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For example, urban populations have many more cars than rural populations per capita. Almost all of the cars in the world in the s were in the United States. Today we have a car for every two people in the United States. If that became the norm, in there would be 5. In China the per capita consumption of coal in towns and cities is over three times the consumption in rural areas.

Economies, therefore, often become more efficient as they develop because of advances in technology and changes in consumption behavior.


And the increased consumption of energy is likely to have deleterious environmental effects. Urban consumption of energy helps create heat islands that can change local weather patterns and weather downwind from the heat islands. The heat island phenomenon is created because cities radiate heat back into the atmosphere at a rate 15 percent to 30 percent less than rural areas. The combination of the increased energy consumption and difference in albedo radiation means that cities are warmer than rural areas 0. Cloudiness and fog occur with greater frequency.

Precipitation is 5 percent to 10 percent higher in cities; thunderstorms and hailstorms are much more frequent, but snow days in cities are less common. Urbanization also affects the broader regional environments. Regions downwind from large industrial complexes also see increases in the amount of precipitation, air pollution, and the number of days with thunderstorms.

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Urban areas generally generate more rain, but they reduce the infiltration of water and lower the water tables. This means that runoff occurs more rapidly with greater peak flows. Flood volumes increase, as do floods and water pollution downstream. Many of the effects of urban areas on the environment are not necessarily linear.

Urbanization: An Environmental Force to Be Reckoned With

Bigger urban areas do not always create more environmental problems. And small urban areas can cause large problems. Much of what determines the extent of the environmental impacts is how the urban populations behave — their consumption and living patterns — not just how large they are. The urban environment is an important factor in determining the quality of life in urban areas and the impact of the urban area on the broader environment. Some urban environmental problems include inadequate water and sanitation, lack of rubbish disposal, and industrial pollution. The health implications of these environmental problems include respiratory infections and other infectious and parasitic diseases.

Capital costs for building improved environmental infrastructure — for example, investments in a cleaner public transportation system such as a subway — and for building more hospitals and clinics are higher in cities, where wages exceed those paid in rural areas. And urban land prices are much higher because of the competition for space. But not all urban areas have the same kinds of environmental conditions or health problems.

Some research suggests that indicators of health problems, such as rates of infant mortality, are higher in cities that are growing rapidly than in those where growth is slower. Since the s, many cities in developed countries have met urban environmental challenges. Los Angeles has dramatically reduced air pollution. Many towns that grew up near rivers have succeeded in cleaning up the waters they befouled with industrial development.

But cities at the beginning of their development generally have less wealth to devote to the mitigation of urban environmental impacts. And if the lack of resources is accompanied by inefficient government, a growing city may need many years for mitigation. Strong urban governance is critical to making progress. As mentioned earlier, the ATP yield obtained from lipid oxidation is over twice the amount obtained from carbohydrates and amino acids. So why don't all cells simply use lipids as fuel? In fact, many different cells do oxidize fatty acids for ATP production Figure 2.

Skeletal muscle cells also oxidize lipids. Indeed, fatty acids are the main source of energy in skeletal muscle during rest and mild-intensity exercise. As exercise intensity increases, glucose oxidation surpasses fatty acid oxidation. Other secondary factors that influence the substrate of choice for muscle include exercise duration, gender, and training status. Another tissue that utilizes fatty acids in high amount is adipose tissue.

Since adipose tissue is the storehouse of body fat, one might conclude that, during fasting, the source of fatty acids for adipose tissue cells is their own stock. Skeletal muscle and adipose tissue cells also utilize glucose in significant proportions, but only at the absorptive stage - that is, right after a regular meal.

Other organs that use primarily fatty acid oxidation are the kidney and the liver. The cortex cells of the kidneys need a constant supply of energy for continual blood filtration, and so does the liver to accomplish its important biosynthetic functions. Despite their massive use as fuels, fatty acids are oxidized only in the mitochondria. But not all human cells possess mitochondria! Although that may sound strange, human red blood cells are the most common cells lacking mitochondria. Other examples include tissues of the eyes, such as the lens, which is almost totally devoid of mitochondria; and the outer segment of the retina, which contains the photosensitive pigment.

You may have already guessed that these cells and tissues then must produce ATP by metabolizing glucose only. In these situations, glucose is degraded to pyruvate, which is then promptly converted to lactate Figure 2. This process is called lactic acid fermentation. Although not highly metabolically active, red blood cells are abundant, resulting in the continual uptake of glucose molecules from the bloodstream. Additionally, there are cells that, despite having mitochondria, rely almost exclusively on lactic acid fermentation for ATP production.

This is the case for renal medulla cells, whose oxygenated blood supply is not adequate to accomplish oxidative phosphorylation. Finally, what if the availability of fatty acids to cells changes?

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The blood-brain barrier provides a good example. In most physiological situations, the blood-brain barrier prevents the access of lipids to the cells of the central nervous system CNS. Therefore, CNS cells also rely solely on glucose as fuel molecules Figure 2. In prolonged fasting, however, ketone bodies released in the blood by liver cells as part of the continual metabolization of fatty acids are used as fuels for ATP production by CNS cells.

In both situations and unlike red blood cells, however, CNS cells are extremely metabolically active and do have mitochondria. Thus, they are able to fully oxidize glucose, generating greater amounts of ATP. Indeed, the daily consumption of nerve cells is about g of glucose equivalent, which corresponds to an input of about kilocalories 1, kilojoules. However, most remaining cell types in the human body have mitochondria, adequate oxygen supply, and access to all three fuel molecules.

