Water and Carbon - AP Biology

In osmosis, water diffuses through selectively permeable membranes to regions where the concentration of solute is higher (hypertonic). Osmosis is not the movement of the solute particles.

Carbon is phenomenally important to life as we understand it. The ability to form bonds with up to four different atoms gives carbon an incredible chemical diversity, and allows for carbon to make long chains and aromatic compounds. The ability to make long chains and aromatic compounds accounts for the formation of nucleic acids, proteins, and lipids (macromolecules that are absolutely essential to life). Binding properties of carbon also relate to the structure and orientation of biological compounds, which are important aspects of organic chemistry.

In its ground state carbon has four valence electrons, two its full s subshell and two in a partially filled p subshell. Normally, this would indicate that carbon forms two bonds, since only two of the electrons are in orbitals that are not already paired. Carbon, however, is able to form hybrid orbitals by combining the three p orbitals and one s orbital to form four identical sp3 orbitals, each containing one electron. This means that carbon can form four bonds, allowing it to achieve a stable octet.

For biology, the important note is that carbon can make four bonds. Organic chemistry is the study of carbon and how these bonds function to create organic and biological materials.

The chemical properties of an element are, in a large part, determined by the number of bonds that element can form with other elements. Silicon, like carbon, can form four bonds with other elements, and thus is the most similar. This can easily be seen on a periodic table as elements with similar properties are grouped together in the same column. Note that these similarities arise from having the same number of valence electrons.

Carbon has four valance electrons, allowing it to form a wide range of bonds with other atoms.

When carbon bonds to four separate substituents, it forms a tetrahedral structure. Because of its ability to hybridize orbitals, carbon can also bond to three substituents by forming a double bond, or to two substituents via two double bonds or the combination of a single bond and a triple bond. This variability in molecular bonding and shape allows carbon to exist in numerous compounds, exhibiting a number of different properties and functions.

Carbon is incapable of forming a quadruple bond, and it is not magnetic. Though carbon has a relatively low atomic mass, one would expect hydrogen to be the most relevant element if low mass was the most pertinent property of carbon. Carbon can form ionic bonds (generally with metals), but is most commonly found in organic molecules where it forms covalent bonds.

Life is "carbon-based" or predominantly carbon because it can form stable bonds with itself, but also with a variety of other types of elements. Electronegativity increases from left to right on the periodic table, but also from bottom to top. While carbon is relatively high and right on the periodic chart, there are still elements like oxygen or fluorine (the most electronegative) that have a great pull for electrons. While carbon makes up a lot of the universe, it pales in comparison to hydrogen which is the most common element (three fourths of the mass of our universe). Therefore ratios do not matter. The polar and nonpolar nature of molecules are important for the functions of life (like membranes), but were it not for the bonding of carbon to itself, the nonpolar molecules would not be able to form. Thus, its bonding versatility is the main reason for life being carbon based.

Water is a polar molecule, and thus can adhere to different surfaces; thus, adhesion is the correct answer here. Cohesion is close, as cohesion describes the ability of water to stick to itself due to its polarity. We want the property that allows water to stick to other surfaces, not to itself. Polymerization involves chains of similar molecules, and does not occur in water. Parsimony is the principle that the simplest explanation is usually the reality of a situation (such as when tracing evolutionary histories). Gravity does not play into the properties of water.

The properties of water make it essential to life. Cohesion refers to its ability to form hydrogen bonds, attracting the molecules together and contributing to its high surface tension. Adhesion refers to water's attractive properties to other substances, and helps processes like absorption through the xylem. Solid ice is less dense than liquid water, allowing life to exist below the frozen surfaces of lakes and ponds. The polarity of water is essential for numerous biological processes and makes it a good solvent for most biological molecules. Finally, the high specific heat of water makes it resistant to temperature change, allowing life forms to maintain relatively constant internal temperatures.

The high specific heat and surface tension of water contribute to its high boiling point, helping to keep it in liquid form for most biological processes.

Nonpolar compounds will not be adequately dissolved in aqueous solutions. Lipids are nonpolar compounds that are mainly insoluble in water. This causes lipids to congregate together, rather than be broken apart in aqueous solutions. Lipids will generally come together to form globs or balls called micelles.

Ions, amino acids, and sugars (carbohydrates) are all polar, and will be adequately dissolved and ionized by water.

These properties of water are all a result of hydrogen bonding. Hydrogen bonds result from the electrical attraction between partially positive hydrogen atoms and partially negative oxygen atoms of adjacent water molecules. The differences in electronegativity between hydrogen and oxygen give rise to the hydrogen bonding and associated properties.

Attraction and polarity in water molecules cause them to "stick" to one another. Attraction between water molecules results in cohesion, and attraction between the water molecules and other compounds in the environment results in adhesion. The high surface tension of water is caused by the "sticking" of water molecules to one another, which keep vapor pressure low.

Hydrogen bonding is a temporary intermolecular force, and is different from covalent or ionic bonding. Covalent and ionic bonding result in permanently joined atoms to build molecular structures.

