AP Biology

## Learning Outcomes

- Explain how water’s polarity allows hydrogen bonds to form between water molecules.
- Define cohesion, adhesion, surface tension, and capillary action in terms of intermolecular attractions.
- Connect cohesion and adhesion to water movement through plant xylem.
- Explain why small organisms or objects can rest on the surface of water.
- Distinguish surface tension from specific heat, ice density, and capillary action in exam questions.
- Use a structure → property → biological function explanation in AP Biology responses.

## AP Biology Syllabus

In AP Biology Unit 1, water’s properties are tested as part of the Chemistry of Life. For this subtopic, you need to connect the structure of water to hydrogen bonding, then connect hydrogen bonding to biological effects such as water transport in plants and the ability of water surfaces to resist breaking.

- Water is a polar molecule because oxygen and hydrogen do not share electrons equally.
- Oxygen has a partial negative charge, while hydrogen atoms have partial positive charges.
- Hydrogen bonds form between water molecules.
- Cohesion is attraction between water molecules.
- Adhesion is attraction between water and other polar or charged substances.
- Cohesion and adhesion together help explain capillary action in narrow tubes such as xylem.
- Surface tension results from cohesive forces at the surface of water.
- AP questions often ask you to identify which water property explains a biological example.

## 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 cohesion and adhesion?
3. How do cohesion and adhesion help water move upward in plant xylem?
4. Why can a water strider or other lightweight insect stand on water?
5. In an AP multiple-choice question, how would you distinguish surface tension from capillary action?

## Introduction

Water is the most abundant compound in living organisms and plays a major role in cells, tissues, and environments. Its unusual properties come from its 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 creates a polar molecule with partial charges.

Because of these partial charges, water molecules attract one another. The partially positive hydrogen atom of one water molecule is attracted to the partially negative oxygen atom of another water molecule. This attraction is a hydrogen bond. Hydrogen bonds are individually weak compared with covalent bonds, but when many water molecules interact at once, they produce major biological effects. Three closely related effects are cohesion, adhesion, and surface tension.

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

### Water Polarity Creates Hydrogen Bonds

To understand cohesion, adhesion, and surface tension, start with the structure of water. The bonds within a water molecule are polar covalent bonds. A covalent bond involves shared electrons, but in water the sharing is unequal. Oxygen has higher electronegativity than hydrogen, meaning oxygen attracts the shared electrons more strongly. As a result, oxygen becomes partially negative, written as $$\delta^-$$, and each hydrogen becomes partially positive, written as $$\delta^+$$.

This uneven charge distribution makes water polar. The positive end of one water molecule can attract the negative end of another water molecule. The attraction between the hydrogen of one water molecule and the oxygen of another is a hydrogen bond.

> **Key Term: polar covalent bond**
> A covalent bond in which electrons are shared unequally because one atom attracts the electrons more strongly than the other atom.
>
> **Key Term: hydrogen bond**
> A weak attraction involving a partially positive hydrogen atom and a nearby electronegative atom, such as oxygen, often on a different molecule.

A common AP Biology skill is explaining the chain of cause and effect:

Oxygen is more electronegative than hydrogen → water has polar covalent bonds → water is polar → water molecules form hydrogen bonds → water has properties such as cohesion, adhesion, surface tension, and capillary action.

This chain matters because AP questions rarely ask only for a memorized definition. They often ask why a property exists or how a property supports life. For example, if asked why water can move upward in the xylem of tall trees, a strong answer should mention hydrogen bonding, cohesion among water molecules, adhesion to xylem walls, and capillary action.

> **Test Tip:**
> In free-response answers, do not stop at “water is polar.” Link polarity to hydrogen bonding, then link hydrogen bonding to the named property and biological effect.

### Cohesion: Water Molecules Stick to Each Other

Cohesion is the attraction between molecules of the same substance. In water, cohesion occurs because water molecules hydrogen bond to other water molecules. Each water molecule can form attractions with neighboring water molecules, so water behaves as if its molecules are connected in a shifting network.

> **Key Term: cohesion**
> The attraction between molecules of the same substance; in water, cohesion is caused by hydrogen bonding between water molecules.

Cohesion explains why water forms droplets. In a droplet, water molecules pull toward one another rather than spreading out completely. Raindrops, dew drops, and beads of water on a waxy leaf all show cohesive behavior. The water molecules are not permanently locked together, but they are attracted enough to cluster.

In plants, cohesion helps maintain a continuous column of water through xylem vessels. Xylem is made of long, narrow tubes that conduct water from roots to stems and leaves. During transpiration, water evaporates from leaf surfaces. As water molecules leave the leaf, they pull on nearby water molecules. Those water molecules pull on others behind them, creating tension through the water column. Cohesion helps prevent the column from breaking.

This is especially useful in tall plants such as redwoods. Water must move upward against gravity from the roots to leaves far above the ground. Cohesion alone is not the whole explanation, but it is one major part of it. It allows one evaporating water molecule to influence other molecules lower in the plant.

> **Exam Warning:**
> Do not confuse cohesion with covalent bonding. Cohesion between water molecules is due to hydrogen bonds, not the polar covalent bonds within a single water molecule.

Cohesion is also related to surface tension, but the two terms are not identical. Cohesion is the general attraction between water molecules. Surface tension is the effect seen at the surface of water, where cohesion makes the surface resist being broken.

### Adhesion: Water Molecules Stick to Other Surfaces

Adhesion is attraction between different substances. Water can adhere to many polar or charged surfaces because its partial charges interact with charges or partial charges on those surfaces.

> **Key Term: adhesion**
> The attraction between molecules of different substances; for example, attraction between water and another polar or charged surface.

A simple example is water sticking to glass. If you place a thin layer of water between two glass slides, the slides can be difficult to separate. This happens because water molecules adhere to the glass surfaces. In a narrow glass tube, water may climb slightly up the sides because water molecules are attracted to the glass.

In plants, adhesion helps water move through xylem. The walls of xylem vessels contain polar substances, including cellulose in plant cell walls. Water molecules adhere to these surfaces. This adhesion helps resist the downward pull of gravity, especially in narrow tubes.

Cohesion and adhesion work together in capillary action. Cohesion keeps water molecules connected to each other, while adhesion helps water attach to the sides of a narrow tube. The narrower the tube, the more contact there is between water and the tube wall relative to the amount of water inside, so capillary effects are stronger.

> **Key Term: capillary action**
> The movement of water through narrow tubes or spaces due to cohesion between water molecules and adhesion between water and the tube surface.

Capillary action is a common AP Biology example. Redwood trees and other tall plants transport water upward through xylem despite gravity. AP multiple-choice questions may describe water moving up plant vessels and ask which property best explains the process. The answer is usually capillary action, or cohesion and adhesion acting together, depending on the choices.

> **Test Tip:**
> If the question describes water climbing up a narrow tube or moving upward in xylem, think “capillary action.” If it describes an insect standing on water, think “surface tension.”

Adhesion also explains the meniscus in a graduated cylinder. Water curves upward at the edges because water molecules are attracted to the polar glass surface. This is not usually the main focus of AP Biology water questions, but it is a useful visual example.

### Surface Tension: Cohesion at the Water Surface

Surface tension is the resistance of a liquid surface to being stretched or broken. In water, surface tension is high because of cohesion among water molecules. At the surface, water molecules are strongly attracted to neighboring water molecules. This creates a “skin-like” effect at the surface, although there is not a real membrane.

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

Water has higher surface tension than many other liquids because hydrogen bonds create strong attraction between water molecules. This property allows small, lightweight insects, such as water striders, to stand or move across the surface without immediately sinking. Their mass is spread over their legs, and the cohesive surface resists rupture.

Surface tension also helps explain why water droplets hold a rounded shape. Molecules at the surface are pulled inward by cohesion, reducing the exposed surface area. A sphere has a low surface area for its volume, so droplets tend to become rounded when other forces are not too strong.

In AP Biology questions, surface tension is often tested through an example rather than a definition. A question might show or describe a lightweight insect on a pond and ask which water property explains it. The correct answer is surface tension. High specific heat, capillary action, and the lower density of ice are all real properties of water, but they explain different biological effects.

> **Exam Warning:**
> Surface tension does not explain why lakes resist rapid temperature change, why sweat cools the body, why ice floats, or why water moves up xylem. Match each property to the correct example.

A useful comparison:

- Surface tension: small insects or leaves can rest on water; water surface resists breaking.
- Capillary action: water moves upward through narrow tubes, including xylem.
- High specific heat: water resists rapid temperature change; coastal climates and aquatic habitats are more stable.
- High heat of vaporization: sweating or transpiration cools organisms because evaporation absorbs heat.
- Lower density of ice: ice floats, insulating water below.

### Connecting Water Properties to Plant Transport

Plant water transport is one of the best biological examples for cohesion and adhesion. Water enters roots from the soil, moves through xylem, and eventually reaches leaves. The movement is not simply “pumping” water upward in the way an animal heart pumps blood. Instead, it depends on physical properties of water.

Transpiration is the evaporation of water from leaves. When water evaporates from cells in the leaf, it creates a pull on the remaining water molecules. Cohesion transmits this pull down the xylem because water molecules attract one another. Adhesion helps the water column cling to the walls of the xylem vessels. Together, these properties help water rise through narrow tubes. This is capillary action, assisted by the transpiration pull from the leaves.

