Biochemistry (Water and Life)
Biochemistry (Water and Life)
Water is found in all forms of life on Earth in some form or another. The human body is about 70% water, and other organisms, such as jellyfish, contain as much as 95% water. All of the oxygen that animals breathe had its origin as water. During photosynthesis (the process of using light to create food energy), plants break water apart to produce oxygen and food.
Water is one of the most abundant molecules on Earth. There are approximately 350 million cubic miles (1.4 billion cubic kilometers) of water on the planet. Nearly 97% of all water is found in the oceans, which cover two-thirds of the surface area of the planet. About 90% of all fresh water is frozen in the ice in the North and South Poles and glaciers (large slow-moving masses of ice). Less than 1% of all the water on Earth is available for consumption, and most of it is found in aquifers (porous rock chambers holding fresh water) underground.
Characteristics of water
Water is a simple, yet extremely important, molecule comprised of one oxygen atom and two hydrogen atoms (an atom is the smallest part of an element that has all the properties of the element, and a molecule is two or more atoms held together by chemical bonds). The water molecule's small size and biochemical properties allow it to bond easily with other molecules. In fact, water is involved in almost every biological reaction.
Water has many chemical and physical properties that make it useful to cells and organisms. Water acts as a solvent (a liquid in which other substances are dissolved). Water sticks to itself and to other things, which allows it to flow slowly and to fill small places. Water is the only material that can exist naturally as a solid, liquid, and gas at Earth's natural temperatures. It takes a lot of energy to change the temperature of water, so water maintains stable temperatures well. Water also transmits light, allowing photosynthesis to occur underwater.
Water is polar
Water is composed of one oxygen atom and two hydrogen atoms. The oxygen atom has eight positively charged particles, called protons, and eight negatively charged particles, called electrons. The protons move about in the nucleus (center of the atom). The electrons spin around the nucleus in what are called electron shells or orbitals. Different orbitals hold different numbers of electrons. The first orbital contains two of these electrons and the second orbital contains six. Hydrogen atoms contain one proton and one electron. When water forms, electrons are shared between each of the hydrogen atoms and the oxygen atom. The sharing of an electron between two atoms forms a covalent bond (this is not a physical bond, atoms do not touch). The covalent bonds result in full outer orbitals for both atoms: eight in the second orbital of the oxygen and two in the first orbital of the hydrogen.
Positive and negative electrical charges attract each other, like two positive charges repel each other. Because oxygen has more protons than hydrogen, it has a greater positive charge. That causes the spinning electrons in the water molecule to be attracted to the oxygen. This results in extra negative charge in the oxygen part of the molecule, and a positive charge on the hydrogen part.
Water on Mars
Water is so important to life on Earth that few scientists assume that life on any other planet is possible without water. In 1984, scientists from the National Aeronautics and Space Administration (NASA) and Stanford University found a meteorite (rock or metal that has fallen to Earth's surface from outer space) from Mars in Antarctica. After analyzing the meteorite, they said that the meteorite came from rocks that formed on Mars about 3.5 billion years ago. At that time, the atmosphere on Mars was similar to Earth's atmosphere today. It contained much carbon dioxide that helped keep the planet warm. Most scientists assume that at that time the planet was warm enough for water to form as a liquid and that an ocean existed on Mars. When scientists cut the meteor open, they found microscopic structures in the rock that look a lot like fossilized bacteria (microscopic single-celled organisms) from Earth. The only difference is that the structures in the Martian rock are about 100 times smaller than bacteria on Earth. This suggested, but did not prove, that bacteria did live on Mars many, many years ago.
Over time, the rocks on Mars absorbed the carbon dioxide from the atmosphere and the planet cooled down. The water from the Martian ocean probably either froze or became bound to rocks. Some scientists say that the water may still be available deep underground. In 2004, NASA sent two spacecrafts, Spirit and Opportunity, to Mars to look for signs of water. Both of these spacecrafts have confirmed that the landscape of Mars once contained water. Two major questions remain: How long ago did the water dry up? Is there any left underground? In the coming years, the development of spacecraft that can return rocks to Earth or that can date the rocks directly on the Martian surface will help answer those questions.
The oxygen molecule takes on a "V"-shape, with the oxygen part of the molecule at the bottom of the "V" and the hydrogens at the arms. The bottom of the "V" has a small negative charge, while the arms of the "V" have a small positive charge. This type of molecule is referred to as a polar molecule, because it has a positive pole (the bottom of the "V") and negative poles (the arms of the "V").
The polarity of water molecules allows them to interact with each other electrostatically (due to their charges). The positive pole of one water molecule will be attracted to one of the negative poles of another water molecule. This sort of attraction is called a hydrogen bond. Hydrogen bonds are weak bonds; they easily form and are broken. Each water molecule has the potential to form four hydrogen bonds with other molecules.
Water dissolves polar substances
Some molecules are made up of ions. Ions are atoms that have either lost or gained electrons. If the atom has lost electrons, it is positively charged. If the atom has gained elections, it is negatively charged. Ionic bonds form between positively and negatively charged atoms. In these molecules, no electrons are shared; instead, the atoms are held together by their opposite charges.
When ions are mixed with water, the positively charged atom is attracted to the negative poles of water molecules and the negatively charged atom is attracted to the positive poles of water molecules. Eventually, the attraction between the different parts of the ion and the water molecules will pull the ion apart, breaking the ionic bond and dissolving the ion into positively charged atoms and negatively charged atoms. The fact that water is effective at dissolving ions makes it a good solvent.
