Topic A1.1 - Water

Standard Level & Higher Level

Water as the Medium for Life

A1.1.1—Water as the medium for life. Students should appreciate that the first cells originated in water and that water remains the medium in which most processes of life occur. 

Water is an essential component for life on Earth. It played a crucial role in the origin of the first cells and continues to be the primary medium in which most processes of life occur. Let's explore the significance of water as the medium for life and its unique properties that make it so vital for living organisms.

The Origin of Cells in Water:

The first cells, believed to have emerged around 3.8 billion years ago, originated in water. This liquid medium provided a stable environment for the complex chemical reactions necessary for the formation of life. The presence of water allowed organic molecules to interact and eventually form primitive cells, marking the beginning of life as we know it.

Water's Role in Biological Processes:

Water serves as a crucial medium for countless biological processes that sustain life. From the simplest organisms to complex multicellular organisms, water plays a fundamental role in various physiological functions. Some key aspects include:

a) Solvent for Biochemical Reactions:

Water's unique properties as a solvent make it an ideal medium for biochemical reactions. It can dissolve and transport a wide range of substances, such as nutrients, gases, and waste products, facilitating metabolic processes within cells.

b) Facilitator of Cellular Functions:

Water enables cellular functions, including nutrient uptake, chemical reactions, and waste removal. It provides a medium for cellular organelles to carry out their specific roles effectively.

c) Temperature Regulation:

Water has a high specific heat capacity, meaning it can absorb and retain a significant amount of heat without undergoing drastic temperature changes. This property allows water to act as a temperature regulator, helping to stabilize the internal environment of organisms and maintain homeostasis.

d) Medium for Transport:

In multicellular organisms, water serves as a medium for the transport of essential substances, such as nutrients, hormones, and gases, throughout the body. It is a key component of bodily fluids like blood, lymph, and sap, ensuring the efficient delivery of vital molecules to cells and tissues.

Water's Unique Properties:

Water possesses several unique properties that contribute to its significance as a life-sustaining medium:

a) High Polarity:

Water is a polar molecule, with oxygen being slightly negatively charged and hydrogen being slightly positively charged. This polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules, facilitating the dissolving and transport of various substances critical for biological processes.

b) Cohesion and Surface Tension:

Due to hydrogen bonding, water exhibits cohesion, meaning water molecules tend to stick together. This property gives water a high surface tension, allowing some organisms to walk on water or enabling capillary action in plants.

c) Density Anomalies:

Unlike most substances, water reaches its maximum density at 4°C. As water cools further, it expands and becomes less dense, eventually forming ice. This anomaly of density changes helps aquatic organisms survive in colder environments by allowing ice to float, insulating the underlying liquid water and providing a stable habitat.

In conclusion, water's role as the medium for life is undeniable. From the origin of cells to the support of vital biological processes, water's unique properties make it essential for the existence and functioning of living organisms. Understanding the significance of water will provide a foundation for exploring further concepts in biology.

Hydrogen Bonds in Water

A1.1.2—Hydrogen bonds as a consequence of the polar covalent bonds within water molecules. Students should understand that polarity of covalent bonding within water molecules is due to unequal sharing of electrons and that hydrogen bonding due to this polarity occurs between water molecules. Students should be able to represent two or more water molecules and hydrogen bonds between them with the notation shown below to indicate polarity. 

The polarity of water molecules arises from the unequal sharing of electrons within its covalent bonds. Understanding this polarity is essential to comprehend the formation of hydrogen bonds between water molecules. Let's explore how the polar covalent bonds within water molecules give rise to hydrogen bonding and how this phenomenon can be represented.

Polarity of Covalent Bonds in Water:

Water (H2O) consists of two hydrogen atoms and one oxygen atom. The oxygen atom has a higher electronegativity than hydrogen, resulting in an uneven distribution of electrons. As a result, oxygen attracts the shared electrons more strongly, acquiring a partial negative charge (δ-) while the hydrogen atoms carry partial positive charges (δ+). This uneven charge distribution gives water its polarity.

Hydrogen Bonding between Water Molecules:

Due to the polarity of water molecules, hydrogen bonding occurs between neighboring water molecules. Hydrogen bonds form when the partially positive hydrogen atom of one water molecule is attracted to the partially negative oxygen atom of another water molecule. These bonds are weaker than covalent bonds but are still significant in determining water's unique properties.

