Their purpose is reproduction.
A) a-ketoglutaric acid and NAD+
Yes! A recent example could be babies that do not survive child birth for a certain reason, are obviously unable to pass on whatever caused them to die in the first place.
Also, once humans started to domesticate cows and drink their milk (I'm not sure why humans drink a cow's milk) their bodies evolved to continue making the enzyme to digest milk even after they were weaned off their mothers milk. Many people of Asian descend do not have this enzyme, since their culture did/does not raise as much dairy product.
Answer:
Humans need energy for just about any type of function they perform. Houses must be heated, energy is required for industry and agriculture and even within our own bodies a constant flow of energy takes place. All processes that provide us with the luxuries of every day live we can no longer live without require energy generation. This is an industrial process that can be performed using various different sources. These sources can be either renewable or non-renewable. Renewable energy sources are replaced in time and will therefore not run out easily. However, non-renewable energy sources are threatening to run out if our standard of usage becomes too high.
Nowadays many renewable energy sources are available for use, for example solar and wind energy and water power. Ironically, we still gain most of our energy from non-renewable energy sources, commonly known as fossil fuels (figure 1). The non-renewability of these sources will probably cause prices to rise up to a point where they are no longer economically feasible.
Figure 1: fossil fuel combustion is part of the carbon cycle (right)
Fossil fuels consist of deposits of once living organisms. The organic matter takes centuries to form. Fossil fuels principally consist of carbon and hydrogen bonds. There are three types of fossil fuels which can all be used for energy provision; coal, oil and natural gas. Coal is a solid fossil fuel formed over millions of years by decay of land vegetation. When layers are compacted and heated over time, deposits are turned into coal. Coal is quite abundant compared to the other two fossil fuels. Analysts sometimes predict that worldwide coal use will increase as oil supplies become scarcer. Current coal supplies could last for 200 years or more. Coal is usually extracted in mines. Since the middle of the 20th century, coal use has doubled. Since 1996 its application is declining again. Many developing countries depend on coal for energy provision because they cannot afford oil or natural gas. China and India are major users of coal for energy provision.
Oil is a liquid fossil fuel that is formed from the remains of marine microorganisms deposited on the sea floor. After millions of years the deposits end up in rock and sediment where oil is trapped in small spaces. It can be extracted by large drilling platforms. Oil is the most widely used fossil fuel. Crude oil consists of many different organic compounds which are transformed to products in a refining process. It is applied in cars, jets, roads and roofs and many other. Oil cannot be found everywhere on earth and consequentially, there have been wars on oil supplies. A well-known example is the Gulf War of 1991.
Natural gas is a gaseous fossil fuel that is versatile, abundant and relatively clean compared to coal and oil. Like oil, it is formed from the remains of marine microorganisms. It is a relatively new type of energy source. Until 1999, more coal was used than natural gas. Natural gas has now overtaken coal in developed countries. However, people are afraid that like oil, natural gas supplies will run out. Some scientists have even predicted this might happen by the middle or end of the 21st century. Natural gas mainly consists of methane (CH4). It is highly compressed in small volumes at large depths in the earth. Like oil, it is brought to the surface by drilling. Natural gas reserves are more evenly distributed around the globe than oil supplies.
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Explanation:
