- 1 十年 前最佳解答
Effects on life
From a biological standpoint, water has many distinct properties that are critical for the proliferation of life that set it apart from other substances. It carries out this role by allowing organic compounds to react in ways that ultimately allow replication. All known forms of life depend on water. Water is vital both as a solvent in which many of the body's solutes dissolve and as an essential part of many metabolic processes within the body. Metabolism is the sum total of anabolism and catabolism. In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g. starches, triglycerides and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g. glucose, fatty acids and amino acids to be used for fuels for energy use or other purposes). Water is thus essential and central to these metabolic processes.
Water is also central to photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. Hydrogen is combined with CO2 (absorbed from air or water) to form glucose and release oxygen. All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and CO2 in the process (cellular respiration).
Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H+, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as hydroxide ion (OH−) to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. Acids have pH values less than 7 while bases have values greater than 7. Stomach acid (HCl) is useful to digestion. However, its corrosive effect on the esophagus during reflux can temporarily be neutralized by ingestion of a base such as aluminum hydroxide to produce the neutral molecules water and the salt aluminum chloride. Human biochemistry that involves enzymes usually performs optimally around a biologically neutral pH of 7.4.
Aquatic life forms
Some of the biodiversity of a coral reefEarth's waters are filled with life. Nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales that also live in the water. Some kinds of animals, such as amphibians, spend portions of their lives in water and portions on land. Plants such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton is generally the foundation of the ocean food chain.
Some marine diatoms - a key phytoplankton groupDifferent water creatures have found different solutions to obtaining oxygen in the water. Fish have gills instead of lungs, though some species of fish, such as the lungfish, have both. Marine mammals, such as dolphins, whales, otters, and seals need to surface periodically to breathe air.
2007-03-27 13:38:38 補充：
2007-03-27 13:40:59 補充：資料來源： ｗｉｋｉ
- 1 十年 前
Water is a chemical substance that is essential to all known forms of life. It covers 71% of Earth's surface. There are 1.4 billion cubic kilometers (330 million mi³) of it available on Earth. It appears mostly in the oceans (saltwater) and polar ice caps, but it is also present as clouds, rain water, rivers, freshwater aquifers, lakes, airborne vapor and sea ice. Water in these bodies perpetually moves through a cycle of evaporation, precipitation, and runoff to the sea. Clean water is essential to human life. In many parts of the world, it is in short supply. Outside of our planet, a significant quantity of water is thought to exist at the north and south poles of the planet Mars, and on the moons Europa and Enceladus.
For drinking，As a solvent，As a thermal transfer agent，Recreation，Industrial applications，Food Processing.
Health and pollution:
Water fit for human consumption is called drinking water or "potable water". Water that is not fit for drinking but is not harmful for humans when used for food preparation is called safe water.
This natural resource is becoming scarcer in certain places, and its availability is a major social and economic concern. Currently, about 1 billion people around the world routinely drink unhealthy water. Most countries accepted the goal of halving by 2015 the number of people worldwide who do not have access to safe water and sanitation during the 2003 G8 Evian summit. Even if this difficult goal is met, it will still leave more than an estimated half a billion people without access to safe drinking water supplies and over 1 billion without access to adequate sanitation facilities. Poor water quality and bad sanitation are deadly; some 5 million deaths a year are caused by polluted drinking water.
In the developing world, 90% of all wastewater still goes untreated into local rivers and streams. Some 50 countries, with roughly a third of the world’s population, also suffer from medium or high water stress, and 17 of these extract more water annually than is recharged through their natural water cycles. The strain affects surface freshwater bodies like rivers and lakes, but it also degrades groundwater resources.
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