AP Biology

## Learning Outcomes

- Explain why water is polar using oxygen’s electronegativity and unequal electron sharing.
- Describe how hydrogen bonds form between water molecules.
- Connect hydrogen bonding to cohesion, adhesion, surface tension, capillary action, and plant water transport.
- Explain why water has a high specific heat and high heat of vaporization.
- Relate water’s hydrogen bonding properties to climate moderation and evaporative cooling.
- Distinguish hydrogen bonds from covalent bonds in water.
- Apply water’s polarity to its role as a solvent for ionic and polar substances.

## AP Biology Syllabus

In AP Biology Unit 1, Chemistry of Life, you are expected to connect the structure of water to its biological functions. Unit 1 usually contributes about 8–11% of the multiple-choice section, and water is one of the first concepts because many later topics depend on it. For this subtopic, exam questions often test whether you can build a clear chain: electronegativity difference → polar covalent bonds → polar water molecule → hydrogen bonding → properties that support life.

- Water contains polar covalent bonds between oxygen and hydrogen.
- Oxygen has a partial negative charge; hydrogens have partial positive charges.
- Hydrogen bonds form between the partial negative oxygen of one water molecule and the partial positive hydrogen of another.
- Hydrogen bonding causes cohesion, adhesion, surface tension, and capillary action.
- Hydrogen bonding contributes to water’s high specific heat and high heat of vaporization.
- Water helps moderate climate and body temperature because it absorbs and releases heat slowly.
- Water’s polarity makes it a useful solvent for many charged and polar substances.
- Exam answers should connect molecular structure to biological examples, such as xylem transport, sweating, and aquatic habitats.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why does oxygen in a water molecule carry a partial negative charge?
2. What is the difference between a polar covalent bond and a hydrogen bond?
3. How do cohesion and adhesion work together during capillary action in plant xylem?
4. Why can water help stabilize the temperature of organisms and habitats?
5. Why does evaporating sweat cool the body?

## Introduction

Water is the most abundant compound in living organisms, and many biological systems exist in water-rich environments. Cells contain aqueous cytoplasm, blood plasma carries dissolved materials, plants move water from roots to leaves, and aquatic organisms depend on the unusual behavior of water across temperature changes. These properties are not random: they arise from water’s molecular structure.

A water molecule has two hydrogen atoms covalently bonded to one oxygen atom. Oxygen attracts shared electrons more strongly than hydrogen does, so the electrons spend more time near oxygen. This unequal sharing gives oxygen a partial negative charge and the hydrogens partial positive charges. Because water molecules have charged regions, they attract one another by hydrogen bonding. Hydrogen bonds are weaker than covalent bonds, but in large numbers they give water several life-supporting properties.

> **Key Term: polar molecule**
> A molecule with an unequal distribution of charge, producing partially positive and partially negative regions.

### From polarity to hydrogen bonding

To understand water’s hydrogen bonding properties, begin with the covalent bonds inside one water molecule. A covalent bond forms when atoms share electrons. In water, each hydrogen shares electrons with oxygen. However, the sharing is unequal because oxygen has a higher electronegativity than hydrogen.

> **Key Term: electronegativity**
> The tendency of an atom to attract electrons in a chemical bond.

Because oxygen attracts electrons more strongly, the oxygen end of the molecule becomes partially negative, written as δ−. The hydrogen ends become partially positive, written as δ+. The molecule as a whole is neutral, but its charge is not evenly distributed. This makes water polar.

> **Key Term: polar covalent bond**
> A covalent bond in which electrons are shared unequally because the bonded atoms differ in electronegativity.

A hydrogen bond forms when the partial positive hydrogen of one water molecule is attracted to the partial negative oxygen of another water molecule. This is not the same as the covalent O–H bond inside a water molecule. The covalent bonds hold each water molecule together. Hydrogen bonds form between neighboring water molecules.