Representation of Hydrogen Bonds:

Hydrogen bonds between water molecules can be represented using a notation that indicates the polarity. The notation involves drawing two or more water molecules and illustrating hydrogen bonds as dotted lines between the oxygen atom of one molecule and the hydrogen atom of another molecule. This representation highlights the intermolecular interactions that contribute to the cohesive and adhesive properties of water.

Example notation:

H H

\ /

O--O

In the notation above, the dashed lines indicate the hydrogen bonds between water molecules, emphasising the electrostatic attraction between the partially positive hydrogen and the partially negative oxygen.

Understanding the concept of hydrogen bonding in water is crucial because it contributes to several of water's unique properties. These properties include its high boiling and melting points, high surface tension, and the ability to dissolve various substances necessary for life.

By comprehending how hydrogen bonds form as a consequence of the polar covalent bonds within water molecules, students can appreciate the significance of these intermolecular forces in shaping the behavior and characteristics of water, ultimately playing a vital role in biological processes.

Cohesion of Water Molecules and its Consequences for Organisms

A1.1.3—Cohesion of water molecules due to hydrogen bonding and consequences for organisms Include transport of water under tension in xylem and the use of water surfaces as habitats due to the effect known as surface tension. 

The cohesion of water molecules, resulting from hydrogen bonding, plays a crucial role in various biological phenomena. This intermolecular attraction has significant consequences for organisms, including the transport of water under tension in xylem and the utilization of water surfaces as habitats due to surface tension. Let's explore these aspects in detail.

Cohesion and Hydrogen Bonding:

Cohesion refers to the attraction between molecules of the same substance. In the case of water, cohesion arises from the formation of hydrogen bonds between water molecules. These hydrogen bonds occur between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another water molecule. The cohesive forces resulting from hydrogen bonding enable water molecules to stick together.

Transport of Water under Tension in Xylem:

Xylem is a specialized tissue in plants responsible for transporting water from the roots to the rest of the plant. Cohesion and the phenomenon of water's high surface tension enable this remarkable process. As water evaporates from the leaves through small pores called stomata, a negative pressure or tension is created within the xylem. This tension pulls water upward in a continuous column due to the cohesive forces between water molecules. This phenomenon, known as the cohesion-tension theory, allows plants to overcome gravity and transport water against it, ensuring the supply of water to all parts of the plant.

Surface Tension and Use of Water Surfaces as Habitats:

Surface tension is the cohesive force acting on the surface of a liquid, resulting from the attraction between molecules at the liquid-air interface. Water exhibits a higher surface tension compared to many other liquids due to its hydrogen bonding properties. This has important implications for various organisms.

a) Aquatic Organisms:

Water's high surface tension enables certain organisms to utilize water surfaces as habitats. For example, some insects like water striders are able to walk or "skate" on water due to the cohesive forces between water molecules. Additionally, small organisms like pond skaters and water spiders take advantage of the water's surface tension to create air pockets for respiration or to capture prey.

b) Capillary Action:

Surface tension also facilitates capillary action, the movement of water against gravity in narrow tubes or structures. In plants, capillary action aids in the upward movement of water from the roots to the stems and leaves, contributing to the overall transport of water and nutrients.

Understanding the cohesion of water molecules and the consequences it has for organisms provides insights into the remarkable adaptations and functionalities observed in nature. The cohesive forces due to hydrogen bonding allow for the transport of water in plants, while surface tension provides opportunities for organisms to utilize water surfaces as habitats.

By comprehending these concepts, students can appreciate the significance of cohesion and surface tension in shaping the behavior of water and its impact on the survival and functioning of various organisms.

Adhesion of Water and Its Impacts for Organisms

A1.1.4—Adhesion of water to materials that are polar or charged and impacts for organisms. Include capillary action in soil and in plant cell walls. 

The adhesion of water, particularly to materials that are polar or charged, is a critical phenomenon with significant implications for organisms. Adhesion allows water to cling to surfaces, and this property plays a vital role in various biological processes. Let's explore the adhesion of water to polar or charged materials and its impacts on organisms, including capillary action in soil and plant cell walls.

Adhesion of Water to Polar or Charged Materials:

Water molecules exhibit a strong attraction, known as adhesion, to materials that possess polarity or charge. This adhesion occurs due to the polarity of water molecules and their ability to form hydrogen bonds with other molecules or ions. When water comes into contact with polar or charged surfaces, the partially negative oxygen atoms are attracted to positively charged ions or regions, leading to adhesion.

