cold, it becomes more viscous and harder to pump. A way to decrease viscosity would be to decrease red blood cells. This would decrease the strain on the heart and blood vessels and allow for a faster rate of flow. But red blood cells are crucial for carrying oxygen. Since the water is so cold in Antarctica there is a very high concentration of dissolved oxygen. Because of this it is easier for the fish to absorb oxygen into their blood and carry it in their blood plasma without using as many red blood cells. They also may be able to absorb some oxygen directly into the blood in their fins or through their skin in some species. Also since most Antarctic fish spend most of their time sitting around instead of swimming they don't need as much energy so they don't need as much oxygen so they can get by with fewer red blood cells. Polar Fish Activity #3 will show a chart comparing red blood cell count in Antarctic fish of varying life styles. SHOW DIAGRAM OF THE ICEFISH HERE.
Resources and Reference Materials
Atwood journal, parcticularly Dec 5 and ...
Atwood Journal Fish Photos Dec 5 and
Book: ANTARCTIC FISH BIOLOGY Evolution in a Unique Environment by Joseph Eastman Academic Press Inc 1993. A technical but very thorough presentation of what is presently known about Antarctic fishes. Lots of data and good drawings.
Reader's Digest Book on Antarctica
Video: "Under the Ice" by Wild South
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