Molecules that are polar are able to dissolve easily in water. These substances are often called hydrophilic (or water-loving). Examples of hydrophilic molecules are table salt and table sugar (glucose). Some molecules, however, do not dissolve well in water. These molecules are not polar and they are termed hydrophobic (water-hating). Examples of hydrophobic molecules are fats and proteins.
The membranes (layers) that surround cells are made up of large fats and proteins that cannot be dissolved in water. However, because water is a small molecule, it can pass through these membranes. As a result, water can transport small nutrients that cells need through cell membranes without destroying any cell membranes and without requiring an input of energy. Similarly, water can transport small waste molecules out of cells.
Water sticks together
The hydrogen bonds formed between water molecules allow water to stick to itself. This is important for many biological purposes. For example, the surface tension (a force that controls the shape of a liquid) of water allows some animals, such as water striders (spidery-like water insects), to walk on its surface. When rain falls onto Earth, the viscosity (resistance to flow) of water slows the rate it flows over the surface, allowing more water to absorb into the soil where it can be used by plants.
Water changes temperature slowly
Oceans and lakes change temperature very slowly due to the amount of energy needed to alter the water's temperature. Thus, as water covers so much of Earth (nearly three-fourths of the planet), the planet has relatively stable temperatures. This means that animals and plants that live in water experience a relatively stable environment. Many animals and plants contain a lot of water in their bodies, which helps them minimize body temperature changes as well.
The energy required to change water from a liquid to a gas is extremely great because many hydrogen bonds must be broken. When a molecule of water gains enough energy to escape all the hydrogen bonds that surround it, it becomes water vapor. As this molecule leaves the liquid water, it takes with it all of its energy. This means the water left behind has less energy. This process is known as evaporative cooling. Many animals (like humans) use evaporative cooling to reduce heat in their bodies. Plants also use evaporative cooling to stay cool in strong sunlight.
Water is found in three states
At the temperatures and pressures found on Earth, water can be found as a gas, liquid, and solid. A notable property of water is that it is densest, and therefore heaviest, at about 39°F (4°C). Water turns to ice at even colder temperatures, 32°F (0°C).
When water turns to ice, it gains a crystal-like structure. In this form, nearly all the water molecules are joined by the maximum number of hydrogen bonds, which is four. [These hydrogen bonds force the water molecules to move away from each other compared to when they are in the liquid state. As a result, water expands when it is frozen. As it expands, it becomes less dense and, therefore, floats on liquid water. As a result, ice is lighter than cold water and so it floats on top of it. If it were not for this unique structure, ice could form in deep water throughout lakes and oceans, making it very difficult for animals to exist there in cold climates.
Camels are well known for the humps on their backs and for being able to go for long periods of time without water. Although it is tempting to think that the humps are large water storage tanks on the camels' backs, they are actually large mounds of fat. In some camels, the humps can weigh as much as 80 pounds (35 kilograms). The fat acts as a food supply for the camel, who can exist for up to two weeks in the desert without eating.
Camels have developed several physical adaptations to manage without much water in hot, dry climates. Camels' bodies normally use about 5 gallons (20 liters) of water a day. However, if water is scarce, camels lose up to 40% of their body weight in water and recover without any damage. Most animals can only lose about 20% of their body weight in fluid each day without threat of dehydration (excessive harmful water loss).
The bodies of camels can undergo large fluctuations in body temperature. Whereas humans maintain their body temperature within a narrow range of about 2°F (1°C), camels can withstand body temperatures that fluctuate about 10°F (4°C). Only at temperatures above 105°F (40°C) does a camel begin to sweat to cool itself, which helps to conserve water. In addition, with the hump as a fat storage tank, camels avoid having fat all over their bodies. Since fat is insulating and holds heat in, this allows camels to lose heat from all the other regions of their body, thereby enabling the camel to remain cooler than other animals would at similar temperatures.
Camels have developed a circulatory system that can handle large water losses. As humans become dehydrated, they lose water from their blood, which lowers blood volume and blood pressure, leading to fainting. Camels, instead, maintain the water in their blood and lose water from their fat tissues. This keeps their blood pressure relatively stable. In addition, the red blood cells of camels, which carry oxygen to the tissue cells of the body, are well adapted for managing large fluctuations in water. When camels are able to replace lost water, their red blood cells can swell to 240% of their usual size. In most other animals, red blood cells can only increase their size by 50%.
Camels have also developed specialized digestive and excretory systems that protect them from large fluctuations in water. When water is scarce, camels produce feces that are so dry they can be burned. The urine produced by camels can be a thick syrupy consistency, with twice as much salt as ocean water. When camels do need to drink large amounts of water to replace losses, their stomachs and intestines are specially designed to manage the large input of water.
Water both transmits and absorbs light
Water has the property of transmitting some types of light, while absorbing or scattering others. The ways that different types of light interact with water benefits life on Earth. Ultraviolet light, which has very small wavelengths, can damage cells. However, water vapor in the atmosphere (mass of air surrounding Earth) absorbs light in the ultraviolet wavelengths, greatly decreasing the amount of ultraviolet light that reaches the Earth's surface. Blue and green wavelengths of light can pass through water relatively easily. These are wavelengths that are most effectively used by plants for photosynthesis. As a result, plants can grow and flourish in underwater environments such as lakes and oceans. Water strongly absorbs red wavelengths of light, which produce a lot of heat. Because water vapor is found throughout the atmosphere, much of the red light that hits the Earth is absorbed by water. This aids in keeping the temperature of the Earth warm enough for life to exist.
Juli Berwald, Ph.D.
For More Information
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Solomon, Eldra Pearl, Linda R. Berg, and Diana W. Martin. Biology. 5th ed. Philadelphia: Saunders, 1999.
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