Capillary Action in Soil:

Capillary action, driven by water's adhesion and cohesion properties, plays a crucial role in soil moisture distribution. Soil contains tiny spaces or pores, and when water is present, it adheres to the soil particles and is drawn upward against gravity. The adhesive forces between water and the polar surfaces of soil particles allow water to climb up narrow capillary tubes or spaces. This capillary action ensures that water can reach plant roots situated deeper in the soil, supporting plant growth and nutrient uptake.

Capillary Action in Plant Cell Walls:

In plants, adhesion of water to the surfaces of cell walls facilitates capillary action within the plant's vascular system. The cell walls of xylem vessels and tracheids, composed of polar molecules like cellulose, attract and adhere to water. As water is transported upward through the xylem, adhesion helps counteract the force of gravity, allowing water to move against it. This upward movement of water, combined with cohesion between water molecules, enables the efficient transport of water and dissolved nutrients from the roots to the rest of the plant.

Impacts for Organisms:

The adhesion of water to polar or charged materials has several impacts on organisms:

a) Nutrient Uptake:

Adhesion helps water adhere to the surfaces of soil particles and facilitates the movement of water and dissolved nutrients towards plant roots. This allows plants to absorb essential nutrients from the soil, supporting their growth and development.

b) Cellular Processes:

Adhesion is crucial for cellular processes that rely on water. For example, in organisms with specialized cells, such as epithelial cells in animals or stomatal cells in plants, adhesion enables the proper functioning of these cells by ensuring the transport of water and ions across cell membranes.

c) Surface Interaction:

Adhesion to polar or charged surfaces influences the behavior of organisms living in aquatic environments. For instance, microorganisms can adhere to surfaces like rocks or aquatic plants, forming biofilms that play important roles in nutrient cycling and ecosystem functioning.

Understanding the adhesion of water to polar or charged materials provides insights into how water interacts with various surfaces in biological systems. The ability of water to adhere to surfaces is essential for processes such as capillary action in soil and plant cell walls, facilitating the movement of water and nutrients critical for the survival and growth of organisms.

Solvent Properties of Water and its Role in Metabolism and Transport

A1.1.5—Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals. Emphasize that a wide variety of hydrophilic molecules dissolve in water and that most enzymes catalyse reactions in aqueous solution. Students should also understand that the functions of some molecules in cells depend on them being hydrophobic and insoluble. 

Water's solvent properties are closely linked to its role as a medium for metabolism and transport in both plants and animals. As a versatile solvent, water has the ability to dissolve a wide variety of hydrophilic (water-loving) molecules, facilitating metabolic reactions and serving as a medium for transportation. However, it is important to note that some molecules in cells rely on their hydrophobic (water-repelling) nature and insolubility for their specific functions. Let's explore the significance of water as a solvent and its impact on cellular processes.

Water as a Solvent for Hydrophilic Molecules:

Water's ability to dissolve hydrophilic molecules is vital for numerous biological processes:

a) Metabolic Reactions:

Most metabolic reactions, including those catalyzed by enzymes, occur in aqueous solutions. Water serves as the medium for these reactions, providing a suitable environment for enzymes to catalyze biochemical transformations necessary for cellular functioning.

b) Transport in Plants and Animals:

Water's solvent properties enable the transportation of various substances within organisms. In plants, water dissolves minerals and nutrients from the soil and transports them through the xylem tissue to different parts of the plant. In animals, water in the bloodstream serves as a solvent for carrying oxygen, nutrients, hormones, and waste products to and from cells and organs.

Hydrophobic Molecules and Insolubility:

While water is an excellent solvent for hydrophilic molecules, certain molecules in cells depend on their hydrophobic nature and insolubility for specific functions:

a) Lipids:

Lipids, such as fats and oils, are essential components of cell membranes and serve as energy stores. These molecules are hydrophobic and insoluble in water. The insolubility of lipids in water allows them to form the hydrophobic core of cell membranes, providing a barrier between the aqueous interior and the external environment.

b) Nonpolar Gases:

Nonpolar gases, like oxygen and carbon dioxide, are relatively insoluble in water. This property is crucial for their exchange across respiratory surfaces, such as the alveoli in the lungs or the gills in aquatic organisms. The low solubility of these gases in water allows for efficient diffusion across these surfaces.

c) Hydrophobic Signaling Molecules:

Certain signaling molecules, such as steroid hormones, are hydrophobic and require transport proteins in the bloodstream to travel through the aqueous environment of the body. These molecules bind to carrier proteins, enabling their transport to target cells where they can regulate gene expression and other cellular processes.