> **Key Term: hydrogen bond**
> A weak attraction between a partially positive hydrogen atom bonded to an electronegative atom and a partially negative electronegative atom nearby.

Hydrogen bonds are individually weak compared with covalent bonds. A single hydrogen bond can form and break quickly. However, water contains huge numbers of molecules, so many hydrogen bonds can act together. This explains why water behaves differently from many small molecules of similar size.

> **Exam Warning:**
> Do not say that hydrogen bonds hold the hydrogen and oxygen atoms together within a water molecule. The O–H bonds inside water are polar covalent bonds. Hydrogen bonds form between water molecules.

A strong AP Biology answer often follows this pattern:

- Oxygen is more electronegative than hydrogen.
- Electrons are shared unequally in O–H bonds.
- Water is polar, with δ− oxygen and δ+ hydrogens.
- Opposite partial charges attract between water molecules.
- Hydrogen bonds form.
- Hydrogen bonding produces a property, such as cohesion or high specific heat.
- That property supports a biological process, such as transpiration or temperature regulation.

### Cohesion, adhesion, and surface tension

Hydrogen bonding causes water molecules to stick to one another. This attraction between molecules of the same substance is called cohesion.

> **Key Term: cohesion**
> Attraction between molecules of the same substance; in water, cohesion results mainly from hydrogen bonding between water molecules.

Cohesion explains why water forms droplets. In a droplet, water molecules pull on nearby water molecules, producing a rounded shape. Cohesion also helps water move through plant xylem. During transpiration, water evaporates from leaves. As water molecules leave the leaf surface, they pull on other water molecules behind them. Because the molecules are connected by hydrogen bonds, a continuous column of water can be drawn upward from roots through xylem vessels.

Adhesion is different from cohesion. Adhesion is attraction between different substances. Water can adhere to polar or charged surfaces, such as glass, cellulose in plant cell walls, or other hydrophilic materials.

> **Key Term: adhesion**
> Attraction between different substances; for water, this often means attraction to polar or charged surfaces.

Cohesion and adhesion together produce capillary action. In narrow spaces, water can climb upward because adhesion pulls water along the surface, while cohesion keeps water molecules connected to each other. This is especially useful in plants, where water must move through narrow xylem tubes against gravity.

> **Key Term: capillary action**
> Movement of water through narrow spaces caused by adhesion to surfaces and cohesion between water molecules.

Surface tension is another result of cohesion. At the surface of water, molecules are pulled inward and sideways by hydrogen bonds with neighboring molecules. Because there are fewer water molecules above the surface, the surface behaves like a stretched film. This allows some lightweight insects to stand or move on water without immediately breaking the surface.

> **Key Term: surface tension**
> The resistance of a liquid surface to being broken, caused by cohesive forces among molecules at the surface.

A common AP Biology multiple-choice item may ask which property allows insects to walk on water. The correct answer is surface tension, not capillary action or specific heat. Capillary action refers to water rising in narrow tubes. Specific heat refers to temperature change.

> **Test Tip:**
> When a question gives a biological example, identify the water property first. Insects on water = surface tension. Water rising in xylem = cohesion plus adhesion causing capillary action. Stable lake temperature = high specific heat.

### Heat-related properties of water

Hydrogen bonds also explain why water resists rapid temperature change. Temperature reflects the average kinetic energy of molecules. When heat is added to water, some of that energy is used to disrupt hydrogen bonding interactions before water molecules move much faster. As a result, water has a high specific heat.

> **Key Term: specific heat**
> The amount of energy required to raise the temperature of a substance by a given amount; water has a high specific heat because of hydrogen bonding.

High specific heat matters for living systems. Organisms are mostly water, so their internal temperature does not change as rapidly as it would if their cells were made mostly of a liquid with low specific heat. This supports homeostasis because many cellular processes, including enzyme activity, depend on temperature staying within a workable range.

