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| 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.
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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.
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.
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| Water sticks together |
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.
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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.
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| Water both transmits and absorbs light |
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.
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.