Understanding water's role as a solvent for hydrophilic molecules is essential for comprehending metabolic processes and transportation within organisms. While water is a versatile solvent, it is equally important to recognize the significance of hydrophobic molecules and their insolubility in water for specific cellular functions. This balance between hydrophilic and hydrophobic properties contributes to the diverse range of molecules and their interactions within living systems.

By grasping the solvent properties of water and the interplay between hydrophilic and hydrophobic molecules, students can gain a deeper understanding of the intricate workings of cellular processes and the importance of water as a medium for metabolism and transport in plants and animals.

Physical Properties of Water and Consequences for Animals in Aquatic Habitats

A1.1.6—Physical properties of water and the consequences for animals in aquatic habitat. Include buoyancy, viscosity, thermal conductivity and specific heat capacity. Contrast the physical properties of water with those of air and illustrate the consequences using examples of animals that live in water and in air or on land, such as the black-throated loon (Gavia arctica) and the ringed seal (Pusa hispida). 

Water possesses unique physical properties that have significant consequences for animals living in aquatic habitats. These properties, including buoyancy, viscosity, thermal conductivity, and specific heat capacity, contrast with those of air and play a crucial role in shaping the adaptations and behaviors of aquatic organisms. Let's explore these physical properties and their consequences, illustrated by examples of animals that inhabit water versus those that live in air or on land, such as the black-throated loon (Gavia arctica) and the ringed seal (Pusa hispida).

Buoyancy:

Water's property of buoyancy allows animals to float or maintain a neutral buoyancy in aquatic environments. The buoyant force exerted by water counteracts the force of gravity on the animal's body. This is particularly advantageous for aquatic organisms, reducing the energy required for movement and enabling them to support their body weight.

Example - Black-throated Loon:

The black-throated loon, a diving bird, relies on buoyancy to swim and dive underwater. Its streamlined body shape, along with its webbed feet and dense bones, helps maintain buoyancy while diving and enables efficient movement through water.

Viscosity:

Water has a higher viscosity compared to air, which refers to its resistance to flow. This property affects the movement of organisms through water and influences their feeding strategies and locomotion.

Example - Ringed Seal:

The ringed seal, an Arctic marine mammal, is well adapted to its aquatic habitat. Its streamlined body, coupled with a thick layer of blubber, reduces drag while swimming through the viscous water. This adaptation allows the seal to navigate efficiently and hunt for prey in its icy environment.

Thermal Conductivity:

Water has a higher thermal conductivity than air, meaning it can transfer heat more efficiently. This property impacts the thermal regulation strategies of aquatic organisms, helping them cope with temperature variations in their environment.

Example - Ringed Seal:

The ringed seal, living in frigid Arctic waters, relies on the thermal conductivity of water to retain body heat. Its thick layer of blubber acts as insulation, reducing heat loss to the cold water and enabling the seal to maintain a stable body temperature.

Specific Heat Capacity:

Water has a high specific heat capacity, meaning it can absorb and store a significant amount of heat energy without a drastic change in temperature. This property helps stabilize aquatic environments and protects organisms from rapid temperature fluctuations.

Example - Black-throated Loon:

The black-throated loon relies on the specific heat capacity of water for its breeding behavior. It builds its nest close to water bodies, ensuring the surrounding water regulates the nest's temperature. If the nest becomes too hot or cold, the parent birds can bring water to wet the nest and maintain a suitable environment for their eggs.

Understanding the physical properties of water and their consequences for animals in aquatic habitats allows us to appreciate the remarkable adaptations and behaviors of aquatic organisms. By contrasting the properties of water with those of air, we can observe how aquatic organisms have evolved specific traits to thrive in their watery environments.

The black-throated loon and the ringed seal exemplify how animals utilize the physical properties of water to their advantage. Whether it is buoyancy, viscosity, thermal conductivity, or specific heat capacity, these properties shape the characteristics and behaviors of organisms and contribute to their survival in aquatic ecosystems.

Advanced Higher Level Only

Extraplanetary Origin of Water on Earth and Reasons for its Retention

A1.1.7—Extraplanetary origin of water on Earth and reasons for its retention. The abundance of water over billions of years of Earth’s history has allowed life to evolve. Limit hypotheses for the origin of water on Earth to asteroids and reasons for retention to gravity and temperatures low enough to condense water. 

The presence of abundant water on Earth over billions of years has played a pivotal role in the evolution and sustenance of life. The origin of Earth's water is believed to have extraplanetary sources, primarily from asteroids, and its retention on the planet can be attributed to the forces of gravity and favorable temperatures for water condensation. Let's explore the hypotheses for the origin of water on Earth and the reasons behind its retention.

