The oxygen cycle is the movement of oxygen through three main components: the atmosphere; the biosphere, which contains all the ecosystems; and the lithosphere, which is the earth’s crust. The largest amount of oxygen is found in the earth’s crust, and the smallest is in the biosphere. The oxygen in the crust is found as silicate, oxide minerals, and mantle. The existence of the oxygen cycle is heavily dependent on photosynthesis. Plants conduct photosynthesis by consuming carbon dioxide from the atmosphere and releasing oxygen into the air. However, this only contributes a very small amount to the oxygen cycle. Respiration and decay due to bacteria are two ways in which the oxygen in the atmosphere is recycled or consumed.

The growth of a population is very dependent on the amount of resources available in the environment. A limiting factor is a resource or condition that can decrease the rate of population growth. When a group of organisms is faced with a limiting factor, they will show a logarithmic growth rate. This represents an S-shaped curve, with the plateau at the top representing carrying capacity. The carrying capacity is governed by the limiting factors and is the maximum number of organisms that can be sustained with the available resources. In the absence of a limiting factor, the growth rate of a population will be exponential (J-shaped). Some common limiting factors are food, nutrients, oxygen, and sunlight. In most cases, an excess of water means that a population can flourish and continue to grow; therefore it is not perceived as a limiting factor.

Water is a very important molecule in order for life to take place. It is the solvent in which chemical reactions take place, as well as a reactant for certain reactions. Water's polarity also causes polar molecules to dissolve and nonpolar compounds to aggregate. As a result, nonpolar molecules are not dissolved in water.

Water is often called the universal solvent and is one of the most important compounds on Earth. The molecular structure of water promotes intermolecular hydrogen bonding, a very strong force between molecules. The result is known as cohesion, the phenomenon of water molecules "sticking" to one another. Cohesion leads to other water properties, such as high surface tension and capillary action, which are essential to supporting life. Hydrogen bonding is only possible in water molecules because of the polarity of the bond between oxygen and hydrogen; oxygen carries a partial negative charge, and hydrogen carries a partial positive charge. Water also have a very high specific heat, which means that it requires a lot of energy to raise the temperature of water. This promotes homeostasis and temperature stability in biological organisms.

In the liquid state, cohesion promotes stability, but in the gaseous state water molecules must separate from one another to move around. Essentially, cohesion must be overcome in order for water to boil. This leads to a high heat of vaporization, rather than a low heat of vaporization.

Molecules that are hydrophobic or "water fearing" will have a hard time dissolving in an aqueous solution. Lipids, which are nonpolar molecules, would not be able to dissolve in water.

In the water cycle, there are abiotic and biotic factors that contribute to the recycling of water. The process by which non-living things return water to the atmosphere is called evaporation. Plants, on the other hand, intake water through their roots and release the water as vapor through tiny openings on leaves called stomata. This process is called transpiration.

When water freezes, its density decreases (most substances do not exhibit this property). This means that ice will float on top of a lake, rather than sink to the bottom. Because the ice floats on top of a lake, it freezes from the top down rather than from the bottom up. The other answer choices are all properties of water, but they do not explain why lakes freeze from top down, and not from bottom up.

Hydrolysis is the reverse of a dehydration reaction; the bond between monomers is broken by the addition of a water molecule, with a hydrogen atom from water attaching to one monomer and the hydroxyl group attaching to the other.  

Water is unique in many ways. Much of its versatility is due to hydrogen bonding and polarity. The electronegativity of the oxygen atom creates a slightly positive charge on the hydrogen atoms—water is polar. Water can share hydrogen bonds and form them with itself. Because of this it has a high boiling point and high heat capacity. It is a good solvent because the oxygen can surround positively charged molecules and the slight positive charge on the hydrogen atoms can surround negative charges.

The cohesive nature of water stems from the fact it forms hydrogen bonds with itself. This allows water molecules to pull on lower molecules as they all travel up the xylem. The adhesive nature comes from the polarity of the molecule, which can be attracted to the walls of the xylem. 

Water actually has a high specific heat, higher than any other common substance. A high specific heat (defined as the amount of heat needed to raise one mass unit of substance by one unit of temperature, usually expressed in  or ) means that water can absorb a large amount of heat without changing temperature. This makes it important for organisms in things like thermoregulation (think prespiration).