> **Key Term: transpiration**
> The loss of water vapor from plant leaves, which helps create a pull that draws water upward through xylem.

For AP Biology, you do not usually need to calculate the forces involved in plant water movement for this subtopic. Instead, focus on explaining the mechanism clearly. A strong response should include:

- Water is polar.
- Water molecules form hydrogen bonds.
- Hydrogen bonding creates cohesion between water molecules.
- Water also adheres to polar xylem walls.
- Cohesion and adhesion contribute to capillary action.
- Transpiration from leaves helps pull the water column upward.

A weak answer would say only, “Water moves up because of capillary action.” That may identify the correct concept, but it does not fully explain the structure-property-function relationship. A stronger answer links capillary action to hydrogen bonding and polarity.

### Common AP Biology Question Patterns

AP Biology often tests this topic with examples. The challenge is choosing the property that best matches the scenario.

If the question asks about an insect walking on water, choose surface tension. The insect does not float because ice is less dense, because water has high specific heat, or because capillary action is occurring. The key idea is that cohesive hydrogen bonding among surface water molecules resists breakage.

If the question asks about water climbing up narrow tubes, choose capillary action. In plants, this usually involves xylem. Cohesion keeps water molecules linked, and adhesion helps them stick to the tube walls.

If the question asks about droplets or beads of water, cohesion is likely central. Water molecules stick together, forming beads or rounded droplets.

If the question asks about water sticking to glass, plant cell walls, or other polar surfaces, adhesion is the main property.

> **Test Tip:**
> When two answer choices seem related, identify the exact object in the question. Water sticking to water = cohesion. Water sticking to another surface = adhesion. Water surface resisting breakage = surface tension. Water rising in a thin tube = capillary action.

Free-response questions may ask you to justify a prediction. For example:

> **Question:**
> A mutation reduces the amount of polar material in the walls of a plant’s xylem vessels. Predict how this could affect water movement and justify your answer.

A good answer could state that reduced polarity in the xylem walls would reduce adhesion between water and the vessel walls. This would weaken capillary action and make it harder for water to move upward against gravity. Cohesion between water molecules would still occur because water molecules can still hydrogen bond with each other, but the reduced adhesion would decrease support from the xylem walls.

Another possible question might ask why nonpolar liquids would be less effective than water in this role. A nonpolar liquid would not form hydrogen bonds in the same way and would have weaker cohesion and adhesion with polar xylem surfaces. Therefore, it would be less able to maintain a continuous column and less able to climb through narrow tubes.

### Avoiding Misconceptions

One common misconception is that hydrogen bonds are the bonds holding hydrogen and oxygen together inside a water molecule. They are not. The hydrogen and oxygen atoms within one water molecule are held together by polar covalent bonds. Hydrogen bonds occur between molecules, such as between the hydrogen of one water molecule and the oxygen of another.

Another misconception is that surface tension is the same as capillary action. Surface tension depends on cohesion at the surface of water. Capillary action depends on both cohesion and adhesion in narrow spaces. They are related, but they are used to explain different observations.

A third misconception is that water’s properties come from a single hydrogen bond. In biology, the power of hydrogen bonding comes from large numbers of these weak attractions acting together. One hydrogen bond is weak and temporary, but many hydrogen bonds can strongly influence behavior at the level of droplets, xylem columns, and water surfaces.

Finally, remember that water is not “sticky” in a vague way. In AP Biology, “sticky” has a molecular explanation: partial charges in polar water molecules allow hydrogen bonds and other attractions to form. The exam rewards precise cause-and-effect language.

## Key Point Checklist

This article has covered the following key knowledge points:

- Water is polar because oxygen attracts shared electrons more strongly than hydrogen.
- Oxygen in water has a partial negative charge, while hydrogen atoms have partial positive charges.
- Hydrogen bonds form between water molecules due to attraction between opposite partial charges.
- Cohesion is attraction between water molecules.
- Adhesion is attraction between water and other polar or charged substances.
- Cohesion helps water molecules remain connected in droplets and in xylem columns.
- Adhesion helps water stick to surfaces such as glass and plant xylem walls.
- Capillary action results from cohesion and adhesion in narrow tubes or spaces.
- Surface tension is caused by cohesive forces among water molecules at the surface.
- Water striders and other lightweight insects can stand on water because of high surface tension.
- Water movement upward in plant xylem is linked to cohesion, adhesion, capillary action, and transpiration.
- AP Biology answers should connect molecular structure to property and then to biological function.

## Key Terms and Concepts

- polar molecule
- polar covalent bond
- hydrogen bond
- cohesion
- adhesion
- capillary action
- surface tension
- transpiration


Structure of water and hydrogen bonding - Cohesion, adhesion, surface tension