Origin of Water on Earth:

The origin of Earth's water is a subject of ongoing scientific investigation. One prominent hypothesis suggests that water was delivered to Earth through extraplanetary sources, such as asteroids or comets, during the early stages of the planet's formation. These celestial bodies contain significant amounts of water in the form of ice.

Asteroids: Asteroids are rocky bodies that orbit the Sun, and some of them contain water-rich minerals. It is hypothesized that impacts from water-rich asteroids could have contributed to the delivery of water to Earth's surface.

Retention of Water on Earth:

The retention of water on Earth can be attributed to two main factors:

a) Gravity: Earth's gravitational force plays a crucial role in retaining water on its surface. Water, being a liquid, is subject to the force of gravity, which prevents it from escaping into space. The gravitational pull of the planet keeps water molecules bound to Earth, allowing for the formation of oceans, lakes, rivers, and other bodies of water.

b) Favorable Temperatures for Condensation: Water exists in various forms across different temperature ranges. On Earth, temperatures are low enough to allow water vapor to condense into liquid water, promoting the formation of bodies of water. The presence of a relatively stable temperature range, thanks to Earth's position in the habitable zone around the Sun, has enabled the retention of liquid water on the planet's surface.

The Role of Atmospheric Pressure: Earth's atmosphere, with its specific composition and pressure, also contributes to the retention of water. The presence of gases, such as carbon dioxide, methane, and water vapor, helps create a greenhouse effect, trapping heat and maintaining temperatures suitable for water to remain in its liquid form.

The combination of extraplanetary delivery of water through asteroids and the retention of water on Earth due to gravity and favorable temperatures has allowed water to persist and accumulate on the planet's surface. This abundance of water has been fundamental in creating the conditions necessary for life to evolve and thrive over the course of Earth's history.

By understanding the extraplanetary origins of Earth's water and the factors that have facilitated its retention, we gain insights into the fundamental role that water plays in supporting and sustaining life on our planet.

The Relationship between the Search for Extraterrestrial Life and the Presence of Water

A1.1.8—Relationship between the search for extraterrestrial life and the presence of water. Include the idea of the “Goldilocks zone”. 

The search for extraterrestrial life is closely tied to the presence of water, as water is considered a crucial ingredient for life as we know it. Scientists and researchers focus their efforts on identifying environments that possess liquid water, as it provides a potential habitat for the development and sustenance of life beyond Earth. This relationship is often discussed in the context of the "Goldilocks zone," also known as the habitable zone.

The Goldilocks Zone:

The Goldilocks zone refers to the region around a star where conditions are neither too hot nor too cold for liquid water to exist on a planetary surface. This zone is often considered the ideal range of distances from a star where a planet could potentially support life.

Importance of Liquid Water: Liquid water is crucial for the chemistry of life as we understand it. It serves as a solvent for biochemical reactions, acts as a medium for transport and interaction within cells, and provides an environment where complex organic molecules can form and interact. Therefore, the presence of liquid water is seen as a key factor in determining the potential habitability of a planet or moon.

Habitability and Water:

The relationship between the search for extraterrestrial life and the presence of water is founded on the understanding that water plays a vital role in supporting life's fundamental processes. Scientists explore various environments in our solar system and beyond, assessing the potential for liquid water as an indicator of habitability. Some key considerations include:

Planetary Conditions: Researchers investigate planetary bodies with characteristics that could support liquid water, such as those within the habitable zone of their respective star systems. This includes planets orbiting within the not-too-hot, not-too-cold range where water can exist in its liquid state.

Moons and Subsurface Oceans: Beyond planets, moons that have subsurface oceans, such as Jupiter's moon Europa or Saturn's moon Enceladus, are of significant interest. These subsurface oceans, kept in liquid form by tidal forces and geothermal activity, offer potential habitats for life.

Extremophiles as Analogues: Scientists study extremophiles, organisms capable of thriving in extreme environments on Earth, to gain insights into the potential for life in harsh conditions. These studies help expand the understanding of the adaptability of life forms and the conditions in which they may exist beyond Earth.

By focusing on the presence of liquid water and identifying environments within the habitable zone, scientists aim to increase the likelihood of finding extraterrestrial life. However, it's important to note that the search for life is not limited exclusively to water-dependent forms but also explores the potential for alternative biochemistries and environments.

Understanding the relationship between the search for extraterrestrial life and the presence of water provides a foundation for exploring habitability in our universe. By investigating the conditions that allow liquid water to exist, scientists broaden their search for potentially habitable environments and increase the chances of discovering life beyond our own planet.