There are three forms of heat transfer: radiation, convection, and conduction. Radiation is the transfer of heat via electromagnetic waves, such as sunlight of microwaves. Convection is the transfer of heat through a fluid medium, such as water or air currents. Conduction is the direct transfer of heat between environments through physical contact. Since your hand is in physical contact with the glass, heat is transferred by conduction.

Heat is always transferred from a body of higher temperature to a body of lower temperature. Since your hand is warmer than the glass, heat is transferred from the hand to the glass. It can be easier to think of heat transfer in terms of concentration. Like molecules, heat energy will travel from a region of high concentration (hotter) to a region of low concentration (colder) in order to reach equilibrium.

To select the correct answer, you must understand the difference between hypertonic, hypotonic, and isotonic. A hypertonic solution, such as ocean water with a high salt content, contains more solute than a normal cell. Water will flow out of the cell and into the environment in an attempt to equalize the amount of solute in the two "compartments." An isotonic solution has the same relative amount of solute as the cell to which it is being compared, so no concentration gradient exists and no net diffusion will occur. Finally, a hypotonic solution contains less solute than the cell too which it is being compared.

Pure water will always have less solute concentration than a cell, creating a hypotonic relationship. The solution is hypotonic to the cell (less solute) and the cell is hypertonic to the water (more solute). Water will flow from the hypotonic environment to the hypertonic cell, causing it to swell in size.

Hydrogen bonding can easily be disrupted by changes in temperature. It is important to note that hydrogen bonding is not a true example of a chemical bond, but rather an intermolecular force. Hydrogen bonds are essential for the formation of protein structure and DNA base pairing. When proteins and DNA are exposed to heat, they degrade as these hydrogen bonds are broken.

Covalent bonds, which include peptide bonds, and ionic bonds are examples of real chemical bonds that require high amounts of energy before they can be easily disrupted. These bonds are considered more permanent interactions than other intermolecular forces.

Water has an unusually high boiling point for a liquid. This is related to the intermolecular forces between water molecules; when a liquid has particularly large intermolecular forces, it will have a higher boiling point. Large intermolecular forces between molecules will favor the liquid state over the gaseous state.

Water is made up of oxygen and hydrogen and can form hydrogen bonds, which are particularly strong intermolecular forces. These strong intermolecular forces cause the water molecules to "stick" to one another and resist transition to the gaseous phase.

Cohesion is the result of increased strength of hydrogen bonding between many water molecules. This increased strength requires a great amount of heat in order to break the hydrogen bonds between molecules, in order for these molecules to become vapor. Cohesion and hydrogen bonding are the cause for water's low vapor pressure, high boiling point, and high heat capacity.

Adhesion is water's property to adhere to a surface, and is the cause of capillary action. Water does have low density as a solid, which allows ice to float, but is not the reason for water's high heat capacity. Water has a high boiling point, considering its low mass.

Water, unlike many other compounds, has several special properties due to its hydrogen bonding between molecules. The hydrogen bonds are relatively strong, leading water to have very low vapor pressure and high surface tension. A side effect of the hydrogen bonding, however, is that when water crystallizes, the molecules will inevitably align so that the hydrogen bonds are maintained. The solid lattice structure of water molecules is, thus, not very tightly packed. The structure is ideal to optimize intermolecular forces, rather than space and volume. The density of the solid (ice) is thus less than the density of the liquid water.

Water vapor (gas) and supercritical water both have lower densities than ice, making liquid water the most dense.

Boiling point is generally determined by a few factors that are directly related to molecular weight and intermolecular forces. In general, lighter molecules have lower boiling points and molecules with stronger intermolecular forces have higher boiling points.

Water is relatively light, but has very strong hydrogen bonding. Hydrogen bonding is the strongest intermolecular force and will act to pull water molecules closer to each other. The result is a dense liquid that does not easily transition into a less dense gas. In order for water to boil these intermolecular hydrogen bonds must be broken, which takes energy. A greater energy input means a higher boiling point.

The shape and composition of water are not particularly relevant to its boiling point, and being a small, light molecule would be conducive to a low boiling point rather than a high boiling point.

Adhesion refers to the attraction between water molecules and foreign particles or surfaces. Cohesion refers to the attraction between one water molecule and another.

Adhesion of water to the cell walls of the xylem vessels and cohesion of water molecules to one another allow for water transport in plants. This is also known as capillary action.

The high heat capacity, low solid density, and polarity of water, as well as its use as a solvent, are all essential to the role of water in supporting life in other ways.

The high specific heat of water means that it requires a large amount of heat to raise the temperature of water. This helps the temperature of the environment from becoming too hot or too cold. Also, humans are about 66% water, thus this property of water helps us regulate our body temperature too. Evaporative cooling helps prevent bodies from overheating. We have evolved to take advantage of this property of water, perspiring through our skin to cool it down during exercise. The fact that ice floats protects lakes from further cooling by cold wind. The fact that water is such a good solvent for polar molecules allows the chemistry of life to occur. Recall that life uses four main biomolecules, and dehydration synthesis and hydrolysis reactions occur constantly.

A hypertonic solution has a higher concentration of solute than the solute in the cell. Water diffuses to the higher concentration of solute, so water diffuses out of the cell, and the cell shrivels up. One way to think of this is that water follows "salt," as salt is the most common biological solute.