Which fuel, then, is preferentially used by each of these cells? Virtually all cells are able to take up and utilize glucose. What regulates the rate of glucose uptake is primarily the concentration of glucose in the blood. Glucose enters cells via specific transporters GLUTs located in the cell membrane. There are several types of GLUTs, varying in their location tissue specificity and in their affinity for glucose.

Adipose and skeletal muscle tissues have GLUT4, a type of GLUT which is present in the plasma membrane only when blood glucose concentration is high e.

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The presence of this type of transporter in the membrane increases the rate of glucose uptake by twenty- to thirtyfold in both tissues, increasing the amount of glucose available for oxidation. Therefore, after meals glucose is the primary source of energy for adipose tissue and skeletal muscle. The breakdown of glucose, in addition to contributing to ATP synthesis, generates compounds that can be used for biosynthetic purposes. So the choice of glucose as the primary oxidized substrate is very important for cells that can grow and divide fast. Examples of these cell types include white blood cells, stem cells , and some epithelial cells.

A similar phenomenon occurs in cancer cells, where increased glucose utilization is required as a source of energy and to support the increased rate of cell proliferation. Interestingly, across a tumor mass, interior cells may experience fluctuations in oxygen tension that in turn limit nutrient oxidation and become an important aspect for tumor survival. In addition, the increased glucose utilization generates high amounts of lactate, which creates an acidic environment and facilitates tumor invasion.

Setting up our environment

Another factor that dramatically affects the metabolism is the nutritional status of the individual — for instance, during fasting or fed states. After a carbohydrate-rich meal, blood glucose concentration rises sharply and a massive amount of glucose is taken up by hepatocytes by means of GLUT2. This type of transporter has very low affinity for glucose and is effective only when glucose concentration is high. Thus, during the fed state the liver responds directly to blood glucose levels by increasing its rate of glucose uptake.

In addition to being the main source of energy, glucose is utilized in other pathways, such as glycogen and lipid synthesis by hepatocytes.

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The whole picture becomes far more complex when we consider how hormones influence our energy metabolism. Fluctuations in blood levels of glucose trigger secretion of the hormones insulin and glucagon. How do such hormones influence the use of fuel molecules by the various tissues? Demands by one cell type can be met by the consumption of its own reserves and by the uptake of fuel molecules released in the bloodstream by other cells. Energy use is tightly regulated so that the energy demands of all cells are met simultaneously.

Virtually all cells respond to insulin; thus, during the fed state cell metabolism is coordinated by insulin signaling. An extraordinary example is how insulin signaling rapidly stimulates glucose uptake in skeletal muscle and adipose tissue and is accomplished by the activity of GLUT4. In the absence of insulin, these transporters are located inside vesicles and thus do not contribute to glucose uptake in skeletal muscle and adipose tissue.

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Insulin, however, induces the movement of these transporters to the plasma membrane, increasing glucose uptake and consumption. As different tissues continue to use glucose, the blood glucose concentration tends to reach the pre-meal concentration Figure 3. Therefore, during fasting, cell metabolism is coordinated by glucagon signaling and the lack of insulin signaling. As a consequence, GLUT4 stays inside vesicles, and glucose uptake by both skeletal muscle cells and adipocytes is reduced. Now, with the low availability of glucose and the signals from glucagon, those cells increase their use of fatty acids as fuel molecules.

Therefore, the use of fatty acids during fasting clearly contributes to the maintenance of adequate blood glucose concentration to meet the demands of cells that exclusively or primarily rely on glucose as a fuel. But, mentioned above, glucose is used at an apparently high rate by the brain and constantly by red blood cells. And, under physiological conditions, blood glucose is maintained at a constant level, even during fasting. Biomagnification is the increasing concentration of persistent, toxic substances in organisms at each trophic level, from the primary producers to the apex consumers.

Many substances have been shown to bioaccumulate, including classical studies with the pesticide d ichloro d iphenyl t richloroethane DDT , which was published in the s bestseller, Silent Spring , by Rachel Carson. DDT was a commonly used pesticide before its dangers became known. In some aquatic ecosystems, organisms from each trophic level consumed many organisms of the lower level, which caused DDT to increase in birds apex consumers that ate fish.

Thus, the birds accumulated sufficient amounts of DDT to cause fragility in their eggshells. This effect increased egg breakage during nesting and was shown to have adverse effects on these bird populations. Figure 4. Numbers on the x -axis reflect enrichment with heavy isotopes of nitrogen 15N , which is a marker for increasing trophic level. Notice that the fish in the higher trophic levels accumulate more PCBs than those in lower trophic levels. Other substances that biomagnify are polychlorinated biphenyls PCBs , which were used in coolant liquids in the United States until their use was banned in , and heavy metals, such as mercury, lead, and cadmium.

These substances were best studied in aquatic ecosystems, where fish species at different trophic levels accumulate toxic substances brought through the ecosystem by the primary producers. The apex consumer walleye has more than four times the amount of PCBs compared to phytoplankton. Also, based on results from other studies, birds that eat these fish may have PCB levels at least one order of magnitude higher than those found in the lake fish.

Other concerns have been raised by the accumulation of heavy metals, such as mercury and cadmium, in certain types of seafood. The United States Environmental Protection Agency EPA recommends that pregnant women and young children should not consume any swordfish, shark, king mackerel, or tilefish because of their high mercury content. These individuals are advised to eat fish low in mercury: salmon, tilapia, shrimp, pollock, and catfish. Biomagnification is a good example of how ecosystem dynamics can affect our everyday lives, even influencing the food we eat.

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