High specific heat also helps moderate climate. Large bodies of water absorb a lot of heat when surrounding air is warm and release heat slowly when surrounding air cools. Coastal areas often have milder temperature changes than inland regions because oceans and lakes buffer temperature swings.

Water also has a high heat of vaporization. Vaporization means liquid water becomes water vapor. For a water molecule to evaporate, it must gain enough energy to escape from hydrogen bonding with neighboring water molecules. Because hydrogen bonds must be disrupted, evaporation requires a relatively large amount of energy.

> **Key Term: heat of vaporization**
> The amount of energy required to change a liquid into a gas; water’s heat of vaporization is high because hydrogen bonds must be disrupted.

This property enables evaporative cooling. When sweat evaporates from skin, high-energy water molecules leave the surface. The energy used for evaporation comes partly from the body, so heat is removed and body temperature decreases. The same principle helps cool plant leaves during transpiration and helps some animals cool themselves by panting.

> **Test Tip:**
> For sweating or panting questions, do not just write “water cools the body.” Explain the mechanism: hydrogen bonds require energy to break during evaporation, and that energy is absorbed from the body surface, reducing body temperature.

A free-response answer about evaporative cooling might say:

“Water molecules in sweat form hydrogen bonds with one another. Evaporation requires energy to disrupt these hydrogen bonds. As sweat evaporates, heat energy is absorbed from the skin, lowering body temperature.”

This answer earns more credit than simply stating that sweat evaporates.

### Water as a solvent and the behavior of nonpolar substances

Water’s polarity also makes it a useful solvent. A solvent is the substance that dissolves another substance, called the solute. In living systems, water dissolves many ions and polar molecules, allowing reactions to occur in cytoplasm and allowing materials to be transported in blood, plant sap, and other body fluids.

> **Key Term: solvent**
> A substance that dissolves solutes to form a solution.

When ionic compounds dissolve in water, the partial negative oxygen atoms of water are attracted to positive ions, while the partial positive hydrogen atoms are attracted to negative ions. Water molecules surround the ions, helping separate them and keep them dispersed in solution. Water can also interact with polar molecules, such as sugars or many amino acids, by forming hydrogen bonds or other charge-based attractions.

Substances that interact well with water are called hydrophilic. They are usually charged or polar. For example, salts, glucose, and many proteins contain regions that can interact with water.

> **Key Term: hydrophilic**
> Water-attracting; describes substances that dissolve in or interact favorably with water, often because they are polar or charged.

Nonpolar substances do not form favorable interactions with water. They are called hydrophobic. Oils and many lipids are hydrophobic because they contain mostly nonpolar hydrocarbon regions. When nonpolar molecules are placed in water, they tend to cluster together rather than mix evenly with water.

> **Key Term: hydrophobic**
> Water-repelling; describes substances that do not interact favorably with water, usually because they are nonpolar.

This difference is central to cell biology. Cell membranes contain phospholipids with hydrophilic heads and hydrophobic tails. The hydrophilic heads face the watery environments inside and outside the cell, while the hydrophobic tails cluster away from water in the membrane interior. Although membrane structure is usually tested later in the course, the reasoning begins with water’s polarity and hydrogen bonding behavior.

> **Exam Warning:**
> Do not assume that all biological molecules dissolve well in water. Polar and charged molecules tend to dissolve or interact with water. Nonpolar molecules, such as many lipids, tend to avoid water.

Water’s solvent properties also connect to osmosis and water potential, topics often linked to plant and cell physiology. Water moves across membranes depending on solute concentration and water potential. Adding solute lowers water potential, making water more likely to move into that solution. Although calculations may use formulas, the basic idea depends on water’s interactions with dissolved solutes.

### Ice, density, and aquatic life

Hydrogen bonding also affects water when it freezes. In liquid water, hydrogen bonds are constantly forming, breaking, and reforming. As water cools and freezes, hydrogen bonds arrange water molecules into a more open, ordered structure. This structure spaces molecules farther apart than in liquid water, so ice is less dense than liquid water.

