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/Efficiency and Living Systems
GoalDiscover that efficiency can be studied in living systems as well as machines.Introduction
You have learned a lot about efficiency in machines. Living systems, like machines, use fuel to provide energy. In your SCIENCEFOCUS 10 textbook, you read that animals are about 38 percent efficient in converting the chemical potential energy stored in glucose to usable energy stored in ATP. You also read that plants are probably less than one percent efficient in converting solar energy to chemical potential energy in glucose and starch. These are just two of many processes that scientists have studied. Three other processes are discussed below.
C3 and C4 Plants
The symbols C3 and C4 represent two major classes of plants, based on how the plants incorporate each new carbon atom from carbon dioxide into glucose and starch. These two groups differ in their efficiency of transforming solar energy into chemical potential energy. All plants use a series of chemical reactions, called the Calvin-Benson cycle, to incorporate carbon dioxide into glucose and starch. The mechanism by which carbon dioxide is transported to the location where these reactions occur differs in C3 and C4 plants.
In C3 plants, carbon dioxide diffuses through pores, called stomata, in the leaves. The name C3 comes from the fact that the first compound to contain the new carbon atom from the carbon dioxide has three carbon atoms. Oxygen and water vapour diffuse out of the pores that let in the carbon dioxide. When the air becomes too hot and dry, the pores close to prevent water loss, which would result in dehydration. Soon oxygen accumulates and carbon dioxide is used up. The excess oxygen then causes the breakdown of the energy storage molecules. C3 plants can loose as much or more than 20 percent of their stored energy under favourable weather conditions. If weather conditions are not favourable, the process can become very inefficient
C4 plants also have stomata that allow carbon dioxide to diffuse into air spaces inside the leaves, and allow oxygen and water vapour to diffuse out of the leaves. As shown the diagram below, however, C4 plants have an extra layer of cells between the air spaces and the cells in which the Calvin-Benson cycle occur. These cells incorporate the carbon dioxide into molecules made up of four carbon atoms. In a sense, these molecules are storing carbon dioxide. The four-carbon molecules move into the cells where the Calvin-Benson cycle is in progress and release carbon dioxide. The carbon dioxide can then be used in the Calvin-Benson cycle. In hot, dry regions, such as arid planes and deserts, the stomata remain closed during the day but open at night when the air is much cooler. The leaves take in carbon dioxide at night and then use the carbon dioxide the next day. During the day, the stomata remain closed to prevent water loss. Since they are not exposed to excess oxygen, the energy storage molecules are never broken down by oxygen.
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/Efficiency and Living Systems(continued)
You might ask why all plants are not C4 plants, because C4 plants never waste stored energy. One possible answer is that C4 plants use a small amount of energy to capture and store the carbon dioxide. In cool, humid regions, the process used by C3 plants is just as efficient as the process used by C4 plants.
Animal Metabolism
Every animal uses energy to carry out the most basic processes that keep the cells, and thus the whole animal, alive. The rate at which this energy is used is called the basal metabolic rate (BMR) in animals that maintain a constant body temperature. This rate is called the standard metabolic rate (SMR) in animals that do not regulate their body temperature internally.
Just as you would expect, larger animals have a higher metabolic rate. The story does not end here, however. A better comparison can be made by determining how much energy is used for every kilogram of living tissue. The graph below, called the “mouse to elephant’ graph, shows both measurements for several mammals. The solid line is a line of best fit for a large number of different mammals, as shown by the points. The dashed line shows how much energy is used for every kilogram of living tissue for the mammals of different sizes. As you can see, keeping one kilogram of mouse tissue alive requires over ten times as much energy as keeping one kilogram of elephant tissue alive.
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BLM 6-3
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/Efficiency and Living Systems(continued)
Another interesting comparison of metabolic rates is between mammals and other classes of animals, such as reptiles. For animals of similar sizes, mammals have a much higher metabolic rate than reptiles. For example, a reptile can live a month on the amount of food that a mammal of the same size eats every day. The most extreme case of high metabolism is the smallest mammal, the shrew. Some shrews eat more than their body weight every day. Without food, they can die of starvation in a matter of hours.
Animal Locomotion
How much energy do animals use for walking, swimming, and flying? When you compare animals of the same class (such as mammals) and of similar size, the results are nearly the same. About the same amount of energy is needed to walk, run, swim, or fly. When you compare animals of the same class but different sizes, however, you get a result that is similar to the basal metabolic rate. In other words, small mammals use much more energy for every kilogram of their body weight than large mammals do when travelling the same distance. For example, while running at the same speed, a chipmunk uses about 15 times as much energy as a horse or a human.
When you keep size constant and compare different classes of animals, you find that fish use less energy to swim than a bird uses to fly the same distance. Both use less energy than a mammal does when running the same distance.
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/Efficiency and Living Systems(continued)
Some researchers have compared specialized mammals to versatile mammals. For example, seals and porpoises are specialized for swimming. Cheetahs are specialized for running. Specialized swimmers use about the same amount of energy to swim a given distance as to runners of the same size. In comparison, versatile mammals, such as muskrats and otters, spend time both swimming and running. They are not specialized for either form of locomotion. Consequently, they require more energy than specialized mammals to travel the same distance. A mammal such as a sea otter uses about two-and-a-half to five times as much energy to swim as a specialized swimmer does.
One unique animal is the kangaroo. Researchers have discovered that a kangaroo uses less energy to hop than it uses to walk the same distance.
What to Do
Choose one of the examples described here, or another example you have read about, and do a more extensive study. Use print resources or the Internet to research your chosen example. Then write a short essay about energy efficiency in a living system.
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