Because ice is less dense, it floats. This is unusual; many solids are denser than their liquids. Floating ice has major biological effects. Lakes and ponds freeze from the top down rather than from the bottom up. The ice layer can insulate the water below, helping aquatic organisms survive during cold periods.

If ice sank, bodies of water could freeze solid more easily, making survival much harder for fish, microorganisms, and aquatic plants. Floating ice also affects seasonal mixing and nutrient cycling in aquatic habitats.

This property is still linked to hydrogen bonding, but do not confuse it with surface tension. Surface tension explains why the surface of liquid water resists breaking. Lower density of ice explains why ice floats.

> **Test Tip:**
> If the question mentions ice floating, frozen lakes, or organisms surviving under ice, choose the property that ice is less dense than liquid water due to hydrogen bonding in the solid structure.

### Building exam-ready explanations

AP Biology rewards cause-and-effect reasoning. A definition alone may not be enough. You should be able to connect molecular structure to a property and then to a biological function.

For example, consider this prompt:

> **Question:**
> Explain how the structure of water allows water to move upward through xylem in a plant.

A strong answer would include:

- Water is polar because oxygen is more electronegative than hydrogen.
- Water molecules form hydrogen bonds with each other.
- Cohesion keeps water molecules connected in a column.
- Adhesion helps water interact with the walls of xylem vessels.
- These forces contribute to capillary action and support upward movement of water.

A weak answer would say only, “Water is sticky, so it moves up plants.” That response uses a vague description and does not explain the molecular cause.

Now consider a heat-regulation prompt:

> **Question:**
> A student claims that water is effective for cooling skin because water has hydrogen bonds. Justify the claim.

A strong answer would include:

- Water molecules form hydrogen bonds with other water molecules.
- Evaporation requires energy to disrupt these interactions.
- Heat is absorbed from the skin as water evaporates.
- This removes heat from the body and cools the skin.

A weak answer would say, “Hydrogen bonds are strong, so water is cold.” That is inaccurate. Hydrogen bonding does not make water “cold”; it affects how much energy is needed to change temperature or phase.

For multiple-choice questions, focus on the trigger words:

- “Sticks to itself” → cohesion
- “Sticks to another surface” → adhesion
- “Rises in narrow tubes” → capillary action
- “Insect standing on water” → surface tension
- “Temperature changes slowly” → high specific heat
- “Sweating cools” → high heat of vaporization
- “Dissolves ions or polar molecules” → polarity and solvent ability
- “Ice floats” → ice is less dense than liquid water

## Key Point Checklist

This article has covered the following key knowledge points:

- Water is polar because oxygen attracts shared electrons more strongly than hydrogen.
- O–H bonds in water are polar covalent bonds, not hydrogen bonds.
- Oxygen in water has a partial negative charge, while hydrogens have partial positive charges.
- Hydrogen bonds form between the δ+ hydrogen of one water molecule and the δ− oxygen of another.
- Cohesion is water sticking to water and helps maintain water columns in xylem.
- Adhesion is water sticking to other polar or charged surfaces.
- Capillary action results from cohesion and adhesion in narrow spaces.
- Surface tension results from cohesive forces among water molecules at the surface.
- Water’s high specific heat helps stabilize body temperatures and climates near large bodies of water.
- Water’s high heat of vaporization allows evaporative cooling by sweating, panting, and transpiration.
- Water is an effective solvent for many ionic and polar substances because it is polar.
- Ice floats because hydrogen bonding creates a less dense solid structure than liquid water.

## Key Terms and Concepts

- polar molecule
- electronegativity
- polar covalent bond
- hydrogen bond
- cohesion
- adhesion
- capillary action
- surface tension
- specific heat
- heat of vaporization
- solvent
- hydrophilic
- hydrophobic


Structure of water and hydrogen bonding - Hydrogen bonding properties